Selecting Gearboxes
From the data sheet to the selection decision: Calculate gear ratios and efficiency, distinguish between different types of gearboxes, and select the appropriate one based on sound reasoning.
5 units · approx. 35 minutes total · Step 1 of 8
Getting Started
This module focuses on selecting the right gearbox for a given application—from the data sheet to the final selection decision. You will learn how gear ratio, speed, torque, and efficiency are mathematically related, and the fundamental differences between worm, planetary, bevel, and helical gearboxes.
Ultimately, the question is when self-locking makes sense—and when it can pose a safety risk if relied upon exclusively. Calculation exercises, a decision tree, flashcards, and a case study bring the theory to life; a self-test at the end shows whether you’ve mastered the material.
Learning Objectives – After completing this module, you will be able to:
- calculate the gear ratio as well as the output speed and torque of a gearbox;
- calculate the overall efficiency of a multi-stage gear train and estimate the power loss;
- Distinguish between worm, planetary, bevel, and helical gearboxes based on efficiency, self-locking capability, and design;
- Select an appropriate gearbox based on a given set of requirements;
- Technically evaluate and contextualize the statement “Self-locking replaces a brake.”
Learning units
Five short learning units (5–10 minutes) with learning objectives, examples, and knowledge checks—standalone learning content for this module, maintained independently of the website’s guides. Progress is saved locally in this browser.
0 of 5 units
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- Unit 1 of 5
Gear Ratio, Speed, and Torque
approx. 7 minutes
Open - Unit 2 of 5
Efficiency and Losses
approx. 8 minutes
Open - Unit 3 of 5
Comparing Gearbox Types
approx. 7 minutes
Open - Unit 4 of 5
Self-Locking: Benefits and Limits
approx. 7 minutes
Open - Unit 5 of 5
Selection in Practice
approx. 6 minutes
Open
Calculation Exercise 1: Gear Ratio
Calculation task
Calculate the output speed n₂ and the output torque M₂ of the following gearbox.
- n₁ = 1,450 min⁻¹ (drive speed)
- M₁ = 12 Nm (drive torque)
- i = 20 (translation)
- η = 0.90 (efficiency)
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Hints:
Use n₂ = n₁ / i to calculate the output speed.
Use M₂ = M₁ · i · η to calculate the output torque.
Use n₂ = n₁ / i to calculate the output speed.
Use M₂ = M₁ · i · η to calculate the output torque.
Show solution
The solution unlocks after your first check.
Worked solution – how to get the result:
n₂ = n₁ / i = 1,450 / 20 = 72.5 min⁻¹
M₂ = M₁ · i · η = 12 · 20 · 0.90 = 216 Nm
Source: Gear Ratio Calculator →Calculation Problem 2: Efficiency Chain
Calculation task
Calculate the overall efficiency (in %) and the power loss (in kW) of the following three-stage drive chain.
- η₁ = 0.98 · η₂ = 0.97 · η₃ = 0.80 (stage efficiencies)
- P_ein = 3 kW (drive power)
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Hints:
The efficiencies of multi-stage chains are multiplied, not added: η_ges = η₁ · η₂ · η₃.
Power loss is calculated as follows: P_V = P_ein · (1 − η_ges).
The efficiencies of multi-stage chains are multiplied, not added: η_ges = η₁ · η₂ · η₃.
Power loss is calculated as follows: P_V = P_ein · (1 − η_ges).
Show solution
The solution unlocks after your first check.
Worked solution – how to get the result:
η_ges = η₁ · η₂ · η₃ = 0.98 · 0.97 · 0.80 = 0.7605 ≈ 76.0 %
P_V = P_ein · (1 − η_ges) = 3 kW · (1 − 0.7605) ≈ 0.72 kW
Source: Wirkungsgrad-Rechner →Decision Tree: Worm Gear or Planetary Gear?
Four quick questions about shaft alignment, self-locking, efficiency, and low backlash—you’ll receive a well-reasoned recommendation with a link to the appropriate guide.
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Questions and answers
- How are the input and output shafts positioned relative to each other? (The axis arrangement may, under certain circumstances, rule out one of the two designs from the outset.)
- Parallel / coaxial – A single shaft for both input and output, or parallel and offset
- Right-angle (90°) – The shafts are at right angles to each other
- Does the gearbox need to hold the load without a brake (self-locking)? (This is important, for example, for lifting mechanisms, dampers, or actuators that are designed to maintain their position on their own.)
- Yes, self-locking is required – The axis should remain stationary without an additional brake
- No, that's not necessary – A brake is either present or not required
- How important is high efficiency? (Relevant for continuous operation, long operating times, or when energy costs are a major factor.)
- Important – Continuous operation, long operating time, energy costs are a factor
- Secondary – Occasional operation; efficiency is not a key criterion
- How important are low backlash and positioning accuracy? (Relevant for servo applications and precise positioning tasks.)
- Important – Precise Positioning, Servo Applications
- Secondary – Positioning accuracy is not critical
Recommendations
The first matching line applies.
- If How are the input and output shafts positioned relative to each other?: Right-angle (90°) → Recommendation: Recommendation: Worm gearbox – In a right-angle shaft configuration, the planetary gearbox is not suitable: it is coaxial in design, with the input and output shafts aligned on a single axis. The worm gearbox, on the other hand, connects the shafts at a right angle (90°) and is therefore the appropriate design for this application.
