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Transmission components carry torque and motion from a motor to the load, and in doing so set the positioning accuracy, torque capacity, efficiency and service life of the machine. In a worm drive, a single stage turns a fast shaft into a slow, high-torque output at 90°, with a reduction ratio that would need several stages of spur gearing.
Working principle
A worm is a screw-shaped shaft. It meshes with a worm wheel whose axis is perpendicular to and offset from the worm axis. Turning the worm one revolution advances the thread by one lead, which pushes the wheel forward by z₁ teeth, where z₁ is the number of thread starts. Contact is sliding rather than rolling, which gives smooth, quiet motion and also the friction that governs efficiency and self-locking.
Interactive worm and wheel model
Change the values. Speed, torque multiplication, efficiency and self-locking update instantly. On-screen rotation is slowed for readability.
Reduction ratio i
40 : 1
Output speed n₂
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Efficiency η
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Torque gain T₂ / T₁
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Self-locking
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Equations used i = z₂ / z₁ · n₂ = n₁ / i · tan γ = L / (π·d₁), where L = lead = z₁ × axial pitch η = (cos αₙ − μ·tan γ) / (cos αₙ + μ / tan γ) · T₂ = T₁ · i · η Self-locking when tan γ ≤ μ / cos αₙ (static friction, no vibration) Assumptions: normal pressure angle αₙ = 20°, constant μ, steady load, no bearing or seal losses. Static μ is usually higher than running μ, and vibration can release a marginal lock, so do not rely on self-locking as a safety brake.
Reduction in one stage
Ratio equals wheel teeth divided by worm starts. A 1-start worm on a 40-tooth wheel gives 40:1, where a spur gear pair would need three or more stages. Multi-start worms give lower ratios and higher efficiency.
Efficiency and heat
Sliding contact converts part of the input power to heat. Efficiency rises with lead angle and falls with friction. Low-ratio, multi-start sets can exceed 90%, while high-ratio sets near the self-locking limit run near 50%.
Self-locking
When the lead angle is small enough that friction exceeds the tangential component of load, the wheel cannot drive the worm. This holds a vertical axis or rotary table position without a brake, and it costs efficiency.
Types and technical differences
Type
Key feature
Backlash control
Typical use
Standard cylindrical
Constant thread thickness
Set by center distance at assembly
General power transmission
Dual lead
Left and right flank leads differ, so tooth thickness varies along the worm
Adjust by axial shift of the worm, with no change of center distance
Rotary tables, indexing axes, tool magazines
Split worm / wheel
Two-part construction with adjustable relative position
Adjust or compensate wear by repositioning the halves
Positioning axes needing repeatable low backlash
ZC profile
Concave-flank thread form giving a larger contact area
Set by center distance and assembly
Higher load capacity and smoother load sharing
Key performance parameters
Ratio and starts: i = z₂ / z₁. Ratios of 5:1 to 100:1 are common in a single stage.
Backlash: the angular free play at the output. It sets bidirectional positioning error and must be low for indexing and servo axes.
Efficiency: depends mainly on lead angle, friction, sliding speed and lubrication.
Torque capacity: limited by wheel tooth surface pressure and root strength, worm shaft deflection, and thermal rating.
Precision grade (DIN 1): the tightest DIN 3974 class, bounding pitch, cumulative and profile errors, which determines smoothness and angular accuracy.
Thermal behavior: sump temperature above about 90 °C degrades oil and accelerates wear, so check the thermal power limit at duty cycle, not only the mechanical rating.
Materials, manufacturing and measurement
The standard pairing is a hardened steel worm (typically case-hardened alloy steel, ground) with a tin bronze wheel. Steel on bronze gives good sliding behavior and lets the softer wheel conform and carry wear, so the worm survives and the wheel is the wear part. The worm is hardened and thread-ground to the required profile, and the wheel is hobbed or generated with a tool matched to the worm. Accuracy is verified by measuring lead, pitch, profile and runout on the worm and pitch and profile on the wheel, plus checking contact pattern and backlash on the assembled pair.
Material grades shown are typical industry practice. Replace with your exact grades and inspection equipment before publishing.
Selection guide
Inputs to collect
Peak and continuous output torque
Input speed and required ratio
Duty cycle and ambient temperature
Maximum allowed backlash
Static holding load or back-drive requirement
Mounting space and shaft interface
Input to choice
Backlash critical and wear expected: dual lead
Hold position with power off: single start, small γ
Efficiency or continuous duty: multi-start, larger γ
High shock or load: larger module, ZC profile
Common failure modes
Pitting: contact stress too high
Scuffing: poor lubrication or sliding speed too high
Overheating: duty above thermal rating
Backlash growth: wheel wear or bearing play
Wear and thermal limits are the usual cause of an undersized set, so size on the thermal rating, then check strength and stiffness.
Applications and the requirement each one sets
Application
Requirement on the gearing
Relevant products
CNC rotary tables
Low, adjustable backlash and high torsional stiffness
