An Internal Worm-gear Wheel is a gear set where the worm meshes with teeth cut on the inside diameter of a ring rather than the outside of a disc. The worm rotates inside the ring and its helical thread wraps further around the internal teeth than it would around an external wheel, increasing tooth contact and load-sharing. This geometry packs a high reduction ratio into a compact, often self-locking drive used in slewing rings, antenna positioners, and stairlift rails — typically delivering 30:1 to 100:1 reduction with 50–75% efficiency.
Internal Worm-gear Wheel Interactive Calculator
Vary worm starts, ring teeth, input speed, torque, and efficiency to see reduction ratio, output speed, output torque, and power loss.
Equation Used
The worm reduction is the internal ring tooth count divided by the number of worm starts. Output speed is reduced by this ratio, while output torque is multiplied by the ratio and the entered efficiency.
- Single internal worm wheel stage.
- Efficiency is entered as an overall stage efficiency.
- Ring advances by the number of worm starts per worm revolution.
- Steady-state speed and torque; dynamic shock loads are not included.
Operating Principle of the Internal Worm-gear Wheel
The mesh is the whole story. In a conventional worm and wheel, the worm sits outside the wheel and wraps around maybe 1 to 2 teeth at a time. Flip the geometry inside-out — cut the wheel teeth on the inside of a ring and drop the worm into that ring — and the worm now wraps around 3 to 5 teeth simultaneously. More teeth in mesh means lower contact stress per tooth, higher torque density, and quieter running. That's why you find this layout in slewing ring drives where a single small worm motor needs to swing a several-tonne platform.
The motion principle is the same as any worm drive: each full turn of the worm advances the wheel by one tooth (for a single-start worm) or by the number of starts. Lead angle controls efficiency. Below roughly 5° lead angle the drive is self-locking — back-drive torque on the wheel cannot spin the worm — and efficiency sits around 40–55%. Push lead angle to 15–25° and efficiency climbs into the 75–85% range, but you lose self-locking and need a brake. Tooth profile is typically involute helicoid, and on premium designs the worm is globoidal — its diameter swells in the middle to follow the curvature of the internal ring — which doubles the contact patch again.
Get the centre distance wrong and the whole thing falls apart. Internal worm meshes are unforgiving on the worm-to-ring centreline tolerance — typically ±0.05 mm on a 200 mm ring. Too tight and you get binding and pitting on the trailing flank within hours. Too loose and you see backlash spike past 0.5°, which on an antenna positioner means the dish wanders off-target in wind. The other classic failure mode is bronze wheel wear from inadequate lubrication — the sliding velocity at the mesh is high, and EP (extreme pressure) gear oil is mandatory, not optional.
Key Components
- Worm (input shaft): Hardened steel screw, typically 20MnCr5 case-hardened to HRC 58–62, with 1 to 4 thread starts. The worm spins at motor speed and its lead angle sets the reduction ratio and self-locking behaviour. Surface finish on the flanks must be Ra 0.4 µm or better — anything rougher tears up the bronze ring within 100 hours.
- Internal worm wheel (ring gear): Centrifugally cast aluminium bronze (CuAl10Fe5Ni5) or phosphor bronze ring with helical teeth cut on the inside diameter. The ring carries the slewing load directly through an integrated bearing race in many slewing drive designs. Tooth count is usually 30 to 100 — fewer teeth raises stress per tooth, more teeth shrinks the worm clearance window.
- Worm support bearings: A pair of angular contact or taper roller bearings on the worm shaft, set with 5–15 µm preload to control axial float. Axial thrust on the worm equals roughly tangential load divided by tan(lead angle), so on a 5° lead worm transmitting 200 Nm, the bearing sees several kN of thrust — undersize them and the worm walks axially and backlash explodes.
- Housing and centre-distance fixture: Cast iron or fabricated steel housing that locates the worm centreline relative to the ring axis to within ±0.05 mm. The housing also carries the lubricant — internal worm drives are almost always oil-bath lubricated with ISO VG 220 to VG 460 EP gear oil.
- Output ring flange: Bolted face on the outside of the worm wheel ring that connects to the driven load — slewing platform, antenna mast, stairlift rail, or solar tracker frame. Bolt pattern is typically 8 to 24 M12–M20 bolts torqued to spec; under-torquing here lets the ring micro-slip on the load and chews the dowel pins.
Where the Internal Worm-gear Wheel Is Used
You see internal worm-gear wheels anywhere a system needs high torque, compact packaging, and ideally self-locking behaviour without a separate brake. The internal mesh wins over external worm drives when load capacity matters more than top speed, and it wins over planetary reducers when back-drive resistance is a hard requirement.
- Construction equipment: Slewing drive on a Liebherr LTM 1090-4.2 mobile crane superstructure, where an internal worm wheel ring rotates the boom and counterweight up to 360° at 1–2 RPM under full load.
- Solar energy: Azimuth tracking drive on a Nextracker NX Horizon single-axis tracker, using a sealed internal worm slew drive (typically a Kinematics SE7 or IMO H-series) to hold panel position against wind loads up to 130 km/h without back-drive.
