Robot joints: The 6 different types explained

What is a robot joint exactly?

Robot joints enable movement in robots by connecting two rigid links. The type of joint determines the range and nature of movement possible.

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These joints allow robots to complete tasks with varying levels of flexibility and dexterity. The most advanced versions enable bots to operate at superhuman levels.

Rotary joints: Movement around an axis

Rotary joints, as the name suggests, enable rotational movement around an axis. These joints allow robots to twist and turn, which is essential for mobility and manipulation.

There are several types of rotary joints:

• Single-axis rotation (revolute joints). The most common type is the revolute joint, which provides rotation around a single axis. Robotic arms typically have more than one revolute joint, allowing them to bend at the elbow and move from side to side.
• Multi-directional mobility (spherical joints). For more complex movements, spherical joints provide rotation in multiple directions. These joints have three perpendicular axes of rotation, similar to a ball and socket in your shoulder. Spherical joints are useful for robotic hands, enabling them to maneuver objects skillfully.
• Combined rotation and sliding (cylindrical joints). Cylindrical joints enable both rotation around an axis and sliding along that axis. They are useful when rotation alone is insufficient, such as in robotic fingers or legs. The combination of revolute and prismatic joints in cylinders allows for grasping, walking, and other coordinated movements that require controlled sliding and rotation.

We will discuss each of these types of robotic joints in greater detail below.

Linear joints: Straight-line motions

Linear joints enable robots to move in a straight line. These joints typically consist of a movable element that slides or glides along a set of rails or guides.

The most common types are prismatic and slider joints:

• Prismatic joints allow for sliding motion in one direction. An example is a drawer—it can only move in and out along one axis. Industrial robots often use prismatic joints to extend and retract robotic arms, allowing the arm to reach farther without needing a larger base.
• Slider joints enable two plates to slide over each other in a plane. They are commonly found in X-Y tables that move in two dimensions. These joints give robots a wide range of motion within a flat surface. 3D printers frequently use slider joints to control the print head. By moving it in the X and Y axes, the print head can access the entire print bed.

Twisting joints: Enabling rotation

Twisting joints, also known as revolute joints, allow for rotation around a single axis. They are one of the simplest yet most useful types of robot joints.

How twisting joints work

A twisting joint has two links connected by a pin, which serves as the rotation axis. One link remains fixed, while the other rotates around the pin. Many twisting joints can rotate a full 360 degrees but often have limits for specific uses.

The joint is powered by an actuator, such as an electric motor, which provides the force needed for rotation.

Applications of twisting joints

Twisting joints are prevalent in robotics and automation.

They are often found in robotic arms and hands, making them ideal for tasks that involve:

• Assembly: Screwing components together, precisely placing parts, or any task that requires a rotating motion.
• Packaging: Opening jars, twisting caps, or rotating items for optimal placement in boxes.
• Machining: Rotating workpieces for cutting/milling, operating grinding tools, or polishing surfaces.

Examples of twisting joints

Here are some specific examples of real-world robots and their use cases:

• RO1 by Standard Bots (machine tending): This collaborative robot is known for its flexibility. Its twisting joints allow it to position both small and large components precisely during machine tending operations.
• ABB IRB (auto assembly): This industrial robot utilizes multiple twisting joints to tighten bolts on car chassis precisely, ensuring consistent quality throughout the assembly line.
• Yaskawa Motoman GP8 (food processing): Equipped with a specialized gripper, this robot uses twisting joints to swiftly remove lids from jars prior to filling on a high-speed production line.