A PTFE-lined self-lubricating bushing is the workhorse of modern robot joints, carrying static loads up to 140 N/mm² while running completely dry from -195°C to +280°C. Robot builders fit this construction in wrist, shoulder, and gripper pivots because oscillation, not rotation, dominates robotic motion, and oscillating joints destroy grease films faster than any other duty cycle.

The DU bushing is built in three bonded layers. A steel backing carries the structural load, a porous sintered bronze interlayer anchors the lubricant, and a PTFE-based overlay provides the sliding surface. According to ISK Bearings' technical guide, the PTFE transfers onto the shaft during the run-in phase and forms a solid lubricating film that prevents metal-to-metal contact for the life of the joint. The name DU itself means dry and unlubricated, which is the point.

Why grease fails in robot joints

Grease attracts dust, migrates out of small housings, and stiffens at low temperature. In a cobot wrist running millions of cycles, reaching the bearing for relubrication often means full disassembly. Solid lubricants avoid all of this. As igus explains in its Engineer's Toolbox, dry-running bearings rely on microscopic platelets of PTFE, graphite, or molybdenum disulfide embedded in the material, so no grease gun, oil change, or automatic relubrication system is ever needed.

Thermal behavior matters too. According to a friction study of robot joint bearings published by Scholar Press, the allowable PV value of a self-lubricating bearing falls as sliding speed rises, because friction heat raises the thermal load on the sliding surface. Designers should check the PV limit against the real duty cycle, not just the peak load.

Selection checklist for robot joint bushings

• PV check: the DU series from Ahcell rates 3.6 N/mm²·m/s with a 60 N/mm² dynamic load limit, enough for most servo-driven joints.
• Oscillation: high-frequency small-angle motion suits PTFE composites; high-load low-speed pivots suit graphite-plugged bronze.
• Stick-slip: PTFE's low coefficient of friction removes stick-slip in slow positioning axes.
• Shaft finish: a hardened, ground shaft with Ra 0.4 or better extends bushing life.

How to select a bushing for a robot arm joint

Start with the PV calculation: multiply the radial load in N/mm² by the sliding speed in m/s and keep the result below the material limit. For a DU bushing from Ahcell, that limit is 3.6 N/mm²·m/s. Then confirm the temperature range and check that the oscillating angle and frequency stay inside the rated surface speed. Wrapped PTFE bushings manufactured to ISO 3547 give dimensional consistency for press-fit housings.

Polymer and composite plain bearings already dominate grippers and linear actuators, where igus reports that lightness, corrosion resistance, and maintenance-free operation are the deciding factors. For heavier payload arms, the same transfer-film principle scales up in bronze-based bushings. Automation keeps pushing toward sealed, oil-free drivetrains, and self-lubricating plain bearings are the component that makes that possible.