- Accessibility equipment: Rack-and-pinion-style stairlift drive on a Stannah 600 curved stairlift, where the worm-driven internal ring climbs the helical tooth rail and the self-locking property prevents the chair from rolling back if power cuts.
- Broadcast and radar: Azimuth positioner on a SATCOM 2.4 m VSAT antenna mount, where backlash under 0.05° and self-locking are both mandatory — an internal worm wheel hits both targets in one stage.
- Wind turbines: Yaw drive backup on smaller-class turbines like the Northern Power 100, using internal worm slewing rings (IMO or Rollix) to rotate the nacelle into the wind and lock against gusts without engaging a brake.
- Material handling: Turntable drive on a Combilift C-Series multi-directional forklift, where a compact internal worm slew drive rotates the wheel modules ±90° and holds position under cornering loads.
The Formula Behind the Internal Worm-gear Wheel
The headline number for any worm drive is the gear ratio, but on an internal worm-gear wheel the practitioner number that actually matters is output torque, because that's what sizes the slewing motor. The formula below ties input torque to output torque through the ratio and efficiency. At the low end of typical lead angles (3–5°) you get high ratio and self-locking but efficiency drops below 50% — half your motor torque becomes heat. At the high end (20–25° lead) efficiency reaches 80% but you lose self-locking and need a brake. The sweet spot for most slewing applications sits around 8–12° lead, where ratios of 30:1 to 60:1 deliver 65–75% efficiency with marginal self-locking that holds static loads but releases under shock.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Tout | Output torque at the worm wheel ring | N·m | lbf·ft |
| Tin | Input torque at the worm shaft | N·m | lbf·ft |
| i | Gear ratio (number of wheel teeth divided by number of worm starts) | dimensionless | dimensionless |
| η | Mesh efficiency (function of lead angle and friction coefficient) | dimensionless (0 to 1) | dimensionless (0 to 1) |
Worked Example: Internal Worm-gear Wheel in a harbour pedestal crane slew drive
You are sizing the slew drive on a 2-tonne harbour-side pedestal jib crane similar to a Palfinger PK 8.501 mounted on a fixed dock. The crane needs to rotate the boom 270° in roughly 60 seconds against a worst-case wind-loaded boom moment of 18 kN·m. The drive is an internal worm-gear slewing ring with a single-start worm, 48 teeth on the internal ring, and you are choosing between a 5° lead (self-locking) and a 12° lead (semi-self-locking) worm. The input motor delivers 0.6 kW continuous at 1400 RPM through a 10:1 primary reducer, so the worm sees 140 RPM input.
Given
- Tin = 40 N·m (after primary reducer)
- Zwheel = 48 teeth
- Zworm = 1 start
- λnominal = 12 ° lead angle
- μ = 0.05 friction coefficient (oil-bath EP)
Solution
Step 1 — compute the gear ratio. With 48 wheel teeth and a single-start worm:
Step 2 — compute mesh efficiency at the nominal 12° lead angle. For a worm drive driving the wheel:
Step 3 — compute nominal output torque at 12° lead:
That is well short of the 18,000 N·m boom moment, so the slew drive needs an additional output-side reduction or a larger motor — but the mesh itself is sized for 1.5 kN·m delivery per the worm wheel alone, which is the number the gearset is rated to. Now sweep the operating range.
Step 4 — at the low end, 5° lead (self-locking design):
At 5° lead you keep self-locking — the boom will not back-drive the worm under wind gusts, so no brake is needed — but you give up about 21% of the output torque to friction heat. On a continuous-duty crane that heat shows up as a bronze ring temperature 30–40 °C above ambient, which is fine with VG 460 oil but pushes lighter VG 220 oils toward thermal breakdown.
Step 5 — at the high end, 20° lead:
20° lead delivers the most torque per watt of motor input — you'd recover roughly 7.5% more output versus 12° — but you've now left the self-locking regime entirely. The crane will free-wheel under the boom's own weight if the brake fails, so a fail-safe spring-applied electromagnetic brake on the motor becomes a hard requirement, not an option.
Result
Nominal output torque at the 12° lead design point is 1,536 N·m at the worm wheel ring. That is the sweet spot for a harbour-crane slew — the operator gets crisp boom response when the motor commands rotation, the drive holds station against modest wind without immediately back-driving, and efficiency stays high enough that the bronze ring runs at safe temperatures over a full shift. Across the operating range, the 5° self-locking variant gives up 21% torque to gain back-drive lock, while the 20° high-efficiency variant gains 7.5% torque but forces you to add a brake. If you measure output torque 15–25% below 1,536 N·m on the bench, suspect three things in order: (1) under-filled oil bath causing boundary lubrication and friction coefficient climbing from 0.05 to 0.12, (2) worm shaft bearing preload lost so the worm walks axially under load and partially disengages from the mesh, or (3) ring-flange bolts under-torqued so the ring micro-rotates on the housing and steals output displacement.
