Gear Coupling Tutorial - Part V: Failure Analysis (with photos)

While gear couplings are a well proven and highly leveraged technology, they are a metal-on-metal torque transfer wearing type of solution. Proper lubrication will greatly extended the life of a gear coupling, but it will still eventually need to be replaced.

Evaluating gear tooth wear and being able to root cause and address coupling failure are both critical to ensuring maximum reliability and up-time for a given mechanical power transmission system. Fortunately, gear couplings often provide "signature" failure modes that can be quickly identified and diagnosed.

Common causes of wear or failure include normal wear (again, gear coupling teeth are designed to wear over time), lack of lubrication, torque overload, misalignment, fatigue (of flange or bolt), and thrust loading (sleeve seal end rings).

Normal Wear

Normal wear is generally characterized by tooth wear localized primarily to the center of the teeth. If your system has been running reliably for some time, and you've properly lubricated it the entire time... you can expect to naturally see this type of wear to occur.

Lack of Lubrication

Lack of lubrication may look similar to normal wear, but the tooth wear will be greatly accelerated relative to a properly lubricated coupling. If your gear coupling teeth look like those pictured at right and below after a short amount of use, you have a serious lubrication issue. You may be using the wrong type of lubricant, have a seal issue, or may have forgotten to use lubricant at all. 

Torque Overload

If there is a peak torque overload that the coupling cannot handle, the most likely failure mode will be a coupling hub burst or crack over a corner of the keyway. Failures are common at this location because it is generally the weak point of the coupling hub (with the least amount of material to carry the load. 

Fatigue (Bolt/Flange)

Fatigue failures are typically due to high start-up or impact loads, typically in combination with reversing or highly fluctuating loads. In these situations the joint may undergo bending fatigue. This type of failure can also be caused by insufficient fastener torque. 

Thrust Loading (Sleeve Seal End Rings)

When presented with excessive thrust loading fractures of fasteners can occur. Such fractures can also be the result of high misalignment, and tooth contact patterns can often be seen on the end rings (as seen in the two pictures below).

To learn more about coupling failure analysis, go to:

 

Coupling Failure Analysis - Jaw Couplings (includes hub & spider photos)

Grid Coupling Failure Analysis (includes photos) 

Coupling Peak Torque Failure at Keyway

Top Reason for a Coupling Failure

To keep learning about gear couplings, go to:

Gear Coupling Tutorial - Part I: Overview

Gear Coupling Tutorial - Part II: Configurations 

Gear Coupling Tutorial - Part III: Mounting the Coupling

Gear Coupling Tutorial - Part IV: Selection & Availability

If you have any further questions or concerns with diagnosing or troubleshooting a gear coupling failure, please do not hesitate to visit the Lovejoy website or contact a Lovejoy application engineer.

Gear Coupling Tutorial - Part IV: Selection & Availability

Up to this point we have been describing the functional parts and modifications of gear couplings. Another attribute that needs to be described is the performance parameters of the unit. Those parameters include two most important items and many more secondary items. (The HercuFlex & Sier-Bath gear coupling product pages & downloadable PDF catalogs are the best place to find this information.) The important items are the bore and torque capabilities in that order. The secondary items can include a whole host of things like speed, misalignment, weight spacer length, inertia, etc. Torque in this case refers to normal operating torque that the coupling must transmit. Bore refers to the nominal shaft size where the coupling will be used. 

A.    Bore and Torque First Pass Selection 

Hercuflex CX Gear Coupling

 The gear coupling selection in most cases will be determined by the nominal shaft size. If it fits it is okay. The second sort would be to check the torque requirement of the application versus the torque rating of the coupling. Normal operating torque is used unless a peak or cyclic torque is known. The application description is also important to see if further investigation is needed. Smooth running 1800 RPM machinery without high torque starting or stopping requirements can be selected with bore size. If the application calls for peak torque or cyclic torque, more care must be taken.

The normal or continuous operating torque is that torque value that is required for the design point operation of the machine on a continuous basis. The every day operating point if you will. Torque can be derived from the required horsepower at the operational speed. Coupling ratings are sometimes listed as HP per 100 RPM. Torque and horsepower can be derived from one another if the speed in RPM is also known. The equations for that can be found in the coupling catalog.

B.    Service Factors

Service factors are applied to the normal torque to account for application variations. The variations can take many forms, but they are usually application specific. Service factors allow selection on of a coupling for torque and perhaps life without going into the details of the situation. While a bore is finite, torque can be many different values. For application factors one can refer to the Lovejoy gear coupling section of the catalog or any other gear coupling catalog. 

Service factors for elastomer couplings may be different from gear couplings when applied to the same system, and it is critical that you pull this data from the specific manufacturer you intend to source from. (If you are looking for an overarching reference document, AGMA 922: Load Classification & Service Factors for Flexible Couplings can help and and provide a sanity check, but it should not trump the manufacturer's specified service factor when sizing that manufacturer's coupling.)

Service Factors are based on empirical data and experience. Safety Factors and Service Factors should not be interchanged or confused with each other. The former is for design work and the latter is for applications work. The method is multiply the normal or continuous application torque by the service factor to obtain the coupling selection torque. 

