Motor Mounting Tolerances: Align the Feet Before the Coupling
Most alignment jobs that fail in the field do not fail because someone measured the coupling badly. They fail because the motor is sitting on a warped foot or a filled base, and no coupling measurement taken afterwards can be trusted. Motor mounting tolerances are therefore not a formality at the end of the installation drawing - they are the foundation on which every later alignment reading rests. A motor bolted down with a two-tenths soft foot will move when the bolts are finally torqued, and every dial indicator reading taken before that moment describes a machine that no longer exists.
This guide sets out the mounting tolerances that matter, in the order they should be checked: the mounting surface, the feet, the bolts, and only then the coupling. It closes with the questions plants ask most often about motor alignment and re-check intervals.
What Motor Mounting Tolerances Actually Cover
The phrase covers four separate checks, and confusing them is the most common source of rework. The first is the mounting surface itself: flatness, level and rigidity of the baseplate, sole plate or steel frame under the motor. The second is foot flatness - whether each motor foot contacts the base without being forced, which is where soft foot enters. The third is the bolted joint: bolt grade, torque, and whether the motor is clamped rather than sprung by uneven tightening. The fourth is the coupling alignment between motor and driven machine, which is the measurement everyone thinks of first but which is only meaningful once the first three are inside tolerance.
The order matters because each check can undo the previous one. Shimming a coupling on top of an unresolved soft foot simply locks the distortion into the housing, and the bearing clearances inside the motor pay for it.
Soft Foot: The Tolerance That Corrupts Every Other Reading
Soft foot exists when tightening one mounting bolt changes the air gap or the coupling reading, because the foot is rocking on a gap or the case is being distorted. The commonly used field limit is a total gap of no more than 0.05 mm (about 0.002 in) under any foot, measured with the other bolts snugged. Anything larger should be corrected by shimming under the foot, not by pulling the foot down against a gap with bolt torque - that bends the motor frame and shifts the bearing housings inside it.
There are two distinct forms, and the correction differs. A parallel soft foot shows an even gap under one foot and is fixed by adding shims of the measured thickness. An angular soft foot shows a tapered gap, front edge to back edge, and adding shims at the thick corner only hides the problem; the real cause is usually a distorted base or a machined-in error in the foot, and the honest fix is a machined or epoxy-filled base. Squaring the foot - checking whether the gap changes when the neighbouring bolt is loosened - tells an experienced fitter which case he is looking at within minutes.
The Soft Foot Check, Step by Step
A reliable sequence takes under half an hour on a typical foot-mounted motor and saves hours of chasing coupling readings that will not stay put.
- Clean the baseplate and every foot contact face; paint scale and welding slag are the most common false soft feet.
- Snug all bolts to hand-tight so each foot just contacts, then zero a dial indicator on the shaft or coupling hub.
- Loosen one bolt and watch the indicator. Movement above 0.05 mm means that foot is carrying a gap.
- Record the gap for all four feet before correcting anything - the pattern matters more than any single number.
- Correct with stainless shims under the offending foot, never by over-torquing the bolt.
- Re-torque all bolts to specification and repeat the indicator check. On a sound installation, tightening any single bolt moves the reading by no more than the same 0.05 mm.
If the pattern will not resolve - correcting one foot creates a gap at another - the problem is the base, and no amount of foot shimming will hold. Epoxy grouting or remachining the mounting surface is then the economical answer, not a compromise to be avoided.
Coupling Alignment Targets After the Feet Are True
Once the motor sits bolted down without soft foot, the coupling alignment itself has clear working targets. The values below are the commonly used field guidelines for flexible couplings; the coupling manufacturer's own table always takes precedence where it is tighter.
| Shaft speed | Offset (radial) | Angular gap difference across hubs |
|---|---|---|
| Below 1,200 rpm | 0.10 mm | 0.10 mm per 100 mm |
| 1,200 - 3,600 rpm | 0.05 - 0.07 mm | 0.05 mm per 100 mm |
| Above 3,600 rpm | 0.03 mm or less | Per coupling maker's table |
Two habits make these numbers achievable on site. Measure with the whole machine at its normal operating temperature where practical, because a cold alignment drifts as the frames warm up. And take readings in at least two rotational positions on both shafts, rotating them together so the coupling geometry cancels out - readings taken with the hubs rotated independently mix in coupling runout and repeat nothing.
Shimming Practice: Small Habits With Large Consequences
Shim work is where good alignments quietly go wrong. Shims should be stainless, full-face under the foot, and no more than three or four per foot; a stack of thin shims acts like a spring, and the alignment that measured perfectly before torquing will not survive the bolt load. Every added shim should be a deliberate decision - when the stack passes five pieces, the correct answer is usually one thicker shim or a machined foot.
