1. What bearing preload means
Preload is an intentional internal load applied to a bearing before the main external service load acts. In practical maintenance language, it means the bearing is assembled with negative internal clearance rather than free clearance. It may be expressed as a force, for example newtons of preload, or as a displacement, for example micrometres of axial displacement or negative endplay. SKF describes preload as a negative operating clearance used where stiffness, precise shaft positioning, or minimum load is needed, and notes that it can be achieved by springs or by mechanical adjustment such as a nut.
The opposite condition is internal clearance or endplay. For tapered roller bearings, Timken defines bearing “setting” as the amount of endplay, meaning axial clearance, or preload, meaning axial interference, in the mounted bearing. Endplay gives measurable axial shaft movement; preload gives no discernible axial shaft movement and creates measurable rolling resistance.
The objective is not simply “tight is good.” The objective is the correct operating setting after the machine reaches temperature. Timken states that many tapered roller applications are assembled with a cold setting that becomes near-zero at stabilized operating temperature, while excessive operating preload must be avoided because it can reduce fatigue life, create lubrication problems, generate heat, and cause premature damage.
2. Axial preload versus radial preload
Axial preload
Axial preload is applied along the shaft axis. It is the normal preload method for:
- Angular contact ball bearings
- Tapered roller bearings
- Some deep groove ball bearing pairs
- Thrust bearings
In angular contact ball bearings and tapered roller bearings, the rolling-element contact line is inclined at a contact angle, so an axial adjustment produces an internal contact load with both axial and radial stiffness effects. SKF notes that single-row angular contact ball bearings and tapered roller bearings are generally preloaded axially by mounting them against a second bearing of the same type and size in back-to-back or face-to-face arrangements.
Radial preload
Radial preload acts perpendicular to the shaft axis. It is usually created by reducing or eliminating radial internal clearance through:
- Interference fits
- Tapered bore mounting on sleeves
- Controlled radial clearance reduction
- Thermal shrink fits
- Matched components in precision arrangements
SKF distinguishes the two directions clearly: cylindrical roller bearings can be preloaded radially because of their design, while thrust bearings are preloaded axially; angular contact ball bearings and tapered roller bearings are normally preloaded axially in opposed pairs.
The key difference is this: axial preload controls shaft end movement and axial stiffness, while radial preload controls radial looseness and radial stiffness. In an angular contact or tapered roller bearing set, axial preload also improves radial guidance because of the contact angle, but the adjustment itself is still axial.
3. Why preload is used
Correct preload can be beneficial when the bearing arrangement requires:
Higher stiffness. Preload reduces elastic displacement under load. SKF lists enhanced stiffness as a primary benefit, and NSK states that preloaded arrangements can have significantly higher rigidity than non-preloaded arrangements. (nsk.com)
Accurate shaft guidance. Gear pinions, pump shafts, machine tool spindles, and high-speed rotors may require precise axial and radial location. SKF specifically gives gear mesh control as an example: preloading differential bearings increases stiffness, limits gear mesh variation, reduces dynamic forces, and reduces noise.
Reduced noise and vibration. With less operating clearance, more rolling elements remain controlled in the unloaded zone, which can reduce noise. SKF lists reduced noise and improved shaft guidance among preload benefits.
Compensation for settling and wear. In adjusted bearing arrangements, mating surfaces settle during run-in, which can create clearance; preload can compensate for this when properly calculated.
Minimum load for lightly loaded bearings. Some rolling bearings, especially high-speed ball bearings and roller bearings, need minimum load to prevent skidding, smearing, or roller instability. SKF notes that springs are also used to provide a minimum load on lightly loaded bearings.
