Machinery's Handbook, 31st Edition
SHAFT ALIGNMENT 2721 When the shafts of two machines require alignment, the process generally calls for one of the machines to be permanently mounted and the other one to be movable. The fixed unit is usually the driven machinery, such as the pump in a pump-motor pair. The second machine (usually the motor) is moved into approximate alignment (by eye and straight edge, for example) in preparation for measurements that will determine the magnitude and direction of moves required to put it in final alignment with the fixed machine. It is the movable machine whose shaft will be aligned with the shaft of the fixed machinery. The position of the movable machine is adjusted vertically by adding and/or removing shims from under its feet, and horizontally by making small lateral moves as required until sat- isfactory final alignment is obtained. Shaft Alignment with Dial Indicators The material that follows describes the process of shaft alignment when dial indicators are used to measure the alignment data. Instruments and Methods.— Numbers of instruments are available for making shaft measurements and calculating moves, but the most important requirement for any shaft alignment system is repeatability of the readings. Dial indicators and lasers are two choice measuring systems. Dial indicators provide accurate and reliable measurement of shaft alignment. They are the most useful because they can be used to measure bearing alignment, shaft runout, and soft foot directly. Measurement accuracy down to 0.001 inch (1 mil) or 0.025 mm may be achieved if care is taken in mounting and reading the indicators correctly, and controlling or accounting for such variables as indicator sag, axial endplay in the shaft, and vibration from outside sources. The data obtained from properly installed dial indicators are con verted by equations described later in this section into the vertical and horizontal move ments required to bring the movable unit into alignment with the fixed unit. Laser measurement systems are another popular choice for shaft alignment work, although the cost of such systems is much greater than dial indicators. Accuracy of 0.0001 inch (0.0025 mm) or greater is possible, and setup and operation is generally faster and simpler than with dial indicators. Many laser systems can perform some or all of the calculations required to obtain the horizontal and vertical moves. Lasers may not be safe for use in explosive environments. Runout Check.— A runout test of either or both couplings is important only if there is a runout problem. Seldom will the runout of a pump or motor coupling be enough to detect with the eye alone. Standard practice is to position a rim and face indicator setup on the uncoupled couplings (one at a time) and roll said couplings through several 360° turns while monitoring the indicators. The indicator bracket should be firmly attached to a static object close to the pump or motor coupling being checked. Usually a magnetic base designed to hold indicators is used for this purpose if the two couplings involved are more than five or six inches apart. For an ordinary pump and motor, however, it is frequently convenient to mount the indicator jig to the motor to check the runout on the pump and then vice-versa for the motor. Fig. 3 shows a profile view on the left and a motor- or pump-eye view (right) of a typical runout check setup involving the use of indicators attached to a magnetic base. Assume the rim indicator to have been adjusted to zero, the coupling being checked is rotated sev- eral full turns. Repeated turns deliver repeated readings of a maximum minus of − 0.XXX inch at one extreme, and a maximum plus of 0.XXX inch at the other extreme. Assuming there is no paint or rust, etc. involved, at least three possibilities exist. 1) the coupling was bored off-center 2) the shaft is bent 3) a combination of the above two conditions
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