Guides / Tolerances
Bearing Fits, Concentricity and Runout: Tolerances That Matter in Motor Parts
Written by the sourcing desk that reads your RFQPublished How we write these
Most motor part drawings that come back with a confusing quote have the same problem. The tolerances are not wrong, they are just not sorted into the ones that decide whether the motor works and the ones that only decide what it costs. This guide separates them.
The three callouts that decide how the motor runs
For a housing, an end bell or a rotor shaft, three things matter more than everything else on the print combined.
1. The fit on the bearing seat
The seat is the interface between your part and a component you did not make. Bearing makers publish which class goes where, and the rule behind their tables is simple: the ring that rotates relative to the load gets an interference fit, the ring that stays still relative to it gets a clearance fit.
For a normal electric motor, with a rotating shaft and a stationary outer ring, that means an interference fit on the shaft journal and a clearance fit in the housing bore. H7 is the ordinary housing bore. The shaft journal sits in the k or m classes depending on diameter.
Two practical notes that the class on its own does not carry:
- H7 and H8 are different jobs on the shop floor. H7 usually means a dedicated finish boring pass and a bore gauge at the machine. H8 often does not. That step is one of the larger single line items in a motor part quote.
- The class is specified at 20 degrees. A motor is not at 20 degrees. More on that below.
The bore limits for any diameter are on the bearing seat fit reference, along with a calculator for the temperature effect.
2. Runout between features that share an axis
A housing with two bearing seats is really one axis with two surfaces on it. If those surfaces are not on the same axis, the shaft is bent between them when the motor is assembled, and no amount of accuracy on either seat on its own fixes it.
This is where the drawing usually goes wrong, and it goes wrong in a specific way: the requirement is real but it is written as concentricity when runout is what is meant.
Circular runout asks how much a surface moves as the part is rotated about a datum axis. You clock it with an indicator. It captures position error and form error together, which is what a bearing feels.
Total runout does the same along the whole length of a cylinder, so it also catches taper.
Concentricity, in the ISO sense, controls where the median points of a feature sit relative to a datum axis. It is a different and much harder measurement, normally a CMM job with a specific evaluation, and for a bearing seat it is almost never the thing you actually need.
If you want the second seat to line up with the first, name the first as the datum and put a circular runout callout on the second. That is cheap to make, cheap to check, and it is the characteristic that matters.
3. Squareness of the shoulder
The face the bearing seats against has to be perpendicular to the bore. A cocked outer ring loads the raceway at an angle, and a perfect bore does not save it. A perpendicularity callout on that shoulder, referenced to the bore, is worth more than one grade of bore tolerance and costs less.
The callouts that mostly decide price
These are the ones to look at first when a quote comes back higher than expected.
Surface finish applied everywhere
A bearing seat wants about Ra 0.8 or better. A shaft journal running in a lip seal often wants Ra 0.4 or below, which normally means grinding. Putting Ra 0.4 on the whole shaft because the title block template said so sends every diameter through the grinding queue, and grinding is a separate machine with a separate schedule.
Mark the finish where it matters. Leave the rest at the general note.
One tolerance applied to everything
A blanket plus or minus 0.01 mm on a drawing where only the bearing seat needs it is the most expensive line in a quote, because it turns every routine feature into an inspected one. General tolerances have a standard for exactly this reason. Use it and call out the exceptions.
A fit class tighter than the bearing table
Going from H7 to H6 because tighter sounds safer is a design change. In a housing it reduces the internal clearance of the bearing once assembled, which can preload it in a way it was not designed for. It also roughly doubles the inspection burden. The bearing maker’s table is the authority here, not instinct.
Geometric callouts with no datum
A flatness callout is self-contained. A runout, concentricity or perpendicularity callout is not, and without a datum it cannot be made or checked. It either stops the quote with a question or gets priced with a margin for the ambiguity.
The one people miss: temperature
This is the callout that is not on the drawing at all.
