A ball plunger is a threaded body with a spring behind a ball. Push the ball in and it resists; let it go and it springs back. That is the entire mechanism, and it does an enormous amount of work in machine design - indexing a rotary table, holding a cover shut, locating a jig plate, taking up play in a slide, or giving an operator a positive click at the right position.
It is also easy to specify badly, because there are two force numbers and people quote the wrong one.
How a Ball Plunger Works
The ball protrudes from the end of the body under spring pressure. When a moving part passes over it, the ball retracts into the body; when a pocket or groove lines up, the ball springs out into it and resists further movement until enough force is applied to push it back in.
The important consequence: the holding force depends on how far the ball is pushed in, and that depends entirely on the geometry of the pocket it drops into. The same plunger in a shallow dimple and in a deep conical seat behaves completely differently.
Initial Force vs End Force
Every ball plunger is specified with two figures, and confusing them is the usual cause of a detent that feels wrong:
- Initial force - the force with the ball fully extended, at the very start of its travel. This is the force the plunger exerts when it has just dropped into a pocket.
- End force - the force with the ball pushed flush into the body, at the end of its travel. This is the force needed to ride over a flat surface.
Specify on the force you actually need. If the plunger is riding on a flat face and taking up play, the end force is what it delivers. If it is dropping into a detent and holding position, the force is somewhere between the two, set by how deep the pocket is. A plunger chosen on its end force alone will feel far too weak in a shallow detent.
Size and Force Chart
Our stocked spring-type range, with the dimensions that matter for designing the mating feature:
| Thread | Body length A | Ball Ø C | Protrusion D | Initial force | End force |
|---|---|---|---|---|---|
| M3 | 7 mm | 1 mm | 0.8 mm | 4 N | 9 N |
| M4 | 9 mm | 1.8 mm | 1.5 mm | 6 N | 20 N |
| M5 | 12 mm | 2.4 mm | 2 mm | 6 N | 20 N |
| M6 | 14 mm | 2.7 mm | 2 mm | 7 N | 20 N |
| M8 | 16 mm | 4 mm | 2 mm | 15 N | 30 N |
| M10 | 19 mm | 4.5 mm | 2.5 mm | 20 N | 35 N |
| M12 | 22 mm | 6 mm | 3.5 mm | 30 N | 55 N |
| M16 | 24 mm | 8.5 mm | 4.5 mm | 45 N | 100 N |
| M20 | 30 mm | 10 mm | 6.5 mm | 60 N | 200 N |
Dimensions and forces for our stocked spring-type range. Initial force is measured with the ball fully extended; end force with the ball pushed flush into the body.
Designing the Detent
The pocket does more to set the feel than the plunger does. Three common forms:
- Spherical dimple, radius slightly larger than the ball. The standard choice. The ball seats on a large area and releases smoothly. Make the radius the same as the ball and the ball wedges; make it much larger and the detent goes vague.
- Conical seat, typically 90° to 120° included. Sharper, more positive engagement, and self-centring. Steeper cones hold harder but need more force to release and wear faster.
- Groove or slot. For linear detents and end stops - the ball locates in one axis only.
Two rules regardless of form:
- Never make the pocket deeper than the ball protrusion. If it is, the ball bottoms in the pocket, the body face contacts the mating surface, and the plunger cannot release at all. Look up the protrusion figure - it is in the chart above - and keep the pocket meaningfully shallower.
- Harden the pocket if it will cycle. A ball running into a soft aluminium or mild steel dimple thousands of times will hammer the pocket oversize and the detent will go soft. Use a hardened insert for anything with real cycle life.
Hole Preparation
- Tap the thread to the correct class and to full depth. A plunger bottoming on an incomplete thread sits proud and the protrusion is wrong.
- Drill deep enough that the plunger body can be adjusted in and out - you often need to trim the protrusion on assembly.
- Chamfer the hole mouth. A burr at the mouth deforms the first thread and makes the plunger sit crooked.
- Keep the axis square to the surface the ball works against. A plunger fitted at an angle side-loads the ball, which wears the body bore oval and eventually lets the ball fall out.
- In thin material, the thread may be too short to hold. Use a threaded insert rather than relying on two or three engaged threads.
Installation
- Wind the plunger in until the ball protrusion is right for your detent depth - not necessarily until the body is flush.
- Check the movement by hand through a full cycle before committing. The ball should retract fully and return freely.
- Lock the setting. A plunger that unwinds changes its protrusion and therefore its force. Use a thread-locking compound, a locknut, or a nylon-patch plunger where vibration is present.
- Avoid driving the ball with a hammer or pressing on the ball to seat the body - you will indent the ball seat and the plunger will never feel right again.
Why Detents Feel Wrong
| Symptom | Likely cause |
|---|---|
| Detent too weak / slips past | Plunger sized on end force; pocket too shallow; plunger wound in too far |
| Detent too strong / will not release | Pocket deeper than ball protrusion; cone angle too steep; plunger oversized |
| Ball sticks retracted | Contamination in the body; side-loading from an out-of-square hole |
| Detent fades over time | Soft pocket hammering oversize; spring settling; ball seat worn |
| Plunger works loose | No thread locking - protrusion drifts and force changes with it |
| Ball falls out | Body bore worn oval by side loading, usually from a crooked installation |
If you know the force you need at the detent but not which size delivers it, send us the pocket geometry and the force and we will match a plunger from stock.
Need Ball Plungers?
Spring-type ball plungers from M3 to M20, ex-stock Mumbai - tell us your detent force and we will match a size.
View Plunger RangeFrequently Asked Questions
What is the difference between initial force and end force on a ball plunger?
Initial force is measured with the ball fully extended, at the very start of its travel - the force the plunger exerts just after dropping into a pocket. End force is measured with the ball pushed flush into the body, and is what the plunger delivers when riding over a flat surface. Specify on whichever condition your application actually works in.
How deep should a ball plunger detent be?
Always shallower than the ball protrusion, which is listed with the plunger dimensions. If the pocket is deeper, the ball bottoms out and the plunger body face contacts the mating surface, so the detent cannot release. A spherical dimple slightly larger in radius than the ball, or a 90° to 120° cone, are the usual forms.
How do I stop a ball plunger unwinding?
Lock the thread. A plunger that unwinds changes its protrusion, which changes both the detent depth it can engage and the force it delivers. Use a thread-locking compound, a locknut, or a nylon-patch plunger where vibration is present.
Why does my detent get weaker over time?
Usually the pocket rather than the plunger. A hardened ball running into a soft aluminium or mild steel dimple hammers it progressively oversize, so the ball no longer seats at the designed depth. Use a hardened insert for the pocket on anything with real cycle life. Spring settling and a worn ball seat are secondary causes.
Can a ball plunger be used as a locating device?
Only for light positioning, not for precision location. The ball is spring-loaded and will move under load, so it holds a position rather than defining one. Where accuracy matters, locate with dowel pins and use the ball plunger only to hold the parts together at that position.
What thread sizes are ball plungers available in?
Our stocked spring-type range covers M3 to M20, with end forces from around 9 N at the smallest size up to 200 N at M20. Ball diameter and protrusion scale with the thread, and both matter when designing the mating detent.