Worm Gearbox Self-Locking: When It Holds | WGT WeiGao
Worm Gearbox Self-Locking: When It Holds and When It Doesn't
Self-locking is why many buyers choose a WP series worm gearbox: the load stays where you put it after the motor stops, with no brake to wire or replace. It is a real advantage — and the most commonly misunderstood property in the gearbox catalogue. This guide explains what produces self-locking, how to tell whether a specific unit has it, and when to fit a brake instead.
What self-locking actually means
A worm gearbox is self-locking when the worm cannot be rotated by the output shaft: disconnect the motor, apply torque to the output shaft, and nothing moves.
The condition is geometric, not a matter of brand or ratio. Two angles decide it:
- Lead angle — how steeply the worm thread is cut, set by the lead and the worm diameter.
- Friction angle — the angle whose tangent is the friction coefficient of the worm and worm wheel pair, which depends on materials, surface finish and lubricant.
If the lead angle is smaller than the friction angle, the thread cannot slide back under load. If it is larger, the load can back-drive the worm — no matter how large the ratio looks on the nameplate.
Two consequences matter more than the definition:
- It is a static property. It describes holding a load at rest, not stopping a moving load, and not holding a load that is being shaken.
- It fights efficiency. The small lead angle that prevents back-driving also lowers forward efficiency, because more input work is lost to sliding friction. A strongly self-locking gearbox is not a high-efficiency gearbox.
Ratio is a clue, not an answer
Buyers often ask for "a self-locking ratio". Ratio matters only because of how it is achieved — two gearboxes at the same ratio behave differently depending on how many starts the worm has.
| Worm type | How the ratio is reached | Lead angle | Self-locking tendency | Typical applications |
|---|---|---|---|---|
| Single start | Many teeth per worm revolution | Smallest | Strongest — most likely to hold | Lifting, positioning, hold-when-stopped duty |
| Double start | Half the teeth per worm revolution at the same ratio | Moderate | Borderline — confirm per unit | Medium duty drives that also hold |
| Multi-start (3+ / high lead) | Few teeth per worm revolution | Largest | Weakest — usually back-drives | Conveyors, mixers, high-efficiency duty |
So "our worm gearbox is 1:60, so it holds" is not a safe statement. A 1:60 unit built with a multi-start worm can be free to back-drive, while a 1:20 single-start unit may hold firmly.
When self-locking holds — and when it doesn't
Self-locking is a property of one gearbox in one condition. Change the condition and the friction that makes it work changes too — the difference between a drive that holds for years and one that drops a load when the ambient temperature climbs.
| Condition | Effect on self-locking | What to do |
|---|---|---|
| Clean, warm, well-lubricated running-in period | Friction is lowest; a marginal unit may creep | Test the actual unit under load, not the catalogue page |
| Long service, run-in flanks | Friction falls as surfaces bed in | Re-check the hold at the first service interval |
| Vibration or repeated shock | The mesh can ratchet and the load creeps | Treat vibration as a back-drive risk; add a brake or backstop |
| High ambient or hot oil | Lower viscosity and lower friction at the mesh | Do not rely on self-locking in hot plants |
| Low-friction or EP additive oils | These do exactly what they say — lower friction | Confirm the lubricant with the gearbox supplier |
| Load applied dynamically | Self-locking does not arrest motion | Dynamic braking needs a brake, not a worm mesh |
| Static load held for long periods | Best case for a self-locking worm unit | Still confirm break-away behaviour for safety cases |
The practical rule: self-locking is reliable enough for positioning and for holding steady loads. It is not reliable enough to be a safety device. If a person or a product can be injured by the load moving unexpectedly, specify a brake or backstop and treat self-locking as a bonus.
Self-locking, a brake, or a different gearbox
There are four honest ways to make a load stay put.
- Self-locking worm gearbox — lowest cost, lowest efficiency. Suits steady loads, modest duty and small-lead ratios. Confirm the specific unit before relying on it.
- Helical or helical bevel gearbox plus a brake. Higher efficiency and torque density, and holding that does not depend on friction. The standard answer for continuous-duty and safety-related drives.
- Worm screw jacks for lifting. They hold through the thread form itself and are engineered for lifting duty, including multi-unit synchronisation. For "raise and hold", a jack usually fits better than a gearbox that happens to self-lock.
- Efficiency first, holding by design. Where hours are long, a helical gear unit with a brake usually wins on total cost despite the higher purchase price.
Holding torque is not the only interface to check. A brake on the motor shaft and one on the gearbox output shaft are not equivalent, and the coupling between them must suit the braking torque as well as the running torque.
What to confirm before you rely on self-locking
Send these five points to the supplier in writing and keep the answer with the order:
- Is this specific unit self-locking? Ask for the lead angle and the back-driving condition, not just the ratio.
- Under what conditions? Ambient range, lubricant type and duty cycle all change the answer.
- Static or dynamic holding? State plainly whether the drive holds at rest only, or must also stop a moving load.
- What is the safety consequence of failure? If people are involved, specify a brake or backstop as well.
- Behaviour after running-in? Ask whether the hold should be re-verified at the first oil change.
Get advice from WGT
WGT WeiGao Transmission has manufactured worm, helical, bevel-helical and planetary gearboxes and reducers since 1988. If you are unsure whether self-locking suits your application, send the load, duty cycle, mounting position and ambient conditions through our contact page. We will tell you plainly whether a self-locking worm unit is suitable, or whether a helical unit with a brake is the safer and cheaper answer — with drawings and confirmed delivery time.
Frequently asked questions
Does a worm gearbox always hold the load when the motor stops?
No. Self-locking depends on the lead angle of the worm relative to the friction of the mesh, not on the gearbox type or the ratio alone. Many worm gearboxes, especially multi-start designs, back-drive freely. Self-locking has to be confirmed for the exact model, lubricant and operating condition.
Can I use a self-locking worm gearbox instead of a brake on a lifting application?
Only if the load cannot injure anyone or damage anything if it moves unexpectedly. Self-locking is a static property and can be lost to vibration, shock, warm oil or running-in. For any safety-related lifting or holding duty, specify a brake or backstop as well and treat self-locking as a secondary benefit.
Does a higher ratio mean a worm gearbox is more likely to be self-locking?
Higher ratios are more often produced with single-start worms, which have the smallest lead angle and the strongest self-locking tendency, so a high ratio is a useful clue. It is not proof: two gearboxes at the same ratio can be built with different numbers of worm starts, and the multi-start version may back-drive.
