Page 190 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Figure 4-20. Chalking fan shaft to deter-
mine balance.

Before balancing a wheel for any reason, check the troubleshooting pro-
cedures in the operating and maintenance manual and also in Volume 2 of
this series.
Portable instruments are available that indicate vibration displacement
in mils (1 mil = .001 in.) or microns in metric system (1 micron =
1 x 106 mm). Use the manufacturer’s manual or generalized data from
Volume 2 to determine when a fan is operating with too much vibration.
nble 4-2. illustrates vibration guidelines as a hnction of fan speed. Note
that vibration velocities give constant parameter independent of shaft
speed, whereas allowable displacements vary with speed.
Wheels can be balanced by using methods described in Volume 3 of
this series.

Wheel Hub Disassembly
Depending upon the application for which the fan is intended and the
operating speed and temperature, major blower manufacturers employ at
least six different methods of hub-shaft attachment.
If the fan wheel must be disassembled from the shaft for service, obtain
information on the type of hub-to-shaft fit and proceed as follows:
1. Standard Clearance Slip Fit-Used for most moderate applications.
Wheel and shaft are fitted together in the factory, but shipped apart
for field assembly unless otherwise specified. Set screws hold the
hub to the shaft.
2. Close Tolerance Fit-While not an interference fit, this design re-
quires assembly at the factory because the very tight clearance be-
tween bore and shaft requires hydraulic jacks for mounting. To ease
assembly, the hub bore and shaft have stepped diameters with the