Normal torque and continuous torque are synonymous in this procedure. The torque developed by the motor may exceed the normal torque of the application, but should seldom exceed the coupling selection torque. It is also acceptable to select the coupling by applying a service factor to the motor nameplate horsepower converted to torque. The selection torque obtained this way can often result in oversize couplings. Sometimes gear couplings are deliberately oversized to extend the tooth life. Other complications may result from doing that.

(Note: Lovejoy has developed a great and easy to use tool for roughly selecting a coupling once horsepower, rpm, and service factor are known.) 

C.    Peak Torque, Continuous Torque and Cyclic Torque

Normal or continuous torque was defined in the previous paragraphs. The torque forces we are describing are those necessary to operate the machinery successfully. They may also be used to size the motor or driver. 

Peak torque can occur under several circumstances, but are usually transitory conditions. A typical peak could occur on starting if high inertia loads must be started, another might occur if an emergency stop happens, or blockage of the machinery occurs, or voltage spikes, or torsional resonance. Peak torque is usually accounted for by limiting the peak cycles to prevent low cycle fatigue in the coupling. 

Cyclic torque on the other hand could occur continuously for the life of the coupling. It might be that the application operates under a "cycle" of load and unload or it might be that start and stop is a routine of the application. The start and stop routine means low starting torque and non-emergency stops. They might be classified as slow and smooth. Cyclic loads are accounted for either in the service factor or in the coupling torque selection. Service or application factors are established to account for these torque variations based on past experience. If exact values are known they could be used to directly select the coupling.

D.    Stretching the Bore

The subject is included to highlight the fact that it is not recommended. That is do not exceed the bore associated with the coupling size and the key type. That means square keys have a maximum bore rectangular keys have another and metric has its own. Do not mix them. In general, the shaft to hub connection can be the weak point of the coupling. When extra shrink is requested, engineering should review the application. If an overbore is requested for low torque applications, engineering must check the application. The gear coupling is the smallest most power intensive coupling as it is designed. Stretching the limits can result in machinery failure as well as coupling failure.

E.    Other Considerations

1.    Speed.

We mentioned that high speed units need to be balanced to reduce vibration and subsequent damage to the connected machinery. Catalog ratings are often accompanied by speed limits in RPM. In the case of this coupling parameter it is possible to increase the value by balancing the coupling. That is mentioned in the catalog with the rating value. Balancing combined with special manufacturing tolerances can increase the speed even more. Even a perfectly balanced coupling will eventually have a speed limit. That limit comes from stress, friction between the teeth, and lubricant breakdown.

2.    Misalignment.

All couplings have a misalignment limit. That limit will vary by the coupling style and can also vary by modification to a standard product. The gear coupling is generally capable of 1 1/2 ° of misalignment per mesh. Special gear couplings can exceed that. The limit can go to 6° or more, however, a special design is required as is engineering involvement. High misalignment limits can reduce the torque capability of the coupling. High misalignment of some types are limited to non-operational conditions.

3.    Special Adaptations

Special couplings, many of which have been mentioned in this blog require engineering or manufacturing involvement before the selection or pricing is possible. When contacting Lovejoy engineering, it is advisable that as much information about the system and application be gathered in advance so that Lovejoy can quickly facilitate coupling selection for you.

4.    Materials of Construction

Standard gear couplings are manufactured from carbon steel, and can be from bar stock, casting, or forging depending on the size and the component. The material is subject to heat treatment to achieve the correct strength and ductility. Alloy steels are also available, and can be made from bar stock, casting, or forging just like the other couplings. Alloy steels are used to achieve higher strength and to facilitate hardening and strengthening of the teeth. Alloy steels, with the proper treatment, will enable Lovejoy to provide a coupling that has a longer service life. The longer service life is a result of hard teeth resisting wear. A finite number for the life is difficult to express. It is also possible to supply gear couplings in stainless steel. This is an unusual occurrence as it becomes an expensive proposition. (Stainless couplings are often required by the food processing industry.)

Steel can be treated in many ways to improve hardness and strength. The hardness is the key to longer life under conditions like high speed or heavy loads. Strength may be the requirement of couplings that have cyclic loads or have impact loads. Each application is reviewed on its own merits. There is no direct relationship that allows the coupling to be re-rated for a material change. Both bar stock and forging have the same strength characteristics and would hit the same basic specification. Cast steel couplings would have their own capabilities that are different and likely lower than bar or forging.

F.    Stock, Modified, & Made-to-Order

Couplings that are designated as stock units are the competitive coupling sizes from 1 to 15 for which we keep sufficient stock to supply most orders. Stock is based on forecast usage and updated regularly. While larger OEM type orders may require raw material sourcing and longer lead times, smaller MRO type order can generally be supplied from stock and are largely available for Quick Ship finishing if not.

1.    Stock Units
 
Stock units include HercuFlex & Sier-Bath flanged and continuous sleeve gear coupling hubs, sleeves, and accessory kits, universal hubs, mill motor hubs, and rigid half couplings. Hubs include pre-bored and keyed units with AGMA bore dimensions. Spacer pieces for predetermined spacer lengths and coupling sizes are available. (Please call your preferred Lovejoy Distributor or Lovejoy's customer service team to confirm the availablilty of the gear coupling components you seek.)

