Page 89 - An Introduction To Predictive Maintenance

Machine-Train Monitoring Parameters 79

machine-train components is that the looseness or slack in the chain tends to modu-
late and amplify the tooth-mesh energy. Most of the forcing functions generated by a
chain-drive assembly can be attributed to the forces generated by tooth-mesh. The
typical frequencies associated with chain-drive assembly monitoring are those of
running speed, tooth-mesh, and chain speed.

Running Speed
Chain-drives normally are used to provide positive power transmission between a
driver and driven unit where direct coupling cannot be accomplished. Chain-drives
generally have two distinct running speeds: driver or input speed and driven or output
speed. Each of the shaft speeds is clearly visible in the vibration profile, and a dis-
crete narrowband window should be established to monitor each of the running speeds.

These speeds can be calculated using the ratio of the drive to driven sprocket. For
example, where the drive sprocket has a circumference of 10 inches and the driven
sprocket a circumference of 5 inches, the output speed will be two times the input
speed. Tooth-mesh narrowband windows should be created for both the drive and
driven tooth-meshing frequencies. The windows should be broad enough to capture
the sidebands or modulations that this type of passing frequency generates. The fre-
quency of the sprocket-teeth meshing with the chain links, or passing frequency, is
calculated by the following formula:

Tooth - Mesh Frequency = Number of Sprocket Teeth ¥ Shaft Speed

Unlike gear sets, a chain-drive system can have two distinctive tooth-mesh frequen-
cies. Because the drive and driven sprockets do not directly mesh, the meshing fre-
quency generated by each sprocket is visible in the vibration profile.

Chain Speed
The chain acts much like a driven gear and has a speed that is unique to its length.
The chain speed is calculated by the following equation:

¥
Number of Drive Sprocket Teeth Shaft Speed
Chain Speed =
Number of Links in Chain
For example:

¥
25 teeth 100 rpm 2500
Chain Speed = = =10 cpm =10 rpm
250 links 250
5.2.2 Couplings
Couplings cannot be monitored directly, but they generate forcing functions that affect
the vibration profile of both the driver and driven machine-train component. Each