Gear-couplings are the king of coupling types. They can do things that many other couplings cannot do, can only do with difficulty or with expensive modifications and de-rating. Gear couplings are more power intensive, offer more modifications, and a wider size, torque bore range than any other type, and can perform at extremely high speeds. Gear couplings have axial slide capability, low speed high torque capability, shifter capability and spindle capability not found in other couplings. They are easily modified for shear pin service, floating shaft type, vertical type, insulated type, limited end float, and can have a brake drum or disc added. While those latter items may be available on other couplings, it is usually easier and less costly to modify the gear coupling. With all these advantages the gear coupling is used on twice as many applications versus the nearest competitor type.
Gear couplings can also perform at extremely high rates of speed. As implied by the name, gear couplings use the meshing of gear teeth to transmit the torque and to provide for misalignment. External gear teeth are cut on the circumference of the hub. Both toothed hubs fit inside the ends of a tubular sleeve that has matching gear teeth cut around its interior circumference, with each tooth extending axially the full length of the sleeve. Hub and sleeve teeth mesh, so torque transfers from the driving hub’s teeth to the sleeve teeth and back to the driven hub’s teeth.
Gear couplings achieve their misalignment capability through backlash in the teeth, crowning on the tooth surfaces, and major diameter fit. Backlash is the looseness-of-fit that results from gear teeth being narrower than the gaps between the teeth. In addition to contributing to the misalignment capabilities, the backlash provides space for the lubricant. The loose fit provides misalignment capability by allowing the sleeve to shift off-axis without binding against the hub teeth. Some gear couplings have more backlash than others. Those with the least (roughly one-half of the backlash present in those with the most) are known as “minimum backlash” couplings. Some users prefer this type, most prefer normal backlash. Crowning, or curving the surface of the hub teeth, further enhances this capability. The crowning can include tip crowns, flank crowns, and chamfers on the sharp edges. This also helps improve tooth life by broadening the contact area along the “pitch line” (where the teeth mate and transfer torque), thereby reducing the pressure of torque forces. In addition, it prevents the sharp squared edges of the tooth from digging in and locking the coupling. Vari-Crown, which varies the curvature radius along the tooth flank, maintains greater contact area between teeth during misalignment compared with standard crowning, and reduces those stresses that cause wear. Note that crowning applies to hub teeth only; sleeve teeth are straight except for a chamfer on the minor diameter edge.
While the hub and sleeve teeth are cut to fit loosely side to side, they are cut to fit closely where the tip diameter of the hub teeth meet the root diameter of the gaps between the sleeve teeth. That is called a major diameter fit. When the coupling is not rotating, those two surfaces rest upon each other if it is a horizontal installation. Minor diameter fits (where the tips of the sleeve teeth meet the root diameter of the hub teeth) are purposely avoided, because a close fit here would preclude suitable misalignment capability and torque transmission capability.
It was noted earlier that gear couplings are power intensive. That means more torque transmitted per pound of coupling weight and per cubic inch of space consumed than other couplings. In many cases the gear coupling has more torque capability than the shaft can transmit. The resulting relatively small size of the gear coupling allows the addition of attachments without having the coupling grow to impracticable proportions. It also allows the OEM designer more latitude to locate the coupling in small, out-of-the-way places with confidence that it will be reliable. Gear couplings eventually wear, but rarely to a catastrophic failure. They can be sized to make sure that wear life is consistent with the rest of the machine design.
Gear coupling sleeves can be a single piece, termed a “continuous sleeve”, or can be split laterally (radially) into two half sleeves, one on each hub. The split version is termed a “flanged sleeve”, because each half has a flanged end, drilled for bolt holes, which allows them to be bolted together.
Because the continuous sleeve needs neither flanges nor bolts, it provides the advantage of making the coupling lighter and smaller in diameter than comparably rated flange types. With that comes lower inertia values, which helps lighten motor load during start-up. Bolt stress, which can be a weak point in some applications, is eliminated. The absence of bolts is an advantage in high-speed applications, because bolts add potential points of unbalance and bolted connections can be another point of non-concentricity.
When two halves of a flanged sleeve are bolted together, the bolting becomes an important part of the power transmission path. Best designs have the power transmitted across the face by friction, in which case the bolts simply provide enough clamping force to provide face friction. Other designs could allow the bolts to carry the load in shear, but those are in the minority. Both cases require a proper analysis of the multiple loads on the bolts. In addition the bolt bodies may provide the centering action to pilot the two halves of the coupling.
Bolts can either be exposed, or shrouded for safety reasons. However, with the advent of OSHA coupling-guard requirements, shrouding becomes unnecessary. The two types also have different windage loss and that affects high speed applications. Windage losses cause a heat generation inside the coupling guard. Note that flange bolts are specially made for their purpose, and should never be replaced with common hardware-store bolts. Flanged sleeve gear couplings built to American Gear Manufacturers Association (AGMA) dimensional standards will mate half-for-half with all other gear couplings made to those same standards.
While AGMA standards are U.S. based, many European manufacturers build to match the dimensions. However, matching dimensions include the interface only, such as outside flange diameter, number of boltholes, bolt hole size, bolt circle, and flange thickness. Although length-through-bore of the hub is often identical as well, torque and bore capability are likely to be different and should be compared carefully.
Flex Planes and Misalignment Capability
Planes of flexibility (“flex planes”) are those pivot points along the shaft-to-shaft connection where rigid components engage but can move independently of each other The standard gear coupling (two toothed hubs engaging opposite ends of the same rigid sleeve) has two flex planes, one at each hub-to-sleeve gear mesh. When both flex planes work together in series, flexing in the same direction, they give the gear coupling an angular misalignment capability of up to 1½° at each flex plane.
