Shaft Alignment Shim Calculator
Calculate front and rear movable-machine shim changes from coupling offset, angular slope, foot spacing, thermal growth target, and allowed shim stack limits.
Choose a common machine train, then adjust the readings to match your latest vertical alignment measurement.
Shim Correction Result
Use these references to compare your result with common alignment tolerances, shim stock choices, and machine geometry ranges.
| Machine Speed | Offset Target at Coupling | Angular Target | Typical Use |
|---|---|---|---|
| Under 900 rpm | Up to 5 mil | Up to 1.0 mil/in | Low speed conveyors, mixers |
| 900 to 1800 rpm | Up to 3 mil | Up to 0.5 mil/in | General pumps and fans |
| 1800 to 3600 rpm | Up to 2 mil | Up to 0.3 mil/in | Precision motors and compressors |
| Above 3600 rpm | Up to 1 mil | Up to 0.2 mil/in | High speed rotating equipment |
| Shim Thickness | Metric Equivalent | Best Use | Stack Note |
|---|---|---|---|
| 0.001 in / 1 mil | 0.025 mm | Final trim | Avoid many thin layers |
| 0.003 in / 3 mil | 0.076 mm | Small repeatable correction | Useful after bolt tightening |
| 0.010 in / 10 mil | 0.254 mm | Common foot moves | Good base layer |
| 0.050 in / 50 mil | 1.270 mm | Large height build | Use with fine top shims |
| Machine Train | Front Distance | Foot Span | Reading Pattern |
|---|---|---|---|
| Close-coupled pump | 5 to 9 in | 10 to 16 in | Small offset creates moderate foot move |
| Long base pump | 8 to 14 in | 16 to 28 in | Angular error dominates rear feet |
| Gearbox input | 6 to 12 in | 12 to 22 in | Thermal target may be specified |
| Skid package | 4 to 10 in | 8 to 18 in | Base twist and pipe strain common |
| Result Sign | Front Feet Action | Rear Feet Action | Field Check |
|---|---|---|---|
| Both positive | Add shim | Add shim | Movable machine is low overall |
| Both negative | Remove shim | Remove shim | Movable machine is high overall |
| Front plus, rear minus | Add shim | Remove shim | Machine tilts down toward rear |
| Front minus, rear plus | Remove shim | Add shim | Machine tilts up toward rear |
Shaft alignment are the process of ensuring that the two shaft are in the correct position in relation to one another. Shaft alignment is a necessity in that without performing the process of shaft alignment, teh motor will eventually fail. Should one attempt to lift one side of the motor, the motor will pivot on its multiple foot.
Consequently, its difficult to move the coupling to the correct position. If one does not account for this pivot, then one will spend a great deal of time adding and removing shim from the motor to obtain the necessary alignment. To accomplish shaft alignment, it is necessary to solve for two variable.
How to Align Motor Shafts
The first variable to solve for is the offset of the motor, which is the distance that the two shafts are from one another. The other variable that must be solved for is the angularity of the motor, or at what angle the two shafts meet. If one corrects the offset but the angularity of the motor is not corrected, then there will be a gap at the top of the motor coupling or the motor will pinch at the bottom of the motor coupling.
If the angularity of the motor is corrected but the offset is not corrected, then the motor will be even with one another but will not be in the same centerline. Thus, one must correct both variables to ensure proper alignment of the motors. In order to calculate these variables, it is helpful to use a calculator.
By inputting the distance of the coupling to the motor’s feet, as well as the span of the feet of the motor, the calculator will indicate the change in shim thickness to each pair of motor’s feet. This is beneficial because the rear feet of motors can act as a lever. A small change in shim thickness at the rear of the motor will result in a larger movement of the motor coupling than changing the same thickness of shim at the front of the motor.
In addition to the calculations of the motors, it is important to consider the effects of thermal expansion. Metals will expand when they are hot, and motors tend to heat up during operation, especially if they are moving rapidly or pumping hot liquid. Thus, if the motor is aligned when it is cold, it will begin to expand and become out of alignment.
To account for this, a thermal target is used. A thermal target involves intentionally positioning the motor incorrectly when it is cold, such that it reaches the proper position when it becomes hot with operation. In some instances, it may be necessary to use shims of various thicknesses.
If one uses numerous thin shims, it is possible that the shims will compress under the motors weight. Additionally, the thin shims may become trapped between grit particle, creating inaccuracies in the motor’s measurements. Instead, it is best to use a few thick shims to provide the majority of the motor’s necessary height, and to use only one thin shim for the last and final adjustment of the motors alignment.
Before calculating the offset and angularity of the motor, it is necessary to ensure that the motor is not experiencing “soft foot.” Soft foot is the phenomenon where the motor is not completely seated on one of its feet. If the motor is soft footed, tightening the motor’s motor bolts will twist the motor. This twisting of the motor will impact the accuracy with which the motor’s measurements are taken.
To avoid this error, shims should be used to even out the gap between the motor and the base, the motor should be ensured to be solid on its feet, and then the process can continue. The degree of precision that is required for shaft alignment is related to the speed of the equipment. Low speed mixers, for instance, dont require the same level of precision as high speed compressors.
One must be aware of the speed of the equipment being used, as that speed will impact the vibration of the motor. Should the motors be moving at a high speed, any offset will result in vibration; excessive vibration will result in bearing failure or a broken motor coupling. Thus, it is important to use a systematic approach to aligning the motors, ensuring that they move in the way that will bring each motor shaft to the correct position relative to each other.
