The Shaft That Walked Out of Its Bearings
We mounted a drive shaft on two deep groove ball bearings. Both bearings were press-fitted on the shaft and fixed in the housing. On the bench, it spun. On the floor, after a day of running, the shaft heated up and grew 0.3 mm longer. The two fixed bearings couldn’t accommodate the thermal expansion. The bearing inner races were pushed off, or the outer races were loaded sideways. The bearings started to whine and fail in two weeks. The mistake was fixing both ends. A rotating shaft needs one fixed end (axially located) and one floating end (free to expand).
Bearing mounting arrangement design is about how the shaft is supported. The wrong arrangement (both ends fixed, or no location) causes premature failure. This article covers the standard arrangements.
The Basic Rule: One Fixed, One Floating
A rotating shaft heats up as it runs. The shaft grows longer (thermal expansion). The bearing arrangement must accommodate this growth.
The standard arrangement:
- Fixed end (locating bearing): One bearing that positions the shaft axially. The inner ring is locked on the shaft, and the outer ring is fixed in the housing. It takes both radial and axial loads. This end “fixes” the shaft’s axial position.
- Floating end (non-locating bearing): The other bearing allows the shaft to slide axially as it expands. The outer ring is free to move in the housing bore (or the inner ring is free to move on the shaft). It takes radial load only.
If both ends are fixed, thermal expansion pushes the bearings apart (preload increases, then failure). If neither end is fixed, the shaft floats axially (end play, misalignment).
Standard Arrangements
Fixed-Floating (Most Common)
One deep groove ball bearing (fixed end), one deep groove ball bearing (floating end). The fixed end takes axial load in both directions. The floating end’s outer ring slides in the housing bore.
Best for: Most drive shafts, conveyor pulleys, general rotating applications.
Pair of Angular Contact Bearings (Fixed End)
For high radial and axial loads (a gearbox output shaft, a spindle), the fixed end uses two angular contact bearings (back-to-back or face-to-face). They take combined radial and axial loads in both directions. The floating end is a deep groove ball bearing.
Best for: Gearboxes, spindles, high-precision shafts.
Both Ends Fixed (Short Shaft)
For very short shafts (under 100 mm) where thermal expansion is negligible (0.01 mm), both ends can be fixed. The expansion is too small to matter. This is common in small motors and gearboxes.
Best for: Short shafts, or shafts with controlled temperature (cooled spindles).
| Arrangement | Fixed End | Floating End | Best For |
|---|---|---|---|
| Fixed-floating (deep groove) | Deep groove ball bearing | Deep groove (sliding outer ring) | General drive shafts |
| Angular contact + floating | Duplex angular contact | Deep groove ball bearing | Gearboxes, spindles, high axial load |
| Both fixed (short shaft) | Deep groove (both ends) | None | Short shafts, low thermal growth |
| Tapered roller (both ends) | Two tapered roller bearings | Preloaded (both fixed) | Heavy loads, wheel hubs |
How to Make the Floating End Float
The floating bearing must allow axial movement. Two ways:
Sliding Outer Ring
The outer ring is not fixed in the housing bore. It’s a sliding fit (clearance). The shaft expands, and the outer ring slides in the housing. The bearing retains its radial position (it’s captured in the housing by a cover, but can move axially).
Note: The outer ring must be captured radially (it can’t fall out). A housing cover keeps it in place. But the cover doesn’t clamp the outer ring axially — it leaves room to slide.
Sliding Inner Ring (Needle Roller)
For needle roller bearings (full complement), the outer ring is fixed, and the inner ring slides on the shaft. The needle rollers are between the outer ring and the sliding inner ring. This is common in planetary gearboxes.
How to Make the Fixed End Fixed
The fixed end must locate the shaft axially (both directions). The inner ring is locked on the shaft (with a nut or shoulder), and the outer ring is clamped in the housing (with a cover).
- Inner ring lock: A shaft shoulder on one side, and a lock nut or circlip on the other. The inner ring can’t move axially on the shaft.
- Outer ring lock: A housing shoulder on one side, and a end cover (bolted) on the other. The outer ring can’t move axially in the housing.
This “locks” the shaft’s axial position. The shaft is located at the fixed end. The floating end allows expansion.
The mounting rule: One end fixed (inner ring locked on shaft, outer ring clamped in housing). One end floating (outer ring free to slide in housing). Never fix both ends on a long shaft — thermal expansion will preload and fail the bearings. For shafts under 100 mm, both ends fixed is acceptable. For longer shafts, fixed-floating is mandatory.
Shaft and Housing Fits
The bearing fits determine the mounting.
- Rotating inner ring (most common): The inner ring rotates on the shaft. The fit is an interference fit (press fit) on the shaft — the inner ring can’t spin on the shaft. The outer ring is stationary in the housing (transition or clearance fit).
- Rotating outer ring: The housing rotates (like a pulley). The outer ring is interference-fit in the housing. The inner ring is stationary on the shaft (clearance fit).
The rule: the ring that rotates should have an interference fit. The stationary ring can be looser (it allows the floating movement).
Bearing Preload (For Precision)
For high-precision shafts (spindles, lead screws), the fixed end uses preloaded angular contact bearings. The preload removes the internal clearance, making the shaft rigid.
Preload is applied by clamping the inner rings against each other (using a lock nut) or by spring-loading the outer ring. Too much preload = high friction, heat, and short life. Too little preload = play and vibration. The manufacturer specifies the preload amount (light, medium, heavy).
Lubrication and Seals
The bearing housing needs lubrication and seals.
- Grease lubrication: Most ball bearings are grease-packed for life. The housing is sealed. No maintenance.
- Oil lubrication: For high-speed or high-temperature bearings. The housing has oil passages. Needs maintenance (oil level, change interval).
- Shaft seals: The housing needs seals (lip seals, labyrinth) to keep grease in and dirt out. For dirty environments (conveyors, outdoors), use heavy-duty seals.
A Bearing Arrangement Checklist
- How long is the shaft? (Short: both fixed. Long: fixed-floating.)
- What radial load? (Deep groove vs angular contact.)
- What axial load? (Is there thrust?)
- Which end is fixed? (Usually the drive/coupling end.)
- Is the fixed end fully locked? (Inner ring and outer ring.)
- Is the floating end free to slide? (Outer ring clearance in housing.)
- What fits? (Interference on rotating ring, clearance on stationary.)
- Is preload needed? (For precision/spindle.)
- What lubrication? (Grease packed, or oil?)
- What seals? (For the environment.)
- Is there a housing cover to capture the floating outer ring?
- Can the shaft be assembled/disassembled? (Press fit access?)
The Bottom Line
Bearing mounting arrangement design is one fixed end and one floating end. The shaft that walked out of its bearings wasn’t undersized — it was fixed at both ends, and thermal expansion destroyed the bearings. Lock the fixed end (inner ring on shaft, outer ring in housing), let the floating end slide. Use angular contact for high axial loads, deep groove for general use. The shaft that runs cool and quiet for years wasn’t the biggest bearing — it was mounted with one end fixed and one end free to expand.
