How coupling types affect bearing forces
When they flex to accommodate misalignment between connected shafts, all flexible couplings produce radial reaction forces on shafts and bearings. But designers seldom consider these forces (loads) when selecting couplings and bearings.
The reaction forces may seem surprisingly high to engineers who never considered them before. In some equipment, especially delicate instruments that have slender shafts running in fragile bearings, such forces can shorten the life of these bearings and shafts.
Radial misalignment of parallel shafts causes larger reaction forces than angular misalignment. However, angular misalignment seldom occurs by itself — without some accompanying radial misalignment.
The magnitude of these forces depends on the coupling design. Flexible couplings must provide both torsional stiffness in the direction of rotation to minimize lost motion due to wind-up, and compliance (low resistance) in the radial direction to accommodate misalignment with minimal reaction forces. Couplings achieve these selective functions through different means including sliding contact, transverse flexure of metallic members, and elastomeric deflection. All of these approaches are commonly used in positioning applications, but some produce lower radial loads than others, thereby offering more bearing protection.
Sliding contact couplings
By sliding laterally, interlocking members of sliding contact couplings accommodate shaft misalignment. The same interlocking members transmit torsional loads through rotation. Reaction forces in these two modes are independent, the first being a function of the coefficient of friction between the sliding surfaces, the second a function of the modulus of rigidity of the members. For this reason, such couplings can offer different torsional stiffnesses independent of radial reaction forces.
The Oldham and universal-lateral types, Figure 1, are two examples of sliding contact couplings. They use floating structural members with bearing surfaces that engage mating surfaces on the two facing hubs. The floating member acts as a two-axis slider on the hubs, aligning first with one hub, then the other, to accommodate misalignment while remaining in full torsional engagement.
With these couplings, friction between the sliding surfaces produces radial forces that are independent of radial deflection as shown by the flat curves in Figure 2. In such a device, antibacklash preload causes radial forces comparable to those of flexural couplings at the lower end of the misalignment range. But, sliding contact couplings are better able to tolerate severe misalignments while keeping reactive bearing forces low.
Flexural couplings
With flexural type couplings, Figure 3, thin metallic (or synthetic) members flex radially, thus differing from elastomeric types, which are discussed later. Some of these couplings have one or more flexible members along the coupling’s length.
The reaction, or resistance, of flexural couplings to radial (and angular) misalignment increases proportionally with shaft deflection, Figure 2. This reaction can be defined as a spring rate and expressed as force per unit deflection.
Because flexural couplings accommodate misalignment in a bending mode, reaction is proportional to the flexing member thickness. In some types, such as the membrane and bellows couplings, the flexing members can be thin because torque is transmitted with the members in shear. Thus, high torsional stiffness is attained with relatively low radial forces.
On the other hand, couplings such as the beam and flexible leaf, transmit torque with the members in bending, the same mode used to accommodate misalignment. Here, relatively thick members are needed to obtain a reasonable level of torsional stiffness. But, making the coupling torsionally stiff causes proportionally higher radial reaction forces.
All of these flexural couplings accommodate radial shaft misalignment by “bending” through two complementary changes of direction. The flexible leaf and membrane couplings both operate like a Cardan U-joint, bending at two fixed points. Beam and bellows couplings, though, have multiple flexing elements (coils and convolutions respectively) and operate like a flexible shaft, bending through two complementary sweeps.
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The magnitude of radial forces depends on the severity of the bend. It follows that keeping bending angles to a minimum does the same for the bearing forces, but inevitably reduces misalignment capability. With flexible leaf and membrane couplings, you can boost the misalignment capability by inserting a central member between two flexible members. The longer distance between flexure points enables them to operate with shallower bends for a given radial shaft offset, and so reduces the radial forces.
Couplings with multiple flexing elements (beam and bellows) can also use this approach, but generally require extensive modifications to divide the flexing elements into two separate clusters. Some short models are available that can be connected by an intermediate shaft.
Flexural couplings can run at higher speed (rpm) than sliding contact couplings. But to accommodate comparable levels of radial misalignment and produce comparable radial forces, flexural couplings must generally be much longer than their sliding contact equivalents. This usually adds to the cost as well.
Elastomeric couplings
Certain coupling types incorporate flexible elastomeric elements and offer a range of torsional damping properties. But their associated characteristics — relative rotation between hubs, and backlash — can be counter-productive in precision motion control applications. Depending on the coupling type, elastomers are loaded in shear, compression, bending, or combinations of these modes.
One example of an elastomeric device, Figure 4, the ubiquitous jaw coupling is probably the most widely used general purpose coupling. This device transmits torque and accommodates misalignment by compressing legs (or lobes) of an elastomeric insert between its jaws.
As with flexural couplings, the reaction forces of elastomeric jaw couplings are proportional to radial shaft deflection, Figure 2. However, the ability of these couplings to handle radial misalignment and minimize bearing forces is generally less than for other types. Therefore, the jaw coupling is best for applications where the connected shafts can be manipulated into nearalignment so as to keep bearing forces within an acceptable range.
Softer elastomer inserts can be used to accommodate radial misalignment and minimize bearing forces. However, the trade-off is a commensurate reduction in torsional stiffness. Two jaw couplings mounted back-to-back will accommodate radial misalignment, but are rarely used this way, usually because of added cost.
Getting the facts
Vendor’s catalogs rarely give couplinginduced reaction forces, consequently designers overlook them as a selection factor. Figure 2 offers general guidelines for these forces. However, for specific coupling types, it is best to request such information from the vendor. If unavailable, set up a simple test rig to determine the values yourself, Figure 5. Two couplings in tandem avoids the need for linear slides to accommodate lateral deflection.
Torsional stiffness is another important factor in selecting flexible couplings, especially those used in precision positioning applications. Figure 6 shows the torsional stiffness curves for the coupling types described.
Wally Walden is the founder and president, Huco Engineering Industries Ltd., Hertford, England, with offices in San Rafael, Calif.