Machinery's Handbook, 31st Edition
SHAFT ALIGNMENT 2723 there is serious coupling runout. A runout check is mainly to have recorded information if excessive vibration occurs. Soft Legs.— One of the most often overlooked precautions relevant to motor alignment is the firming up of the soft legs. For the motor in Fig. 5, one leg (D) is 1 ⁄ 2 inch short. In align ment jargon, the short leg is said to be soft . It is also clear that if a 1 ⁄ 2 inch shim were placed under the short leg, all four motor feet would firmly support the motor. On the other hand, if the motor were tightened down without the use of proper shims, its support would be very spongy and would produce non-repeatable indicator readings.
1/2” GAP
A
B
D
D
C
MOTOR BASE
(a) (b) Fig. 5. Profile (a) and Overhead (b) Views of Motor with Soft Foot
On an ordinary electric motor any of the four motor feet may be either raised or lowered via the placement or removal of shims, as applicable, to render the motor equally sup- ported by all four feet. Geometry may allow a certain freedom of choice, so it is practical to be very selective about which motor foot or feet will receive additional shims during the firming up of the motor. When a soft leg involves only a few thousandths of an inch, it is much less obvious than in Fig. 5. By using what is known to some mechanics as the rock-a-bye method, it is possi ble to detect a slight amount of instability in the motor supports. With the bolts inserted, but only partly threaded in, alternately apply heel-of-the-hands pressure on or above two motor feet at a diagonal. If no rock-a-bye is evident when heel-of-the-hands pressure is alternately applied above feet A and D, switch to B and C. If rock-a-bye is found, remove applicable shims if possible or add shims under one or both of the unstable feet. For the 1 ⁄ 2 inch (12.7 mm) gap shown in Fig. 5, downward heel-of-the-hands pressure at points A and D would yield more movement than the indicators could handle. On more subtle soft feet, it takes an experienced feel to detect only a few mils of rock-a-bye at a motor foot. When feel becomes too little to be certain, it is time to apply indicators. On units where 0.010 inch (0.254 mm) or more of movement is evident, a tapered wedge gauge can be inserted under the motor foot as the motor is gently rocked. When the motor will no longer rock, the approximate thickness of the wedge gauge under the motor foot will be the amount of shim material to insert under that foot. If a single foot requires more shim thickness than is desirable, put half the thickness under the soft foot and the other half under the opposite diagonal foot; then, a second slight amount of shim placed under one of the two feet should remove any unwanted slack. During the rock-a-bye test, trial and error shimming will eliminate any obvious soft feet in a few tries. The careful step- by-step elimination of unwanted slack in the support feet of a motor or other movable unit should result in a very stable movable unit. On larger motors, leverage proportional to resistance should be used, involving any- thing from a small pry-bar to hydraulics. When the rock-a-bye method gets so involved, it is advisable to have dial indicators mounted at the applicable motor feet and/or at the motor coupling, with the indicator button on top of the pump coupling or comparable static object, to register any movement.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online