Guide to Preventative Maintenance of Industrial Robots: Measuring Joint Clearance, Inspecting Wire Harnesses and Replacing Grease

1. Scope and Objective

This technical guide is intended for maintenance technicians to perform critical preventative operations on industrial robots with articulated arms commonly deployed in the aerospace and energy sectors. It specifically covers measuring joint clearance, inspecting wiring harnesses and replacing grease in axle gearboxes. The aim is to ensure the precision, repeatability and longevity of robotic systems, thereby minimizing unplanned downtime and optimizing production performance. This maintenance must be carried out in accordance with the manufacturer's recommendations and NF EN ISO 10218-2. standards.

2. Safety Precautions

WARNING: Before any intervention, it is imperative to apply the lockout/tagout procedure (Lockout/Tagout) of the robot and the robotic cell. Make sure all power sources (electrical, pneumatic, hydraulic) are turned off and locked out. Check the absence of tension and residual pressure. The intervention must be carried out by qualified and authorized personnel. Failure to follow these instructions could result in serious injury or death and irreversible property damage.

Mandatory Personal Protective Equipment (PPE):

• Cut-resistant and chemical-resistant protective gloves (NF EN 388, NF EN 374)
• Safety glasses with side protections (NF EN 166)
• Safety shoes (NF EN ISO 20345)
• Adapted work clothing, without floating elements
• Hearing protection if ambient noise level requires it

Specific risks:

• Electrical energy: Danger of electric shock. Cut and log the general power supply to the robot.
• Mechanical energy: Residual movement of the axes, trapping. Use safety supports if necessary for heavy arms.
• Chemical products: Greases and solvents. Handle with care, ensure adequate ventilation.
• Heat: Some areas of the robot may be hot after use. Allow to cool before intervention.

3. Tools and Materials Required

4. Pre-Maintenance Inspection Checklist

5. Walkthrough

5.1. Preparation and Recording

1. Place the robot in the maintenance or neutral position according to the manufacturer's recommendations.
2. NOTES: Cut off the general power supply to the robot at the control cabinet. Lock the circuit breaker in the 'OFF' position and affix the lockout label. Check the absence of voltage at the controller input terminals with a digital multimeter (measurement < 5V).
3. Confirm the discharge of all capacitors and the release of residual pneumatic/hydraulic energy.
4. Install mechanical supports (if required) to secure the robot arms against unexpected movement, especially for large robots.

5.2. Inspection of Internal and External Wire Harnesses

1. Access the wiring harnesses by removing the protective covers from the robot arms, starting at axis J1 and working toward the end effector (J6). Do not force the covers, use the appropriate tools.
2. Visually inspect each cable segment (power, data, signals) for:
• Signs of wear, friction or cuts in the insulation.
• Pinch or crush points.
• Degradation due to heat, chemicals or UV.
• Excessive twists or non-compliant bend radii.
• Common mistake: Ignoring hidden or hard-to-reach cable sections. Use an inspection mirror and flashlight.
3. Check the integrity of the connectors at both ends of the cables (robot and cabinet). Make sure they are securely engaged with no corrosion or bent pins.
4. Check for proper cable routing. Cable ties must be present and properly tightened to prevent unwanted cable movement during robot operation.
5. Replace any wiring harness that is irreparably damaged. If a cable is cut or severely damaged, continuity should be checked with the multimeter in ohmmeter mode. Infinite resistance indicates an outage.
6. Document observations (photos) and corrective actions.

5.3. Measuring Joint Play (Backlash)

Measuring clearance is critical for accuracy. It is generally carried out on axes J1 to J6, using a dial indicator or a laser measuring system.

