problems, which include shaft misalignment, pulley misalignment, belt wear,
belt resonance, belts too tight, belts too loose, pulley eccentricity and
bent shafts, can be relatively straight forward to detect but can be far
more difficult to specifically diagnose and correct. That is mainly due
to the wide variety of problems that can occur in the installation and
assembling of the belt drive, the difficulty of doing field testing on
belts and the possibility of other influences (i.e. the base) having some
|It is important
to realize that some of the belt-drive vibration problems
listed above do NOT cause vibration at belt related frequencies.
Problems due to the shafts or pulleys (misalignment, eccentricity, etc.)
cause vibration at 1x rpm of the component with the problem (i.e. eccentric
pulley on the fan causes vibration at 1x rpm of the fan). Worn belts, on
the other hand, will cause vibration at harmonics of belt running speed.
|The good news,
especially in the case of component (belt and pulley) wear, is that belts
and pulleys are typically relatively easy to inspect and inexpensive to
replace. The bad news is that outside of that, they're often difficult
to correct. One positive development in recent years has been the availability
of laser alignment units for belt drives for a moderate price. Unfortunately,
in more cases than not the old string & straight edge is still the
alignment method used for belt drives. The first step to identifying a
belt problem is to determine the belt speed.
the Belt Speed:
belt speed can be a bit difficult but there are a few tricks. Some methods
are listed here:
It can be calculated
mathematically if you know some of the variables: belt length, pitch diameters,
center distances, etc.; but usually that is not the case. The formulas
are listed below.
it. With a bit a practice and understanding
of a simple technique, an analyst can actually extract the
probable belt rpm from the spectrum. One important requirement for this
technique to be successful - there must be at least some
vibration at belt-related frequencies. The following steps should be used:
Detecting it with
a strobe light is very difficult since it is usually a slow flash rate
and the mark used may be unreliable (lettering on the belt, etc.).
A photoeye will
be very accurate but will require proper setup and a mark applied to the
would be the best option for an accurate belt rpm since it does not require
a traditional "mark" - a good one will operate on pattern recognition.
any driver and driven related peaks (1x rpm and harmonics). Label them
or make a mental note of which peaks they are.
cutting the belt in half and wrapping it around one of the pulleys. How
many times will it wrap around - twice ? three times ? This will give you
a very rough estimate in your mind of the belt speed (if it wraps 3 times,
the speed would be 1/3 of that pulley's speed).
Finally, do the
Display your velocity
spectrum on a logarithmic scale.
Move your spectrum
cursor to your estimated belt rpm and turn on the harmonics.
Move your cursor
left and right in the smallest increments possible (some software allows
movement of 1/10th of a line of resolution - this helps with identifying
harmonics) and try to get the harmonics to line up on top of any significant
but previously unidentified amplitude peaks.
If there are significant
belt related peaks on the spectrum, you should be able to get them lined
up at some point.
If you cannot
find any pattern of previously unidentified peaks of significant
amplitude, that means one of two things:
Either you do
not have the spectrum resolution necessary, or;
There is no significant
belt vibration (in which case, why do we need to know the belt rpm ?).