This standard configuration is called “full flex” or “double engagement” coupling.
The full-flex gear coupling, with two flex planes in series flexing in opposite directions, allows for parallel (radial) misalignment of 0.055 to 0.165 inches in standard models with short sleeves. The longer the sleeve (i.e. the greater the axial distance from one flex plane to the other), the greater the parallel misalignment. The greatest parallel capability results from floating shaft, spacer and spindle versions, described later, which greatly lengthen the distance between flex planes.
Gear couplings can be configured with only one flex plane, for applications where parallel misalignment capability is unwanted. In flanged type couplings, this is accomplished by using a single-piece flanged hub with no teeth, as the rigid half, bolted to a flexible half that uses a standard flanged sleeve with teeth and a standard hub with crowned teeth. These are called “flex-rigid”, “single-engagement” or sometimes “half couplings”. In continuous-sleeve couplings, a flex-rigid configuration is accomplished by mating the sleeve at the rigid end with a hub having straight teeth that fits into the sleeve like a spline shaft into a spline hub. While the full flex design is the most popular in gear couplings, flex rigid designs are often useful in systems with three bearings or floating shafts. Sometimes one flex-rigid coupling is used in series with another flex-rigid coupling at a distance to allow much more parallel misalignment.
While gear couplings will normally provide from ½° to 1½° of angular misalignment per flex half, they can be designed for up to 6° with reduced load capability and with accommodating grease seals.
Gear couplings naturally accommodate axial (in-out) shaft movement better than other competing designs, because their hub teeth easily slide along their sleeve teeth with no effect on coupling operation or torque load capability. Axial movement often results from thermal expansion/contraction of the shaft, as in hot applications, or a rotor seeking its magnetic centers (floating rotor). Thrust bearings can limit or prevent shaft movement at the coupling end, but if positioned at the far end of the machine, they can force the shaft movement back toward the coupling. The amount of axial displacement the gear coupling can handle depends primarily on the length of the sleeve, and specials are available for long sliding application.
The Gear Coupling Tooth
The gear coupling tooth has evolved over many years. The first gear couplings had straight teeth, and depended purely on backlash to achieve misalignment. Later improvements included tooth crowning that increased misalignment capability and coupling life. The original tooth form followed the spur gear form with modification. Various pressure angles were used that walked the line between life and strength. The 40° pressure angle tooth was chosen for strength. It proved to have problems with wear life and with reactionary loading on the machinery. Eventually the 20° pressure angle tooth became the standard, and it still is the standard. Some 25° teeth are used to achieve added strength for special designs. The additional strength of today’s materials alleviates the need for 40° teeth and still provides low sliding friction.
The gear coupling tooth, like the spline tooth, is not a full height tooth. Where the spline is 50% height, the gear coupling tooth is about 80%. Gear coupling teeth do not need full height because the torque load is carried at the pitch line of the tooth and many teeth are in contact with each other in the hub and the sleeve to carry the load. The number of teeth in contact is a function of the true form of the teeth. If all teeth in the hub and sleeve are identical the maximum number will be in contact. As the teeth wear into place the more teeth come into contact. Therefore initial tooth wear makes the coupling stronger, but can increase the friction loading too.
The strength of the gear tooth is the subject of many questions in determining the amount of load to be carried. The tooth is the strongest of all the elements of a gear coupling. The tooth strength is calculated as a bending moment at the root of the tooth, the shear strength at the pitch line, and the Hertzian loading at the contact surface. All of these forces act concurrently.
The most likely failure mode of a gear coupling tooth is that which comes from wear rather than any other factor. As the teeth wear, they move from being the strongest element to being the weakest element.
Severe misalignment that causes a lock up of the teeth will also result in premature failure. Most other loading on the coupling will not result in failed teeth.
Lubricant must always be available in the tooth mesh. The lack of lubricant will, of course, cause the coupling to fail almost instantly. The gear coupling is fitted together so as to prevent the lubricant from leaking. Most gear couplings are lubricated with grease. The sleeve to hub interface at the boundaries will need elastomer O-rings, gaskets, or labyrinths to prevent grease leakage. (Note that O-ring material might limit the coupling’s ambient temperature capability.) When oil lubrication is used, it is usually a continuous flow through the tooth mesh, but can be a batch lube in some applications. Oil lubrication is a special case.
Misalignment may allow grease to leak out the seal surface, or some modifications may need a wiper seal rather than an O-ring. One type of flange coupling uses a high misalignment seal with more flex than the regular seal. The seals can be held in place by several means. The Oring is the simplest; it fits into a groove in the sleeve.
The continuous sleeve coupling seal is held in place by a spiral ring. The seal has stiffeners molded into the inside face. It is a U or C shape that stays closed under load. It also provides the movement limit for the coupling and is actually rated to withstand an axial force.
Sometimes the seal holder is bolted to the coupling sleeve. This is always the case on couplings larger than size 9. It makes the assembly of the coupling to the shaft easier, and makes replacement of seals easier. The couplings with bolt on seal carriers are designated heavy duty (HD). Flange series couplings size 7 through 9 can be either the “HD” version or the plain version.
Remember that the coupling grease is not ordinary grease but is specially formulated so the oils do not separate from the soaps. The result is that the lubricant is contained within the needed space and sludge is not allowed to accumulate. Oil and soaps separate in ordinary lubricants because of centrifugal forces on the heavier particles. Use only coupling grease for best results.
