Clevis & Actuator Gap Tolerance Stack-Up
Clevis and rod-eye joints connect actuators, linkages, and control systems throughout mechanical assemblies. The axial gap between the clevis ears and the rod eye determines free play, binding risk, and assembly ease. This guide walks through the tolerance stack-up for this common configuration.
The Clevis Assembly
A typical clevis joint consists of:
- Clevis (yoke) — A U-shaped bracket with two parallel ears and aligned holes
- Rod eye (tang) — The end fitting on the actuator rod or linkage, with a single hole
- Clevis pin — Passes through all three holes, held by a cotter pin or retaining ring
The rod eye sits between the clevis ears. The gap of interest is the total axial clearance—the sum of the gaps on both sides of the rod eye.
Assembly Sketch
┌───────────────────────────────────┐
│ CLEVIS (YOKE) │
│ ┌─────┐ ┌─────┐ │
│ │ EAR │ ┌─────┐ │ EAR │ │
│ │ 1 │ │ ROD │ │ 2 │ │
│ │ │ │ EYE │ │ │ │
│ │ (○) │◄──►│ (○) │◄──►│ (○) │ │
│ │ │Gap1│ │Gap2│ │ │
│ └─────┘ └─────┘ └─────┘ │
└───────────────────────────────────┘
│
PIN passes
through all 3
Total Gap = Gap1 + Gap2 = Clevis Width − Rod Eye Thickness
Building the Signed Loop
The gap calculation is straightforward: the space inside the clevis minus what the rod eye occupies.
But the "clevis inside width" often isn't a single dimension. It's typically:
Sign Convention
Using the standard approach (positive = increases gap):
| Dimension | Effect | Sign |
|---|---|---|
| Clevis overall width | Larger → more space → bigger gap | + |
| Ear 1 thickness | Larger → less space → smaller gap | − |
| Ear 2 thickness | Larger → less space → smaller gap | − |
| Rod eye thickness | Larger → occupies more space → smaller gap | − |
Typical Contributors
Beyond the basic dimensions, real clevis assemblies may include:
| Contributor | Where It Appears | Sign |
|---|---|---|
| Washers / spacers | Between rod eye and ears | − |
| Bushings (flanged) | In clevis ear holes | − (flange thickness) |
| Pin head / shoulder | One end of assembly | Usually outside stack |
| Retaining ring | Other end of pin | Usually outside stack |
| Coating thickness | All mating surfaces | − (both sides of rod eye) |
Pin Clearance Is Separate
The clevis pin fit (clearance in the holes) is a separate radial stack-up. This guide focuses on the axial gap. If the pin is too tight, see the Pin-Bushing Clearance guide.
Worked Example: Hydraulic Cylinder Clevis
A hydraulic cylinder has a clevis mount. The rod eye must fit between the clevis ears with 0.2–0.8 mm total gap for easy assembly and acceptable play.
Given Dimensions
| Dimension | Nominal (mm) | Tolerance | Sign |
|---|---|---|---|
| Clevis overall width | 50.00 | ±0.15 | + |
| Ear 1 thickness | 12.00 | ±0.10 | − |
| Ear 2 thickness | 12.00 | ±0.10 | − |
| Rod eye thickness | 25.50 | ±0.10 | − |
Nominal Gap Calculation
Gap = 50.00 − 12.00 − 12.00 − 25.50
Gap = 0.50 mm
Worst-Case Analysis
Total WC tolerance = 0.15 + 0.10 + 0.10 + 0.10 = ±0.45 mm
Max Gap = 0.50 + 0.45 = 0.95 mm
Min Gap = 0.50 − 0.45 = 0.05 mm
Spec Check: Problem Found
The spec requires 0.2–0.8 mm gap. WC range is 0.05–0.95 mm. Both limits are violated. At minimum gap (0.05 mm), assembly may bind. At maximum gap (0.95 mm), excessive play.
RSS Analysis
RSS tolerance = √(0.15² + 0.10² + 0.10² + 0.10²)
RSS tolerance = √(0.0225 + 0.01 + 0.01 + 0.01) = √0.0525 = ±0.229 mm
RSS Max = 0.50 + 0.229 = 0.73 mm ✓
RSS Min = 0.50 − 0.229 = 0.27 mm ✓
RSS range (0.27–0.73 mm) falls within spec. If production processes are well-centered and volumes are high, RSS may be acceptable. For safety-critical applications or low volumes, tighter tolerances are needed.
Fixing the Stack
Options to meet spec at worst case:
- Tighten rod eye tolerance — From ±0.10 to ±0.05 (highest sensitivity)
- Tighten clevis width tolerance — From ±0.15 to ±0.08
- Add shim at assembly — Select shim thickness to center the gap
- Increase nominal gap — Reduce rod eye nominal from 25.50 to 25.40
Common Pitfalls
- Forgetting both ears — If the clevis is machined as a unit, both ear thicknesses may be controlled by the same dimension (inside width). Check the drawing.
- Ignoring parallelism — Non-parallel ears create uneven gap. In extreme cases, the rod eye may bind on one side while having excess gap on the other.
- Missing bushing flanges — Flanged bushings take up axial space. Include their contribution.
- Confusing radial and axial — Pin-hole clearance is radial. Ear-to-eye gap is axial. Different stacks, different concerns.
- Symmetric assumption — The gap splits between both sides, but not necessarily evenly. The stack predicts total gap, not per-side distribution.
Key Takeaways
- Gap = Clevis Width − Ear 1 − Ear 2 − Rod Eye — The basic loop for most clevis joints
- Include all axial contributors — Washers, bushing flanges, coatings
- Check both gap limits — Too small binds, too large creates play
- Rod eye tolerance is often most sensitive — It affects gap 1:1
- Consider shimming for critical applications — Adjust gap at assembly