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I've heard that both of these coupling types introduce a degree of kinematic error in the driven shaft. What does it actually mean in practice? Is it that the driven shaft doesn't turn uniformly even when the input shaft does? But is this error arbitrary or the same amount of error is experienced for every revolution?

In fact, in what sort of application should one be used but not the other?

Any thought appreciated.

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  • $\begingroup$ It is the same error in each revolution - except that under light loads, the uneven speed may also cause chatter in the clearances between parts of the system (not necessarily in the coupling itself). The uneven speeds and load transmission can cause torsional vibration resonances at particular RPMs, which may accelerate fatigue failure even if they are not creating any "obvious" problems. $\endgroup$ – alephzero Mar 12 '16 at 2:43
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Both of these types of joints do not provide a constant velocity to the output shaft because the resistance to rotation of the joint varies throughout one rotation. This is an inherent feature of the geometry rather than a materials or manufacturing issue.

However a joint composed of two universal joints back to back will give a much better approximation of constant velocity, these are often found in steering columns. There are also designs which stack two universal joints concentrically.

Universal joints do have the advantage that they are simple to manufacture and tend to be quite rugged.

There are a variety of designs of constant velocity (CV) joints used for different applications for example the CV joints used in front wheel drive vehicles often consist of a spherical inner and outer shell joined by ball bearings located in grooves in both shells.

Key design considerations for selecting a particular joint type include :

  • The angle, or range of angles at which the joint needs to work
  • The speeds and loads of the system
  • Whether the joint will be subject to shock loading
  • Acceptable levels of vibration
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The short answer is that universal joints are tolerant of angular misalignment while Oldham couplings are intended for use where the shafts are misaligned but parallel.

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