Most bearings that fail early are not worn out - they were damaged while being fitted. Mounting is the one operation where a fit that is a few micrometres too tight, a force applied through the wrong ring, or a seal cooked by an oil bath can shorten a bearing life from years to weeks. The rules are simple and unforgiving, and they apply whether you use a press, a heated oil bath or an induction heater.
This guide covers cold, warm and induction mounting, adapter sleeves and hydraulic nuts, setting preload on angular contact and tapered pairs, and dismounting without damage. It closes with a step-by-step procedure for both directions.
- Never transmit mounting force through the rolling elements. Push only on the ring that has the interference fit - inner ring for a shaft fit, outer ring for a housing fit.
- Heat is the standard way to close an interference fit. An oil bath at 80-100 degC is typical, with 120 degC the practical ceiling for standard heat-stabilised bearings.
- Induction heating is cleaner and controllable. It heats the ring directly, but the bearing must be demagnetised and the temperature measured, not guessed.
- Dismounting force damages too. A puller applied to the wrong ring, or a shock load, brinells the raceway before the bearing is even inspected.
The forces rule
A bearing is a precise assembly of hardened rings separated by rolling elements. If mounting force passes from one ring to the other through the balls or rollers, those elements press indentations into the raceways and the bearing is damaged before it turns. The rule is therefore absolute: apply the force to the ring that carries the interference fit, and never let it cross the rolling elements.
Cold mounting with a press
Cold mounting suits small and medium bearings with a light interference fit, typically a few tens of micrometres.
The drive-up sleeve
Use a drive-up sleeve or an arbor that contacts the ring face evenly, ideally over the full ring circumference. A sleeve that touches only part of the face tilts the ring and skims metal as it goes.
Where the force is applied
For a shaft fit, the force goes on the inner ring face. For a housing fit, it goes on the outer ring face. For a simultaneous shaft-and-housing fit, apply force to both rings together with a matched sleeve, so that neither ring is loaded through the other. A press that is not square to the shaft will cock the bearing and score the seat.
| Fit | Force applied to | Result if applied to the wrong ring |
|---|---|---|
| Inner ring on shaft | Inner ring face only | Brinelling of the raceway through the rolling elements |
| Outer ring in housing | Outer ring face only | Raceway indentations and a ring that seats skewed |
| Both rings interference | Both faces with a matched sleeve | Cocked bearing and a scored seat |
Warm mounting and the oil bath
When the interference is too great for a cold press, heat the bearing to expand it, fit it while hot, and let it shrink onto the seat. This is the normal method for medium and large bearings.
Temperature limits
The heating temperature depends on the bearing and its heat-stabilisation class. A general guide is given below.
| Bearing / class | Typical heating temperature | Absolute ceiling |
|---|---|---|
| Standard (S0) | 80-100 degC | 120 degC |
| S1 heat-stabilised | 100-120 degC | 150 degC |
| Sealed / shielded | Check the seal limit | Often 80-100 degC |
| With polyamide cage | Check the cage limit | Often 100-120 degC |
Two traps are common here. First, oven heating is preferred over an oil bath for sealed bearings because a hot oil bath can attack the seal compound. Second, never heat a bearing above the stabilisation temperature of its class, or the dimensional stability it was heat-treated for is lost.
The shrinkage-recovery arithmetic
The expansion is predictable. For a steel inner ring, the thermal expansion coefficient is about 11.5×10-6 per degC, close to 0.0000115 mm per mm per degC. A 40 mm bore heated from 20 degC to 100 degC expands by 40 × 0.0000115 × 80, about 0.037 mm on diameter. That is roughly 37 micrometres of clearance from heating alone, which is why heating lets you mount a fit that a cold press could not close. The bearing then shrinks back as it cools and grips the seat.
Induction heaters
An induction heater is the modern method for medium and large bearings. It heats the bearing by eddy currents, so there is no oil, no contamination and a fast, controllable rise. Good practice is to use a heater with temperature control and a probe, to stop at the set temperature, and to demagnetise the bearing afterwards - a magnetised bearing attracts metallic debris to the raceway. Flame heating is banned because it is uncontrollable, leaves soft spots and chars seals. The induction heater guide covers the settings.
Hydraulic nuts and adapter sleeve drive-up
Adapter sleeves mount a tapered-bore bearing on a straight shaft, and hydraulic nuts close the fit with controlled force rather than a hammer.
The clearance-reduction method
On a tapered seat, the axial drive-up of the sleeve converts into a radial interference and a reduction of internal clearance. The correct amount of drive-up is therefore measured as a clearance reduction, not as a force. With the bearing on the shaft, measure the internal clearance with a feeler gauge at the unloaded position, drive the nut the specified distance, then re-measure until the clearance has reduced by the value the catalogue gives for that size and clearance class; typical figures run from about 15 to 25 micrometres on small units to several times that on large ones. Drive past the target and the bearing is preloaded and will overheat. The locknuts and adapters reference gives the tables, and internal clearance explains where the starting value comes from.