Protection against vibration while stationary. Schaeffler notes that bearings exposed to vibration at low speed are mounted clearance-free or even with preload to avoid rolling elements impacting the raceways. (schaeffler.com)
4. Bearings where preload is commonly required during mounting
Angular contact ball bearings
Single-row angular contact ball bearings are normally used in pairs or sets, because each bearing supports axial load primarily in one direction. Schaeffler states that single-row angular contact ball bearings can support axial force in one direction and high radial force, and must be axially adjusted against a second bearing in a mirror-image arrangement. (schaeffler.com)
They are common in pumps, compressors, machine tools, high-speed gear shafts, and electric motors where axial location and stiffness matter. Preload may be provided by:
- Matched duplex bearings
- Ground spacers
- Locknuts
- End covers
- Shims
- Springs
- Manufacturer-preloaded universal bearing sets
For high-speed service, a spring preload is often safer than a rigid preload because it accommodates thermal growth better. NSK distinguishes position preload, which gives high rigidity, from constant-pressure preload, which is suited to high-speed rotation, axial vibration prevention, or thrust-bearing use on horizontal shafts. (nsk.com)
Tapered roller bearings
Tapered roller bearings are especially suited for combined radial and axial load. Schaeffler states that tapered roller bearings can support axial loads in one direction and high radial loads, and must normally be axially adjusted against a second bearing in a mirror-image arrangement. (schaeffler.com)
They are used in gearboxes, pinions, wheel hubs, crushers, heavy pumps, rolls, and differentials. Their setting may be:
- Endplay
- Line-to-line, or zero clearance
- Light preload
- Designed operating preload
Timken emphasizes that tapered roller bearings can be set during assembly to a desired axial or radial clearance, and that this adjustment is a key advantage because it allows the designer to optimize bearing and system performance.
Deep groove ball bearings
Deep groove ball bearings are not usually the first choice for deliberate heavy preload, but they can be axially preloaded in certain arrangements, especially with springs or where light axial positioning is needed. SKF notes that deep groove ball bearings can be axially preloaded when larger radial internal clearance, such as C3 or C4, is selected so that a contact angle is obtained.
Cylindrical roller bearings
Cylindrical roller bearings are primarily radial-load bearings. They may be radially preloaded in special precision arrangements, but they are usually selected with controlled radial internal clearance rather than axial preload. Excessive radial preload in a cylindrical roller bearing can cause heat, smearing, edge stress, and premature spalling.
5. How preload is set during mounting
The important point is that preload must be set deliberately, not by “feel” unless the OEM procedure explicitly allows it. Schaeffler states that angular contact ball bearings and tapered roller bearings are mounted in pairs, and that axial internal clearance and radial internal clearance are set during mounting according to operating requirements. (schaeffler.com)
Common methods include:
Axial displacement method. Measure the axial movement with a dial indicator, then use shims, spacers, or nuts to reach the specified negative displacement. SKF describes this method as measuring shaft endplay relative to a fixed surface, typically with a dial indicator, then adjusting shims or spacers to produce the desired negative distance.
Frictional moment or rolling torque method. Tighten or shim the assembly until the measured rotating torque corresponds to the specified preload. SKF notes that preload and frictional moment are related, but that frictional moment also depends on lubricant, preservative, and sealing method.
Direct force or spring method. Springs apply a more constant preload force as parts expand. This is useful in high-speed ball bearing arrangements but may not give the same stiffness as rigid position preload. NSK describes constant-pressure preload as suitable for high-speed rotation and axial vibration prevention. (nsk.com)
Matched bearing sets. Some bearings are manufactured so that a predetermined preload or clearance is obtained after mounting. SKF lists universally matchable angular contact ball bearings and matched tapered roller bearings as bearing types supplied for preloaded arrangements.
Locknut, end cover, shim, or spacer adjustment. Schaeffler describes the transition from clearance to preload by tightening or loosening the locknut, inserting calibrated plates, rotating the shaft during adjustment, and using a dial gauge to check shaft movement. (schaeffler.com)
For tapered roller bearings, rotating the shaft during adjustment is especially important. SKF states that, when adjusting tapered roller bearings, the shaft must be turned several times so the rollers are not skewed and the roller ends contact the guide flange; otherwise the final preload may differ from the target.