Variations to Gear Couplings
1. Fill the Space between Shafts
Couplings often must fill a space between shafts as one of their primary attributes. It would seem a simple enough task, but not all couplings offer flexibility doing that job. This is another reason why the gear coupling is very popular.
The distance between shaft ends (BSE) will vary with different machine systems to accommodate design standards, product line alternatives,different motor frames and maintenance needs. The “BSE” dimension is important for all couplings. Gear couplings have the advantage of allowing a variable “BSE”. That variation can be achieved by machining the hub face or can be achieved by reversing one or both of the hubs. An infinite number of possibilities can be obtained from catalog minimum to catalog maximum. Note this gap (BSE) does not always affect the distance between flex planes unless the hubs are reversed. A combination of facing and reversing is possible too. All couplings have a certain “BSE” dimension variability, but few are able to tolerate as great a variance as gear couplings can.
3. Spacer Couplings
Spacer couplings consist of two flexible hub and sleeve assemblies i.e. a half coupling on both the driving and driven shafts. These are connected by a tubular center section of various lengths that can easily be removed to allow space for removal of the hub or other components on one side of the system without disturbing the hub or component mountings on the other side. The tubular center section can have flanged ends for bolting to hub flanged sleeves, or toothed ends that mate with hubs using continuous sleeves. Spacers are built to the standards of the rotating machinery builders. Pumps have several standard spacers such as 3½ inches, 7 inches and others. Compressors could have a different set of standard spacers. Spacers can serve to separate the flex planes and can be part of the torsional tuning of a coupling.
They have practical limits on length in regard to cost, weight and critical speed. The flanged hollow tube is machined to varying tolerances depending on speed and balance. As the tube gets longer, deflection of the unsupported center section forces the cylinder walls to be made thicker. As the walls get thicker the cost grows more and so does the weight. The weight then reduces the critical speed. That is a cross combination of events that eventually makes the spacer a poor choice. When the spacer becomes impractical, the next step is to use a floating shaft coupling to achieve the necessary spacing.
Floating shaft couplings consist of flex rigid couplings on both driving and driven shafts connected by a piece of solid shafting between the couplings. Usually the coupling hubs on the equipment ends are rigid while the two center hubs connected to the floating shaft are flexible.
While these two can be used to provide service spacing, the primary reason for a long floating shaft is to allow for greater radial misalignment between shafts. The secondary reason is to reach a long distance between the driver and the rotating equipment. Weight and critical speed are important considerations for floating shafts. They are found on bridge cranes and steel rolling mills.
The couplings and center shaft are designed as a unit to suit their specific application. The parameters include the usual torque and bore, but must include length and speed because, as in any spacer, critical speed and deflection are interrelated. These issues may require a larger diameter center shaft to reduce deflection. In that case the rigid hubs are on the floating shaft, taking advantage of the rigid hub’s greater bore capability, to accommodate the oversized center shaft.
Otherwise, the center shaft may need to be necked down (reduced) to fit a flex coupling hub. The rigid hub could also be placed on the outside to fit a shaft that is made larger than is necessary to carry torque, as would be the case with bending problems. The center shaft would be smaller to carry torque only and thus fit the flex hub. When the flex hub is on the center shaft it is called a marine style coupling. When the flex hub is on the equipment shaft it is called a reduced moment style. The floating shaft designer must always balance the effects of weight (which causes deflection) and diameter (which determines torque capacity and resists deflection but increases weight and cost).
5. Limited End Float Couplings
Gear couplings can be modified to allow shaft growth in the axial direction or to limit movement in the axial direction. Limiting the movement calls for a plate and possibly a button to be inserted between the coupling halves. As the shaft tries to move in the axial direction, it is stopped after moving a predetermined distance.
These are called limited end float couplings. They are necessary with sleeve bearing motors, a design commonly found in larger sizes of 200 horsepower or more. The same plates and
buttons are used on vertical couplings as explained below.
In addition to thermal growth, gear couplings can be arranged to slide great distances. Extra long sleeves enable the hub to slide 10 inches or more, either at rest or while in operation, to serve applications where equipment must be temporarily removed from the system and the coupling is the most suitable point of movement. Refiners, Jordan machines, and roll winders found in paper mills utilize this sliding capability. The Jordan coupling is a special variation that can move its hub relative to its shaft with a clamping mechanism.
Two dimensions are important when considering the slider coupling. One is the minimum BSE and the other is the total amount of slide. Those are in addition to the usual gear coupling requirements. If a Jordan is involved the amount of clamping movement is necessary to know.
Spindle couplings are special floating shaft gear couplings that are used in rolling mills. They are designed for high torque, shock loads, and high angular misalignment. They have replaceable wear parts and customized accessories. Spindle couplings also have some slide capability to adjust to the installation or operational requirements of rolling mills. The spindle coupling uses the continuous sleeve principal to reduce the overall outer diameter.
Gear couplings can also be equipped to block galvanic (electrical) currents, which can cause pitting and corrosion at the close running fits of gear teeth and other mechanical components. One half of the coupling is electrically insulated from the other half by adding insulating plates and bushings. It is not necessarily a high voltage insulator as found in wiring systems.
Modification for Special Needs
Both continuous sleeve and flanged sleeve gear couplings can operate in the vertical position with the addition of a vertical kit, which is a limited end float plate or plate-and-button that supports the loose weights above the coupling. The button is rounded to allow the load to transfer under misalignment. Therefore, load is transferred to the lower shaft and ultimately supported by a thrust bearing in that equipment. Since a gear coupling is normally a shrink or interference fit, the upper hub is fixed to the shaft as is the lower one. In a vertical-floating shaft coupling, where both outer hubs are rigid and inner hubs are flexible, the entire center rotor is loose weight that needs to be supported by a plate in the upper coupling and a plate and button in the lower coupling.