Mounting into housings and setting angular contact pairs
Housing fits follow the same logic in reverse. Where the outer ring has the interference, drive on the outer ring face; where a split housing is used, seat the bearing into the housing half with the locating shoulders, then close and tighten the cap evenly, following the manufacturer bolt sequence. For angular contact and tapered roller bearings, mounting includes setting the bearing to the intended end play or preload: use shims, a spacer whose width is adjusted, or a torque method where the nut is tightened until the friction torque reaches the specified value. The preload in bearing sets guide covers both approaches. Interference and fit values themselves come from the shaft and housing fits tables.
Dismounting without damage
Dismounting is mounting in reverse, and it is where brinelling usually happens. The rules:
- Support the correct ring. A puller must react on the ring that has the fit, not through the other ring or the rolling elements.
- Avoid shock and use an even puller. A hammer blow travels through the races and brinells them, and an uneven puller tilts the ring on the seat and scores it.
- Heat when needed. A bearing that will not come off cold should be warmed, not forced harder.
- Consider the seat first. If the shaft or housing is being sacrificed, cutting the ring off with a controlled method beats distorting a good seat.
After dismounting, judge re-use carefully. A bearing removed without force and without brinelling can be re-fitted if it has no visible damage and was not overheated. A bearing pressed off with force or hammered off should not be re-used, whatever it looks like.
| Removal method | Brinelling risk | Re-use decision |
|---|---|---|
| Even puller on the fitted ring | Low | Re-use if undamaged and not overheated |
| Warm removal within the class limit | Low | Inspect, then re-use if clean |
| Pressing off with force | High | Do not re-use |
| Hammer or shock removal | Very high | Do not re-use; treat as scrap |
Mount and Dismount: a Step-by-Step Procedure
Mounting first.
- Inspect and clean. Check the shaft and housing for burrs and damage, confirm the fit dimensions, and confirm that the bearing part number, closure and clearance class match the drawing.
- Measure the clearance on tapered seats and record the starting value if the fit will be set by clearance reduction.
- Choose the method. Cold press for a light fit, heat for a heavy fit, induction where available.
- Apply force to the correct ring with a square, even sleeve, then seat the bearing fully against the shoulder or to the specified clearance reduction.
- Verify after mounting. Re-check clearance with a feeler gauge and turn the shaft by hand to confirm free rotation.
Dismounting next.
- Plan the reaction and warm the fit ring where possible, within the class limit, to break the interference.
- Pull evenly with a matched puller and avoid shock loads.
- Inspect and record immediately, then decide re-use on the basis of how it came off, not only how it looks.
Common mistakes
- Hammering the outer ring to fit an inner-ring fit. The force crosses the rolling elements and brinells the raceways.
- Heating a sealed or shielded bearing in an oil bath without checking the seal limit, which softens the lip and later lets contamination in.
- Re-using a bearing that was pressed off with force, because the brinelling caused during removal will appear as a defect frequency in a few weeks.
- Over-driving an adapter sleeve, or leaving a heat-fitted bearing magnetised. Past the target clearance reduction the bearing is preloaded and runs hot, and an un-demagnetised bearing collects metallic debris.
Clearance after mounting is measured to order under ISO 5753 and DIN 620; the envelope itself is fixed by ISO 15. The damage that incorrect mounting produces is the subject of the failure analysis guide, and the way clearance shifts with temperature is covered in the temperature and clearance reference. For specific fits, see our deep groove ball bearing listings and the NSK range, or send the shaft and housing drawing through the contact form for a fit recommendation. Large self-aligning positions are covered under self-aligning roller bearings, where adapter-sleeve mounting is the norm.
Frequently asked questions
How do I mount a bearing without damaging it?
Apply the mounting force only to the ring that has the interference fit, using an even drive-up sleeve. Never let the force pass through the rolling elements. For a heavy fit, heat the bearing to expand it and fit it while hot, then let it shrink onto the seat.
What temperature should I heat a bearing to before mounting?
For a standard heat-stabilised bearing, 80 to 100 degC is typical and 120 degC is the practical ceiling. A higher class such as S1 allows more. Never exceed the stabilisation temperature of the class, and for sealed or shielded bearings check the seal and cage limits first, which are often lower.
Why is flame heating a bearing not allowed?
A flame cannot be controlled, so it overheats local areas, produces soft spots in the hardened steel and chars seals and cages. It also leaves no reliable temperature measurement. An oil bath, an oven or an induction heater gives a controlled, uniform rise that keeps the bearing within its stabilisation limit.
What is the clearance-reduction method for adapter sleeves?
On a tapered seat the axial drive-up of the sleeve produces a radial interference that reduces internal clearance. Instead of tightening to a force, you measure the internal clearance with a feeler gauge, drive the nut a specified distance, and stop when the clearance has reduced by the catalogue figure for that size and class.
Can a dismounted bearing be re-used?
Only if it came off without force or shock and shows no damage or overheating. A bearing that was pressed or hammered off may carry brinelling from the removal itself, which will appear later as a defect frequency. When in doubt, replace it.
How do I set preload on an angular contact pair?
Use shims or an adjustable spacer to set the axial position so that the resulting friction torque matches the value the manufacturer specifies, or set the bearing to the intended small end play. The target is defined by the bearing pair and application, and over-tightening raises friction and temperature sharply.