A gear reducer functions like the "tendon" of industrial machinery, connecting a servo motor to an actuator. A reducer, motor, and sensors form the joint unit of a robot. This article explains the role, basic structure, and common types of gear reducers used in robotics.
What is a gear reducer?
A gear reducer is a key component for improving the positional accuracy and output torque of a mechanical arm joint. It is mainly composed of transmission elements (gears or worms), shafts, bearings, a housing, and related accessories.
Basic structure
Commonly, a reducer is composed of three basic parts: 1) gears, shafts, and bearings assembly; 2) the housing; and 3) reducer accessories.
Function of a gear reducer
When the prime mover's output cannot directly meet the low-speed requirements of a driven mechanism, a reducer matches speed and transmits torque between the prime mover and the driven machine or actuator. In practice, this reduces speed and increases torque. For robots, a reducer allows the servo motor to operate at an appropriate speed and precisely reduce rotational speed to the levels required at different robot joints. This improves the robot's rigidity while enabling a higher output torque.
Six-axis robots commonly use two RV reducers plus four harmonic drives.
Common types of gear reducers
Common reducer types include harmonic drives, RV reducers, planetary reducers, and cycloidal pinwheel reducers. Brief descriptions follow.
Harmonic drive
A harmonic drive consists of three basic elements: a wave generator, a flexible gear, and a rigid gear. It transmits motion and torque by controllable elastic deformation of the flexible gear produced by the wave generator, which then meshes with the rigid gear. Harmonic drives are increasingly used in fields such as power electronics equipment, mining and metallurgy, transportation, lifting machinery, petrochemical machinery, and medical devices. They are especially suitable for high-dynamic-performance servo systems.
Advantages: high gear ratio, high load capacity, high transmission accuracy, high efficiency, simple structure, compact size, light weight, and the ability to transmit motion into confined spaces. Disadvantages: the flexible gear is prone to fatigue damage, rotational inertia and starting torque can be large, and manufacturing is difficult with complex processes.
Figure 1 Harmonic drive
RV reducer
An RV reducer combines a front-stage planetary gear reducer with a rear-stage cycloidal pinwheel reducer. It is characterized by a compact structure and a large transmission ratio. Under certain conditions, an RV reducer can exhibit self-locking. It is one of the most commonly used reducer types.
Advantages: low noise, low energy consumption, ability to carry relatively large loads, and a high reduction ratio that supports high precision. Disadvantages: performance is sensitive to manufacturing and assembly quality; it requires high wear resistance and rigidity in the gears, as well as specialized component machining and precision assembly techniques.
Planetary reducer
A planetary reducer consists of planet gears, a sun gear, an internal ring gear, and a planet carrier. The planet gears revolve around the sun gear. Planetary reducers enable power splitting and multiple-tooth meshing. They are compact, lightweight, and capable of high load capacity. Additional advantages include long service life and smooth operation, making them common in lifting, excavation, and construction applications.
Cycloidal pinwheel reducer
The cycloidal pinwheel reducer uses cycloidal pin meshing and a planetary transmission principle, so it is also called a planetary cycloidal reducer. The outer gear profile is a hypocycloid, while the inner gear has circular pins that mesh internally. Its structure is shown in Figure 2.
Advantages: small size, light weight, wide range of reduction ratios, smooth transmission, good efficiency, and reasonable precision. Disadvantages: limited ability to withstand very large loads; disassembly can be complex, and maintenance costs may be relatively high.
Figure 2 Cycloidal pinwheel reducer animation