A special vertical coupling is the rigid adjustable pump coupling. This coupling is designed for use with vertical circulating pumps that need clearance adjustments in the impeller. As indicated in the coupling name, it is a rigid coupling with no teeth in either half, with no provision for misalignment. The entire rotor weight is hung from the motor or driver bearings. Special designs of hanging load gear couplings can provide misalignment capability.
Other Gear Coupling Special Configurations
Gear couplings can be configured to do special jobs. Possibilities include the shear pin, cutout shifter, and brake coupling. Shear pin couplings disconnect when subjected to predetermined torque overloads thus protecting other equipment. Torque overloads could come from stalls or cyclic overloads.
Cutout couplings allow the driving/driven halves to be disengaged without disassembling the coupling. They use a special sleeve in which the teeth are interrupted at one end by a flat-bottomed annular groove. When the sleeve is shifted axially to align the groove with the teeth of one hub that hub spins freely, disengaged from the torque transmission path. A cutout pin (set screw) holds the sleeve in engaged or disengaged positions. They can be used on a dual drive machine to isolate the unused driver, or for a turning gear that rotates heavy equipment when it is off line, and helps prevent a permanent set in the shaft. Automatic cutout is available for temporary disconnect “on the fly” to allow adjustment of relative position between driving/driven halves.
Brake Drums and Brake Discs
Gear couplings are easily modified for the attachment of a brake, which saves system space by eliminating a separate brake. In other situations putting the brake at the coupling prevents the high cyclic torque from reaching low torque shafts. Brake wheel couplings are often attached near the gearbox shaft since high gear inertia is in the box. The brake drum or disk is a piece of metal, machined to standard brake sizes and clamped between the coupling’s bolted flanged sleeves, requiring longer bolts. The coupling manufacturer does not include the brake and actuator.
Whenever a brake is installed in the system, it flags the need to check the stopping torque requirements. Stopping torque, like starting torque, depends on the amount of time that is available to stop or start. See the section on torque for a torque formula.
Moderate and High Speed Applications
As noted earlier, gear couplings are capable of very high speeds and high torque together. The limits have always been the need for lubrication of the mating gear surfaces and the need for balance. While high speeds increase the wear rate and can be the cause of high stresses within the coupling, the bigger issue is balance. Couplings operating at high RPM or high rim speed will cause vibration problems if they are not in balance.
A full discussion of balance will be found in another section of this handbook so only a few issues that relate specifically to gear couplings will be referenced here. Balance concerns itself with how the weight of the rotating mass or inertia is positioned or displaced relative to the center of rotation. If that weight is perfectly distributed around the center of rotation, the coupling is in balance. Since nothing is perfect in couplings, there is always a potential unbalance.
Coupling balance is achieved through design, manufacturing and remedial balancing machines. Off center bores, out of round circumferences, non-parallel sides, or even loose fits lead to mass displacement. In castings some of the potential unbalance could come from voids or air space internal to the casting. When a coupling consists of an assembly, the component design and the assembly process can result in an unbalance condition.
If the hub OD is not perfectly concentric with the hub bore the center of mass and center of rotation will be different. This means the gear teeth must be carefully cut with a pitch diameter concentric with the bore. That is controlled by the arbor or mandrel used on the hobbing machine and the concentricity of the pilot bore. The hub face must be perpendicular to the bore or to the hub OD. If it is not it becomes a trapezoid. Trapezoidal hubs have poor weight distribution and therefore unbalance.
Sleeves must likewise be concentric with the hub bore at the pitch diameter, the OD and at the pilot fits if any exist. Flanged sleeves must have a concentric bolt circle as well as a proper hole size and location. Flange-to-flange alignment before bolting will have a big effect on the balance of the assembled coupling.
Once the equipment is designed and the tolerances are established, it is possible to calculate the mass displacement of each component. The mass displacement of each component is added algebraically by a method that is called the square root of the sum of the squares. The total mass displacement can then be called the potential unbalance of the coupling. That total unbalance of the coupling could then be compared to recognized standards to see if it is acceptable. Refer to AGMA standard 9000-C90 for more on this subject.
Component & Assembly Balancing
It is unlikely that calculation of the mass displacement would be sufficient to satisfy a high-speed specification. That leads to the next process. Each component or piece of the coupling could be subjected to a balancing procedure on a balancing machine. Single-plane or two-plane balancing is also a consideration. If the coupling’s width-to-diameter ratio is 1:1, or greater in diameter, single-plane balancing is sufficient. If width (axial dimension) is greater, two-plane balancing is needed. (See chapter on balancing for more information.) Machine balancing results in adding or subtracting weight from the piece to counter the unbalanced weight and lessen the unbalance. The remaining unbalance of the part while on the balance machine is called the residual unbalance. The coupling can be assembled after component balancing and left at that potential unbalance. The total unbalance of the assembly at that point would depend on the distribution of the individual high points within the assembly. The worst case would be to end up with all heavy points in one quadrant.
For further reduction of unbalance, the coupling could be assembled and returned to the balance machine, again with corrective adding or subtracting of weight. The result would be called an assembly balanced coupling. With these, all individual pieces are match marked before the coupling is disassembled so they can be reassembled exactly the same way on the users equipment.
A gear coupling is not easily assembly balanced. First the coupling must be assembled with tight fits between the hub teeth and sleeve teeth so that loose parts will not fool the balancing machine. After the coupling is balanced, the teeth are relieved so the coupling can be installed in a system with possible misalignment.
