Selecting a flexible coupling for a machine is probably the easiest part of a drive design. First check your company's past practices to see which couplings have been the most reliable in similar applications. Then, using a design guide or catalog, plug in the application data and pick an appropriate size. Be sure to apply a service factor so that the selected coupling will be large enough for the application.

At this point, your work may appear to be done. But now consider how the selected coupling may affect the life of other components. Coupling failures are rare, so you probably wouldn't suspect that they contribute to machine failures. However, they generate forces that are absorbed by shafts and bearings, which often shortens bearing life. Some of the most common factors that contribute to shorter life are coupling imbalance, torsional resonance, and shaft misalignment.

Imbalance

At or below 1,800 rpm, coupling imbalance is rarely a problem. This is especially so with units made entirely of relatively rigid materials and essentially concentric - such as disk, gear, grid, or elastomeric jaw (spider) couplings. In many cases, these devices run safely at speeds up to 3,600 rpm without excessive forces or vibration from imbalance.

Above 3,600 rpm, it's difficult to ensure reliability. For example, imbalance of elastomeric tire couplings may cause excessive machine vibration. Most of the imbalance results from manufacturing variations. Either the elastomeric inserts are not well balanced or the clearances between the inserts and mating hub are too large. Therefore, couplings running at such speeds should be balanced prior to assembly in a machine.

Also, check the balance where couplings are mounted on long flexible shafts, where they contribute most of the bearing loads, and where they are used in machines with flexible foundations.

You can achieve balance in several ways: through tighter tolerances on parts and fits, by balancing individual components (for couplings with clearances between parts), and by balancing a complete assembly (for couplings without clearance between parts).

AGMA standard No. 9000-C90 offers guidelines for the degree of balancing in terms of a balance class number based on the operating speed range. ISO standards 2372-1974 and 10816-1 (1995) give more general information on machine vibrations.

Torsional resonance

Torsional vibration occurs in a drive system when a component operates at its natural frequency or critical speed. Typically, two portions of a rotating component don't maintain the same relative position as the piece rotates. During one revolution of a shaft, for example, a point on the output end may lead and then lag the corresponding point on the input end due to twisting.

All components have torsional resonances, and they can occur at multiples of the critical torsional frequencies. The resultant problems sometimes show up as failed coupling elements, broken shafts, or even broken gear teeth in speed reducers. More often they appear as greatly reduced bearing life because of fatigue forces on the ball or roller cages.

Before selecting a coupling, determine its torsional stiffness, which is usually available from the manufacturer, plus the rotational inertia of the drive components. For an elastomeric coupling, you also need its damping factor. From this data, calculate the drive system's torsional critical frequencies, using an FEA-based simulation software package. Also ask the manufacturer what variations in critical frequency take place with changes in load, temperature, and coupling gap spacing. This type of analysis is particularly important for couplings that are torsionally "soft" because their critical frequencies occur at lower speeds that are likely to fall within the operating range.

Torsional vibration can sometimes be avoided either by shifting the operating speed out of the critical speed range, or by changing the weight and inertia of components to move the critical speed away from the operating speed. In most cases, however, these methods are impractical. Therefore, a different type of coupling is used to move the critical speed out of the operating range.

Flexible couplings offer a wide range of torsional stiffness and damping properties to protect equipment against shock and vibration. For example, elastomeric couplings come in two basic types, compression and shear. Those that use elastomeric elements in compression can transmit high loads, whereas those with elements in shear offer high damping of torsional vibration.

Misalignment

When couplings flex to accommodate misalignment between connected shafts, they produce radial reaction forces that are resisted by support bearings and shafts. Catalogs usually provide guidelines on how much misalignment is tolerable, but they rarely give the reaction forces that result from misalignment. Consequently these forces are often overlooked as a selection factor. Ask the vendor for such information. If unavailable, you may have to test the couplings to get the values.

These reaction forces can shorten the life of shafts and bearings. Therefore, determine what forces will be generated and how they will be absorbed before selecting a coupling. After getting these forces for the initial static misalignment of the machine, simulate variations in operating conditions such as loads and temperatures to find the range of reaction forces during normal use. As with vibration analyses, a computer simulation may be helpful.

The magnitude of these reaction forces depends on the coupling design and how it flexes to accommodate misalignment. Couplings flex through different methods, including lateral sliding, radial flexure of metallic members, and elastomeric deflection.

Sliding couplings generally produce low reaction forces that are nearly independent of radial shaft deflection. Those with flexible metallic elements tend to produce higher reaction forces that are proportional to the amount of misalignment. Jaw couplings, which are widely used elastomeric types, also produce reaction forces proportional to radial shaft deflection. However, these forces are generally higher than with the other two types. Soft elastomeric inserts can be used in these couplings to minimize bearing forces when shafts are misaligned.

Neville Sachs is the president of Sachs, Salvaterra & Associates Inc., Syracuse, N.Y.

Cutting component life

To better understand how coupling factors can reduce the life of other components, consider these basic points:

  • Ball and roller bearing life is inversely proportional to the 3rd power (ball bearings) or 3.3 power (roller bearings) of the load. Doubling the load on a ball bearing reduces its life by a factor of eight.
  • Radial acceleration forces, such as those from imbalance, are proportional to the square of the rotational speed. Therefore, doubling the speed increases the force by four times.
  • An 1,800 rpm motor, running 24 hr/day, 365 days/year rotates almost a billion times per year.