6. Back-to-back, face-to-face, and tandem arrangements
Back-to-back, DB or O arrangement
In a back-to-back arrangement, the load lines diverge. This gives a wide effective support span and better resistance to overturning moment. SKF states that the distance between pressure centres is longer in back-to-back arrangements than in face-to-face arrangements, allowing larger tilting moments with smaller elastic displacement.
This arrangement is common in gear pinions, pump thrust bearing pairs, and precision shafts.
Face-to-face, DF or X arrangement
In a face-to-face arrangement, the load lines converge. It is somewhat more tolerant of misalignment than back-to-back but has a shorter effective support span. It is used where the housing arrangement or mounting sequence favors it.
Tandem, DT arrangement
In a tandem arrangement, two bearings face the same direction and share axial load in one direction. A tandem set must usually be opposed by another bearing or bearing set to take axial load in the reverse direction. Tandem angular contact ball bearings appear in pump designs where axial thrust is high in one dominant direction.
7. How preload changes during operation
The preload set at ambient temperature is not necessarily the preload during operation. Thermal expansion, shaft speed, lubricant churning, housing temperature, external axial load, and component deflection all change the final condition.
SKF explains that if the shaft temperature is higher than the housing temperature, the preload adjusted at ambient temperature changes; radial thermal expansion of the inner ring increases preload, while axial thermal expansion can increase or reduce preload depending on whether the bearings are face-to-face or back-to-back.
Schaeffler gives a practical warning: high loads and high speeds increase temperature at the bearing position, thermal expansion changes the clearance set during mounting, and a test run with temperature checks is needed where close shaft guidance is required. (schaeffler.com)
A correctly mounted bearing can therefore fail from preload if the operating preload becomes too high because of:
- Too little cold endplay
- Excessive locknut tightening
- Incorrect spacer or shim thickness
- Shaft growing hotter than the housing
- Housing distortion
- Excessive interference fits
- Bearing outer ring trapped where it should float
- Pump thermal growth between casing and shaft
- Gearbox housing growth different from shaft growth
- Incorrect replacement bearing internal clearance class
- Grease overfill or oil starvation causing heat, which further increases preload
SKF specifically warns that overtightening the adjustment device can create excessive preload, excessive operating temperature, and increased frictional moment in tapered roller or angular contact bearing arrangements.
8. When preload becomes a cause of failure
Preload becomes damaging when it exceeds the bearing’s required minimum and moves the bearing into excessive internal stress. The first sign is often temperature rise. Then the lubricant film thins, rolling/sliding friction increases, and fatigue life falls.
SKF states that stiffness increases only marginally after an optimum preload value, while friction and heat increase and can substantially reduce bearing service life.
Timken states that excessive preload can generate a large amount of heat and can produce damage similar to inadequate lubrication; it can also cause premature subsurface fatigue spalling even when the lubricant is strong enough to prevent direct scoring. (assets.wellertruck.com)
Typical failure symptoms from excessive preload include:
- Abnormally high running temperature
- Rapid grease oxidation or oil darkening
- Brown, blue, or straw discoloration
- Heavy polished raceway bands
- Smearing or scuffing
- Roller-end and rib distress in tapered roller bearings
- Cage wear or cage fracture
- Full-circumference raceway distress rather than a normal limited load zone
- Premature spalling
- Seizure in severe cases
NTN lists insufficient clearance, excessive load including excessive preload, insufficient lubrication, roller skewing, and abnormal temperature rise among causes associated with seizure, discoloration, softening, and abrasion on raceways. (ntnglobal.com)
9. Too little preload is also a problem
Insufficient preload or excessive endplay can also cause failure. The shaft may move axially, gear teeth may lose correct contact, balls or rollers may skid, and rolling elements can impact the raceways during vibration.