The final balance, after the coupling is removed from the machine, will be affected by the concentricity run-out and bearing surfaces of the mandrels, arbors and mounting devices of the balance machine. The coupling, unlike machinery rotors, is not balanced on its own shaft. A half coupling might be balanced on the equipment rotor, but then the two half couplings from the two different rotors must be joined together. Why should one worry so much about balance? The balance is critical on high-speed applications to prevent destructive vibration. Different applications have different definitions of high or low speed, but generally for couplings, anything greater than 3000 RPM is high speed.
High Speed Gear Couplings
Most high-speed gear couplings are spacer types, that would acknowledge the need for maintenance on the connected equipment. Two important attributes of high-speed couplings are lightweight and low inertia. If the coupling is to be accelerated from zero to 10,000 or 15,000 rpm the torque required to reach those speeds quickly is substantial if inertia is allowed to be too high. High-speed machines are sensitive to overhung weights too. Everything is built for speed, which means small, light and precise.
We mentioned that high-speed couplings are precision made to tight tolerances. They are also made with ground bores, body fitted bolts and reamed holes in the flanges. Since the couplings are highly stressed the materials are magnetic particle inspected to make sure of the integrity of the piece. The material may be standard 4140 steel, but it often has papers to prove its strength and chemical composition. Hubs are attached to the shaft by hydraulic fits on a taper in the really high-speed units. That eliminates keys and keyways that could affect the balance. Other methods might include an integral flange on the rotor that bolts up to a marine style spacer coupling.
Sometimes the need for maintainability or rigidity forces the coupling to be a marine type of spacer coupling. Marine style refers to the tooth location not the application. In a marine type unit the gear teeth are on the spacer section not the hub section. This increases the overhung moment so a trade off is being made.
Materials for High Speed Units
While balance is most important to high speed gear couplings, it must also be noted that high speed has the potential for high wear of the teeth. For that reason extreme high-speed units utilize hardened teeth to extend the coupling life. However, this requires material that will be compatible with induction hardening, carbonization, or nitride hardening. The hardened tooth must retain its strength to carry the torque. Iron carbides and carbon or other nitrides provide the surface hardness. While AISI 1045 carbon steel is the most popular for gear couplings, AISI 4140 high alloy steel is used on high-speed units. Coupling materials and hardening will be discussed more thoroughly a bit later in this chapter.
Lubrication of High Speed Units
High-speed couplings are lubricated with oil rather than grease. The oil, which is circulated through filters and coolers, is sprayed into the sleeve on one side of the teeth and drained from the sleeve on the other side of the teeth. Circulating oil has the advantage of constant renewal, but even with the circulation it is necessary to prevent sludge build-up in the coupling. Sludge will prevent oil from reaching the necessary surfaces that need lubrication. Anti-sludge features in a coupling prevent the build up by putting drains and dams in the passages.
Grease-lubricated high-speed couplings are limited in their application possibilities. Even though grease labeled as “coupling type”, will resist separation of soaps and oils, it is not enough for the true high-speed application. Another problem with grease is temperature build up. Oil that is circulating is also cooled. Grease that is static would heat up from the rubbing friction at the high speeds.
Mounting the Gear Coupling in a Shaft System
Metric Versus English Units
The metric and English systems of size and tolerance were developed without a desire to interchange with each other. Simple conversions are not satisfactory because different bore dimensions are used, along with different tolerances and different formulas defining tight and loose fits. Metric bores are defined in ISO standards while English bores are defined in AGMA and ANSI standards. Those standards are also summarized in coupling manufacturers’ catalogs.
There are several methods to fasten the hub to the shaft. In all cases the objective is to have a joint that facilitates the transfer of torque from shaft to hub, is easy to install or remove, and does not make the alignment more difficult.
Loose fits are easiest to manufacture and to install. But, loose fits are not the first choice for gear couplings, except low torque applications or some nylon sleeve applications. The loose fit does not provide sufficient restraint for the forces found in gear couplings, so interference fits are used. Loose or clearance fit hubs use a keyway and a loose fit key to transmit torque, with a setscrew to hold the hub tight to the shaft and key to prevent wobble and fretting wear. The key and setscrew also help if some cyclic loading is present. Since that is the only means of transferring the torque, the length through bore for clearance fits is longer than that of other fits. The preferred length is 1.25 to 1.5 times the diameter of the bore. Keyways on clearance fit bores are a square cross section. Key sizes are matched to shaft sizes to ensure sufficient surface is available for the torque transfer. The key also has a loose fit within the keyway.
Interference (or Shrink) Fits
The interference or shrink fit is the hub mounting choice in the majority of gear couplings. It utilizes a hub bore diameter that is slightly smaller than the shaft diameter under all tolerance combinations. There are many combinations to the amount of interference, but a popular number is .0005 inches per inch of shaft diameter.
The interference fit installation is accomplished by heating the hub to the point where it expands enough to fit over the shaft. Heating can be done in ovens, oil baths or by induction. The induction method is popular as a hub removal method too. A temperature of 300° F to 350° F is sufficient to do the job. Excess heat may change the metallurgical properties of the hub, and excess shrink or interference may split the hub.
The interference fit hub has a straight bore with a keyway so the friction between shaft and hub and the key are not used to transmit torque. The key is the main means of torque transfer, and may be either a loose or interference fit. Again a square key is used, and most times a radius is included in the keyway and on the key to reduce stress concentrations.