1. Preparation: Clear the area around the axis to be measured. If possible, uncouple the elements which could distort the measurement (for example, the effector).
2. Installation of the comparator: Firmly fix the magnetic support of the comparator on a stable surface of the robot (not on the moving axis) near the joint. Place the indicator button perpendicularly on the moving part of the axis, as far as possible from the center of rotation to maximize the measured travel.
3. Measuring procedure:
• Unlock the brake of the axis concerned (if manual or via the controller in maintenance mode).
• Apply a slight constant force on the axis in one direction, then read the value on the indicator (0 mm).
• Apply the same force in the opposite direction until firm resistance is felt, then read the new value. The difference between the two readings represents joint play.
• Repeat the measurement 3 times for each axis and take the average.
• Common error: Applying inconsistent force or not securing the comparator rigidly enough.
4. Acceptance criteria:

Axis	Maximum allowable clearance (mm)	Maximum allowable clearance (arc-min)
J1 (Basic)	0.05 - 0.10	<2.0
J2 (Lower arm)	0.05 - 0.08	<1.5
J3 (Upper arm)	0.04 - 0.07	< 1.2
J4 (Wrist rotation)	0.03 - 0.06	< 1.0
J5 (Wrist pitch)	0.03 - 0.06	< 1.0
J6 (Wrist roll)	0.02 - 0.05	< 0.8

More than specified clearance indicates excessive gearbox wear. Replacement planning is required.

5.4. Replacing Grease in Axle Reducers

Grease replacement is essential for the life of gearboxes.

1. Identify the lubrication points for each axis by referring to the robot's OEM documentation. Each reducer has drain and fill plugs (or a single bleed/fill screw).
2. Place the recovery tray under the drain point of the reducer.
3. Remove the drain plug. Allow the used grease to drain completely. Gently warming the shaft (if possible and safe) can facilitate flow.
4. If the reducer is equipped, remove the filler cap to facilitate ventilation.
5. Once draining is complete, clean the holes and surrounding areas with a lint-free cloth and a degreasing solvent.
6. Inject the new grease using the grease gun, via the filling point, while rotating the axle manually to facilitate purging of the old grease and distribution of the new. Continue injecting until clean grease comes out of the drain port.
7. Indicative quantities of grease for an industrial robot axis reducer (must be checked with the OEM documentation):

Axis	Typical amount of fat (grams)
J1 (Basic)	500 - 1500
D2, D3	300 - 800
J4, J5, J6	100 - 300

8. Close the drain and fill caps. Tighten to the torque specified by the manufacturer (typically between 15 Nm and 30 Nm for standard caps).
9. Clean any grease around the holes.
10. Proceed in the same way for all axes of the robot.
11. Common error: Using an improper grease or mixing incompatible greases, which can lead to rapid degradation and gearbox failures. Underfilling or overfilling are also critical.

5.5. Reassembly and Initial Check

1. Replace all the protective covers of the robot arms. Make sure all latches and screws are in place and torqued.
2. Remove the mechanical safety supports.
3. LOCKING OUT: Remove the lockout label and unlock the circuit breaker. Restore the general power supply to the robot.
4. Initialize the robot and its controller. Follow the manufacturer's specific start-up procedures.
5. Execute slow, controlled movements of each axis individually, then of the entire robot. Check for abnormal noises, vibrations or jerky movements.
6. Check the absence of grease leaks from the gearboxes.

6. Post-Maintenance Verification Checklist

7. Troubleshooting Guide

8. Recommended Maintenance Schedule

9. Spare Parts Reference

To order the necessary spare parts specific to your robot model, please consult the UNITEC e-catalogue: https://www.unitecd.com/e-catalog/

10. References

• NF EN ISO 10218-2:2011 Robots and robotic equipment - Safety requirements for industrial robots - Part 2: Robot systems and integration.
• NF EN 60204-1:2018 Safety of machines - Electrical equipment of machines - Part 1: General requirements.
• NF EN ISO 9283:1998 Industrial manipulating robots - Performance characteristics and associated test methods.
• Technical documentation and service manuals from the robot manufacturer (ABB, KUKA, FANUC, Yaskawa, Staubli, etc.).
• AFNOR internal standards for industrial maintenance.