What is a "fit"?
How tightly one part sits on another. A fit is set by the tolerances on the two parts. A letter and a number, such as h6 or H7, give the size band the part may fall in. Capital letters (H7, M7) are for holes, such as a housing bore. Small letters (h6, m6) are for shafts. An interference fit means the shaft is a little bigger than the hole, so it must be pressed in. A clearance fit leaves a small gap.
How are the outer ring and the inner ring fitted?
The outer ring is normally pressed in. The inner ring depends on the load.
Outer rings of spherical plain bearings are generally pressed into their housings. This keeps the split in the outer ring closed and stops the ring turning under heavy rotating or oscillating loads. Inner rings may have an interference or a clearance fit on the shaft, depending on the application. An outer ring split by cracking is slightly out of round on its own, and its roundness comes back once it is pressed into a housing bore of the right size. So an accurate housing bore matters.
The rule behind these choices is that the ring which turns relative to the load direction needs the tighter seat. A ring that sees the load sweep around it would creep on a loose seat and fret it away, while a ring whose load always lands on the same spot can sit looser and slide axially when needed.
What fits does the maker's table suggest?
For the shaft:
| Condition | Steel/steel (needs maintenance) | Maintenance-free |
|---|---|---|
| Clearance or transition fit, loads of all kinds | h6, hardened shaft at least 45 HRC | h6 or g6 grades |
| Interference fit, loads of all kinds | m6 | k6 |
For the housing:
| Condition | Steel/steel | Maintenance-free |
|---|---|---|
| Light loads, axial movement required | H7 | H7 |
| Heavy loads | M7 | K7 |
| Light-alloy housing | N7 | M7 |
THK gives its own table, arranged by which ring turns. With the inner ring turning under a normal load it suggests a k6 shaft and an H7 housing. With the outer ring turning under a normal load it suggests g6 and M7. For loads whose direction is not clear, it suggests m6 and H7 (inner ring turning) or h6 and N7 (outer ring turning). "Inner ring turning" means the load rotates relative to that ring. N7 is also recommended for light-alloy housings.
What do those letters mean in micrometres?
Here is an example using standard ISO 286 tolerance bands (1 micrometre, µm, is 0.001 mm).
| Part | Size band | Fit | Allowed deviation |
|---|---|---|---|
| Shaft, 25 mm | 18 to 30 mm | h6 | 0 to −13 µm |
| Shaft, 25 mm | 18 to 30 mm | k6 | +2 to +15 µm |
| Shaft, 25 mm | 18 to 30 mm | m6 | +8 to +21 µm |
| Housing bore, 50 mm | 30 to 50 mm | H7 | 0 to +25 µm |
| Housing bore, 50 mm | 30 to 50 mm | K7 | −18 to +7 µm |
| Housing bore, 50 mm | 30 to 50 mm | M7 | −25 to 0 µm |
| Housing bore, 50 mm | 30 to 50 mm | N7 | −33 to −8 µm |
Read it this way: an h6 shaft of 25 mm can be up to 13 µm smaller than 25 mm, but never larger. An m6 shaft is always larger than 25 mm, by 8 to 21 µm, so it will be pressed in. What the fit comes to depends on the bearing's own bore and outside diameter tolerances, which are in the maker's table, not in these bands.
How hard and smooth should the shaft be?
If the inner ring is fitted loosely and the load is heavy, the shaft can slip in the inner ring's bore and wear away. The same maker's guard against that is a shaft hardness of 58 HRC or higher and a surface roughness of Ra 0.80 µm or less, with a hardened shaft surface whenever the inner ring will turn on a clearance fit. The first table asks for at least 45 HRC for the h6 steel/steel clearance fit. The two sources set different numbers, so use the figure that goes with the bearing you buy. For the surface finish of the housing bore, ask the maker.
What helps when you install it?
A lead chamfer of 10° to 20° on the ends of pins or shafts and on the edges of housing bores lets the bearing go in easily and lowers the risk of skewing and damaging the mating surfaces. For a bearing with a split outer ring, the split should lie at 90° to the main load direction, otherwise life will be shorter, especially under heavy load. This usually also puts the lubrication holes in the load zone, where grease is most needed; check where the holes sit on your bearing. More on installing is in Q1-10.
What changes for a bearing in an aluminium arm, a tube-end cup or a bolted bracket?
Three things: the housing material, how the housing was made, and what passes for the "shaft".
Aluminium expands about twice as much as steel (roughly 23 against 12 millionths per kelvin), so a press fit in an aluminium arm loosens as it warms. Illustrative: with arm and bearing both 60 K warmer, a 50 mm bore opens by about 50 × (23 − 12) × 10⁻⁶ × 60 ≈ 33 µm relative to the steel outer ring, about as much as the whole interference of an N7 housing fit at this size (8 to 33 µm in the table above). That is why the maker's table gives tighter letters for light-alloy housings, and why the fit should be judged at the highest housing temperature, not on the bench.
A welded or thin-walled housing can go out of round. The pressed-in outer ring takes the shape of the bore, and the clearance follows (Q1-07). Finish the bore after welding, or use a cup whose bore is finished after it is made (Q1-10).
On a vehicle the "shaft" is usually a bolt that clamps the inner ring between spacers or bracket faces, not a ground pin. The clamp must hold the inner ring so that the motion happens in the ball, with faces flat and square, and the bolt a close fit through the bore (small clearance, not an interference fit). Ask the maker what finish and fit it wants when a bolt is used this way. For cup-housed formats see SYZ's uniball cups and uniball cup buyer's guide.