Reduced keys, known as shallow, half height or rectangular keys can be used to allow greater shaft diameters within the hub limits. All are wider than they are tall. Metric keys are of the reduced or rectangular key variety. When using reduced keys, torque capability must be carefully assessed. On large couplings and shafts two half-height keys are sometimes used to strengthen torque transmission. Interference fit hubs use a 1 to 1 ratio between the hub contact length and the shaft diameter. That ratio may vary in applications prone to high cyclic loads or sudden peaks in the torque from transitory conditions.
Tapers and Mill Motor Bores
Two types of taper bores are also common on gear couplings. One type is the tapered and keyed mill motor bore. This hub fits a standard mill motor shaft that has a like taper. As the hub slides up the shaft it forms a tight fit with the shaft. A shaft end nut is used to hold it in place. This method achieves good torque transfer, with a tight fit. It is an easy assembly or disassembly feature. Tapered shafts of this type can be used with machinery other than mill motors.
Another type of taper bore is the shallow taper hydraulic type. In this type there is no key. The hub is expanded by hydraulic pressure and pushed up the shaft to a predetermined point. When the pressure is removed, the hub shrinks to the shaft. The shaft can have a nut or plate attached to the end for retention of the hub. Removal also is accomplished by hydraulic pressure. The hubs have oil grooves machined in the bore to facilitate the application of oil pressure. Taper bore shaft hub combinations require a very complete match between the hub and shaft. The contact area of the hub bore to a gage acting as a shaft is measured in the manufacturing of the hubs to make sure a proper fit will be obtained when the hub is mounted on the shaft. Standards have been established to use as a guide for percentage of contact.
Shrink fit and hydraulic fit hubs are the choice for the heavy torque applications. One of the weak points in the power transmission train is the interface between hub and shaft. It is also the place where cyclic loads and peak loads can cause slippage or fretting damage. The tightness of the fit contributes to a more secure connection for torque transmission.
Sleeve to Sleeve Interface
Gear couplings from size 1 to size 9 will match up half for half with other flange type gear couplings made to AGMA standard dimensions. However, while the dimensional standard ensures compatibility of the face to face match between sleeve flanges, it does not assure matching torque capability or bore. This should always be checked. When a labyrinth seal coupling is matched to an O-ring-sealed coupling, the bore capability and torque may both be different despite the fact that their flanges match and bolt together.
Flange bolting is important to coupling reliability, as bolting can be a potential weak point. Most designs use a friction basis for transferring the load across the face to face match of the two coupling halves. Bolts are designed for tension loading, and primarily serve the purpose of clamping the two flanges together to enable face friction to transfer torque. In fact, the maximum outer diameter of the flange on flanged sleeve couplings is partially determined by the needs of space for bolts and surface for friction. Although friction is the main means of torque transfer, if the coupling is overloaded to the point of overcoming friction, it becomes a shear load on the bolts before becoming a coupling failure. Since the bolts are loaded by several types of forces one must be sure the bolt threads are not in the shear plane between the flanges.
Other specifications could allow body fit bolts to carry the load in shear, although from an engineering standpoint the concept of carrying load on bolts in shear is not favored. The body fit bolt has a tight fit to the bolt holes that keep the two halves concentric. To carry that method to the extreme one would drill and ream the boltholes at assembly and then match mark the two coupling halves.
Bolting will also affect and be affected by balance requirements. Balanced couplings may require weigh-balanced bolts. In addition, bolting can provide a means of piloting the two half couplings. To use the bolts as a pilot, the boltholes must be drilled to a close tolerance or line reamed at assembly.
Remember that the continuous sleeve coupling is not affected by any of the issues associated with bolting. The continuous sleeve coupling provides a bolt-free method of transferring torque through a continuous cylinder of metal with the additional advantage of a smaller outside diameter.
Although alignment is covered in another section of this handbook, the gear coupling has some special alignment considerations that should be noted here. As mentioned in the bolting section, it is necessary for the two halves of flanged type to have some sort of piloting for best alignment practice. That can be achieved by piloted bolts or better achieved by pilot rings or rabbet fits. The alignment needs depend on the connected machinery and the speed of operation. High-speed operation always needs close alignment. Always refer to the machinery specification’s first, not the coupling specifications, when setting the alignment parameters. Since continuous sleeve couplings do not have bolts, alignment is done hub face to hub face.
Once in a while there is a call for an “indexing” coupling. That type of coupling aligns two shafts in a rotational circular position that is the same each time. To accomplish that, the hub keyway is cut to be in line with a tooth or a space. The second hub is cut the same way. If it is a continuous-sleeve coupling, the continuous sleeve might be marked to identify the same tooth or space on both ends of the sleeve. The procedure on flanged sleeve couplings is more complex. In addition to the keyway meeting the hub tooth or space, a bolthole on the flange also must be lined up with a tooth or space. The mating flange must be drilled the same way so that when it is assembled the unit will be aligned or indexed. Of course, to make this work, the shaft keyway must also be aligned with a significant part of the machinery. Indexing is done to a specified tolerance on the location of that alignment.
Additional indexing is accomplished with floating shaft couplings when the coupling on each end of the unit has a different number of teeth. The indexing can then have a number of set points equal to the product of the two numbers of teeth.
Gear coupling selection parameters include two very important items and many more secondary items. The most important items are the bore and torque capabilities, in that order. Bore refers to the nominal shaft size where the coupling will be used. Torque in this case refers to the normal operating torque that the coupling must transmit. The secondary items can include a whole host of things like speed, misalignment, weight, spacer length, inertia, etc.
