The catalog says the arm carries 20 kg. You bolt on a gripper that weighs 3 kg, pick a 10 kg part, and call it good. The wrist gears disagree.

Rated load is a best case, not a budget

Robot payload numbers are measured at a specific mounting point on the wrist, with the part’s center of gravity close to that flange, at a defined speed and acceleration. Move the center of gravity out, and the allowable mass drops fast. It’s not linear.

A typical 20 kg robot might have a rated moment on the wrist of 100 N·m. A gripper that extends 150 mm out from the flange, carrying a 10 kg part, generates 14.7 N·m just from the static weight. Add acceleration at 0.5 g and the moment roughly doubles. Before you know it, you’re asking the wrist to deliver 30 N·m in a dynamic move, which is fine until you look at the speed-dependent derating curve.

The derating curve nobody reads

Every robot datasheet has a graph: allowable payload vs. speed and center-of-gravity distance. At slow speeds and a CG close to the flange, you get the full rating. At full speed with a CG extended, the same arm might be limited to half. Integrators who size off the catalog number and ignore the graph find out during commissioning, when the robot faults on wrist torque mid-cycle.

The correct process is to model the complete end-of-arm tooling: gripper mass, cables, pneumatic lines, the part, and their CG positions. Feed that into the manufacturer’s load-analysis software. If you skip the software and do it by hand, you’re guessing, and the guess is usually optimistic.

The weight people forget: cables

External cable sets add 2 to 5 kg depending on length. They also flex, which means their CG moves through the motion cycle. A dress pack routed along the arm can add a dynamic moment that the static calculation misses. On a light-payload robot, the cable set can be 20 percent of the rated load. Specify internal routing when the application allows it; it removes a chunk of this problem.

Gripper weight is on you

The rated payload includes everything bolted to the flange. The gripper, the fingers, the sensors, the vacuum generator or the valve block. A pneumatic gripper with a valve mounted on the wrist can weigh 1.5 kg by itself. That’s 1.5 kg you can’t use for the part.

This is why light end-of-arm tooling pays back fast. An aluminum gripper saves 500 g over a steel one, and that 500 g translates directly into payload margin. On a 10 kg robot, 5 percent margin is the difference between a cycle that runs and one that faults every other shift.

Speed is the hidden derater

Payload and acceleration are a trade. Run the same cycle at half speed and the wrist has headroom. Run it at full speed and the torque spikes. If you’ve sized the arm close to the limit, you’ve also sized yourself into a slow cycle. The customer wants the tact time, not the datasheet number.

When sizing a cell, ask what cycle time is actually required, then pick the arm that hits that cycle with the real payload at the real acceleration. Don’t pick the arm that lifts the part in slow motion and then discover you can’t make it move fast enough.

Vibration and bearing life

An overloaded arm doesn’t always fault. It just shakes a little more, wears the wrist bearings faster, and drifts in position over months. A robot that’s running at 110 percent of its allowable dynamic load can look fine on day one and start showing repeatability drift at six months. The load-analysis report would have flagged this; the catalog number wouldn’t.

Bottom line

Treat the catalog payload as a starting point, not a design budget. Run the load analysis with the real tooling, the real part, and the real speed. If the software says you’re close, step up a frame size. The cost difference between a 20 kg and a 30 kg robot is small compared to a wrist failure in year two.