# 3D-printed wheel or gear slips on the motor shaft: D-shaft and set screw fixes

> Stop a printed wheel or gear from spinning on a motor shaft: match the D flat, add a set screw or a metal hub, and avoid a loose bore.

Canonical: https://roboticsengineering.in/fix/3d-printed-wheel-or-gear-slips-on-motor-shaft/ | Published: 2026-11-03 | Updated: 2026-11-04

**A round bore on a shaft with a flat will slip.** Print the D shape of the shaft and add a set screw against the flat.

## What the message means

The torque goes through the contact between the hub and the shaft. A bore a little too large or without the flat has no grip.

## Fix it in this order

1. Measure the shaft diameter and the flat with calipers.
2. Print a D-shaped bore and test the fit. Allow a small clearance for your printer.
3. Add a nut trap and a set screw that presses on the flat of the shaft.
4. Use a metal hub or a coupling for high torque.
5. Avoid glue on the shaft if you may need to remove the wheel.
6. Check that the hub wall is thick enough, so it does not crack.

## Why it happens

A hub transmits torque through the contact between its bore and the shaft. A plain round bore on a smooth shaft relies on friction alone, and if the bore is slightly too large there is almost no grip.

A D-shaped bore that matches the flat on the shaft locks the two together by their shape, and a set screw against the flat holds the hub in place. Printers differ in how much a hole shrinks, so print a small test piece to find the clearance for your machine.

## If you see this, do this

| You see | Do this |
| --- | --- |
| Motor turns, wheel does not | Slipping on the shaft. |
| Hub cracks at the screw | Thicker wall or a metal hub. |

## Related

[servo torque calculator](https://roboticsengineering.in/servo-torque-calculator/), [robot drifts to one side](https://roboticsengineering.in/fix/robot-drifts-to-one-side-motor-speed-mismatch/).

Last reviewed: November 2026. These are the usual causes and the order that finds them fastest. Test on your own board; clones and cables differ.

## Why this happens

The hub transfers torque through two things: the shape of the bore and the clamp of the screw. On a smooth round shaft, torque is carried only by friction between plastic and metal. If the bore is a little loose, that friction is close to zero and the shaft spins inside the hub. On a D-shaft the flat gives a shape lock, and the set screw presses on the flat so the hub cannot slide along the shaft.

Printed holes usually come out smaller than the model, and by an amount that depends on your printer and settings. That is why a bore drawn at the exact shaft size may be too tight on one printer and loose on another. Layer direction matters as well. A printed hub is weaker between layers than along them, so a screw boss that is printed lying on its side can split along the layer lines.

## How to confirm the cause

- Measure the shaft diameter and the flat depth with calipers, at two or three places. Write the numbers down.
- Print a thin test ring with the D-bore and try it on the shaft before printing the whole wheel. It should push on with a little force and not rock.
- Draw a line with a marker across the shaft and the hub, run the motor briefly, and look at whether the marks stay aligned. If they separate, the hub slips; if they stay together, the wheel is slipping on the floor or the tyre is slipping on the rim.
- Check the set screw. If its tip is not seated on the flat, or it bottoms out before it clamps, there is no grip.
- Look for cracks around the nut trap after a test run, which show the wall is too thin.

## Worked example

A small clearance test, assuming a 3.0 mm shaft with a flat. The sizes are examples; measure your own shaft.

```
Shaft diameter (calipers)     : 3.00 mm
Print 4 test rings, bore      : 3.00, 3.10, 3.20, 3.30 mm
Result on this printer        : 3.10 mm = firm push fit
Use in the model              : 3.10 mm, D-flat scaled the same
Set screw                     : M3, nut trap in the hub wall
Print orientation             : hub axis vertical (layers form rings)
```

## Common mistakes

- Threading a screw straight into the plastic, which strips quickly. Use a nut in a trap or a heat-set insert.
- Making the wall around the screw boss too thin, so it cracks when tightened.
- Over-tightening the screw until the hub splits.
- Gluing the hub to the shaft, which makes the wheel impossible to remove or adjust later.
- Using PLA on a hot motor mount. PLA softens at fairly low temperatures, so check the part does not warm up in use.

## If this did not fix it

- Check that the motor actually delivers torque. A stripped gear inside a geared motor gives the same symptom.
- Check that the tyre is not slipping on the rim, or the wheel on the floor.
- Check the battery. A weak supply can make the motor turn but not push.