Bore and Torque: First Pass Selection
The gear coupling size in most cases will be determined by the nominal shaft size. The nominal shaft size is a mixed number of units and fractions that represent a specific diameter of shafting. The actual shaft is the decimal equivalent of that number plus .000 minus .0005 or .001 inches. Nominal sizes are not just any number, but are chosen from a list of preferred numbers. Preferred numbers can also be metric in origin. This is part of our discussion is limited to inch numbers. That nominal number would also be the coupling bore with the actual size as a function of the class of fit.
Gear couplings typically use interference fits, so the coupling bore usually is smaller than the shaft size. The amount of interference varies by the designer’s requirements, but a value of .0005 inches per inch of bore diameter is often used. For details on shaft size for interference or clearance fits refer to AGMA 9002-A86. That is an inch series document; if metric is of interest, refer to “Preferred Metric Limits and Fits” ANSI B4.2 1978 reaffirmed 1984.
If the nominal shaft size is equal to or less than the published coupling bore capability, the gear coupling is usually okay for the service. “If it fits it is okay” is the gear coupling motto. For example smooth running, 1800 RPM, machinery without high starting torque or stopping requirements can use bore size to select the coupling.
The second step in gear coupling selection is to check the torque requirement of the application vs. the torque rating of the coupling. Normal operating torque is used unless a peak or cyclic torque is known. If the application calls for peak torque or cyclic torque, more care must be taken. The application description is also important to see if further investigation is needed. At this point, the nominal torque requirements of the system times an application factor that could be used to select the coupling.
The normal or continuous operating torque of the system is that torque value that is required for design point operation on a continuous basis. Coupling ratings are sometimes listed as HP per 100 RPM, but torque and horsepower can be derived from one another if the speed in RPM is also known.
Service factors (sometimes called Application Factors) are applied to the normal torque to account for variations that are typical of specific applications. They are based on a combination of empirical data and experience, and provide a quick reference to guide selection of a coupling for torque, and perhaps life, without going into the details of the application. Service Factor tables usually are provided in coupling catalogs, and will be different for different types of couplings. Another source of Service Factors (application factors) is AGMA standard 922-A96. Factors of Safety and Service Factors should not be confused with each other or interchanged. The former is for design work and the latter is for applications work.
This subject is included to highlight the fact that it is not recommended. Never exceed the bore associated with the coupling size and the key type. Square keys have a maximum bore, rectangular keys have another and metric has its own. Do not mix them. When extra shrink is requested, or an over bore is requested for low torque applications, engineering should review the application. The gear coupling is the most power intensive coupling as it is designed, but the shaft to hub connection can be the weak point of the coupling. Stretching the limits can result in machinery failure as well as coupling failure.
There are several magic numbers when it comes to gear couplings. One is the size cut-off between big and small. That number is arbitrarily set at 7, but could be 9. The AGMA dimensional interchange goes to size 9 for gear couplings, but once the size rises to 7 and above, the number of applications become very limited. A size 7 gear coupling has a bore capability of nine or more inches (depends on key size too) and a torque of one million inch pounds. That torque corresponds to 16,000 horsepower at 1,000 RPM. Not many applications go that far and when they do the situation is special or low speed. Generally, big gear couplings are used on very low RPM and very high torque applications such as those found in the steel and aluminum rolling mills, crushers, rubber processors or mine concentrators.
For an idea of how big the gear couplings can be made, the catalogs will show gear couplings up to size 30. Loosely, the number equates to half the pitch diameter for flanged sleeve couplings. That means the coupling overall diameter will exceed sixty inches. Continuous sleeve coupling numbers are roughly equal to the maximum bore.
When the coupling size reaches the double-digit numbers, the torque rating is nebulous. Couplings are often re-rated based on improved materials, heat treating, and hardening. In reality the user and designer are trading wear life for torque rating. The torque rating can be used as a peak load or cyclic high and not always as the normal operating torque.
Not many modifications are made to these large coupling sizes. At this size, added functions are too expensive to build into the coupling and may be available as a separate device. Torque limiters fall in the latter category as they replace shear pins. The weight of the coupling and the other pieces of the rotating system also may preclude the desire for modifications. We should point out that large coupling bores are not always the ordinary bore and keyway because they may have special shapes and non-standard dimensions.
Catalog ratings are often accompanied by speed limits in RPM. It is possible to increase the RPM limit by balancing the coupling to minimize vibration. Balancing combined with special manufacturing tolerances can increase the speed even more. However, a perfectly balanced coupling will eventually have a speed limit set by stress, friction between the teeth, and lubricant breakdown.
All couplings have a misalignment limit. The standard gear coupling is capable of 1½° angular misalignment per mesh. Specially designed gear couplings can push that limit to 6° or more, per mesh. However, high misalignment limits can reduce the torque capability of the coupling.
Misalignment accelerates tooth wear, because it causes the hub and sleeve to rub harder against each other. Sometimes high misalignment capability is sought for and limited to non-operational conditions, such as moving a shaft aside for maintenance.
Modifications used to achieve high misalignment capability in gear couplings include increases in backlash (tooth gap), additional crowning, 25° or more tooth pressure angles, hardened wear surfaces, modified grease seals, increased clearance between sleeve and hub (makes the teeth look taller), and a torque de-rating. High misalignment couplings may also have modifications to make coupling maintenance easier or less expensive such as replaceable wear surfaces.
Materials of Construction
Gear couplings are typically made of two common steels, AISI 1045 carbon steel, and AISI 4140-alloy steel. Alloy steel means elements other than carbon have been added to give additional properties to the steel.
Standard gear couplings use AISI 1045 steel. It can be bar stock or forging depending on the size and the component. Couplings needing higher strength or hardness for greater wear resistance are made from AISI 4140 which also can be bar stock or forging.