2.    Non-Stock and Modified

These units are often made from stocked components. The component stock may include both RSB hubs and partially machined parts and raw forgings ready to go to final machining once it is determined what final result is required. In this category are special bore and keyway requirements, metric bore requirements, hubs that are faced or counter bored, hubs that require clearance fits, some mill motor or taper bore hubs, and add on pieces like limited end float or vertical plates. Non-standard spacer lengths for smaller sized gear couplings are made from stock raw material. Short sliders can be made by facing hubs and making long tooth, standard length sleeves.

2.    Major Modification & Made-to-Order

Major modification couplings are a combination of standard parts, modified parts, special standardized parts and designed to order parts. Included with those designs are floating shafts, insulated couplings, long sliders or Jordan couplings, cutout couplings of two types, shear pin couplings, brake wheel couplings, and moderate speed units where careful machining and component balance will suffice. 

The line between major modification couplings and total made-to-order couplings is blurred. The size range from 9 to 15 is in the gray area, but sizes above 15 are definitely made to order. Precision high speed gear couplings are made to order. Some other gear couplings were designed for a specific application like subway car trucks or spindle rolls. (i.e. - A spindle coupling design by Lovejoy was limited to the smaller applications associated with aluminum rolling mills.)

To keep learning, go to:

Gear Coupling Tutorial - Part I: Overview

Gear Coupling Tutorial - Part II: Configurations 

Gear Coupling Tutorial - Part III: Mounting the Coupling

Gear Coupling Tutorial - Part V: Failure Analysis (with photos)

Note: This article series is an updated & modified version of a legacy Lovejoy training document. The blog writer is not the original source author.

Gear Coupling Tutorial - Part III: Mounting the Coupling

A.    Metric versus English Units

The metric and English systems of size and tolerance developed without a desire to interchange with each other. That is simple conversions will not be satisfactory as not only are different bore dimensions used, but also different tolerances and different formulas are used for tight and loose fits. Metric bores are defined in ISO standards while English bores are defined in AGMA and ANSI standards. Bore standards are generally summarized in coupling manufacturers’ catalogs, but can also be reviewed and downloaded in their entirety. 

B.    Hub to Shaft Interface

In addition to the English versus metric methods there are several other 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.

1.    Clearance or Loose Fits

Loose fits are not the first choice for gear couplings except low torque applications or some nylon sleeve applications. Loose fits are easiest to manufacture and to install. A keyway is used with the loose fit and a set screw is also necessary. The set screw holds the hub tight to the shaft and key to prevent wobble and fretting wear. It also helps if some cyclic loading is present. Hubs with a bore and key use the key for torque transmittal. Since that is the only means of transferring the torque the length through bore for clearance fits is longer that that of other fits. Length is preferred at 1.25 to 1.5 times the diameter of the bore. Keyways on clearance fit bores are classified as square. The key has 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.

Another form of loose fit is the spline shaft to spline bore. In a sense the spline becomes a series of keys and keyways to transfer the torque. While there are several classes of fit for spline shafts, there is only one hub class. Some fits are tighter than others however, the hub always slides over the shaft without interference. There are thousands of spline connection possibilities depending on the equipment under consideration. SAE classifies them and publishes the dimensional standards. Two important ones for a coupling manufacturer are "ag" splines and SAE pump shaft splines. The "ag" splines are parallel side type with 4, 6, 10, or 16 teeth. Hydraulic pump shafts are involute side classified as A, B, C, D, E, and F sizes. Involute splines are further classified as flat root side, filet root side or major diameter fit. Spline shaft connections have contact lengths between hub and shaft as low as .75 when all the splines transmit the torque equally. Spline shaft and hub applications are rare on gear couplings.

2.    Interference Fits

The interference fit has a hub bore diameter that is slightly smaller than the shaft diameter under all tolerance combinations. The interference is variable, but a popular number is .0005 inches per inch of shaft diameter. Interference tit is the hub mounting choice in the majority of gear couplings. It is a straight bore with a keyway so both the friction between shaft and hub, and the key are used to transmit torque. The key mayor may not be an interference tit too.  Again a square key is used, and most times a radius is included in the keyway and the key.  That is to reduce stress concentrations. Reduced keys or 1/2 height keys are used to allow greater shaft diameters within the hub limits. Torque capability must be assessed to allow the shallower key. Sometimes on the large couplings and shafts two half height keys are used.  Metric keys are of the reduced or rectangular key variety. Rectangular keys, shallow keys and half height keys are all wider than they are tall. Interference fit hubs use a 1 to 1 or less ratio between the hub contact length and shaft diameter. That may be changed with high cyclic loads or sudden peaks in the torque from transitory conditions. Like spline fits, there are many variations to interference fits. There are also many variations to the amount of interference.

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 400° F maximum 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. 

(For a more general, non-gear coupling specific discussion on interference versus interference fit bores, please see "Clearance vs Interference Fit Couplings - Which Hub Fit is Better?")

