Selecting for Cost, Duty and Life Expectancy
The couplings manufactured by Huco fall into two classifications: mechanical and flexural. The former work through sliding contact, while the latter rely on the flex of the constituent material. The pertinent issues relating to cost, duty and life expectancy are given below.
Oldham or Uni-Lat couplings should be considered when:
- Cost is the paramount consideration
- The backlash-free life requirement is within the coupling’s backlash-free life expectancy or backlash can be tolerated
- The coupling is expected to transmit only incremental or periodic rotation
- The duty is 50% or less, i.e. the coupling is stationary for half of the time or more
- Radial misalignment is severe and the available space is limited
- Radial misalignment is difficult to predict or maintain
- Slight torsional damping is beneficial
- A three-piece coupling is advantageous. With the Oldham coupling the drive can be connected/disconnected with the hubs in place. The wear element is renewable
- Electrical isolation of shafts is required
- The coupling is required to transmit longitudinal motion (push/pull)
Bear in mind that Uni-Lats have a more pronounced damping characteristic, lower torque capacity and generally run more quietly than Oldham couplings. They also have a greater angular misalignment capacity, though this is only useable at low speeds. Oldham-type couplings, though, are more robust and the replaceable wear element can be supplied in both heat and radiation resistant plastics. The hubs on the standard series are blind bored to a controlled depth, while the X-Y series couplings have through bores and have two-three times the backlash-free life. Torque discs are solid but can be specified with a bore to allow the passage of a shaft, although this will reduce the torsional stiffness of the coupling.
Both Uni-Lat and Oldham general-purpose couplings are suitable for position control. Specifically Uni-Lats are better suited to full and half step motor drives and Oldhams are suited to micro-stepper and closed loop systems.
Huco-Flex disc, bellows or multi beam couplings should be considered when:
- Torsional stiffness is a critical parameter
- The backlash-free life requirement is beyond the capacity of the Oldham or Uni-Lat
- Speeds are typically higher than 3000 revs/min
- Rotation is continuous or the duty-cycle exceeds 50%
- A coupling with axial compliance is required to protect fragile bearings from thrust load
- There is little risk of the alignment errors exceeding prescribed limits during initial installation or on subsequent replacement of the motor, encoder, etc.
- The environmental conditions favour an all metal coupling
Although the life expectancy of the Huco-Flex bellows coupling is not as high as the comparable Huco-Flex disc coupling, size for size it offers the highest torsional stiffness ratio and provides a high level of translation accuracy. This makes the bellows-type coupling ideally suited to intermittent applications.
Huco-Flex disc couplings have a greater reliability and near infinite life when used within their torque and misalignment ratings. They also provide a high level of translational accuracy and their spring rates can be modified through varying the number and thickness of the stainless steel membranes. However users must be aware that couplings designed around a flexural system can fail with little or no warning, causing immediate loss of drive. The cause of these failures is due mainly to metal fatigue caused by sustained flexure above the coupling’s recommended torque and compliance factors.
Failure in mechanical couplings is more subjective and useful life can vary depending on individual applications. For instance, in zero backlash applications the coupling is deemed to have failed as soon as backlash is in evidence. In other applications the failure threshold may be 2 degrees of backlash.