For tapered roller bearings, too much endplay can allow roller skew, poor load sharing, impact loading, cage instability, and gear mesh movement. For angular contact ball bearings, insufficient preload can permit ball skidding, vibration, noise, false brinelling, and unstable axial location.
This is why the correct setting is usually not “as much preload as possible,” but the minimum preload necessary to maintain stiffness, load sharing, and stability under operating conditions.
10. Ball bearings versus roller bearings
Angular contact ball bearings
Angular contact ball bearings use point contact, so they have lower friction and are better suited to high speed than comparable roller bearings. They are sensitive to excessive preload because contact stress rises quickly at the ball-race contact. Excessive preload produces heat, lubricant breakdown, ball/raceway smearing, cage distress, and fatigue spalling.
They are commonly used where speed is high and axial thrust is moderate, such as centrifugal pumps, electric motors, compressors, and machine tool spindles. In pumps, angular contact ball bearings are often installed as single-row pairs or double-row bearings to carry axial thrust.
Tapered roller bearings
Tapered roller bearings use line contact, giving high radial and axial load capacity. They are well suited for gearboxes, pinions, wheel hubs, heavy-duty pumps, and industrial transmissions. However, because the rollers contact ribs and raceways, excessive preload can rapidly increase frictional torque and heat.
Tapered roller bearings are very adjustable, but that adjustability is a risk during maintenance. A small shim, spacer, or locknut error can move the bearing from endplay to damaging preload. Timken notes that bearing setting in tapered roller bearings is primarily dependent on the axial location of one bearing row relative to the opposite row.
11. Preload in gearboxes
Gearboxes use preload mainly to control shaft position and gear mesh. Helical gears, bevel gears, spiral bevel gears, worm gears, and hypoid gears generate axial thrust, so the supporting bearings must control both radial and axial displacement.
Typical gearbox preload applications include:
- Pinion shafts with tapered roller bearings
- Bevel gear shafts with back-to-back tapered roller bearings
- High-speed gear shafts with angular contact ball bearings
- Differential pinions
- Planetary gear supports
- Worm shaft thrust arrangements
Correct preload helps maintain gear tooth contact pattern, reduces backlash variation, reduces noise, and improves gear life. SKF gives a differential example where preload increases stiffness, limits gear mesh variation, reduces dynamic forces, and reduces noise.
However, gearboxes often experience large thermal gradients. The shaft, gears, bearings, and housing heat at different rates. For that reason, many gearbox tapered roller bearing arrangements are assembled with controlled cold endplay so the operating condition becomes near-zero or slight preload. Timken states that the ideal operating bearing setting is generally near-zero to slight preload, and that many bearings are set with cold endplay at assembly to approach that target at stabilized temperature.
A gearbox bearing preload failure often presents as:
- High bearing temperature after rebuild
- Increased input torque or motor current
- Gear whine changing after warm-up
- Oil darkening or burnt smell
- Tapered roller rib/roller-end distress
- Spalling across a wide loaded arc
- Bearing temperature higher at one end of the shaft
- No measurable axial endplay when endplay was specified
Timken’s gearbox example shows a practical check: assemble without a shim and perform a spin check; if the shaft does not spin freely, the bearings are preloaded and shim correction is required.
12. Preload in pumps
Centrifugal pumps frequently use angular contact ball bearings because pump rotors develop axial thrust from impeller hydraulic forces. Common arrangements include:
- Back-to-back angular contact ball bearing pairs
- Double-row angular contact ball bearings
- Tandem angular contact ball bearing sets
- Tapered roller bearing sets in larger or slower pumps
- Spherical roller thrust bearings in very high thrust applications
SKF notes that angular contact ball bearing pairs support loading in either axial direction, and that its pump bearing set combines 40° and 15° contact-angle ball bearings for pump axial load acting predominantly in one direction; SKF also notes that matched tapered roller bearings and spherical roller thrust bearings are used in larger, slower-speed pumps where greater load capacity is needed.