3.    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. The pressure is removed and the hub shrinks to the shaft. The shaft can then either have a nut or plate attached to the end for retention of the hub. Removal is accomplished by hydraulic pressure. The hubs have oil grooves machined in the bore. Taper bore shaft hub combinations require a very complete match between the hub and shaft. Contact area as a percentage of total area is measured in the applications.

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.

4.    Between Shaft Ends (BSE) Dimension

The BSE dimension is important for all couplings. It is the distance from one shaft end to the connected shaft end. Gear couplings have the feature of a variable BSE. That variation can be achieved by facing the hubs and can be achieved by reversing or both of the hubs. A combination of facing and reversing is possible too. The catalog describes the maximum and minimum BSE with the combinations. All couplings have a BSE dimension, but few are able to vary that dimension greatly to make it easy for the designer or user to package and standardize a line of rotating equipment.

C.     Hub to Hub Interface

1.    Interchangeability

Hercuflex FX Gear Coupling

Sier-Bath & HercuFlex 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. While the dimensional standard ensures compatibility of the face to face match, no assurance of torque or bore compatibility is made by the standard. The Sier-Bath & HercuFlex gear couplings will meet most all torque requirements or exceed them (particularly in the case of HercuFlex), but a user should still check to make sure. When a labyrinth seal coupling is matched to an rubber sealed coupling, the bore capability and torque may both be different. However, they will still bolt together.

2.    Bolts and Torque

The bolts can be either shrouded or exposed but not a combination of the two. The bolting is important to the coupling reliability. The bolting might be class 5 or class 8 depending on the designer's choice and standardization. The bolting will also be affected by the balance requirements. Balanced couplings may require weigh balanced bolts. Bolting also provides a means to pilot the two half couplings. To use the bolts as a pilot, the bolt holes must be drilled to a close tolerance or line reamed at assembly. Since the bolts are transferring torque it is generally advisable that the threads are in not the flex plane. This is a complex subject as the bolts may also be used to damp the hubs for friction transfer of torque.

3.    Alignment                                                             

It is not the intention of this post to detail the means and methods for aligning gear couplings, which is a much bigger subject. The gear coupling does have some alignment considerations that should be noted. As mentioned under the bolting paragraph, 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 specifications' first, not the coupling specifications, when setting the alignment parameters. Since "C" and "CX" couplings do not have bolts, alignment is done at the hub face to hub face. Unless high speed is involved, you can assume standard couplings have suitable alignment capability. If there is any doubt, contact an application engineer.

4.    Filler Pieces

When the coupling gap is not sufficient to span the shaft to shaft space some sort of filler piece is used. For short spans that piece is the spacer. For long spans the piece is the floating shaft. If we are not dealing with a radial displacement of the shafts, then, choice between a spacer and a floating shaft is one of economics. Sometimes the space is filled with devices like torque meters. Test stands often have them. It is a situation that should be referred to Lovejoy's engineering team.

5.    Indexing Couplings

Once in a while you will see a call for an "indexing" coupling. That type of coupling aligns two shafts in a circular position that is the same each time. For example, the keyways are located at 1800 to each other. 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 "C" coupling, the continuous sleeve might be marked so the tooth or space on either side matches up to each other. The procedure on "F" or flange sleeve couplings is more complex. In addition to the keyway meeting the tooth or space, a bolt hole on the flange is also lined up with a tooth or space just as the hub was done. A mating half is done 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 in line with a significant part of the machinery. Indexing is done to a specified tolerance on the location of the line up.

To keep learning, go to:

Gear Coupling Tutorial - Part I: Overview

Gear Coupling Tutorial - Part II: Configurations 

Gear Coupling Tutorial - Part IV: Selection & Availability

Gear Coupling Tutorial - Part V: Failure Analysis (with photos)

Note: This article series is an updated & modified version of a legacy Lovejoy training document. The blog writer is not the original source author.

Gear Coupling Tutorial - Part II: Configurations

A. Hubs and Sleeves

Flanged Sleeve Gear Coupling

Gear couplings are made up of hubs which attach to the machinery shafts, and sleeves that span the gap from one hub to the next. Sometimes the sleeve is one piece as in the Sier-Bath & HercuFlex continuous sleeve couplings and sometimes each hub has its own sleeve which in turn bolts to the other half or other side of the coupling. The gear teeth are found on both the hub and the sleeve of the flexible unit. The rigid or non flexing piece could be a flange without teeth as in the Sier-Bath "F" & new HercuFlex "FX" type or could be a hub with straight teeth that acts like the fit of a spline shaft to a spline hub as in the Sier-Bath "C" & new HercuFlex "CX" type.

Continuous Sleeve Gear Coupling

The flexible half coupling consists of a flexible hub and a sleeve. The hub that attaches to the shaft is also the part with the crowned teeth. The crowning includes tip crowns, flank crowns, and chamfers on the sharp edges. Crowning helps improve tooth life as well as misalignment capability.  By crowning the teeth we improve the contact area from tooth to tooth and reduce the pressure of the torque forces. It also prevents the sharp edges of the tooth from digging in and locking the coupling. Sleeve teeth are straight except for a chamfer on the minor diameter edge.

