Technical
Shaft Couplings - Correct Installation
|
 |
| |
Elastomeric Element Failure
|
Why do elastomeric flexible coupling
elements fail:
Excessive misalignment or parallel offset can destroy an elastomeric
coupling element very quickly.
- Excessive misalignment and parallel offset tends to stiffen the composition
of the couplings element and set-up increased internal forces
and stress levels the element has to work with. Cyclic variation is then
created causing vibration which will eventually destroy the element
structurally.
- The inherent balance and working conditions in a driving system can
cause dynamic vibration. It is a question of how much imbalance the
coupling element can tolerate working in these conditions. It is also
about how different flexible elements react to the severity of vibration.
A comparison can also be made between vibration and optimum
operating speeds.
- The material that coupling bodies are manufactured from can cause
variable operating conditions. When making a coupling selection
it is important to check the speed values of the various types of
couplings(rpm). Depending on the selection criteria different materials
will run smoother with less vibration and handle higher or slower
speeds than others. An example of this is the inherent speed restrictions
in some cast irons as opposed to steel. Balancing issues could
be quite critical at the higher operating speed end.
- Stress can also be set-up by reverse loadings or excessive dynamic
torque caused by frequent stops and starts. All of these conditions
have an effect on vibration and the ability of the flexible elements to
absorb and dampen these forces.
The information above is a brief overview of how misalignment and vibration
can affect the operation of flexible couplings. It can require a lot of
analytical skill and the help of vibration measurement equipment to be
fully conversant with this type of study. However the above information
gives the reader an idea of how important it is to maintain the correct
relationship in coupling alignment.
Bore and key fits can also be a cause for concern. Incorrect tolerances
can either loosen or tighten a shaft fit creating assembly and running
problems causing premature failure in the bore to shaft fit. This is why
it is so important that the fits are correct so as to prevent damage to the
coupling or shafts. As a guide couplings are usually bored so they are a
hammer assisted fit (H7/H8).
The failure of a flexible coupling can also be looked upon as a mechanical
fuse. Usually it is the cheapest part in a machinery drive system.
Coupling failure could also indicate that there could be other associated
mechanical problems.
Metallic Membrane Coupling Failures:
The same principles of misalignment and excessive
vibration can cause the premature failure of the steel
elements in metallic membrane couplings. The elements
tend to wear and break or suffer from stress which can
also cause breakage.
An example of the effects that force caused by misalignment has on four different coupling types
An example of the effects that vibration displacement has on four different coupling types
|
| |
|
| |
|