Slotted shims deserve one caution: insert them fully, with the slot completely under the foot. A shim half-installed under a foot spreads load on one edge and creates the very soft foot the shimming was meant to cure. After the final torque, a last indicator sweep across all four bolts confirms the joint is clamped, not sprung.
The Mounting Surface: Where Half of All Problems Begin
No foot measurement means anything on a base that is not rigid. Skid-mounted sets on undersized steelwork, motors bolted to fabricated frames with weld shrinkage in the panel, and sole plates sitting on ungrouted concrete all produce the same symptom: alignment readings that change every time a bolt is touched or the weather changes. The check is simple - indicator on the coupling, then push the motor casing with hand force. A reading that swings a tenth of a millimetre under hand pressure on a large motor describes a base that will never hold an alignment.
Concrete bases should be grouted under the sole plates, not left with the plate spanning a void, because the void compresses and the motor walks. Steel bases need diagonal bracing close to the motor feet; a cantilevered panel under one foot is a spring in the measurement path. Where a gearmotor is mounted on machinery frame, the same rule applies to the driven side - the input flange register and its pilot take the place of the feet, and the register's condition decides everything.
Thermal Growth: The Alignment That Moves After Start-Up
Steel grows by roughly twelve hundredths of a millimetre per metre for every ten degrees of temperature rise, and a motor running warm against a cold driven machine therefore changes its alignment between shutdown and steady state. In most foot-mounted pump and fan sets the effect is small, but on hot processes - gearboxes on kilns, dryers or steam lines against ambient motors - the cold alignment should deliberately bias the coupling so the hot position lands on target. The machine builder's thermal growth sheet, where one exists, states the expected rise per foot; without one, the practical approach is to re-check the alignment hot after the first weeks of running and record the drift for the next installation.
Gearmotors and Flange-Mounted Units
Not every drive has feet to shim. On a gearmotor such as the H226 series gear motor, the motor is factory-assembled to the gearbox, and the mounting tolerances move to the interface with the driven machine: the flange register, the pilot spigot, and the mounting face flatness of the receiving structure. The practical checks change rather than disappear - the pilot should enter its register by hand without rocking, the flange bolts should pull the joint metal-to-metal evenly, and any shimming happens under the gearbox feet or the machine frame, never by forcing the flange joint.
Flange-mounted sets have one advantage worth using: because the motor-to-gearbox concentricity is machined rather than field-built, a flange reading that is badly out almost always points at the receiving mount, not at the drive. Treating that as a mounting-surface finding instead of endlessly re-measuring the coupling is the faster diagnosis.
FAQ
How much soft foot is acceptable on a motor?
The widely used field limit is 0.05 mm (0.002 in) of total gap under any foot. Larger gaps should be shimmed or corrected at the base. What matters nearly as much as the number is the pattern: if correcting one foot creates a gap at another, the base itself is out of condition.
Should motor alignment be measured on the coupling or the shaft ends?
Both surfaces are used, and good practice checks them against each other. Dial indicator and laser systems measure on the coupling hubs or on the exposed shaft ends; the readings agree only when the hubs are concentric and the shafts are clean. Runout on the measuring surface adds directly to the alignment error, so a quick runout check before aligning saves an hour of inexplicable numbers.
How often should a motor alignment be re-checked?
A sensible routine checks alignment at commissioning, after the first weeks of running when the base and grout have settled, and then at each planned shutdown of the driven machine. Sets that see frequent starts, high torque transients or wash-down environments drift faster and justify shorter intervals. Any machine that receives new bearing work or a coupling change should be realigned before restart.
Does a flexible coupling make motor mounting tolerances unnecessary?
No. A flexible coupling tolerates small residual misalignment in running, but it does not correct soft foot, and running a flexible coupling continuously near its misalignment limit transfers the load into the bearings of both machines. The coupling's rating is a margin for thermal and load movement, not a substitute for a correctly mounted motor.
Why does alignment change between the shop and the installation?
The mounting surface is different, the temperature is different, and the base rigidity is different. An alignment performed at the factory bench describes that bench. The only readings that matter are the ones taken on the installed base, at operating temperature, with the bolts torqued to specification.
None of the tolerances above are difficult to achieve on a sound installation, and all of them are expensive to chase on a poor one. When a drive package is being specified, it is worth deciding at the same time how the motor and gearbox will be mounted, what base they will share, and who checks the feet before the coupling is measured. Our online selection system lets you size the drive around the mounting arrangement as well as the duty, and our team can advise on base design and alignment practice for the specific unit you order.