The gear teeth are narrower than the gap between the teeth. That space is called the backlash or if you will the looseness of the fit. Gear couplings always have some backlash. In addition to contributing to the misalignment capabilities, the backlash also provides space for the lubricant. Lubrication is necessary for gear couplings to work well. Some gear couplings have more backlash than others. In addition to the backlash there is another matching fit on the hub to sleeve interface. That is the "major diameter fit". Gear couplings are made to fit closely at the major hub diameter and the root diameter of the sleeve. When the coupling is not rotating, those two surfaces rest upon each other if it is a horizontal installation. In operation the teeth mate at the pitch line, and that is where torque is transmitted. Minor diameter fits would preclude suitable misalignment capability and torque transmission capability.

Rigid hubs are basically a flange, fitted to the shaft and bolted to the adjacent ½ coupling. Rigid "C" and "CX" couplings utilize one hub with straight teeth that mate with the continuous sleeve.  The straight tooth rigid hub fits very tightly into the sleeve because there is very little backlash. We do not want the rigid hub to flop around as it could cause vibration problems. 

The hub and sleeve of the gear coupling are also fitted together so as to prevent the lubricant from leaking out. Most gear couplings are lubricated with grease. When oil lubrication is used it is usually a continuous flow through the tooth mesh. Oil lubrication is a special case. The sleeve to hub interface at the boundaries can have elastomer rings, gaskets, or labyrinths to prevent grease leakage.

The two halves of a flanged type gear coupling are bolted together. The bolting is an important part of the power transmission path. Some designs have the power transmitted across the face by friction in which case the bolt provides enough clamping force to provide face friction. Other designs allow the bolts to carry the load in shear. 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. Sier-Bath originally promoted the shrouded bolt as standard for safety reasons. With the advent of OSHA and required coupling guards, some of the reason for the standard goes away. The two types also have differing windage loss that affects the installation. Bolts are not the common hardware store standard. (Please do not try to use them!)

The continuous sleeve or "C" and "CX" type of gear coupling is not bolted together. That is an advantage in that the coupling can be made smaller and lighter. Smaller, lighter couplings also have lower inertia values. Lower inertia is an advantage when starting the machinery. Another advantage of no bolts is no bolt stress. The bolts can be the weak point in some applications.  Under some conditions of balance and high speed the bolts are also detrimental.

Lovejoy's Sier-Bath and HercuFlex flanged sleeve ("F" and "FX") gear couplings are built to American Gear Manufacturers Association (AGMA) dimensions up to size 9. That means a Sier-Bath or HercuFlex flanged series coupling will mate half for half with all other gear couplings built to AGMA standards. While AGMA standards are US based, many European manufacturers build to match the dimensions.  Matching dimensions include the interface only, such as outside flange diameter, number of bolts, bolt size, bolt circle and gap. Many times the length through bore of the hub is identical. No promise is made about torque or bore capability. Lovejoy like other gear coupling manufacturers use similar nomenclature to identify the coupling sizes. Each company publishes an interchangeability chart up to size 7 at least.

B. Planes of Flexibility

We could look at the plane of flexibility as a pivot point on the connection of shaft to shaft.  An elastomer coupling has one flex plane where the elastomer distorts to provide the angular and parallel capability. A full flex gear coupling has two pivot points at the two gear meshes. A disc coupling has two sets of flexible metallic elements that distort to provide the misalignment. Except for elastomer couplings that distort in two directions, a flex plane only provides for angular flexing. In a gear coupling that angular flexing is generally 1 1/2°. Special design spindle couplings have more capability. For other types of couplings you should refer to the literature as no two coupling types are alike.

1.    Radial or Parallel Misalignment

Two flex planes, each providing some angular flex, are placed in series to obtain parallel misalignment. The greater the axial distance from one flex plane to the next, the greater is the radial or parallel capability. Spindle couplings, and floating shaft couplings provide the maximum capability. Spacer couplings are also good for extra radial displacement and close coupled couplings such as a standard flanged sleeve gear coupling provides the minimum.

2. Angular Misalignment

Flex-rigid gear couplings provide only angular misalignment. There is only one flex plane. No radial capabilities are provided by the flex rigid coupling. The teeth can tilt within the mesh, but that is all the displacement allowed although backlash could allow for some radial misalignment too. Single element disc couplings also provide for angular misalignment only. Single element elastomeric couplings may distort enough to displace both radial as well as angular misalignment.

3. Axial Misalignment

Axial misalignment is also handled by the gear coupling. Axial displacement is available from either the full flex or the flex rigid unit. The amount available depends on many factors, and in fact specials are available for long sliding applications. Axial misalignment or movement is often associated with thermal shaft growth and floating rotors. It is accommodated by the sliding action of hub tooth within sleeve tooth. The axial movement can be stopped with a plate or a button. Other types have little or no capability in that plane of misalignment; some elastomeric couplings even have difficulty stopping the axial f1oat after a specific distance and may fall apart.

