Page 128 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Rotating Machinery: Practical Solutions

Although bolts often fail in the thread area, since this is the
minimum cross-sectional area and also an area of stress concentra-
tion, Figure 8-3 shows an exaggerated effect of over torquing.
Using 72% of the minimum tensile strength as a limit, the 1-
1/4 inch grade 8 fastener with a cross-sectional area of .9646 and
a tensile strength of 150,000 psi would safely produce a clamping
force of 104,209 pounds. If this fastener were stressed beyond its
elastic limit such that there was a reduction in its diameter of .15
inches, its effective cross-sectional area would be reduced to .7212
square inches, and its clamping force limit would be reduced to
77,844 pounds, or 74.7% of its original effectiveness.
A table of recommended torque values is provided in the
appendix section. Always use the proper grade fastener and fol-
low the equipment manufacturer’s recommended torque values.
The table of torque values in the appendix is provided as a general
guide only.

COUPLINGS

When utilizing this method of alignment, it is not necessary
to disassemble or remove the coupling. However, a complete in-
spection of the coupling is highly recommended. The coupling
and its associated components are potential sources for machine
unbalance. Refer to Figure 2-8 for potential sources of coupling
unbalance.
First, inspect all the coupling bolts to assure they are all the
same length, diameter and grade, and that they all have the same
number of nuts and washers. Second, inspect the keys and key-
ways of the coupling, driver, and driven equipment. Most equip-
ment manufacturers balance their rotating assemblies with a full
half key in place. That is they completely fill the key way with key
material, but only to the surface of the shaft or bore. If equipment
is assembled with a full-length, full-height key or a full-height
partial-length key, unbalance may be introduced into the machine.
Although this may seem like splitting hairs, recall from the
examples in Chapter 2 that it is the seemingly minor things that