A servo motor coupling that came loose after 3 months. The coupling was clamped with an M4 screw. The motor shaft was 14 mm. The coupling bore was 14 mm H7. Under load, the coupling crept on the shaft. The customer thought the set screw was loose. It was tight. The issue: H7 clearance fit on a clamped coupling doesn’t grip enough for torque transmission. This is about shaft-to-bore fits for couplings.
The three fit classes
| Fit | Bore tolerance (for 14 mm) | Shaft tolerance | Clearance/Interference | Use for |
|---|---|---|---|---|
| H7 | +0.018 / 0 | h6: -0.011 / 0 | Clearance 0-0.029 mm | Sliding parts, loose fits |
| JS7 | +0.009 / -0.009 | k6: +0.018 / +0.002 | Light interference 0-0.027 mm | Clamped couplings, general |
| K7 | +0.002 / -0.018 | k6: +0.018 / +0.002 | Interference 0-0.036 mm | Press fits, keyed joints |
What H7 means for couplings
An H7 bore is the most common — it’s a “clearance fit.” The bore is slightly larger than the shaft. When you clamp the coupling, the bore collapses around the shaft. But the clamping force only acts through the screw contact area. For a 14 mm bore with one M4 screw, the contact pressure is local. Under torque, the coupling can creep — the shaft rotates inside the bore.
The coupling that crept was H7 on a 14 mm motor shaft. The motor delivers 1.2 N·m peak. The M4 clamp screw provides about 500 N clamping force. The friction torque: T = μ × F × r = 0.2 × 500 × 7 = 700 N·mm = 0.7 N·m. That’s less than the 1.2 N·m peak. The coupling slips.
What I changed
1. Specified K7 bore. With K7, the bore is slightly smaller than the shaft. The coupling must be pressed on (or heated for a shrink fit). When clamped, the interference adds to the friction. The effective friction torque jumps to 2-3 N·m. No slip. The coupling holds.
2. Used a dual-screw coupling. Instead of one M4 screw, I used a coupling with two M3 screws at 90° apart. The clamping force doubles (1000 N). The friction torque doubles (1.4 N·m). With H7 bore, that’s marginal. With K7 bore, it’s solid.
3. Added a keyway. For high-torque or shock loads, I specify a keyway on both shaft and coupling. The key carries the torque mechanically, not through friction. The clamping screw positions the coupling; the key transmits the torque. No creep, no slip.
The motor shaft tolerance
Servo motor shafts are typically k6 or j6 (not h6). A k6 shaft is slightly larger than nominal. When paired with an H7 bore, you get a transition fit (not pure clearance). That’s why most couplings work with H7 — the motor shaft is already oversized. But if the shaft is h6 (ground to nominal), the H7 bore has real clearance. I always check the shaft tolerance before specifying the bore.
For bellows couplings
Bellows couplings are zero-backlash but have low clamping force. The bellows is thin — you can’t torque the screw too hard (it crushes the bellows). For bellows couplings, I always spec K7 or interference fit. The friction from the interference is what holds it, not the screw. For aluminum bellows couplings, I use a split clamp design that compresses evenly around the shaft.
The fit I specify: K7 for clamped servo couplings, H7 only for slip-on pulleys that need to slide off. The coupling that crept wasn’t loose — it was H7 on a h6 shaft with one screw. K7 bore or two screws fixes it. For high-torque, add a keyway.
