Gearboxes for cobots: Which one is right for your project?

The collaborative robot market is growing at a steady rate of over 20% annually. However, one of the most common—and most costly—mistakes in cobot projects remains the improper selection of the gearbox. An improperly specified gearbox compromises the precision, safety, and lifespan of the entire system.

In this article, we explain which types of gearboxes are actually used in cobots, what their technical differences are, and how to select the right one for your application.

Why is the gearbox the critical component of a cobot?

In collaborative robotics, the gearbox is the component that determines positioning accuracy, collision detection capabilities, smoothness of motion, and operator safety. Unlike traditional industrial robotics, cobots must balance strength with sensitivity, which places very specific demands on the type of gearbox used.

The three types of gearboxes used in cobots

Contrary to what is sometimes oversimplified, cobots do not use a single type of gearbox. Depending on the axis, the payload, and the size of the robot, three main technologies are used:

1. Harmonic gearboxes (Strain Wave)

They are by far the most widely used in cobots, especially in the distal axes of the arm (elbows and wrists). In many small models, they make up all of their joints.

Their operating principle is based on a rigid outer ring and a flexible steel inner ring that is deformed by an elliptical bearing. This geometry allows them to achieve large reduction ratios (50:1 up to 160:1) in a very compact space.

Their main advantages in cobot applications:

• Virtually zero backlash, which guarantees very high repeatability.
• Unbeatable torque-to-weight ratio: ideal for lightweight arms.
• Extreme compactness, essential for wrist joints.

Note: On the downside, their main limitation is a lower resistance to impacts and torque peaks compared to cycloidal gearboxes.

2. Precision planetary gearboxes

Precision planetary gearboxes have gained increasing prominence in the new generation of cobots and in mobile robotics (humanoids and quadrupeds). Their design—featuring a central sun gear, planetary gears, and a ring gear—gives them relevant advantages for collaborative robotics:

• High backdrivability: they allow the operator to move the arm manually with ease, simplifying programming through hand-guiding.
• Excellent mechanical efficiency: facilitates current-based force control, allowing the system to detect collisions without external torque sensors on every axis.
• Higher resistance to torque peaks and fatigue than harmonic gearboxes.

Important technical note: The term "helical" does not designate a type of gearbox, but rather the geometry of the gear teeth (helical, spur, etc.). A planetary gearbox can perfectly well have helical teeth—which is common in precision versions. Comparing a "planetary gearbox" with a "helical" one as if they were equivalent categories is a conceptual error that should be avoided in any technical specification.

3. Cycloidal gearboxes (RV type)

These are the dominant technology in heavy industrial robotics (over 75% of the traditional market), but in cobots, their use is reserved for the base axes (axes 1 and 2) of models with higher payloads (from 15-35 kg upwards).

Their ability to absorb overturning moments, heavy loads, and impacts is unsurpassed, making them essential when a large-format cobot operates at maximum extension with heavy loads.

‍

Key technical criteria for gearbox selection

1. Torque: nominal, acceleration, and emergency stop

In robotics, gearboxes do not operate at a constant torque. Three distinct values must be calculated:

• Nominal torque (T2N): the continuous torque during normal operation.
• Acceleration torque (T2α): the peak that occurs during every hard start or stop. The gearbox must withstand this thousands of times throughout its lifespan.
• Emergency stop torque (T2NOT): the collision scenario. The gearbox must absorb this impact without the gear teeth failing.

2. Angular backlash

Angular backlash is the clearance between the gear teeth, measured in arcminutes (arcmin). In cobots, this is a critical parameter:

• Precision applications require gearboxes with reduced or ultra-reduced backlash (≤ 3 arcmin, ideally ≤ 1 arcmin in the axes closest to the base).
• A backlash of 3 arcmin at the base can translate into several millimeters of deviation at the end of the arm, due to the lever effect.
• Harmonic drives offer virtually zero backlash; precision planetary gearboxes achieve very low values; cycloidal gearboxes have reduced backlash but slightly higher than harmonic ones.

3. Weight and power density

Every additional gram on the distal axes reduces the true payload of the cobot. Designs with lightweight aluminum housings or constructions optimized for robotics should be prioritized.

4. Backdrivability

This is the ability to move the gearbox from the output shaft (pushing the arm manually). It is essential for:

• Hand-guiding (teaching the cobot by leading the arm by hand).
• Collision detection via current control, without external torque sensors on each axis.
• Note: Precision planetary gearboxes excel in this aspect. As the reduction ratio increases (> 50:1), backdrivability decreases.

5. Torsional stiffness

A gearbox with low torsional stiffness causes the arm to act like a spring when stopping abruptly, generating vibrations that degrade precision and force sensors.

Selection process: step by step

1. Define the cycle dynamics: required acceleration, tool weight, payload to be transported, and distance to the axis (overturning moment).
2. Determine the reduction ratio (i): in cobots, typical ratios range between 10:1 and 100:1 depending on the axis. A higher reduction means more torque but lower speed.
3. Filter by angular backlash according to the precision required for that specific joint.
4. Verify mechanical compatibility: mounting type, dimensions, weight, and working environment (clean, humid, corrosive).
5. Consult with the manufacturer, providing the complete load cycle, not just the nominal torque. Proper sizing is the best guarantee of long-term reliability.

Market trend: integrated joint modules

The industry is moving towards all-in-one joint modules, where the manufacturer supplies a single compact block: a frameless motor, brake, encoder, control electronics, and gearbox.

The gearbox integrated into these modules is usually harmonic (for maximum precision) or planetary (for dynamic cobots with high backdrivability). In CLR we know that this integration simplifies design, reduces the number of components, and improves the repeatability of the assembly, although it limits the flexibility of individually selecting each component.

‍