A ball-detent torque limiter couples two rotary members through spring-loaded balls in pockets. Sufficient torque pushes the balls up the pocket ramps against the spring, allowing relative rotation. Use this calculator to estimate a frictionless release threshold and compare it with an entered load torque.
Release Rotary Motion Interactive Calculator
Enter the total axial preload on the whole ball set, ball count, pitch radius and ramp angle. Then enter load torque to compare it with the ideal threshold. Force per ball is calculated at the threshold, not at the entered load.
Equation Used
- Entered spring preload is the total axial force on the ball set.
- All balls share the total preload equally and use identical ramps.
- Contact is rigid and frictionless in the threshold estimate.
- The displayed axial separation is schematic rather than a calculated travel.
How the ball-detent limiter works
Two coaxial members contain matching pockets on one pitch circle. A spring applies total axial preload, and the balls share that load. Below the threshold, the balls remain seated and transmit torque. Above it, relative rotation drives the balls up the ramps and separates the members axially.
The drawing combines a top view with an axial section so the pitch circle, equal ball spacing, spring direction, ramp and separation remain explicit.
Comparing release settings
Use the ideal estimate to explore how total preload, pitch radius and ramp angle affect release torque. Increasing preload or radius increases the threshold in direct proportion. Increasing ramp angle raises it according to tan(α).
Ball count changes the estimated force at each contact while leaving the summed threshold unchanged under equal loading. Compare the load with the threshold using the signed margin and load percentage.
Use manufacturer ratings and adjustment instructions when selecting or setting a real limiter. This estimate does not calculate contact stress, spring travel, friction, wear, temperature or allowable speed.
Ideal frictionless equal-load threshold
If total axial preload F is shared equally among n balls, axial force per ball is F/n. With alpha measured above the tangential direction of relative rotation, the ideal tangential force per ball at release is (F/n) tan(alpha). Summing n balls at pitch radius r gives T = F r tan(alpha).
Ball count cancels from the ideal summed threshold but changes the force per ball. Use radius in metres to obtain torque in N·m; the calculator converts the entered millimetres. Signed margin is threshold minus load, and load percentage is 100 × load / threshold.
Worked example
With 400 N total axial preload, six balls, 25 mm pitch radius and a 30-degree ramp, the ideal threshold is about 5.77 N·m. At release, axial load is 66.7 N per ball and tangential force is 38.5 N per ball. A 30 N·m load gives a −24.23 N·m margin and is about 520% of the threshold.
Where the ideal estimate stops
The calculation assumes identical straight ramps, equal load sharing, rigid contacts and no friction. It uses the entered preload as a fixed force; it does not follow changes in spring force while the plates separate.
Actual release and reset behavior depend on pocket shape, ball diameter, spring characteristics, friction, lubrication, tolerances, inertia and operating speed. Those effects require a fuller design or the manufacturer’s data.
The views are schematic. Above the ideal threshold, playback repeats a small relative movement and a plate gap; below it, the balls are held in the seated arrangement. Neither the gap nor the playback rate is a calculated physical travel or shaft speed.
Ball-detent release questions
Why does ball count not multiply the threshold?
The preload input is the total force on all balls. Adding balls divides that same force among more contacts. Under equal loading, their combined ideal torque threshold stays unchanged, while force per ball falls.
Is Tangential Force / Ball the force at my entered load?
It is the ideal tangential force per ball at the release threshold, calculated from preload and ramp angle. Changing the load input changes the margin and percentage, not this threshold-force result.
How is ramp angle measured?
It is the angle of the ideal ramp above the tangential direction of relative rotation. Axial rise divided by tangential travel equals tan(alpha). A real curved pocket may have a changing local angle.
What does a negative margin mean?
Margin is ideal threshold minus entered load. A negative result means the load exceeds the estimate. A positive margin only means the load is below this ideal threshold; it is not a component strength rating.
Does the threshold include friction or spring compression during release?
No. It uses the entered axial preload as a fixed force and assumes frictionless contact. It does not calculate force changes as the spring compresses, or the effects of friction and pocket curvature.
What does the animation show?
Below the threshold the balls remain stationary in their seated arrangement. At or above it, the diagram repeats a small relative ball movement and plate separation. This is a schematic release illustration, not a prediction of shaft speed, release travel or reset behavior.
Technical references
- US8382594B2 — spring-biased balls engage and disengage detents at a torque threshold.
- US9039538B2 — pocketed plates, axial spring loading and overload separation.
- US11585398B2 — preloaded spring, ramped pockets and axial motion on excess torque.
- RINGSPANN, Overload Clutches — ball-principle release at preset limit torque.
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