Tolerances and thermal growth
Machines are aligned cold but run hot. Metal grows as it warms, and the growth is rarely the same on both machines, so the cold alignment has to be set off on purpose.
How much does it grow?
Steel grows about 0.0000065 in per inch per degree F (about 0.012 mm per metre per degree C). A pump and a motor differ in height, in material and in temperature, so they grow by different amounts and the shaft centrelines drift apart.
| Quantity | Formula |
|---|---|
| Growth of a support | Growth = height from base to shaft centre × temperature rise × growth coefficient |
Worked example
A motor shaft sits 400 mm above its feet and runs 40 °C above the cold state. A pump sits 300 mm above the base and its casing is 90 °C above the cold state.
| Machine | Height | Temperature rise | Growth (0.012 mm / m / °C) |
|---|---|---|---|
| Motor | 0.40 m | 40 °C | 0.40 × 40 × 0.012 = 0.19 mm |
| Pump | 0.30 m | 90 °C | 0.30 × 90 × 0.012 = 0.32 mm |
The pump rises about 0.13 mm (5 mil) more than the motor. Set the pump about 0.13 mm lower than the motor in the cold state, so the two are level when hot. Use real surface temperatures, not the pumped-fluid temperature alone.
Where the numbers come from
- Maker data. Many pump and gearbox manuals give a cold target in mils or mm. Use these first.
- Your own measurement. Check the alignment hot, right after shutdown, and note the change. Build up a table per machine.
- Calculation. The example above, with measured casing temperatures.
- Special cases. Water-cooled machines, long steam lines and heated frames grow in different directions. Use maker data or hot checks.
Hot check
A hot check is the only way to know. Shut down, lock out and measure within minutes: the machine begins to cool at once. If you cannot get the sensors on in time, use an online measurement system or a stand-off method. Always record the temperature at which the reading was taken.
Tolerances by speed
| Speed | Angular error at the coupling | Offset at the coupling |
|---|---|---|
| 3,600 rpm | 0.5 mil per inch (0.05 mm per 100 mm) | 2 mil (0.05 mm) |
| 1,800 rpm | 0.7 mil per inch (0.07 mm per 100 mm) | 4 mil (0.10 mm) |
| 1,200 rpm | 1.0 mil per inch (0.10 mm per 100 mm) | 6 mil (0.15 mm) |
Common values for short flexible couplings at the coupling centre. Express the target after any thermal offset, so a machine with a cold offset of 0.13 mm should be measured against that target, not against zero.
Spacer shafts and long couplings
Divide the allowed offset by the spacer length. A long spacer allows more offset at the coupling ends, but the angle at each end is what counts. Check the limit with the maker.
When tolerances conflict
The tightest of the following applies: coupling maker limit, machine maker limit, site standard. A seal or a gear mesh often sets the real limit, not the coupling. Many sites define two bands, such as acceptable and excellent, and align new installations to the better one.
See also the knowledge page on alignment and thermal growth for how it shows up in vibration.
Questions
Why do machines need a cold offset?
Because they grow by different amounts when hot. A cold offset makes the shafts line up at running temperature rather than at rest.
How do I calculate thermal growth?
Multiply the height from the base to the shaft centreline, the temperature rise and the expansion coefficient of the material. About 0.012 mm per metre per °C for steel.
What is a typical alignment tolerance?
For short flexible couplings, about 0.05 mm offset at 3,600 rpm and 0.10 mm at 1,800 rpm, with matching angular limits. Your maker data comes first.
Do I need to check alignment when hot?
It is the only way to know. Take the reading as soon as possible after shutdown, record the temperatures and adjust your cold target accordingly.
General good practice for horizontal machines driven through a coupling. The machine maker's manual, the coupling data and your site standard come first. Lock out and isolate the machine before any alignment work.