Gear couplings can be specified in 303 SS, but that is expensive and usually done only when required for the food processing or the pulp and paper industry.
Steel can be treated in many ways to improve hardness and strength. Hardness is the key to improving wear resistance for longer life under increased friction from high speed or misalignment, because gear couplings typically wear out under load rather than break. Strength provides resistance to the impact and cyclic loads.
The terms heat treatment, hardening, annealing, quenching and tempering are used in conjunction with the materials. Each of these terms represents a process that conditions the steel. Heat-treating is the general description that includes variations of all the others. Heat-treating does not have to mean hardening of the steel although it is usually taken in that context. Hardening of steel can mean in-depth hardened or surface hardened, which is also called casehardening. Hardness is measured in Brinnell units or Rockwell units, abbreviated as Bhn or Rc. The Rockwell Rc method of measurement is more popular on hardened surfaces of gear couplings while Bhn is used for overall hardness of a batch of steel.
For AISI 1045 steel, expected properties of strength for gear couplings would require a range of 190-260 Bhn. For AISI 4140 the range would extend up to 300 Bhn in the higher strength versions of the steel. The basic process in simple terms is that the steel is heated to a critical temperature held for a period of time and then rapidly cooled. After the rapid cooling the steel has a very hard structure that may need further tempering or annealing to trade hardness for strength. Rapid cooling is called quenching. Tempering or annealing is heating to a temperature and then cooling at a predetermined rate that is slower than a quench. The intent of these processes is to obtain a strong hard material that is ductile and tough.
For wear resistance we want to increase the surface hardness to 50 Rc or better. That requires an additional process known as hardening, case hardening, or nitriding. The process is to load the surface with iron carbides by exposure to carbon and heat or carbon nitrides and other nitrides by exposure to nitrogen and heat. The heat is provided by a heat treating furnace and the other elements are provided by the atmosphere in the case of nitriding or by packing the piece in carbon in the case of carburizing. The base steel has to be suitable for the process. In the case of nitriding the end product retains the original dimensions, but in the case of carburizing the end product grows and needs to be ground if the original dimensions are to be held. There are many methods, beyond these mentioned, which can harden steel surfaces. It is a complex subject. The process of hardening the surface of gear coupling teeth can extend the useful life of gear couplings.
Gear Coupling Applications
Reduced Moment, Three Bearing and Four Bearing Systems
The weight of the coupling and any reactionary forces all act at the center of the flex plane and cause a bending moment on the equipment shaft. When the coupling is placed close to a support bearing, the close support reduces that bending moment arm and the coupling can be called a “reduced moment” coupling. Reduced moments mean smaller loads and less wear on the equipment bearings. Placing the flex point close to a bearing also helps keep the system stable. Increasing the distance between flex point and bearing invites vibration, or wobble. For the most part, a three-bearing system has one bearing in the driven equipment and two bearings in the driver. The one-bearing side of the equipment is given a rigid half coupling without a flex plane. The two bearing side of the equipment, which is more stable, is given a flexible half coupling. With only one flex plane, this type of system can only have angular misalignment. Three bearing systems are commonly found in motor generator sets, and a long-shaft situation such as bridge crane traction drives.
The more common system is the four-bearing system with two bearings each in the driving and driven equipment. The system is more expensive and usually needs two flex planes because two bearings on each shaft make shaft locations rigid, usually in parallel misalignment.
Standard Couplings vs. Spacers
The simplest application for a coupling is a pump, compressor or centrifuge or the input side of a gearbox. These usually involve an electric motor drive mounted on the same base plate as the driven equipment. The coupling connects the two shafts and the most complicated issue is usually the BSE dimension. As the gear coupling has some range in BSE, the equipment designer can use a common size base plate for many different models of his equipment. The torque requirement of this type of rotating equipment is usually a smooth curve from zero to full speed and does not have any cyclic content. The coupling can be selected by torque and bore with a minimum service factor.
When the designer wants to make his equipment easier and cheaper to maintain, a spacer is installed between the two flex halves of the coupling. When the designer needs to span a long gap between driving and driven equipment (as when reaching up to a big-diameter roll, removing a large piece of equipment from an on-line position, or extending through a wall or bulkhead) a floating shaft is needed. This arrangement is often used with pinion stands, where the output is a double shaft that drives a meshing pair of rolls or mixers that are part of a large machine, such as a rolling mill.
Separating the Driver & Driven
Rotating equipment such as fans, pumps and compressors can have two separate drivers on the same piece of equipment. The drivers might be an electric motor for start-up and a steam turbine for running. That occurs on co-generation applications where steam is available and the operator wants to conserve electricity or use the electricity for other purposes.
Sometimes the equipment has an electric motor for normal purposes and some other device like an internal combustion engine for emergency operation. Other times the equipment sits idle but the driver runs. While these sound like applications for clutches, they also can be places where cut-out gear couplings might be the wiser choice. The gear coupling in many cases is less expensive and takes less space in the system than a clutch.
Save the Equipment from Torque
Rotating equipment shafts are often oversized because they are designed to limit deflection, which can lead to oversize couplings. Motors are sized as the next larger standard unit compared to the application requirements. Those issues plus a service factor can result in a drive system that has torque capability well in excess of the driven equipment needs. In such systems, torque spikes or overloads are easily passed to components that are not designed to withstand them and may be severely damaged. To prevent that, a torque limit device is installed in the drive train. The gear coupling, which probably is needed in the system for other reasons anyway, can provide the same protection at much lower cost than many devices sold as torque limiters, with the simple addition of a shear pin.