Aluminium expands about twice as fast as the bearing steel inside it. The gap between the two opens up by the difference in expansion coefficient, times the diameter, times the temperature rise. It is pure geometry and nothing about the machining changes it.
Work a real case. A 62 mm bearing seat in an aluminium housing, H7, so 62.000 to 62.030 mm on the bench. The motor runs at 90 degrees, a 70 K rise. The bore grows about 50 micrometres more than the outer ring does.
That is 166 percent of the entire H7 tolerance band. At temperature this seat is a clearance fit no matter what class was printed, and the outer ring can creep.
The fix is not a tighter bore. It is axial clamping, a wave spring, or a housing material closer to the bearing steel. What matters for the drawing is that the decision gets made and written down, because a shop quoting a 62 H7 bore has no way to know your part runs hot unless you say so.
Cast iron and steel housings sit close to bearing steel and barely move relative to it. This is an aluminium problem, which means it is a problem for most modern motor housings.
Runout is not balance
These two get confused on drawings often enough to be worth separating.
Runout is geometry. It is measured on a stationary part with an indicator against a named datum, it is a machining characteristic, and it belongs on the machining drawing.
Balance is mass distribution. It is measured on a spinning rotor after assembly, and it is an assembly characteristic. A shaft can be balanced to G2.5 and still have a journal that runs out. A shaft with perfect runout can be out of balance because of a keyway or a cross drilled hole.
Specify both, separately, on the right drawings. Asking a machining supplier to guarantee a balance grade on a bare shaft is asking for something outside its control, and the good ones will tell you so.
A drawing that quotes cleanly
For a two bearing housing, this is roughly the minimum:
| Feature | What to write |
|---|---|
| First bearing seat | Nominal size and fit class, for example 62 H7 |
| Second bearing seat | Same, plus circular runout to datum A |
| Datum A | The first seat, named explicitly |
| Both shoulders | Perpendicularity to datum A |
| Bearing seats | Surface finish, Ra 0.8 or as required |
| Seat roundness | Called out separately if it matters |
| Everything else | General tolerance per the title block |
| A note | Operating temperature at the bearing seat |
Nine lines. A quote against that drawing will come back faster, closer to the real price, and with fewer questions attached, because there is nothing left for the shop to guess about.
What to do before the RFQ goes out
Read your own drawing once with a single question in mind: for each tolerance on it, what breaks if this is loose. Anything that has no answer is probably a habit rather than a requirement, and habits are the cheapest thing on a drawing to remove.
Standards and references
- ISO 286-1, ISO code system for tolerances on linear sizes. The fit classes and IT grades referred to throughout.
- ISO 1101, Geometrical product specifications (GPS). Geometrical tolerancing. Definitions of concentricity, circular runout and total runout.
- SKF, Tolerances and resultant fits. Which ring gets the interference fit, and the recommended classes by diameter.
- ISO 1940-1, Balance quality requirements for rotors in a constant (rigid) state. Referenced in the section on what runout does not cover.
Figures on this page are typical ranges drawn from the standards above and from ordinary shop practice. They are not a specification for your part. The shop that quotes you confirms what it can hold on your geometry.
Frequently asked questions
Should I use concentricity or runout on a bearing seat?
Runout, almost always. Concentricity in the ISO sense controls the position of the median points of a feature, which is expensive to verify and rarely what the motor cares about. Circular runout controls how far the surface moves as the part turns about a datum axis, it is what a bearing actually experiences, and it is measured with an indicator in a minute.
What happens if I leave the datum off a runout callout?
The callout is incomplete and the shop has to guess which feature to clock from. In practice you get one of two outcomes: a question that delays the quote, or a price with a risk margin in it. Both cost you more than naming the datum would have.
Is a tighter fit always safer?
No. Too much interference in a housing bore squeezes the outer ring and reduces the internal clearance of the bearing, which can preload it beyond what it was designed for. Bearing makers publish a recommended class for each load case, and going tighter than the table is a design change, not a safety margin.
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