There are many combinations of angles and spacing which can be calculated by plane geometry, to obtain the ideal situation for an application. Always keep in mind that equipment should be aligned to the rotating equipment manufacturers' standards and requirements, not only the coupling manufacturers. When operating misaligned, the coupling could transmit reactionary loads or vibrations that are within the coupling capabilities, but not the equipment capabilities.

C. Hardware and Accessories

 

Nuts and bolts, grease seals, back up rings and gaskets are all needed for the gear coupling.  Except for the bolts, these items are used for holding grease in the coupling. Sometimes these items limit the coupling application. For example, the temperature may be limited to the o-ring capabilities. Misalignment may allow grease to leak out the seal surface, or some modifications may need a wiper seal rather than an o-ring. The seals can be held in place by several means. The o-ring is the most simple, and it fits into a groove in the sleeve.  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. These couplings with bolt on seal carriers are designated FHD. "F" series couplings 7 through 9 can be either the "HD" version or the plain version.

The "FA" type of Sier-Bath & "FX" HercuFlex couplings uses a high misalignment seal with more flex than the regular seal. The "C" coupling seal is held in place by a "spirol" ring and it 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 end force.
Grease of course is an important gear coupling accessory. 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.
 

D. Variations to Standard 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 of rotating machinery must vary to accommodate design standards, product line variations, different motor frames and maintenance needs. A gear coupling can meet those needs in a variety of ways.

If the gap between shafts is small the coupling can utilize its capability to reverse the hubs or face off the hubs. An infinite number of possibilities is can be obtained from catalog minimum to catalog maximum. Note this gap does not affect the distance between flex planes unless the hubs are reversed. One or both hubs can be reversed. Sometimes a spacer piece is used to allow for maintenance space or machine removal.

Spacers are built to standards for the machinery builders. Pumps have several standard spacers such as 3 1/2 inches, 7 inches and others. Compressors would have a different set. Spacers use two ½ couplings and one flanged hollow tube to connect the coupling halves. Spacers can serve to separate the flex planes and can be part of the torsional tuning of a coupling.

Spacers have practical limits on length that are associated with cost, weight and critical speed. When the spacer becomes too costly, the next step is to use a floating shaft coupling to achieve the necessary spacing. The primary reason for a long floating shaft is to achieve greater radial misalignment between shafts. The secondary reason is to reach a long distance between the driver and the rotating equipment. The floating shaft coupling consists of two flex rigid couplings connected by a piece of solid shafting. Floating shafts are found on bridge cranes and steel rolling mills. Weight and critical speed are important considerations for floating shafts.

Gear couplings can be modified to allow shaft growth in the axial direction and to limit growth in the axial direction. Limiting the growth 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. That is what we mean by "limited end float". It is necessary with sleeve bearing motors. Sleeve bearings are often used in the large motors over 200 horsepower. Those same plates and buttons are used on vertical couplings.

Sometimes it is not movement that we want to stop, but the electrical current known as galvanic current. To do that we offer an insulated coupling. One half of the coupling is electrically insulated from the other thereby interrupting the flow of current. It is done by adding insulating plates and bushings. Galvanic currents are the cause of pitting and corrosion at close running fits of mechanical equipment and of the gear teeth. It is not necessarily a high voltage insulator as found in wiring systems.

On the other hand, the gear coupling can be arranged to allow axial float, as would be required by thermal growth of a shaft in a hot application. Many couplings can be set to allow some thermal growth, but only the gear coupling freely slides back and forth without adding to the load on the flex element. In addition to thermal growth, gear couplings can be arranged to slide great distances. The long sliders are used for removing equipment from the system where the coupling is the most suitable point of movement. Medium sliders are used when the coupling must adjust axially while the machinery is in motion doing its job. Refiners, Jordan machines, and roll winders found in paper mills utilize the sliding capability. Spindle couplings also have some slide capability to adjust to the installation or operational requirements of rolling mills.

2. Modify the Coupling to Satisfy Some Customer Needs

Some applications extend beyond the traditional coupling requirements of torque transfer, misalignment, and filling space. A simple case is the vertical coupling. When the coupling is mounted vertically, the forces and weights need to be accounted for, since, we cannot allow them to interfere with the misalignment action. For gear couplings we must account for the sleeve weight and potential movement. That is accomplished by adding a plate and button as mentioned before under the limited end float discussion. The button is rounded to allow the load to transfer under misalignment. The load or weight is transferred to the lower shaft and thence to a thrust bearing in that equipment. Hanging load couplings in turn transfer the load upward to a bearing in the upper coupling. In a vertical floating shaft coupling the entire floating assembly rests on the lower shaft and must be accounted for by the designer.

Gear couplings can be configured to do special jobs. We already discussed the slider possibilities, other possibilities include the shear pin, cutout, 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. Cut-out couplings, which can be automatic or manual and include pins to hold them in position, function as a disconnect or a connect coupling. They could be used on a dual drive machine to isolate the unused driver. They could be used for a temporary connection for adjustment of a rotary shaft, in which case they connect and disconnect on the fly. They could also be used for a turning gear that rotates heavy equipment when it is off line, and helps prevent a permanent set in the shaft. The cutout pin holds the coupling in one position or the other. A brake wheel or brake disc coupling includes a brake device on the coupling. It is a matter of space conservation in some systems to put the brake on the coupling. 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 gear box shaft since the gear inertia is in the box.

