Selection Criteria for Couplings

Selection Criteria

What does what?

FLEXIBLE SHAFT COUPLINGS

Flexible shaft-type couplings compensate for radial and angular misalignment though the flexure of a varying number of compliant elements. This type of coupling includes the multi-stage bellows, helical beam and radial slit concepts.

Points to bear in mind:

  • The greater the number of elements, the greater the angular and radial misalignment capacity and the lower the torsional stiffness.
  • The forces required to effect compliance are broadly proportional to the torsional stiffness. The stiffer the coupling in torsion, the higher the resulting bearing loads.
Membrane Disc Couplings

MEMBRANE (DISC) COUPLINGS

Thin pressed spring steel membranes act as the pivotal media in disc couplings. These are attached alternately to the drive and driven members, and provide flex to compensate for misalignment. Any torque is resolved to simple tensile stresses in the opposing segments of the membranes, which are free of residual stresses as no secondary forming operations are involved in their manufacture. Another advantage of this type of coupling is their near-infinite life and dynamically balanced construction, making them suitable for applications where high rotational speed and high-level motion integrity are required. Typical applications include closed loop servo systems in machine tools, robots, scanners, centrifuges, turbines and dynamometers. When selecting a disc coupling, the user can specify modified spring rates, longer/shorter intermediate members and either keywayed or ‘D’ bore.

Bellows Couplings

BELLOWS

The characteristics of the bellows coupling can be modified by varying the number and/or the wall thickness of the convolutions of the bellows. This type of coupling generally has high torsional stiffness and may be used in any drive system where high levels of motion integrity are essential. Typical applications include encoder drives in closed-loop servo systems. Coupling options include modified spring rates, along with keywayed and ‘D’ bores.

Two Stage Bellows Couplings

TWO-STAGE BELLOWS

As opposed to the multi-stage bellows, the two-stage version, as the name implies, has only two convolutions. Removing the additional convolutions has the effect of increasing torsional stiffness. By virtue of this benefit this type of coupling has limited misalignment capacity and is, therefore, ideally suited to high precision, high-resolution applications. These include main axis drives in closed loop velocity and position control systems, encoders, resolvers and tachogenerators. Options include keywayed and ‘D’ bores.

Flexible Beam Couplings

FLEXIBLE BEAM COUPLINGS

The beam coupling is made from one piece of material achieving its flexibility in all three modes; angular, radial and axial, by means of a slot or slots machined through the wall of the material. Most commonly, the slots are machined helically around the circumference of the coupling. Straight radial slots are also sometimes used. Helical beam couplings may have one two or three start helices, a three-start helix providing the highest level of torsional stiffness and hence signal accuracy.

Even higher torsional stiffness can be achieved with straight radial beams, however, this is at the expense of radial and angular flexibility.

As with other types of coupling, increased radial compliance is achieved by joining together two flexible coupling ‘stages’ separated by a spacer.

Double Loop Couplings

PLASTIC DOUBLE LOOP COUPLINGS

This type of coupling uses a moulded plastic element permanently swaged to steel or stainless steel hubs to form an effective two-stage coupling with exceptional flexibility in all three modes. Ideal for transmitting rotation in small drives, this type of coupling works without any friction, wear or noise, although its low torsional stiffness makes it less suitable for high precision positioning applications.

Jaw Couplings

JAW COUPLINGS

Most commonly used to provide some flexibility and misalignment compensation in high power transmission systems, the jaw type coupling achieves its flexibility by means of a plastic element sandwiched between metal hubs. If the hubs are not in true alignment the element deforms to accommodate this difference. Flexibility is somewhat limited as the plastic is compressed, storing energy like a spring, which creates a high resistive force that can cause excessive radial loads to be transmitted to the shaft bearings.

LATERAL DISPLACEMENT COUPLINGS

By using an intermediate member that slides in a plane perpendicular to the axis of rotation, lateral displacement couplings accommodate both radial and angular misalignment. By virtue of its orbital motion the coupling aligns one hub, then the other: in effect, straddling the misalignment. The use of a mechanical sliding contact means that quite large radial errors — which are proportionate to the coupling’s diameter — can be overcome without the penalty of a long coupling.

Points to bear in mind:

  • The greater the distance between the pivotal planes (see figure 17 below), the greater the radial misalignment capacity.
  • Torsional stiffness reduces marginally depending on the length and stiffness of the intermediate member.
  • Angular misalignment capacity cannot be increased beyond the coupling’s basic capability, irrespective of the distance between pivotal planes.
Figure 17

Fig.17 Two single-stage couplers locate a fully-floating shaft on a stable axis of rotation.

Oldham Couplings

THE OLDHAM

This three-part coupling transmits rotation through a central plastic disc that slides over the tenons on the hubs under controlled pre-load to eliminate backlash. The disc can be manufactured from a variety of engineering polymers to suit many different applications. These range from the incremental control of fluid valves to positional systems in machine tools, robots and slide tables. They can also be applied to microstepper and closed loop servo systems and, to a lesser extent, half and full step motor drives. They are available with keywayed or ‘D’ bores in through bore types, and also with radiation and heat resistant torque discs and free running discs (no pre-load)

Uni-Lat Couplings

THE UNI-LAT

To combat angular and radial misalignment this coupling type combines the sliding mechanism of the Oldham (see above) with the pivotal action of the universal joint. The process uses a series of integral pins engage a pair of injection moulded annular rings that feature controlled pre-load to eliminate backlash. The main features of the UNI-LAT are the generous angular and radial misalignment capacity, along with the fact that they are electrically isolating. The application area for these couplings is found in general purpose, light-duty stepper (half and full step) encoder, resolver and tachogenerator drives, and light pull/push duties. Normally supplied with ‘D’ bores, the UNI-LAT can also have other features machined into the hubs.

Universal Joints

UNIVERSAL JOINTS

Universal joint couplings use a mechanical pivotal action controlled by radial bearings. In the case of the Huco-Pol range, these couplings are injection moulded in Acetal and benefit from controlled pre-load to eliminate backlash. This type of coupling has a large offset capacity, along with good torsional damping, water resistance and the added benefit of being lubrication-free. Universal couplings are ideal general-purpose units used typically in light-duty drives in the food, textile, paper handling and packaging environments. Connection options include gears, pulleys, square, keyways and hexagonal bores, and ‘D’ bores with spring clips and with non-pre-loaded bearings.

Cardan couplings also use a two-stage pivotal action with defined fulcrums to handle radial and angular misalignment.