Two Gearbox Types, Fundamentally Different Strengths
Worm gearboxes achieve efficiencies of 40-90% and offer self-locking from a ratio of i=30; planetary gearboxes reach 90-98% and are the better choice for dynamic servo applications. Both transmit torque and reduce speeds, but rely on fundamentally different principles — with direct implications for efficiency, size, noise and cost.
This comparison highlights the most important differences and provides clear guidance on which gearbox is the better choice in which situation.
Key takeaway
Worm gearboxes excel at self-locking and high gear ratios. Planetary gearboxes stand out for their efficiency, compactness and dynamics. The right choice depends on your specific load case.
Operating Principle at a Glance
Worm Gearbox
In a worm gearbox, a helical worm shaft drives a worm wheel. The input and output axes are at right angles to each other. Due to the high sliding component in the tooth contact, this principle generates comparatively high friction — which on one hand reduces efficiency, but on the other hand enables the desired self-locking.
- 90-degree axis offset between input and output
- High single-stage ratios (up to 100:1) possible
- Sliding friction as dominant contact mechanism
- Self-locking from approx. 30:1 ratio (depending on design)
Planetary Gearbox
The planetary gearbox consists of a sun gear, several planet gears on a carrier and a ring gear. The coaxial arrangement (input and output on one axis) enables a very compact design. The load is distributed across multiple tooth meshes, resulting in high torque densities and excellent efficiency.
- Coaxial design (input and output on one axis)
- Load distribution across 3-5 planet gears
- Ratios per stage typically 3:1 to 10:1, multi-stage up to 100:1
- No self-locking — holding brake required
Direct Comparison: Worm Gearbox vs. Planetary Gearbox
The following table summarises the decisive differences between both gearbox types:
| Criterion | Worm Gearbox | Planetary Gearbox |
|---|---|---|
| Efficiency | 40-90 % | 90-98 % (per stage) |
| Ratio (single-stage) | 5:1 to 100:1 | 3:1 to 10:1 |
| Self-Locking | Yes (from approx. i=30) | No |
| Torque Density | Medium | Very high |
| Noise Level | Quiet | Medium to quiet |
| Backlash | Medium to high | Low (backlash-reduced) |
| Axis Arrangement | Right-angle (90°) | Coaxial |
| Size | Medium | Very compact |
| Heat Generation | High (sliding friction) | Low |
| Price Level | Low | Medium to high |
At a Glance
Worm gearboxes are more affordable and offer self-locking, but lose considerable efficiency through sliding friction. Planetary gearboxes are more compact and efficient, but require an additional holding brake for vertical loads.
Design: Power Loss and Standards Reference
Efficiency determines how much of the input power is delivered as useful output at the driven shaft — the remainder is dissipated as heat. Power loss follows directly from input power and efficiency:
PV = Pin · (1 − η)
PV = power loss (heat) in W · Pin = input power in W · η = efficiency
In the worm gearbox, gear-mesh efficiency depends primarily on the lead angle γ of the worm and the friction angle ρ′ of the lubricated steel-bronze pairing:
ηz = tan γ / tan(γ + ρ′)
ρ′ = arctan μ′ (friction angle) · μ′ ≈ 0.03–0.08 for lubricated worm/worm wheel
Small lead angles (high ratio, single-start worm) reduce efficiency and lead to self-locking; large lead angles with multi-start worms raise it above 90%. Planetary gearboxes have no comparable sliding component — their stage efficiency remains virtually load-independent at 0.97–0.99.
Calculation example: heat loss at 4 kW input power
- Worm gearbox, η = 0.75: PV = 4000 W · (1 − 0.75) = 1000 W heat loss
- Planetary gearbox, η = 0.95: PV = 4000 W · (1 − 0.95) = 200 W heat loss
- The fivefold heat loss of the worm gearbox must be dissipated via housing and oil — in continuous operation the dominant reason for temperature limits and cooling requirements.
Standards reference
DIN 3975 defines terms and parameters for cylindrical worms and worm wheels; DIN 3996 provides the calculation procedure for load capacity, efficiency and thermal rating of cylindrical worm gearboxes. The tooth meshes of planetary stages are verified according to ISO 6336 (or DIN 3990). All efficiency figures are reference values — binding are the manufacturer specifications or the standardised calculation for the actual operating point.
Which Gearbox When? Decision Guide
The choice depends strongly on the specific application. The following guidance helps with the pre-selection:
Choose a worm gearbox when:
- Self-locking is required — e.g. for hoists, flaps or actuators that must hold their position without a brake
- High single-stage ratios are needed (up to 100:1 in one stage)
- Low noise is a priority — worm gearboxes run inherently quieter
- A right-angle axis offset is structurally desired or necessary
- The budget is limited and the application does not require high efficiency
Choose a planetary gearbox when:
A planetary gearbox is the right choice when high efficiency, a compact coaxial design, low backlash and high dynamics are required — that is, for servo applications and continuous operation. How these selection criteria translate into gear geometry, number of stages and accuracy class is covered in the dedicated guide Planetary gearboxes: design, function and selection.
In some cases, a combination also makes sense: for example, a planetary gearbox as a first stage with a downstream worm gearbox to combine high ratios with acceptable efficiency and self-locking.
Practical Tip from TEA:
In our consulting work we often see the self-locking of a worm gearbox being planned in as a full replacement for a brake — which is critical from a safety standpoint. Static self-locking (from approximately i = 30) prevents the gearbox from starting to move out of standstill; however, under vibration, reversing operation or oscillation the friction angle drops, and dynamic self-locking can be lost. For hoists and vertical loads, therefore, always state in your enquiry whether the self-locking is safety-relevant — in that case we specify an additional holding brake rather than relying on the gearing alone.
Those comparing acquisition costs, energy consumption and maintenance over the full service life will find a structured method for both gearbox types in the total cost of ownership guide for drive trains. Calculate the efficiency of a specific gearbox arrangement step by step in the gearbox efficiency calculation guide.
TEA Recommendation: Finding the Right Gearbox
Technische Antriebselemente offers both gearbox types in numerous sizes and configurations. Our range includes worm gearboxes in aluminium and cast iron housings as well as single-stage and multi-stage planetary gearboxes for servo and industrial applications. Worm wheels to drawing — also available as custom gears in customer-specific dimensions.
We are happy to advise you on the selection and size the gearbox to match your motor, your application and your budget. On request, we deliver motor-gearbox combinations as complete units — including flange adaptation and shaft configuration.
Conclusion
For cost-conscious applications with self-locking: worm gearbox. For maximum efficiency, dynamics and compactness: planetary gearbox. Unsure? Talk to us — we will find the right solution.
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