A rolling-element pulley bearing lets a pulley turn around its supporting axle. In the arrangement shown here, the pulley carries the rotating outer ring while the inner ring stays on the fixed axle. Balls roll between the rings and their cage travels around the axle. The calculator compares the basic fatigue rating life of a ball bearing under three separately entered steady loads.
Anti-friction Bearing for Pulley Interactive Calculator
Vary bearing rating, radial loads, and pulley RPM to see L10 bearing life for low, nominal, and high-load conveyor conditions.
Equation Used
L10 estimates the basic rating life of a ball bearing in hours. The life rises with the cube of the catalog dynamic rating C divided by the equivalent radial load P, then converts total revolutions to hours using pulley speed N in RPM.
- Deep-groove ball bearing load-life exponent is 3.
- Radial load is steady for each operating case.
- Catalog dynamic load rating C and equivalent radial load P use the same force units.
- The 30k Margin compares nominal life with a fixed 30000-hour reference, not a prescribed service interval.
How the pulley bearing turns
The belt or rope loads the pulley rim. That load passes through the pulley hub, outer bearing ring, rolling elements and inner ring to the axle and its support. The pulley and outer ring rotate together; the inner ring remains stationary in this example.
The balls both spin about their own axes and orbit around the axle. A cage separates them. The animation uses ideal rolling geometry and enlarged bearing details; changing the catalog rating does not select or resize a manufactured bearing.
Parts to identify
- Outer ring: seated in the pulley hub, with an internal raceway for the balls.
- Inner ring: supported by the axle, with an external raceway.
- Balls and cage: carry load between the rings while remaining separated.
- Seals, shields and lubricant: manage contamination, lubricant retention and contact conditions according to the bearing’s construction.
Fits, internal clearance and sealing must suit the actual bearing, load direction, temperature and mounting arrangement. A fit appropriate to a rotating shaft is not automatically appropriate to a pulley whose outer ring rotates around a fixed shaft. Bearing bore means the inner-ring diameter; it is not the pulley’s housing-bore diameter.
Where pulley bearings are used
Conveyor idlers, belt tensioners and rope sheaves use bearing arrangements to support a rotating rim or drum. The load on an individual bearing depends on the belt or rope forces, pulley weight, support spacing and any axial force. Belt tension alone is not necessarily the equivalent bearing load.
Ball, needle and other roller bearings offer different packaging and load capabilities. Some sheaves use plain bushings instead. Selection includes speed, environment, alignment, lubrication and maintenance as well as the catalog load rating.
This calculator addresses basic ball-bearing fatigue life. It does not establish the suitability of an elevator, hoist, vehicle tensioner or any other complete pulley assembly.
Basic ball-bearing rating life
For the entered equivalent dynamic bearing load P and catalog dynamic rating C, the model uses L10h = (C/P)³ × 10⁶/(60N), with speed N in rpm. C and P must use the same force units; the controls use newtons. The factor 10⁶ converts million revolutions into revolutions, and 60N converts revolutions into hours.
Basic L10 is a statistical fatigue rating: 90% of a sufficiently large group of apparently identical bearings are expected to reach or exceed it under the stated conditions. It is neither an individual failure date nor a replacement interval. Lubrication, contamination, mounting and other damage mechanisms require separate assessment.
The calculator uses exponent 3 for ball bearings. Roller bearings use a different basic exponent, 10/3, which is not implemented here. The entered P should already represent the appropriate equivalent load; the tool does not derive it from radial and axial force components.
Low, nominal and high loads produce independent steady-load comparisons. They are not combined into a duty-cycle life. The 30k Margin is 100(L10h,nom/30000 − 1)%. Its fixed denominator is a comparison reference.
Within this formula, doubling P reduces life to one eighth, while doubling rpm halves the life expressed in hours. At P = C, basic life is one million revolutions; the corresponding hours therefore depend on speed.
Worked example: three steady-load comparisons
Use an assumed ball-bearing dynamic rating C = 13500 N and speed N = 380 rpm. These are illustrative values, not a specification for a particular bearing part number.
At nominal load 1800 N, C/P = 7.5 and its cube is 421.875. The conversion factor is 10⁶/(60 × 380) = 43.859649 hours per million revolutions. Nominal L10 is 18503.29 hours, and 30k Margin is −38.32%.
At low load 1200 N, the separate result is 62448.60 hours. At high load 2500 N, it is 6906.32 hours. Each assumes that its own load persists throughout the comparison; these are not predictions for a conveyor alternating among those loads.
For comparison, entering C = 19500 N with the same high load and speed gives 20813.68 hours, still below the 30000-hour reference. Increasing the rating alone therefore does not demonstrate that every load case reaches the reference. A real bearing selection must also satisfy dimensional, speed, static-load and installation requirements.
Comparing bearing arrangements
| Arrangement | What to compare |
|---|---|
| Deep-groove ball bearing | Radial and permissible axial loads, speed, sealing and available internal clearance. |
| Needle or other roller bearing | Radial space, raceway requirements, load capacity and any separate provision for axial load. |
| Plain bushing | Sliding wear, lubricant, pressure and surface-speed limits, and shaft finish. |
| Self-aligning bearing arrangement | Specified alignment capability and the resulting mounting and load requirements. |
Friction, temperature, life and price cannot be assigned universal values from bearing type alone. This ball-bearing formula does not calculate bushing wear or select among these constructions.
Pulley-bearing questions
The sequence depends on the fits and assembly. Follow the bearing and equipment instructions. Apply mounting force to the ring being fitted; do not transmit it through the rolling elements.
No. It is a statistical fatigue rating, not a guaranteed service interval. Inspection and maintenance requirements depend on the actual equipment and operating conditions.
Damage from contamination, lubricant problems, mounting, corrosion or other causes is not predicted by this basic formula. A low observed life requires investigation; the calculator cannot diagnose a cause or justify a universal derating factor.
No. C3 describes greater internal clearance than normal clearance. The appropriate clearance depends on the bearing, fits and temperature conditions; it is not a general load-capacity upgrade.
The controls would still use the ball-bearing exponent 3. This calculator does not implement the roller-bearing exponent 10/3, so it should not be used as a roller-bearing life calculation.
No. Each is evaluated separately as a steady load. The tool contains no time fractions or combined variable-load calculation.
Rough or stiff rotation can have several causes. It does not uniquely identify housing size or establish that a bearing is fit for continued service.
No. Their contact, lubricant-retention and contamination-control characteristics differ. Select the construction for the actual environment and speed rather than treating a suffix as a universal guarantee.
References and further reading
SKF: Rolling bearings catalog explains basic rating life, equivalent loads, mounting and bearing selection. Use the current data and instructions for the particular bearing and pulley assembly.
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