Types of Robotic Joints and Their Freedom

When you're evaluating a robotic arm, you'll run into terms like "6-axis," "degrees of freedom," "revolute joint," and "prismatic joint" pretty quickly. These aren't just spec-sheet vocabulary, they determine what motions the robot can actually perform, which tasks it's suited for, and which it can't do at all.

Here's a straightforward breakdown of robotic joint types, what "degrees of freedom" means in practice, and why it matters when you're choosing automation for your operation.

What Are Degrees of Freedom in Robotics?

Degrees of freedom (DOF) refers to the number of independent directions in which a robot can move. A fully free object in three-dimensional space has six degrees of freedom: three translational (move left/right, forward/backward, up/down) and three rotational (roll, pitch, yaw).

A robotic arm achieves different combinations of these movements through its joints. Each joint typically contributes one degree of freedom. A 6-axis robot arm has six joints, meaning it can position and orient its end effector in any position and angle within its reach envelope. This is why 6-axis arms are the standard for most collaborative robot applications: they match the full range of motion of a human arm.

The Main Types of Robotic Joints

Revolute joints (also called rotational joints) rotate around a single fixed axis, think of a door hinge, or your elbow. They provide one degree of freedom: rotation. The vast majority of joints in industrial and collaborative robot arms are revolute. When a robot is called "6-axis," that typically means six revolute joints stacked in a kinematic chain, each adding one rotational DOF. The UFactory xArm 6, for example, uses six revolute joints to achieve full-range positioning with ±0.1 mm repeatability.

Prismatic joints (linear joints) allow sliding movement along a single axis, like a piston or a drawer. They provide one degree of translational freedom instead of rotational. Cartesian robots and gantry systems rely heavily on prismatic joints. SCARA robots combine revolute joints for horizontal movement with a prismatic joint for vertical positioning, making them fast and precise for assembly and packaging tasks.

Spherical joints (ball-and-socket joints) allow rotation around multiple axes from a single point, similar to a human shoulder or hip. They technically provide three degrees of freedom in one joint. In practice, most industrial arms replicate spherical motion by stacking three revolute joints, because that's easier to manufacture, control, and maintain than a true spherical joint under load.

Cylindrical joints combine both rotation and linear translation along the same axis, two degrees of freedom in one joint. They appear in some specialized robotic configurations where reach-and-rotate motion is needed from a single mechanism.

Planar joints restrict movement to two dimensions, like a tabletop X-Y positioning system. They're common in SCARA robots and precision assembly applications where the task is inherently flat.

Why the Number of Axes Matters for Your Application

More axes mean more flexibility, but not every application needs maximum flexibility.

A 4-axis or 5-axis arm is faster and simpler for tasks that don't require full wrist rotation, like palletizing boxes that all face the same direction. A 6-axis arm handles more complex orientations, rotating a part to inspect all sides, reaching into confined spaces, or following a curved weld path. A 7-axis arm adds a redundant joint that allows the arm to reach around obstacles while keeping the end effector on the same path, which is valuable in tight work cells or research applications.

For most SMB automation, pick and place, machine tending, case packing, a 6-axis cobot is the right balance of capability and cost. The UFactory xArm 6 at $9,500 covers the majority of these applications. If you're working in a tighter space with simpler task requirements, the UFactory Lite 6 delivers six axes at a lighter payload in a more compact form factor, starting at $3,500.

Joint Type and Robot Configuration

The arrangement of joint types defines a robot's "configuration" and the shape of its reachable workspace:

• Articulated robots (all revolute joints): the most flexible, human-arm-like configuration, what most cobots use

• SCARA robots (revolute + prismatic): fast and precise for horizontal assembly tasks

• Cartesian robots (all prismatic): simple, high-precision linear motion, common in CNC and 3D printing

• Cylindrical robots: rotating base with linear extension, used in material handling and machine tending

  
  

Not sure which configuration fits your process? Our Cobot Selector filters by application type and can point you toward the right arm geometry. You can also run the numbers on your process with our Automation Analysis Tool.

CONCLUSION

Robotic joints define the vocabulary of a robot's motion. Revolute joints rotate; prismatic joints slide; spherical and cylindrical joints combine these in different ways. Most production cobots use six revolute joints to achieve full six-axis motion, enough to match nearly any human arm task within their payload range. Understanding this helps you evaluate arm specs with more confidence and connect robot capabilities to your actual production needs.

FAQ:

Q: What's the difference between a 6-axis and 7-axis robot arm? 

A: A 7-axis arm has a redundant joint that allows it to reach around obstacles while keeping the end effector on path. Most SMB applications don't require this, 6-axis is the practical standard.

Q: Do more degrees of freedom mean better accuracy? 

A: Not directly. Accuracy is a function of joint precision and repeatability, not the number of joints. More axes add flexibility, not necessarily precision.

To keep up with this constantly evolving field, consider speaking to a robotics expert from Blue Sky Robotics today.