7.3 Removing and Installing Bearings: Fits, Heating, and Mounting
Key Takeaways
- Mounting force must always be applied to the ring being fitted, never transmitted through the balls or rollers.
- Induction heaters and clean oil baths are the accepted heating methods, and bearing temperature should not exceed roughly 250 degrees Fahrenheit for standard bearings.
- An open flame must never be used to heat a bearing, because it destroys the heat treatment and the lubricant.
- Bearing pullers must grip the ring that is being removed from its interference fit, so an inner ring puller grips the inner ring.
- Shaft and housing dimensions must be measured before mounting, because an out-of-tolerance seat, not the bearing, causes most repeat failures.
Measure first, mount second
A new bearing does not fix a worn shaft. Before anything is removed, and again before anything is installed, measure:
| Dimension | Tool | Why |
|---|---|---|
| Shaft seat diameter | Outside micrometer, at several points and angles | Confirms interference fit; catches taper and out-of-round |
| Housing bore | Telescoping gauge and micrometer, or inside micrometer | Confirms clearance fit; catches a bell-mouthed bore |
| Shaft shoulder squareness and radius | Square and radius gauge | A shoulder that is not square cocks the bearing; too large a fillet radius prevents the ring from seating |
| Shaft runout | Dial indicator on V-blocks or in place | A bent shaft will destroy any bearing |
Recall the rule from Chapter 4: the ring that rotates relative to the load direction gets the interference fit. On a normal motor or pump this is the inner ring on the shaft, with a clearance fit for the outer ring in the housing. On a machine where the load rotates with the shaft — a rotating housing, an eccentric load — the relationship reverses.
If the shaft seat is undersized, the fix is a repair sleeve, metal spray and regrind, or shaft replacement. Knurling or center-punching a worn shaft to "raise" it is not a repair; it creates high points that collapse under load and guarantees a fretted, loose bearing within weeks.
Removing a bearing
- Support the shaft or assembly so it does not drop when the bearing releases.
- Pull on the ring that is tight. An inner ring pressed onto the shaft must be pulled by the inner ring. Gripping the outer ring and pulling forces the load through the balls or rollers and brinells the races — which does not matter for a bearing you are scrapping, but does matter when you are removing a bearing you intend to reuse or inspect, and it can shatter a ring and throw fragments.
- Choose the tool for the access: two- or three-jaw puller, bearing separator (knife-edge splitter) behind the inner ring, hydraulic puller for large interference, or a press with the assembly supported under the ring being removed.
- Heat can help. An induction heater or a bearing heater applied to the outer ring of a housed bearing expands it off its seat. Never apply an open flame.
- Protect yourself. Bearing removal stores energy in the puller. Use a shield or a blanket, keep the puller square, and never stand in line with the jaws.
- Keep the failed bearing for analysis. The wear pattern is the diagnosis, and Chapter 10 depends on it.
Mounting: cold methods
For light interference on smaller bearings:
- Use a bearing fitting tool — a sleeve or driver that contacts only the ring being fitted — with a soft-face hammer, or press the bearing with an arbor press through the same kind of sleeve.
- The mounting force must never pass through the rolling elements. Driving on the outer ring to seat an inner ring on the shaft puts the entire load through the balls and dents both raceways. Those dents are true brinelling, and the bearing will be noisy from the first hour.
- Drive square. A cocked bearing binds on the shaft and is then very difficult to move either direction.
Mounting: heat methods
For medium and heavy interference fits, expand the inner ring instead of forcing it:
| Method | Notes |
|---|---|
| Induction heater | Preferred; fast, clean, controlled, and demagnetizes on completion |
| Clean oil bath with thermostat | Traditional and effective; the bearing must sit on a screen, never on the bottom of the tank |
| Electric hot plate or oven | Acceptable with good temperature control |
| Open flame | Never. Uncontrolled, overheats locally, destroys heat treatment and lubricant, and contaminates the bearing |
The working rule is to heat the bearing roughly 150°F above the shaft temperature, which is normally enough for the ring to slide on freely, and to never exceed the manufacturer's limit of about 250°F (120°C) for standard bearings. Above that limit the hardened rings begin to temper and lose hardness permanently, and sealed or shielded bearings lose their grease charge.
Slide the hot bearing onto the shaft in one continuous motion until it seats against the shoulder, then hold it against the shoulder while it cools, because the ring contracts axially as well as radially and will pull away from the shoulder if it is not held.
A sealed or shielded bearing must never be put in an oil bath, because the oil contaminates the grease charge. Use an induction heater with the bearing's temperature monitored.
After mounting
- Confirm the bearing is fully seated against the shoulder with a feeler gauge all the way around.
- Check for free rotation: a correctly mounted bearing turns smoothly with no notchiness and no drag.
- Verify the axial arrangement — one located end, one floating end. The floating bearing must actually be free to move in the housing.
- Install locknuts and lockwashers correctly: bend the tab into a slot, not against the face, and use the correct spanner rather than a punch and hammer.
- Lubricate before starting, to the correct fill, and set the relubrication interval in the maintenance system.
A mechanic drives a new bearing onto a shaft by resting a length of pipe against the outer ring and striking it with a hammer. What damage is being done?
Which method is unacceptable for heating a bearing prior to mounting?
After sliding a heated bearing onto the shaft, why must it be held firmly against the shaft shoulder while it cools?