기어의 종류 4 - 특수기어

Let’s take a look at special gears, the final type of gear.

Planetary Gears

A planetary gear system consists of a central sun gear and an outer ring gear, with several planetary gears arranged between them, all connected by a carrier. It is named "planetary" because its structure resembles the movement of planets orbiting the sun.

Its most distinctive feature is that the reduction ratio and direction of rotation vary depending on which of the three elements (sun gear, carrier, and ring gear) is fixed and which serves as the input. With the same structure, reduction, speed increase, and reverse rotation can all be achieved, and it is also used in automatic transmissions.

The advantage is that the load is distributed across multiple planetary gears, allowing it to transmit much greater torque than a standard spur gear of the same size.
Since the input and output shafts are coaxially arranged, shaft alignment is simple and space efficiency is high; furthermore, by connecting stages in series, very high reduction ratios (1:1000 or higher) can be achieved.

The disadvantages include a complex structure with many components, making manufacturing and assembly difficult.
If the load is not evenly distributed among the planetary gears, stress may concentrate on certain gears, potentially shortening their service life.

Robot Joint Reducer This is the most common high-performance gear system used in nearly all fields requiring high load capacity, high reduction ratios, and coaxial input and output.

Harmonic Drive

A harmonic drive is a special type of reducer that transmits power by utilizing the elastic deformation of metal.
Unlike conventional gears, which operate through the meshing of rigid components, a harmonic drive differs in that it creates meshing by deforming an elastic component.

Operating Principle

The number of teeth on the flex spline is designed to be two fewer than that of the circular spline. When the wave generator deforms the flex spline into an ellipse, the teeth of the two splines mesh only at both ends of the ellipse’s major axis, while they disengage near the minor axis.

When the wave generator rotates one full revolution, the meshing point moves one full revolution, and as a result, the flex spline rotates in the opposite direction by only the difference in the number of teeth (2).
In other words, while the input rotates one full revolution, the output rotates only 2/202 -> 1% — achieving a high reduction ratio of 1:50 to 1:320 in a single stage.

Advantages

1. Zero backlash — Due to the elliptical deformation, multiple teeth are always in contact, and elastic preload absorbs any backlash. This is a decisive advantage for robot joints where precise position control is essential.

2. High reduction ratio + compact — Achieves a reduction ratio of approximately 1:100 in a single stage using a thin disk design. It is significantly lighter and thinner than a multi-stage planetary gear structure with the same reduction ratio.

3. Coaxial input/output — Since the input and output are on the same axis, it integrates seamlessly into robot joint structures.

4. High torque density — Since multiple teeth are in simultaneous contact, the torque per unit volume is very high.

Disadvantages

1. Dependence on elastic deformation → Low torsional stiffness — Fine positional errors (windup) occur under high loads. Torque-angle compensation is required for precise control.

2. Fatigue limit of flex splines — Since elastic deformation repeats with every rotation, service life is limited by the number of cycles.

3. High cost — Precision machining is essential, and the market is effectively a monopoly (Japan’s Harmonic Drive Systems Inc.), resulting in very high prices.

Main Applications

They are the de facto standard as joint reducers for industrial and collaborative robots. Most collaborative robots, such as those from Universal Robots, KUKA iiwa, and Doosan Robotics, use harmonic drives in each joint.
They are also widely used in fields requiring high precision and low backlash.

Harmonic drives are such a standard component in robotics that when you disassemble a collaborative robot joint, you’ll find they account for about 80–90% of the parts.
It’s helpful to understand that the characteristics of this component—low stiffness, high reduction ratio, and zero backlash—represent trade-offs that must be considered during the modeling phase.
In particular, the “flexible joint model” is a solution derived from harmonic drives; to explain briefly,

Cycloid gear

A cycloidal reducer is a type of reducer that transmits power through the eccentric rotation of a disk machined with a cycloidal curve.
It utilizes epicycloidal or hypocycloidal shapes and is used as a core component in industrial robots and precision reducers.

When the eccentric cam on the input shaft rotates, the cycloid disk performs an eccentric motion and meshes with the ring pins. The number of teeth on the disk is designed to be exactly one less than the number of ring pins; during one revolution of the eccentric cam, the disk rotates in the opposite direction by exactly one tooth.
Generally, a reduction ratio of 1:10 to 1:200 is achieved in a single stage.

Advantages

1. 
   Multiple teeth in simultaneous contact (60–70%) — The cycloidal disk is in contact with more than half of the total teeth on the gear ring at any given moment. This contrasts with standard spur gears, which bear loads on only one or two teeth, resulting in very high torque density and resistance to shock loads.

2. Virtually no backlash — Thanks to simultaneous contact of multiple teeth and precision machining, backlash can be reduced to less than 1 arcminute.

3. High torsional stiffness — Unlike harmonic drives, there are no elastic deformation elements, resulting in behavior close to that of a rigid body. Vibration is low and phase lag is minimal even during rapid acceleration.

4. High efficiency (~95%) — Since tooth contact is primarily rolling contact, there is little sliding loss.

5. Impact resistance — Since multiple teeth share the load, the teeth do not break even under momentary overloads. This is a critical characteristic in industrial environments.

Disadvantages

1. 
   Vibration caused by eccentric rotation — Since the input shaft creates eccentric rotation, unbalanced inertial forces (unbalance) occur. To reduce this, two cycloid discs are typically arranged with a 180° phase difference, but it cannot be completely eliminated.

2. Complex structure and numerous components — Manufacturing costs are high due to the large number of parts requiring precision machining (eccentric cam, disks, go-pin, output pin).

3. Weight and size — Thicker and heavier than harmonic drives. Disadvantageous for applications where distributed mass is critical, such as in humanoids.

4. Unsuitable for high-speed rotation — Since the inertial force of the eccentric motion increases proportionally to the square of the speed, vibration and noise increase at high RPM inputs. Typically used at input speeds of 3000 RPM or less.

Must be lightweight and thin → Harmonic (collaborative robot joints)

Must withstand shocks and have high rigidity → Cycloidal (base joints for industrial robots)

Main Applications

It is the de facto standard for shoulder and waist joints in industrial 6-axis robots. Major industrial robot manufacturers such as ABB, FANUC, KUKA, and Yaskawa use cycloidal reducers for large joints (J1, J2, J3). For the wrist section (J4, J5, J6), where weight is a critical factor, harmonic drives are sometimes used. They are used in applications requiring high load capacity, high rigidity, and shock resistance.