Industrial Robotics
Payload vs Reach: Why a Bigger Robot Arm Gets Slower
Payload and reach fight each other through joint torque. Here is the arithmetic, the derating you should expect at full extension, and how to size an arm without buying twice.
Reach costs payload because a robot joint is limited by torque, and torque is force times distance. Doubling the reach of an otherwise identical arm roughly halves what it can carry at the same joint torque, and it also lowers the achievable acceleration, because the moment of inertia of the arm rises with the square of length. That is why a 1,800 mm arm is never a longer version of a 900 mm arm; it is a heavier, slower and more expensive machine.
The three quantities that fight
Every axis has a torque budget set by its motor and gearbox. Three demands compete for it.
- Static load. Payload mass times gravity times the horizontal distance from the joint. A 10 kg payload at 1.5 m produces about 147 Nm at the shoulder before anything moves.
- Dynamic load. Inertia times angular acceleration. Inertia grows with the square of the distance, so moving the same mass twice as far out demands four times the torque for the same acceleration.
- Self weight. The arm's own links, motors and gearboxes. On long arms this is the dominant term, and it is the reason a 20 kg-class arm at 1,750 mm weighs 62 kg to 68 kg while a 16 kg-class arm at 950 mm weighs about half that.
The consequence is a rated payload that is honest only under stated conditions: a defined centre of gravity offset, usually 50 mm to 100 mm from the flange, at rated speed and duty. Every real installation violates at least one of them.
What the datasheet does not print
| Effect | Typical loss | Why it happens |
|---|---|---|
| Centre of gravity moved 60 to 200 mm off flange | 30 to 50 % | Wrist joints see torque, not mass |
| Operation at full horizontal extension | 10 to 25 % | Shoulder and elbow torque peak |
| High acceleration profile | 15 to 30 % | Inertial term dominates static load |
| Wrist rotated to a torque-unfavourable pose | 10 to 40 % | Axis 5 and 6 are the weakest joints |
| High duty cycle at elevated ambient | 5 to 15 % | Motor and gearbox thermal limits |
| Collaborative operation | varies | Effective mass caps permissible contact speed |
These stack. A 10 kg-rated arm carrying a gripper whose centre of gravity sits 220 mm from the flange, at full extension, on an aggressive profile, can be genuinely limited to 4 kg to 5 kg of part. The controller will usually let you try, then report a torque or following error at the worst point in the path, three weeks after commissioning.
How manufacturers actually resolve the trade
Instead of one scalable design, builders offer families that sit at different points on the curve. Reading a range this way makes selection much faster.
| Payload | Reach band | Arm mass | Design emphasis |
|---|---|---|---|
| 3 to 7 kg | 600 to 900 mm | 25 to 55 kg | Speed and repeatability |
| 10 to 20 kg | 1,400 to 1,800 mm | 150 to 260 kg | General handling, welding |
| 50 to 80 kg | 2,000 to 2,300 mm | 550 to 750 kg | Spot welding, machine tending |
| 120 to 240 kg | 2,600 to 3,100 mm | 1,100 to 1,700 kg | Palletising, press tending |
| 500 to 1,000 kg | 3,100 to 4,700 mm | 4,500 to 8,000 kg | Body handling, foundry |
| above 1,500 kg | 3,000 to 3,500 mm | above 10,000 kg | Heavy assembly, rail and aerospace |
Note how arm mass grows far faster than payload. Going from the 20 kg class to the 240 kg class multiplies payload by 12 and arm mass by roughly 7, and the foundation, the floor loading and the cost of moving the machine grow with it.
A sizing method that does not fail late
- Sum part, gripper, coupler and anything the gripper carries incidentally.
- Measure or estimate the centre of gravity offset from the flange face. If it is above 150 mm, expect to need the next class.
- Divide the total by 0.8 to leave headroom. That is the rated payload you buy.
- Check reach from the mounting point to the furthest work position, then add 10 % for approach and retract vectors that are not straight lines.
- Verify the worst pose, not the average one. Full extension with the wrist rotated is where torque peaks.
- Run the manufacturer's sizing tool with the real inertia tensor before ordering.
Step 4 catches a classic mistake. A cell drawn with the robot 1,250 mm from the far pick point looks fine against a 1,300 mm arm, until the approach vector requires the tool to stand off 120 mm and the arm cannot reach the position in a feasible pose.
Frequently asked questions
Why does a longer robot carry less?
Because joints are torque-limited. Static torque rises linearly with distance and inertia rises with the square of it, so the same motor and gearbox deliver less usable payload as reach grows, and the arm's own mass consumes a larger share of the budget.
How much payload does the centre of gravity offset cost?
Moving the load centre of gravity from about 60 mm to 200 mm off the flange typically costs 30 % to 50 % of rated payload, because the wrist axes are torque-limited rather than mass-limited. Long vacuum bars and offset nests are the usual cause.
Can I run at 100 % of rated payload?
Only under the datasheet conditions. In practice, size to about 80 % of rating so that speed, duty cycle and an unfavourable pose still fit inside the torque budget.
Does reach affect repeatability?
Yes. Repeatability is quoted at a defined test pose, and longer arms generally publish a larger figure because joint angle error is amplified by link length. Compare like with like, and check the test condition, not just the number.
What if I need long reach and high payload?
Move the robot instead of extending it. A shorter, stiffer arm on a linear seventh axis usually delivers better accuracy, higher speed and lower cost than a very long arm sized for the same envelope.
Sources
- ISO 9283, manipulating industrial robots, performance criteria and test methodsDefines the conditions under which payload, pose accuracy and repeatability are stated
- ISO 10218-1:2025, Robotics, Safety requirements, Part 1International Organization for Standardization, definitions of rated payload and workspace
- World Robotics 2025, industrial robotsInternational Federation of Robotics, application mix by payload class