Schaeffler states that double-row angular contact ball bearings are particularly suitable for pumps, can support axial loads in both directions and high radial loads, and are suitable where rigid axial guidance is required. (schaeffler.com)
Pump preload failure often occurs after maintenance when:
- The wrong bearing clearance class is installed
- A matched pair is installed in the wrong orientation
- DB/DF direction is reversed
- Spacers are mixed or ground incorrectly
- A shaft sleeve or locknut is overtightened
- The non-locating bearing is accidentally axially trapped
- Thermal growth from the hot pump casing removes clearance
- Hydraulic thrust is higher than design because of operating point, recirculation, or impeller wear-ring condition
Schaeffler also warns that tapered roller bearings and angular contact ball bearings are not suitable for a floating bearing arrangement because they must be adjusted to run correctly. (schaeffler.com)
13. Practical mounting rules
For angular contact ball bearings and tapered roller bearings, use this sequence:
- Confirm the bearing arrangement. Identify DB, DF, tandem, locating/non-locating, and whether the OEM requires endplay, zero clearance, or preload.
- Confirm bearing markings and orientation. Universal angular contact bearings and matched tapered roller bearings must be mounted in the correct arrangement.
- Measure shaft and housing fits. Excessive interference reduces internal clearance and can create unintended radial preload.
- Assemble clean and lubricated. Contamination or dry assembly can distort rolling torque readings.
- Seat the bearing correctly. For tapered roller bearings, rotate the shaft in both directions during adjustment so rollers settle against the guide rib. Schaeffler gives the same instruction for tapered roller bearings during mounting. (schaeffler.com)
- Measure endplay or preload. Use a dial indicator, displacement method, rolling torque method, or OEM-specified shim calculation.
- Run and monitor temperature. Schaeffler recommends test runs and temperature checks when close shaft guidance is required, to ensure clearance does not become too small and running temperature does not rise excessively. (schaeffler.com)
- Recheck after run-in. Grease distribution, seating, and thermal stabilization can change the setting.
14. How to distinguish preload failure from other failures
A suspected preload failure should be checked against these observations:
| Evidence | Suggests excessive preload | Other possible causes |
|---|---|---|
| High temperature immediately after assembly | Strong | Grease overfill, misalignment, seal drag |
| No axial endplay where endplay is specified | Strong | Incorrect measurement method |
| High rolling torque before operation | Strong | Seal drag, lubricant viscosity |
| Blue/brown heat discoloration | Strong | Lubrication failure, electrical damage |
| Full-circumference heavy raceway band | Possible | Rotation/load pattern, misalignment, overload |
| Tapered roller rib and roller-end scoring | Strong | Oil starvation, wrong lubricant |
| Spalling after short service | Possible | Contamination, overload, misalignment |
| Cage damage | Possible | Lubrication failure, vibration, misalignment |
NTN notes that observing rolling paths can clarify whether a bearing experienced radial load, axial load, combined load, moment load, large load, or mounting error, making raceway pattern analysis important in damage investigation. (ntnglobal.com)
15. Key conclusion
Preload is necessary when the machine requires controlled shaft position, high stiffness, stable rolling-element motion, reduced vibration, or compensation for settling. It is most common in opposed angular contact ball bearings and opposed tapered roller bearings.
Preload becomes a failure cause when the mounted or operating condition produces excessive internal load. In gearboxes this often comes from wrong shim/endplay setting, thermal growth, or locknut over-tightening. In pumps it often comes from incorrect angular contact bearing orientation, trapped non-locating bearings, thermal growth, or excessive hydraulic thrust.
For most rotating equipment, the best practical rule is:
Set the bearing to the OEM-specified cold endplay or preload, then verify that the operating condition reaches the intended temperature, torque, vibration, and axial movement. Never replace a specified clearance with “extra tightness” in the name of rigidity.
Rolling Bearing Reliability 