The Detent Ball That Wouldn’t Hold
We used a spring-loaded detent ball (a ball in a tapped hole, pushed by a spring) to hold a rotary handle in position. The spring was a standard compression spring from the catalog. On the bench, the ball held the handle. On the floor, the handle slipped — the ball popped out of its detent notch under a light load. The problem: the spring force was too low. The detent ball needs enough force to hold the load, but not so much that the operator can’t move the handle. We picked the spring by eye (a “medium” spring), not by calculation. We calculated the required force: the detent notch is a 30° V, the ball is Ø6 mm, and the axial force needed to hold is about 15 N. The spring delivered only 5 N. We upsized the spring (higher rate) to deliver 15 N at the installed length. The detent held. The mistake was not calculating the detent force.
Spring selection for detents and plungers balances holding force against actuation force. This article covers the numbers.
How a Detent Works
A detent (positioner) holds a mechanism in a position until a deliberate force moves it. Common types:
- Ball detent: A ball (Ø4–10 mm) in a spring-loaded hole. It snaps into notches or dimples on a rotating or sliding part. Holds light loads.
- Plunger detent: A spring-loaded pin (with a flat or rounded tip). For heavier holding.
- Roller detent: A spring-loaded roller. For sliding tracks (less wear than a ball).
Step 1: Required Holding Force
The detent must hold the load against unintended movement. What tries to move it? Vibration, gravity (a hanging handle), a light spring return. Estimate the holding force F_hold. For a handle held in notches, the holding force is the axial force needed to pull the ball out of the notch.
The geometry matters. For a ball in a V-notch (angle α):
F_hold = F_spring / sin(α/2)
For a 90° V-notch (α = 90°): F_hold = F_spring / sin(45°) = F_spring / 0.707 = 1.41 × F_spring. The notch multiplies the spring force. For a 60° notch: F_hold = F_spring / sin(30°) = 2 × F_spring.
For F_hold = 15 N (required) and a 90° notch: F_spring = 15 × 0.707 = 10.6 N. The spring must deliver about 11 N at the installed length.
Step 2: Spring Force at Installed Length
A compression spring’s force is:
F = k × x
Where k is the spring rate (N/mm) and x is the deflection (mm). The installed length is shorter than the free length. The spring is preloaded (compressed) by x_preload. When the ball moves out of the notch (additional deflection x_work), the force increases slightly.
For F_spring = 11 N and the spring compresses 5 mm from free length to installed: k = 11 / 5 = 2.2 N/mm. When the ball rides up out of the notch (another 1 mm), the force becomes 2.2 × 6 = 13.2 N. That’s acceptable (the force change is small). If k is too high (stiff spring), the force jumps a lot as the ball moves — the detent feels harsh. If k is too low, the holding force is weak.
The detent rule: Calculate F_hold from the notch geometry, then F_spring = F_hold × sin(α/2). Pick a spring that delivers F_spring at the installed length. The detent that wouldn’t hold had a 5 N spring against a 15 N requirement. Upsize to deliver 11 N. Too stiff and the operator can’t move it; too soft and it slips.
Step 3: Actuation Force (Operator Effort)
The detent must release when the operator deliberately moves it. The actuation force (the force to push the ball out of the notch) is F_hold. The operator should feel a definite click (not too hard, not too soft).
- Manual handle: F_actuation = 5–20 N (comfortable).
- Frequent adjustment: 5–10 N (light).
- Fixed position (rarely moved): 20–50 N (positive hold).
If the actuation force is over 30 N, the operator struggles. If under 3 N, the detent doesn’t feel secure.
Step 4: Spring Fatigue
The spring cycles every time the detent clicks. For frequent detents (a rotary indexer that detents every revolution), the spring must survive millions of cycles. Use music wire (high-quality spring steel) and keep the stress below the fatigue limit (about 40% of the tensile strength). For occasional detents (a set-and-forget handle), fatigue isn’t an issue.
Step 5: Ball Material and Wear
The ball and notch wear over time. For light use (occasional), a steel ball in an aluminum notch is fine. For frequent detents, use a hardened ball (or a carbide ball) and a hardened notch (steel). A steel ball in a soft aluminum notch will wear (the notch rounds over time, and the detent weakens).
| Element | Selection |
|---|---|
| Notch angle | 60° (strong hold) to 90° (easy release) |
| Ball diameter | Ø4–10 mm (bigger = more force, more wear) |
| Spring rate k | F_spring / installed deflection |
| Actuation force | 5–20 N (manual) |
| Ball material | Hardened steel or carbide (for frequent use) |
A Detent Spring Checklist
- What tries to move the detent? (Vibration, gravity, load?)
- Required holding force F_hold?
- Notch angle α? (Determines the force multiplier.)
- Spring force F_spring = F_hold × sin(α/2)?
- Spring rate k? (F_spring / deflection.)
- Is the actuation force comfortable? (5–20 N?)
- Ball diameter? (Matches the notch?)
- Ball and notch material? (Hardened for frequent use?)
- Is the spring fatigue-rated? (Frequent cycles?)
- Is there a set screw to adjust preload? (Fine-tune the force?)
The Bottom Line
Spring selection for detents is the balance between holding and actuation. The detent that wouldn’t hold had a weak spring (5 N vs 11 N required). Calculate F_hold from the notch geometry, then F_spring = F_hold × sin(α/2). Pick a spring that delivers F_spring at the installed length. Keep the actuation force at 5–20 N for manual operation. Use a hardened ball and notch for frequent cycling. The detent that clicks positively every time wasn’t the stiffest spring — it was calculated.
