The Robot That Faulted Under Load
We mounted a gripper on a 6-axis robot. The payload spec said “10 kg.” The gripper + part weighed 8 kg (under the 10 kg). On the floor, the robot faulted (“overload”) during a fast move. The problem: the payload rating is not just the weight. The robot’s wrist can’t handle the moment (torque) from an offset load. The gripper was mounted 200 mm from the wrist. The 8 kg load at 200 mm creates a moment (8 × 0.2 = 1.6 N·m). Plus the gripper’s own offset moment. The total wrist moment exceeded the rating. We either shortened the gripper reach (mounted the load closer to the wrist), or upsized the robot. The mistake was checking the weight, not the wrist moment.
Robot payload and wrist moment rating is about the load AND its offset. This article covers the calculation.
Payload vs Wrist Moment
A robot’s spec sheet lists:
- Rated payload (kg): The maximum weight at the wrist face (zero offset).
- Wrist moment (N·m): The maximum torque (at axes 4, 5, 6) the wrist can take. This is limited by the wrist bearings and gears.
The rated payload assumes the load is centered on the wrist face (zero offset). If the gripper (and part) sticks out (offset), the weight creates a moment. The wrist moment is often the limiting factor (not the weight).
The Moment Calculation
The wrist moment (at axis 5, the pitch axis) is:
M = m × g × d
Where m is the load mass (kg), g is 9.81 m/s², and d is the distance from the wrist face to the load’s center of gravity (m).
For m = 8 kg, d = 0.2 m: M = 8 × 9.81 × 0.2 = 15.7 N·m. Plus the gripper’s own moment (the gripper’s weight at its own CG). Add it up.
Compare the total M to the wrist’s rated moment (from the robot’s spec sheet). For a small robot, the wrist moment rating might be 20 N·m. If you’re at 15.7, you’re close. Add acceleration (the dynamic moment during fast moves) — the moment is higher when accelerating. Add a safety factor (1.5–2×).
The payload rule: Don’t just check the weight. Calculate the wrist moment M = m × g × d. The robot that faulted had 8 kg at 200 mm (15.7 N·m) on a wrist rated for 20 N·m — too close, plus acceleration. Mount the load closer to the wrist (shorten d), or upsize the robot. Keep the total moment under 50–70% of the rating (with acceleration).
Step 1: The Load’s CG
The load’s center of gravity (CG) matters. If the gripper is long (the part is 300 mm from the wrist), the moment is high. Shorten the gripper (mount the part closer). Use a shorter tool. The closer the load’s CG is to the wrist, the lower the moment.
Step 2: All Three Wrist Axes
The robot has three wrist axes (4 = roll, 5 = pitch, 6 = yaw). Each has a rated moment. The load’s offset creates moments on all three axes (depending on the direction). Check all three (not just the pitch). A side load (offset sideways) loads axis 4 or 6.
Step 3: Acceleration (Dynamic Load)
During a fast move (high acceleration), the dynamic moment is higher (F = m × a). The robot spec sheet lists the moment at a rated acceleration. If you run the robot at 100% acceleration, the moments are at the max. If you slow the acceleration (80%), the moments drop. For a high-offset load, slow the acceleration.
Step 4: The Payload Data Sheet
Most robot manufacturers have a payload diagram (a graph) that shows the allowable load at different offsets. Plug in your gripper weight, the part weight, and the offset. The diagram tells you if you’re within the rating. Use it (don’t guess).
| Parameter | Example |
|---|---|
| Gripper weight | 3 kg |
| Part weight | 5 kg |
| Total load (m) | 8 kg |
| CG offset (d) | 0.2 m |
| Wrist moment (M = m·g·d) | 15.7 N·m |
| Robot wrist rating | 20 N·m |
| With acceleration (×1.5) | 23.6 N·m (OVER!) |
A Robot Payload Checklist
- What is the total load? (Gripper + part?)
- What is the CG offset? (Distance from wrist?)
- Calculate M = m × g × d?
- Check all three wrist axes (4, 5, 6)?
- What is the acceleration? (Dynamic load?)
- Is the total moment under 50–70% of the rating?
- Can you shorten the gripper? (Reduce d?)
- Can you slow the acceleration?
- Did you use the manufacturer’s payload diagram?
- Does the robot fault during fast moves? (Test?)
The Bottom Line
Robot payload and wrist moment rating — the weight is not the limit; the moment is. The robot that faulted had 8 kg at 200 mm (15.7 N·m) on a 20 N·m wrist. Calculate M = m·g·d, add acceleration (×1.5), and stay under 50–70% of the rating. Shorten the gripper (reduce the offset). Use the manufacturer’s payload diagram. The robot that never faulted wasn’t the biggest one — it had the load close to the wrist.
