Shims, washers and hold-down bolts
Shims, bolts and washers look like small parts, but they are what holds the machine to the floor at the end of the job. Good ones make an alignment quick. Bad ones make it impossible.
Shim rules that save time
| Rule | Why |
|---|---|
| Use the fewest shims, as thick as possible. Four or five is the usual ceiling; three or fewer is better. | Every extra layer is a place for dirt and a source of spring. |
| Measure each shim, and measure the whole stack. Do not trust the printed value, and do not add up marked values. | Sheet shims vary. One 250 mil requirement came out about 4 mil short, and a 125 mil shim measured 11 mil thin. |
| Micrometer shims of 1.3 mm (0.050 in) and thicker, and discard shims that vary by more than about 0.05 mm (2 mil) across the piece. | Thick shims show the most variation, and uneven shims create a soft foot of their own. |
| Use new, flat, burr-free shims. Do not reuse rusty, bent or paint-stuck ones. | Corrosion hides the true thickness, and a bent shim acts as a spring. |
| Keep front and rear totals similar side to side. | Large differences from side to side tilt the casing and can create angular soft foot. |
| Slide the shim in until it touches the bolt, then back about 6 mm (¼ in). | This keeps it clear of the threads, so tightening cannot bend its edge. |
| Lay shims on a clean cloth, and use the motor cooling fins as a rack. | Stops dirt getting under the foot and keeps the pairs in order. |
How big should a shim be?
Pick shims that sit evenly around the bolt (except the slot). Good practice is for the shim to cover at least half the foot area, and more if you can. Some references ask for 80% contact. Shim makers also publish sizes by motor frame. Use slotted shims with a tab, not shims with round holes, because a round hole lets the shim rotate round the bolt and hide under the foot.
Hidden shims
On machines that have been shimmed before, check for old, undersized shims that have rusted or been painted to the foot or the base. They hide because they are smaller than the foot, have no tab and may be spun under the foot. They are a classic reason for movement that no soft foot check finds.
Washers and bolts
- Washers should be sized to the bolt, flat, and not cupped, cracked or rusted. A thin washer on a large hole is the usual cause of cupping.
- Doubling thin flat washers, or choosing thicker ones, is a simple fix. Very hard washers can also crack, so ask your engineer.
- Do not use lock washers under the head on a machine foot that you will keep moving.
- Check bolts and bolt-hole condition. A worn hole adds slop.
Tightening pattern
Pick a cross pattern and tighten in three passes: hand tight, about half, then full. Use the same pattern every time the feet are loosened and tightened. If you do not, the machine rocks into a slightly different place each time and readings do not match.
When a bolt is binding in its hole
This is called bolt-bound. Before anyone turns a bolt down on a lathe, try these in order:
- Prevent it: at installation, slide each machine to one end of its bolt holes and mark it, then to the other end, and set it at the middle. That gives the most room both ways.
- Make an alternate move: treat different feet as the fixed reference (see the procedure chapter).
- If the bolt threads into a tapped hole, a nut can be dropped to a smaller bolt only if the engineer allows it.
- If you must remove metal, use a purpose-made turned-down bolt. Reducing only the thread diameter keeps most of the strength. Turning a shank down further loses a lot: a 3/4 in grade 5 bolt turned to 5/8 in loses roughly 30% of its shear capacity.
Jacking bolts
Jacking bolts make horizontal moves smooth and controlled. A threaded rod with a nut tack-welded to the base is enough. Count how much the machine moves per turn of a flat on the bolt head to plan the move, then back them off when you finish. Never hit a motor foot directly with a hammer: the blow can mark the bearing races and crack a frame. Use pry points drilled in the base, a pinch bar or a small jack.
Questions
How many shims can I use under a motor foot?
Use as few as possible. Four or five is a common upper limit, and three or fewer is better. Use the thickest shims that give the thickness you need.
Why should I measure shims?
Sheet shims vary in thickness, and the variation is largest on thick shims. A shim marked 125 mil can measure 114 mil. Measure the whole stack, not each marked value.
What size shim should I use under a machine foot?
Cover at least half the foot area, and more where possible. Keep the shim evenly around the bolt and use slotted shims with a tab.
Does a torque wrench improve alignment accuracy?
Not by itself. Readings that change as you tighten come from soft foot or debris, not torque. A consistent cross pattern in three passes matters more.
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.