- If Does the gearbox need to hold the load without a brake (self-locking)?: Yes, self-locking is required → Recommendation: Recommendation: Worm gearbox – If self-locking is required, the worm gearbox is the preferred choice: For single-start worms, static self-locking is common starting at approximately i = 30 and becomes pronounced starting at approximately i = 60; the manufacturer’s specifications are binding, and where a dropping load could endanger people, a brake is still required. A planetary gearbox offers no self-locking capability and requires a separate holding brake for the same task. (More on this: Self-locking in gearboxes)
- If How important is high efficiency?: Important → Recommendation: Recommendation: Planetary gearbox – When it comes to high efficiency or low backlash, the planetary gearbox has the edge: 90–98% efficiency per stage (compared to 35–90% for worm gearboxes) and significantly lower torsional backlash. Note: We currently do not carry planetary gearboxes in the TEA product line—however, from a technical standpoint, this is still the more suitable design.
- If How important are low backlash and positioning accuracy?: Important → Recommendation: Recommendation: Planetary gearbox – When it comes to high efficiency or low backlash, the planetary gearbox has the edge: 90–98% efficiency per stage (compared to 35–90% for worm gearboxes) and significantly lower torsional backlash. Note: We currently do not carry planetary gearboxes in the TEA product line—however, from a technical standpoint, this is still the more suitable design.
- In all other cases → Recommendation: Recommendation: Worm gearbox – Without any special requirements regarding shaft alignment, self-locking, efficiency, or low backlash, the worm gearbox is the more economical standard solution—cheaper to purchase, even though its efficiency (35–90%) is lower than that of a planetary gearbox (90–98%).
Flashcards: Gear Terminology
Flashcards
Tap to see the definition.
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- Gear ratio
- Ratio of input speed to output speed (i = n₁/n₂)—the key parameter of a gearbox that determines the extent to which speed is reduced and torque is amplified. Source: Glossary “Gear ratio”
- Gearbox efficiency
- Ratio of output power to input power (η = P_ab / P_an). A key measure of a gearbox’s mechanical efficiency. Source: Glossary “Gearbox efficiency”
- Self-locking
- A property of a gearbox whereby a load acting on the output side cannot cause the gearbox to reverse—however, in hoists, this does not replace a standard-compliant brake. Source: Glossary “Self-locking”
- Hypoid gear
- A bevel gearbox in which the axes of the two bevel gears do not intersect at a single point but are axially offset. Higher gear ratio and load capacity, but a higher proportion of sliding (EP lubrication required). Source: Glossary “Hypoid gear”
- Center distance (a)
- Distance between the axes of rotation of two meshing gears. For standard gearing without profile shift, a = m·(z1+z2)/2 (covered in more detail in Module 4). Source: Glossary “Center distance (a)”
- Output torque
- The torque delivered at the output side of a gearbox—the usable torque after accounting for the gear ratio and efficiency. Source: Glossary “Output torque”
- Helical gearing
- Helical gear teeth—smoother, quieter operation and higher load-carrying capacity than spur gearing, but axial forces are generated as a result. Source: Glossary “Helical gearing”
All cards in this unit have been shown. The unknown ones are now in the review stack.
Case Study: A Lifting Table Without a Brake?
A customer in the material handling industry is planning a lift table designed to lift loads of up to 200 kg vertically and hold them at any height—sometimes with people in the work area. His design engineer proposes a worm gearbox with a gear ratio of i = 40 and would like to avoid using an additional holding brake: The self-locking mechanism of the worm gearbox should hold the load securely in position, saving costs and installation space. The lifting table is moved several times per shift and remains in a held position most of the time. The customer asks TEA whether this design can be approved as is.
Key Questions
- Is the self-locking capacity of a worm gearbox sufficient, based on calculations, to hold the load at rest when i = 40?
- Can the design of a power-driven lifting mechanism rely solely on this self-locking feature?
- What operational factors can weaken or negate self-locking in everyday use?
View Worked Solution
1. Generally speaking, yes, but it’s not certain: For single-start wormboxes, static self-locking typically begins at around i = 30 and becomes pronounced only at around i = 60 (lead angle below approximately 5°). At i = 40, the lead angle ranges from about 5–7.5° depending on the size: When at a standstill, the gear usually holds the load, but it can come loose under vibration. The manufacturer’s specifications for size and reduction ratio are binding.
2. No, not necessarily. DGUV Regulation 54 (Winches, Lifting and Towing Equipment, DA to § 14(2)) explicitly lists self-locking drives as one way to meet the requirement for an automatically acting braking device—it does not prohibit self-locking. However, gear manufacturers point out that self-locking does not replace a brake: Without verifiable proof of load-bearing capacity for the specific application, the lifting table additionally requires a self-acting (e.g., spring-actuated) holding brake.
3. Wear, heating (decreasing oil viscosity, decreasing coefficient of friction), and vibration can reduce the effective friction angle and weaken or eliminate self-locking. Self-locking is therefore not a reliable safety feature and is no substitute for a standard-compliant brake.
Technical basis exclusively: Self-locking in gearboxes: When is it desirable? (opens in a new tab)
Self-test
10 questions for this module. You see right after each answer whether it was correct.
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Before each answer, you state how confident you feel. This helps distinguish knowledge gaps from uncertainty. It has no effect on the score.
Question 1 of 10
Advanced Topics (optional)
The website’s six gear guides, in case you’d like to delve deeper. Reading them is not required for the learning units and self-test in this module.
6 Min. reading time
Planetary Gearboxes: Design and Selection (opens in a new tab)7 Min. reading time
Bevel Gearboxes: Designs and Criteria (opens in a new tab)6 Min. reading time
Calculating the Gearbox Efficiency (opens in a new tab)7 Min. reading time
Self-locking: When Is It Desirable? (opens in a new tab)6 Min. reading time
Helical Gearboxes: Fundamentals and Design (opens in a new tab)7 Min. reading time