Machinery's Handbook, 31st Edition
2726
SHAFT ALIGNMENT
For the motor, the change in shaft height is: .
. e H T 0 0000063 12 140 75 0 004914 inch # # ∆ = α ∆ = − = ^ h
These calculations indicate that the pump and motor shafts, if they were in alignment at ambient temperature, would be misaligned at operating temperature by approximately 0.0078 inch difference in elevation. Indicator Bracket Sag: Indicator sag, indicator bracket sag, or jig sag are all terms com monly used to describe the effect on indicator readings of the weight of the dial indicator(s) in combination with the length, weight, and orientation of the indicator bracket. Dial indi cators are designed to be used in the vertical position and indicator bracket sag is espe- cially significant in measurements that are made when the dial indicator is inverted in the vertical plane. Consider a rim indicator and bracket mounted on a shaft with the indicator zeroed at the 12 o’clock position; when the shaft and fixture are rotated to put the dial indi - cator in the 6 o’clock position, the reading will indicate a non-zero negative value, which is the indicator fixture sag factor. Indicator bracket sag is dependent on the mounting arrangement of the indicators, and is difficult to measure accurately on equipment that is being aligned because the effect of shaft misalignment will combine with the indicator bracket sag to produce incorrect results. To measure indicator fixture sag, set up the indicator and mounting components on the machinery to be aligned exactly as if doing alignment, and tighten everything down.Then, remove the bracket and indicator from the machine and remount in exactly the same man ner on a rigid fixture, such as a piece of pipe supported by V-blocks. Zero the indicator at the 12 o’clock (reference) position, then without changing the setup carefully rotate the fixture and read the indicator at the 6 o’clock (reading) position. At the reading posi - tion, the indicator displays the total indicator reading (TIR) and this value is the total sag factor . The actual sag in the indicator bracket is equal to one half of the total indicator reading. When the indicator bracket sag has been determined, it can be accounted for in measure ments taken during the alignment procedure. One procedure is to add the indicator bracket sag factor to the indicator value when the indicator is “zeroed” at the reference position, so that the sag factor is accounted for throughout subsequent calculations. For example, if an indicator is zeroed at the 12 o’clock position and reads 0.006 inch at the 6 o’clock position, set the indicator to positive 0.006 at the 12 o’clock reference position to account for the sag factor in further measurements. If the reference position used for checking indicator sag is the 6 o’clock position (indicator zeroed at 6 o’clock position), the positive reading found at the 12 o’clock position is the sag factor that must be subtracted from indicator value when the indicator is zeroed at the 6 o’clock (reference) position. Alignment Procedures for Machinery.— The face and rim method and the reverse dial indicator method are the techniques most commonly used for aligning machinery shafts. The usual shaft alignment procedure is to establish one machine as fixed, and the other as the movable machine. The driven machinery is usually the fixed unit. The shaft of the movable unit is positioned, as required, to align with the shaft of the fixed unit. Most align - ing specialists recommend vertical alignment first, and then horizontal alignment. It is important to verify in advance that the movable unit has sufficient horizontal clearance and freedom of movement to obtain the required horizontal position before starting on the vertical alignment. Vertical Moves: Vertical adjustments are accomplished by adding or removing shims from under the machine feet. Use as few shims as possible. Too many shims under a foot may cause it to act like a soft foot by creating a spring effect; usually a maximum of four shims are allowed. Shims should be measured by micrometer before placing. If possible, apply a single shim that is the total thickness required under the foot. Insert the shims all the way until the shim slot bottoms out on the bolt, then pull the shim back about a quarter inch before tightening the bolts. If alignment is not possible with four shims, then use fewer,
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online