Cobot Payload Classes from 3 kg to 30 kg, Compared

Pick the class by gripper plus part plus safety margin, not by part weight. A 5 kg cobot handling a 3 kg part with a 1.8 kg gripper is already at 96 % of rating, and above roughly 80 % most arms lose speed, repeatability or both. In practice the useful classes are 3 to 5 kg for assembly, 10 to 16 kg for machine tending, and 20 to 30 kg for palletising.

What the rated payload actually covers

Rated payload is the mass the wrist flange can carry at the specified centre of gravity, usually a short offset of 50 mm to 100 mm from the flange face, at full extension, at rated speed, for the rated duty cycle. Every one of those qualifiers can be violated by a real installation.

• The gripper counts. A compact two-finger electric gripper weighs about 0.8 kg to 1.1 kg, a larger one 1.5 kg to 2.4 kg, and a four-cup vacuum head with valve block and mounting plate lands between 1.2 kg and 3 kg.
• Centre of gravity is the silent killer. Moving the load's centre of gravity from 60 mm to 200 mm off the flange can cut permissible payload by 30 % to 50 % on a small arm, because the wrist joints see torque, not mass.
• Speed derates. Many controllers reduce maximum joint speed above 70 % to 85 % of rated payload to stay inside torque and thermal limits.
• Collaborative operation derates again. Effective mass rises with the payload, and the permissible contact speed falls with it. The worked example in ISO/TS 15066 Annex A drops from 2,000 mm/s toward an upper leg at 1 kg effective mass to 500 mm/s at 20 kg.

The six classes and where each one belongs

Two patterns are worth reading out of that table. First, reach and payload are not correlated in the way buyers expect: a 16 kg arm often reaches less than a 10 kg arm because the manufacturer traded length for wrist torque. Second, arm mass roughly triples between the 5 kg and 20 kg classes, which changes what the mounting structure has to be. A 20 kg-class arm on a wheeled trolley needs a base mass on the order of 150 kg to 300 kg before it stops walking across the floor during hard decelerations.

Sizing it properly in four lines

Do this arithmetic before any vendor conversation. It takes two minutes and it settles the class.

1. Part mass at its heaviest variant, in kg.
2. Plus end effector mass, including coupler, sensor and cabling: add 0.8 kg to 3 kg.
3. Plus anything the gripper carries incidentally, such as a fixture plate or a nest.
4. Divide by 0.8. That result is the class you buy.

Worked example: a 4.2 kg casting, a 2.1 kg pneumatic gripper with an ISO 50 coupler, and a 0.4 kg blow-off ring gives 6.7 kg, divided by 0.8 is 8.4 kg. The 10 kg class is correct. The 5 kg class fails, and no amount of controller tuning fixes it.

What each step up costs

List prices vary by region and volume, but the shape of the curve is consistent across suppliers. Moving from the 5 kg class to the 10 kg class typically adds 25 % to 40 % to the arm price. Moving from 10 kg to 20 kg often adds another 40 % to 70 %, because the larger classes carry heavier gearboxes and brakes rather than a scaled version of the same joint.

The arm is rarely the dominant cost. Across integrated cells, the arm is commonly a quarter to a third of the installed total, with end effector, guarding, fixturing, vision, integration labour and commissioning making up the rest. Buying one class larger than needed adds a few thousand to a project that is measured in tens of thousands. Buying one class too small usually means buying the arm twice.

Frequently asked questions

Sources