By Bob Boyle, Director – Power Transmission Products, Applied Industrial Technologies
Shaft alignment is a critical component in keeping a coupling aligned. While most couplings can tolerate some misalignment, they operate much more efficiently and with much longer service lives when simple precautions and procedures are in place. One area of potential misalignment – yet one that is fairly simple to address – is that of thermal movement. Heat buildup from the friction generated by moving parts can cause machinery to expand which, in turn, can adversely affect shaft alignment.
The amount of thermal movement can be predicted by knowing the following factors:
- The construction material of the machinery
- Amount of temperature change
- Distance between shims and shaft centerline
Thermal movement can be calculated from:
G = TLC
where G = Thermal growth, mils; T = Temperature change, degrees; L = distance between shims and shaft centerline, inches; C = growth factor.
Common growth factors for various machinery materials are:
- Aluminum = 0.0126
- Bronze = 0.0100
- Cast iron = 0.0059
- Stainless steel = 0.0074
- Mild steel = 0.0063
Fig 1. Thermal growth for a pump/motor assembly.
For example, consider the simple pump/motor assembly shown above in Fig. 1. Both components are constructed from mild steel; therefore, growth factor C = 0.0063. For a temperature change T of 40 degrees and centerline distance L of 14 inches, thermal growth is:
G = 40(14)(0.0063) = 3.53 mils or 0.0035 inches.
Most machinery must be misaligned cold so that the shafts are collinear at normal operating conditions. Graphing the move above shows that the pump does not move (no change in temperature) and the motor rises an estimated 0.0035 in. (3.53 mils).
There are many ways to move machinery to align rotating equipment, and all must be controlled and accurate. Any alignment method, regardless of its accuracy in measuring misalignment, is useless if proper precautions are not taken and correct procedures are not followed.
Alignment specialists continually discuss whether vertical or horizontal moves should be made first. Typically, during the rough adjustment stage, once the machine is adjusted to its proper vertical alignment, slight horizontal adjustments will not change vertical alignment. This helps support the case that vertical alignment be done first.
Once rough vertical adjustments have been made, a horizontal move sometimes must be made to provide correction for alignment errors introduced during the adjustment stage. Any alignment process may involve some alternating from vertical to horizontal until precision alignment is reached.
Vertical moves: Dial indicators are not required to determine any change in the vertical direction, but they can be helpful to check for any spring effect or soft foot created when moving the machine. No shim should be installed without first verifying its thickness with a micrometer. Regardless of any information printed, stamped or etched on the shim, use a micrometer to double-check thickness.
Fig. 2 Foundation with jack screws installed.
Fig. 3 Moving large machines horizontally without jack screws.
Fig. 4 Indicator positioning to monitor moves.
When making vertical adjustments, never loosen all hold-down bolts at the same time. Rather, loosen only two bolts at a time. Loosening all of the hold-down bolts at one time may change the entire foot configuration if the machine is raised. This is particularly true if any of the feet have been corrected for soft-foot conditions using a tapered or step shim.
Also, leaving two hold-down bolts in place reduces the chances for an uncontrolled move. Uncontrolled movement of the machine can change its horizontal, axial or vertical positioning, negating any previous adjustments.
Loosen the bolts on either the left or right side of the machine. Then, raise that side just enough to make the shim change. Raising the machine too high can bend the feet that are still bolted down. Once the machine is raised enough to make the shim change, remove the shims from beneath each foot and add or subtract the number of shims required to offset the thermal misalignment.
It is always a good idea to use the fewest shims under a foot as is practical. The more shims under a foot, the more likely the chance of creating a spring effect, or soft foot. If possible, make or use a single shim that is the total thickness required under the foot. If this is not possible, a maximum of four shims should be the limit, giving the foot a solid resting place and reducing the possibility of anything getting between individual shims.
Make the necessary shim change, and tighten the bolts to their proper torque value. Then, repeat the process for the other side of the machine. Always tighten bolts to their prescribed torque on the first tightening to avoid any movement when working on the other side of the machine.
When making shim changes for vertical alignment, shims must be placed beneath any shims already present to correct for soft foot. Placing shims used to correct vertical alignment on top of those used to prevent soft foot can create additional soft foot problems.
Insert the shim all the way in until the shim slot bottoms out on the bolt. Then, pull the shim back about a quarter of an inch before tightening the bolt. If the shim is left inserted all the way into the threads, tightening the bolt will pinch the end of the shim, possibly affecting the accuracy of the shim change.
After completing the shim changes and tightening the hold-down bolts, take another set of readings with the method used in the alignment calculation. If the move is within the specified tolerance, proceed with the horizontal moves. If a second vertical move is required, determine the necessary shim change and make the correction.
Horizontal moves: Base plates installed with jack bolts are the easiest to move. In many situations, it is best to install jack bolts at the time of alignment rather than trying to move the machine some other more difficult and time-consuming way.
Where no jack bolts are present, small hydraulic jacks can be used to move a machine if it is backed up to a solid structure such as a beam, solid wall, adjoining pedestal or base. A chain fall or come-along hoist also can be used. The machine must be secured to something solid or immovable and operated one click at a time.
Another effective method is to use pony clamps or pipe clamps attached and operated to move the machine smoothly in the desired direction as shown below in Fig 3.
Where jack bolts are installed, all must be loosened before loosening any of the hold-down bolts. There is no need to completely back off or remove the hold-down bolts because very little pressure is required on the jack bolt to move the machine horizontally.
Constructing a graph to determine the position of the misaligned shaft will show exactly which direction to move the machine. Another way to avoid confusion is to look from the fixed component toward the movable component. Everything to the right – or three o’clock – position is in the positive direction. Everything to the left – or nine o’clock – position is in the negative direction.
The preferred and most accurate method for measuring the horizontal move is to place dial indicators around the machine in the planes of the feet. It is important to position the indicators on the machine in the location used in the alignment calculation. This is usually the center of the hold-down bold and at the approximate shaft height.
The procedure for performing a horizontal move on a machine is:
- Position the base-mounted dial indicators in the location of the machine’s feet or at the point determined from the alignment calculation
- Adjust the dials to zero
- Move the machine in the proper direction by the determined amount, making sure all of the dial indicators agree
Once all horizontal moves have been completed, a final set of readings should be taken to verify both vertical and horizontal alignment. Occasionally, a horizontal move can change the machine’s vertical position. If another vertical move is required, recheck the horizontal alignment.
The most important consideration throughout all of these steps is to maintain control of the machine. The most accurate and precise measurements can be lost in an instant with the wrong method or technique.
The Bottom Line...
Thermal movement is an often-overlooked factor in misalignment.
Control and accuracy are the keys to proper alignment.
A dial indicators is the most effective way to measure horizontal move.