A worm wheel wears during service, and the free play at the output increases accordingly. A dual-lead worm allows you to eliminate this excess play with a minor axial adjustment, without opening the housing and without replacing the worn worm wheel.
Among transmission components used in machine-tool applications, the worm-and-wheel pair determines the repeat positioning accuracy of a rotary axis. It operates via sliding contact, delivering smooth and quiet running. However, this sliding action also causes gradual wear of the softer bronze worm wheel. Such wear manifests as increased mesh backlash, which directly impacts the output-side lost motion of the worm wheel: the angular distance the wheel output can rotate freely before the worm input re-engages to transmit drive torque.
Why backlash matters
For CNC rotary tables, indexing axes or tool magazines, rotary axes require bidirectional driving in both clockwise and counterclockwise directions. Any backlash in the transmission will turn into positioning error at each direction reversal, which in turn contributes to dimensional errors of parts machined on this rotary axis. The tooth thickness of standard cylindrical worms remains constant. Therefore, its backlash is determined once during assembly by the housing centre distance and assembly tolerances. If backlash increases later, the common remedy is to reopen the housing, replace the worn worm gear, or reduce the internal centre distance of the housing via mechanical adjustment to restore the required backlash value.
The dual-lead idea
With a dual-lead worm, the tooth flanks on either side have slightly different leads, creating a lead difference between the left and right flanks. As a result, the tooth thickness increases gradually along the axial direction. The mating worm gear is machined using a dedicated worm-gear hob designed for this worm. For this matched worm and worm gear set, the more the worm shifts toward its thicker-tooth end, the smaller the backlash at the meshing point.
Meanwhile, the centre distance between the worm and worm gear can remain unchanged, and the meshing condition stays consistent. No additional mechanism for centre-distance adjustment needs to be designed into the housing.
Choosing between adjustment methods
The options differ in what sets the backlash:
| Type | How backlash is set | Best Suitable Applications |
|---|---|---|
| Standard Cylindrical Worms and Wheels | Housing centre distance during assembly | Where backlash requirements are loose, or worm gear replacement as a wearing part is acceptable |
| Dual Lead Worms and Wheels | Axial displacement of the worm; housing centre distance remains constant | Applications with anticipated wear requiring sustained low backlash and high precision: CNC rotary tables, indexing axes, tool magazines |
| Split Worms and Wheels | Adjustment of relative position between two worm halves | Applications with anticipated wear requiring sustained low backlash and high precision under shock load conditions, medium and large-sized CNC rotary tables |
Tooth profile is a separate decision. Profiles such as ZA, ZN, ZI, ZK and ZC (Niemann) affect load capacity, efficiency and how smoothly the load is shared, but they do not change the way backlash is taken up; the backlash is still set by the housing center distance, or taken up by a dual-lead or split design. ESSOR can design and machine all of the profiles above.
Practical notes for engineers
Measure before you adjust
Check backlash at several wheel positions. Wear is rarely even, and the tightest position sets how far you can take it up without binding.
Watch the thermal limit
Eliminating backlash reduces the volume of lubricant entering the tooth surfaces, leading to a rise in oil temperature. When the sump temperature exceeds approximately 90 °C, the lubricant degrades and wear accelerates. For this reason, verify the operating temperature rise under your actual operating conditions after every adjustment.
Plan for the bearings
The worm must allow controllable axial movement and subsequent stable locking. Bearing clearance on the worm contributes to backlash just as tooth surface wear does, so it must also be inspected.
Common questions
Does a dual-lead worm change the reduction ratio?
No. The ratio is still wheel teeth divided by worm starts. The lead difference only varies the thread thickness.
Can a double-lead worm be retrofitted into a standard housing?
Only possible when the housing enables axial adjustment and locking of the worm, and the wheel is cut by a double-lead hob. Standard-hobbed ordinary worm wheels are non-conjugate and cannot mesh properly. Double-lead worm sets must be made as matched pairs. Submit drawings or installation limits for our evaluation.
Is self-locking still available?
It depends on the lead angle and friction, not on the dual-lead form. Do not rely on it as a safety brake, because vibration can release a marginal lock.
Need low, adjustable backlash?
Browse worms and wheels, complete worm drives, or send a drawing or your torque, ratio and space limits for a quote. Related reading: What is a Worm Drive?
