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
|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.
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 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.