E. Moderate and High Speed Applications

At the beginning of our discussion of gear couplings we mentioned that gear couplings are capable of very high speeds. They can do speed and torque at the same time. The limit has always been the need for lubrication of the mating gear surfaces. While high speeds increase the wear rate and can be the cause of high stresses within the coupling, the big issue is balance. Couplings operating at high RPM or high rim speed will cause vibration problems if they are not in balance. Balance and radial deflection also plays a big role in the issue of lateral critical speeds.

Coupling balance is achieved through design, manufacturing and balancing machines. Without going through a complete dialog of motion mechanics, let us say that balance concerns itself with how the weight of the rotating mass or inertia is positioned or displaced relative to the center of mass. If that weight is perfectly distributed around the center of rotation and the center of the coupling, the coupling is in balance. Since nothing is perfect in couplings and some other issues in life, there is always a potential unbalance. Some of the potential unbalance is a result of machining tolerance. Off center, out of round, non-parallel 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 design and the assembly process can result in an unbalance condition. To have the best balance it is necessary to be balance proactive in the design and tolerances, and to have very tight tolerances. The final step then becomes one of putting the coupling or its components on a balance machine to measure the unbalance and to take some countermeasures. Countermeasures usual involve removing material in certain areas to compensate for the mass displacement. Balance is also accomplished in some cases by addition of material. In the end there will always be some residual unbalance.

Residual unbalance specifications and balance methods are the subject of several standards.
The standards apply to all types of couplings even low speed units in some cases.  Specifications have been developed by AGMA, ISO, and API to name three. Individual OEM customers may have their own standards that are usually taken from the recognized standards listed previously. Imperial unit unbalance is measured in inches (usually micro inches), or in oz-inches. In the former case the dimension locates the center of mass relative to the rotation, while in the latter case the unbalance is identified in both weight and distance. Many specifications base the standard on operational speed as well.

Gear couplings present a special case for balance problems. A gear coupling has a backlash and loose fit between the teeth so it cannot be spun on a balance machine while assembled without modification. The gear coupling is assembled with a slight interference on the major diameter, balanced, disassembled, ground at the tooth tips and reassembled for use in the application. Gear couplings will not work without the looseness at the major diameter.

While balance is very important to high speed gear couplings, it must also be noted that high speed has the potential for high wear of the teeth. Extremely high speed units utilize hardened teeth to extend the coupling life. Slider couplings have hardened teeth in some applications too. In a misaligned gear coupling the hub teeth and sleeve teeth constantly rub together. That is the wear mechanism that eventually causes the demise of the coupling. It is necessary to change materials of construction to harden the couplings. The material to be used must be compatible with induction hardening, carbonization, or nitride hardening. When the tooth is hard it must retain its strength to still carry the torque. Iron carbides and carbon nitrides provide the surface hardness. While 1045 carbon steel is a popular standard steel for gear couplings, Lovejoy also uses 4140 and other high alloy steels.

Lubrication is certainly necessary to decrease the wear and reduce the friction between the mating teeth. High speed couplings have oil lubrication. The oil, which is circulated through filters and coolers, is sprayed into the area of the teeth on one side and drained from the coupling on the other side of the teeth. Grease would pose temperature rise problems, might centrifuge out of the area needing lubrication, and would break down requiring re-lubrication. Circulating oil has the advantage of constant renewal.

 

F. Bigger Than Size 7

Large Gear Coupling

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 nine for gear couplings. Once the size rises to 7 and above, the number of applications and therefore sales are very limited. A size seven gear coupling has a bore capability of 9 or more inches (depends on key size too) and a torque of approximately 1 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. Usually big gear couplings are used on very low RPM and very high torque applications as found in the steel and aluminum rolling mills, mine concentrators, crushers, or rubber processors.

While Lovejoy's gear coupling catalogs will show gear couplings up to size 30, Lovejoy's Downers Grove, IL and South Haven, MI facilities make and stocks gear couplings up through size 15. Going above a size 15 (very rare & very large couplings) requires Lovejoy's coordination with an outside machine house and will add to a given product's lead time.

Very large gear couplings (i.e. - above size 15) 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 the normal operating torque. The transmission shaft is loaded with torque only on these applications so shaft capability is the same as or greater than the coupling. We can offer that re-rate service, which in turn reduces the coupling size, as long as the bore capability is not exceeded.

To keep learning, go to:

Gear Coupling Tutorial - Part I: Overview

Gear Coupling Tutorial - Part III: Mounting the Coupling

Gear Coupling Tutorial - Part IV: Selection & Availability

Gear Coupling Tutorial - Part V: Failure Analysis (with photos)

Note: This article series is an updated & modified version of a legacy Lovejoy training document. The blog writer is not the original source author.