Choosing the Internal Worm-gear Wheel: Pros and Cons
An internal worm-gear wheel is one of three common ways to build a high-ratio, compact slewing drive. The other two are external worm-and-wheel sets and planetary slewing reducers. Each wins on different axes — pick the one that matches your dominant constraint.
| Property | Internal Worm-gear Wheel | External Worm and Wheel | Planetary Slewing Reducer |
|---|---|---|---|
| Typical reduction ratio (single stage) | 30:1 to 100:1 | 20:1 to 80:1 | 3:1 to 10:1 per stage |
| Mesh efficiency | 50–80% (lead-angle dependent) | 45–75% | 92–97% per stage |
| Self-locking capability | Yes, below ~5° lead angle | Yes, below ~5° lead angle | No — always back-drives |
| Backlash (typical) | 0.05–0.2° | 0.1–0.3° | 0.05–0.5° depending on stages |
| Output speed range | 0.5–10 RPM | 1–20 RPM | 10–100 RPM |
| Load capacity per kg of drive mass | High — teeth share load 3–5 at a time | Medium — 1–2 teeth in mesh | High but spread across multiple stages |
| Cost (relative, like-for-like torque) | 1.0× | 0.7× | 1.4× |
| Typical service life on continuous duty | 15,000–25,000 hours | 10,000–18,000 hours | 20,000–30,000 hours |
| Best application fit | Slewing rings, stairlifts, antennas, solar trackers | Conveyor drives, gate openers, light hoists | Robot joints, machine tool axes, high-speed indexing |
Frequently Asked Questions About Internal Worm-gear Wheel
The internal mesh has more teeth in contact at any moment — typically 3 to 5 versus 1 to 2 on an external set — so the total sliding contact area is larger. More sliding area means more friction heat generated at the same torque, even though the per-tooth contact stress is lower.
That's actually working as designed. The trade is heat for tooth life. Compensate by stepping up to ISO VG 460 EP gear oil if you were running VG 220, and check that the oil-level sight glass shows the worm half-submerged — under-fill is the single biggest cause of runaway temperature on internal worm drives.
Use the static torque ratio test. Calculate the back-drive torque the load applies to the wheel under worst-case static conditions (boom weight, panel wind load, stairlift passenger plus chair). Divide by the gear ratio to get the back-drive torque seen at the worm shaft. If that number is less than the static friction torque of the worm bearings plus seals — typically 0.3 to 1.0 N·m on a small drive — you can use a 12–15° lead and skip the brake.
If the back-drive torque at the worm exceeds bearing friction, you need either a sub-5° lead (true self-locking) or a separate brake. Don't trust marginal self-locking in the 6–10° range for safety-critical holds — vibration and oil viscosity changes can break the lock unpredictably.
Not without compensation. Even a precision-ground internal worm set holds backlash around 0.05° (3 arc-minutes) and that is your hard limit on bidirectional repeatability. The internal mesh is better than external for backlash because more teeth share the load, but the bronze wheel still wears and backlash grows over service life.
For sub-arc-minute work, either add a closed-loop encoder on the output ring (not on the motor) and command unidirectional approaches to every target, or switch to a duplex worm design where two slightly offset worms preload the mesh from opposite flanks. Duplex worms hold backlash under 0.5 arc-minute but cost roughly 3× a standard internal worm drive.
Almost always one tooth on the internal ring is damaged — usually a single chipped or pitted flank from a foreign object that entered the oil bath, or from a one-time shock load that exceeded yield on that tooth. Because the worm wraps around 3–5 teeth simultaneously, a single damaged tooth still transmits load but generates an audible click each time it enters and exits the mesh.
Pull the inspection cover and rotate the ring slowly by hand. You'll find one tooth with a visible chip or surface pit. If it's only surface pitting under 0.5 mm deep you can usually run on for several thousand hours; deeper damage or a clean chip means the ring is on borrowed time and you should plan replacement before the damage spreads to neighbouring teeth.
On a typical 200 mm internal ring, the worm-to-ring centreline tolerance is ±0.05 mm. At ±0.10 mm you're already seeing measurable backlash growth — typically doubling from 0.1° to 0.2°. At ±0.20 mm the mesh either binds (centre too tight) or skips under load (centre too loose).
Check this with a dial indicator on the worm shaft housing referenced to the ring face when commissioning a rebuild. The number that matters is not the assembled centre distance — it's the variation around one full ring rotation, because a warped ring or out-of-round housing bore makes the effective centre distance change as the ring turns. Variation should stay under 0.03 mm over a full revolution.
Pick globoidal when load capacity per unit volume is the deciding constraint and budget allows. The globoidal worm — its diameter swells in the middle to wrap the curvature of the internal ring — roughly doubles the contact patch and increases torque capacity by 40–60% in the same envelope. It also shifts wear distribution more evenly across the worm flank.
The cost is manufacturing complexity: globoidal worms require specialised hobbing and centre-distance tolerance tightens to ±0.025 mm. Pick cylindrical when you want field-replaceable worms, easier alignment, and the load case fits within standard envelope sizing. Most stairlift and solar tracker drives use cylindrical worms; heavy crane slews and large antenna positioners often justify globoidal.
References & Further Reading
- Wikipedia contributors. Worm drive. Wikipedia
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