Trying to maintain smooth, precise motion can be an insurmountable challenge in the presence of vibration, which causes unwanted machine movements, loss of register, and improper operation of electronic controls.

Typically, vibration is reduced by balancing the rotating component (rotor) of the driving or driven machine, detuning system natural frequencies to avoid operating speeds, or adding dampers (flexible machine mounts). But, vibration can also be controlled by redirecting machine misalignment so that coupling forces counteract shaft motions. This technique can be more economical than disassembling and rebalancing the equipment.

Unbalance starts it all

Any rotating component (rotor) of a machine has some residual unbalance, even after balancing is performed. This unbalance produces vibration inside bearings within the bearing clearances. These clearances range up to 1 mill (0.001 in.) for rolling-element bearings and 3 to 8 mills in plain bearings, depending on shaft size.

Essentially, couplings influence machine vibrations in two ways: by design, and through misalignment.

Design influence

For analysis purposes, a machine shaft can be divided into two parts: the portion between bearings, and the portion that protrudes outside the machine housing. Computer analysis of most machine shafts show that the influence of couplings on vibrations of the portion between bearings is negligible, particularly when the two portions have different diameters.

Most machine systems (two machines connected by a coupling) are designed to operate at least 20% away from any resonant frequency. This reduces the chance of operating at resonance speed, even with adjustable-speed equipment, and reduces the machine’s sensitivity to unbalance.

To ensure smooth operation, machinery manufacturers perform precision balancing of machine rotors. But, these manufacturers have little control over the portion of the shaft that protrudes from the housing. The responsibility of coupling selection and vibration analysis for the protruding shaft rests with the packager — the shop that connects two or more machines through couplings. Unfortunately, few packagers give sufficient attention to this aspect of installation; few couplings are even balanced.

In a typical coupling application, Figure 1, the resonant frequency of the protruding shaft is estimated by:

f = (0.4 × 10⁶ × d²)÷√WL³

where
f = Resonant frequency, cpm
d = Shaft diameter, in.
W= Coupling weight, lb
L = Overhung distance between coupling cg and bearing centerline, in.

Because coupling weight rests on the shaft through a flexing element (gear mesh, disk pack, or rubber element), it is safe to assume the cg is at the flexing-element centerline.

When the calculated frequency falls too close to the machine speed, designers can select a different coupling, which is either lighter or has a cg closer to the bearing.

Problems can occur later if the selected coupling is replaced with a different type without considering the possible change in resonant frequencies. Consider two similar couplings, Figure 2, which have flexible disk-pack elements. The overhung distance, L1, differs between the two because in (a), the disk pack is placed on the hub, whereas in (b), it is outside the hub. Replacing coupling (a) with coupling (b) lowers the resonant frequency of the shaft, and could make the machine more sensitive to rotor or coupling unbalance. Users that do not recognize this problem might spend (unnecessarily) a lot of time and money rebalancing the machine to restore smooth operation.

Coupling reactions

Misalignment produces two effects in couplings:

• Reaction forces that try to restore alignment. For most non-lubricated couplings, restoring force is proportional to the misalignment. Reactions also depend on the type of flexible element (rubber, urethane, or metal disk) and coupling size. In lubricated couplings, reactions are torque dependent, but can also vary with the lubricant type and coupling wear. In either case, restoring forces are resisted by the machine bearings.

• Heat. In lubricated couplings, misalignment causes the hub teeth to rub on other surfaces, which generates heat. A larger misalignment increases the sliding distance, thereby generating more heat. In elastomeric couplings, misalignment causes flexing, and the inherent damping of elastomers produces heat. Larger misalignment increase flexing, thus generating more heat. High temperature of the flexing elements has a negative effect on the ability of all couplings to transmit torque: heat softens elastomers and degrades the lubricating properties of grease.

Continue on Page 2