Gear Coupling Tutorial - Part I: Overview

Gear couplings like all other shaft coupling devices perform the basic functions of connecting two shafts to transmit torque and compensate for misalignment. Gear couplings though are the king of the coupling types. While each type of coupling has its own niche, gear couplings are more power intensive, have more modifications, and a wider size, torque, and bore range than all the others. Gear couplings can also perform at extremely high rates of speed. As inferred by the name, gear couplings use the meshing of gear teeth to transmit the torque and to provide for misalignment.

To give you some idea of the differences between gear couplings and other types look at the sales by size and the torque capability per pound of coupling. While some may argue that the sale of gear couplings is not growing compared to other types, there are so many gear couplings installed in rotating machinery that the replacement business keeps the product sales robust.  Actually gear couplings can do things that many other couplings cannot do or can only do with difficulty or with expensive modifications and de-rating. 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 to shear pin service, floating shaft type, vertical type, insulated type, limited end float, and can have a brake drum or disc features added. While those latter items may be available on other couplings, it is usually easier and less costly to modify the gear couplings.

Gear couplings are power intensive. That means more torque transmitted per pound of coupling weight and per cubic inch of space consumed. That allows space and weight for attachments without having the coupling grow to unusable proportions. Power intensity and space savings mean the original equipment manufacturer (OEM) can bury the coupling in small out of the way places. When the OEM does that, it can be done with the confidence that the coupling will not fail. The gear coupling has more torque capability than the shaft can transmit. The gear coupling eventually wears without a spectacular failure. Gear couplings can be sized to make sure that wear life is consistent with the rest of the machine's design. The Sier-Bath "C" or continuous sleeve gear coupling has long been a champion of OEM service for its small size, easy installation, and large torque ratings, and the HercuFlex "CX" continuous sleeve gear coupling promised to take this performance to the next level.

Gear couplings have been likened to a one-to-one gear box, that is, torque transfers from hub teeth to sleeve teeth and across the shaft gap with no change in RPM. The gear coupling can be configured with two flex planes to achieve parallel misalignment as well as angular and axial, or it can be configured with one flex plane and one rigid plane to limit the misalignment to angular and axial only. The two flex plane version is the most popular, but there are many applications for the single flex plane. Those applications appear in three bearing systems or in floating shafts. Many times the single flex coupling is used in series with another single flex unit to give much more parallel misalignment capability.

Gear couplings achieve their misalignment capability through backlash in the teeth, and crowning on the tooth faces. Gear couplings also utilize a major diameter fit which helps the misalignment and assembly capability. Teeth on gear couplings normally use a 20° pressure angle but can also be made with 25° and 40°. The 20° tooth evolved over the years as the most wear resistant and strongest form. The 25° tooth is used on spindles to improve the strength at some sacrifice in wear, and the 40° tooth form is the strongest but is rarely offered anymore.

To keep learning, go to:

Gear Coupling Tutorial - Part II: Configurations 

Gear Coupling Tutorial - Part III: Mounting the Coupling

Gear Coupling Tutorial - Part IV: Selection & Availability

Gear Coupling Tutorial - Part V: Failure Analysis (with photos)

Note: This article series is an updated & modified version of a legacy Lovejoy training document. The blog writer is not the original source author.

Coupling Peak Torque Failure at Keyway

Gear Coupling Hub Burst Due to Over Torque

Coupling hubs failing, cracking, or bursting over the keyway corners is a common failure mode that clearly indicates the coupling hit a peak torque that it was not designed or built to take. The reason the coupling hub breaks at this point is because it is generally the weakest point on the hub. (There is less material between the corner of the keyway and outside diameter of the hub than any other point on the inside diameter of the hub.)

The root cause for this over-torque failure may be that the coupling was undersized for the given application (perhaps the wrong factor of safety was used when calculating required torque), that the application saw a massive unexpected peak load (and perhaps the coupling failing may have saved more expensive equipment from failing), the coupling bore and/or keyway were oversized relative to the maximum bore and keyway combination specified from the manufacturer for the given product size, or the coupling had excessive misalignment.

Jaw Coupling Hub Burst Due to Over Torque

Identifying the peak torque load and either eliminating it from re-occurring and/or ensuring the replacement coupling is designed to handle the peak torque is critical to ensuring this failure mode is not seen again. (Unless there was a material defect in the hub, which, at least for Lovejoy manufactured couplings, is extremely unlikely... simply replacing the hub will likely result in another hub failure.)

It is also important to note that, while undersized/underrated couplings are not good and can cause failures... it is also a mistake to significantly oversize a coupling. There are a number of drawbacks to this approach, including excessive overhung loads, potentially reduced performance, and cost.... and should be avoided.

For further information on sizing a coupling, please consider reading  "Coupling Service Factors - Best Practices" and "Coupling Sizing Torque - How to Quick Calculate". For further information on misalignment, please see "Top Reason for a Coupling Failure". 

To learn more on coupling failure analysis, go to:

 

Coupling Failure Analysis - Jaw Couplings (includes hub & spider photos)

Gear Coupling Tutorial - Part V: Failure Analysis (with photos)

Grid Coupling Failure Analysis (includes photos) 

Top Reason for a Coupling Failure

And, if you still have questions or concerns... please feel free to contact a Lovejoy application engineer or specialist.