“Reach” and “payload” appear in every robot arm spec, but manufacturers rarely explain what the numbers mean in practice — or how quickly they fall apart at full extension. Here’s how to read arm specs honestly and check whether a servo is up to the job.
Reach: what the arm can actually access
Reach is the maximum distance from the arm’s rotation axis to the tool tip when the arm is fully extended. A 300 mm reach means the tool can touch a point 300 mm away from the base — but only at that single, fully-stretched configuration.
The working envelope (or workspace) is the full 3D volume the tool can reach across all joint configurations. For a 6-DOF arm this is roughly a hemisphere; for a SCARA arm it’s a ring-shaped cylinder. The key insight: the usable workspace is always smaller than the geometric workspace because:
- Joint limits cut off the far edges.
- The arm needs room to approach from a useful angle, not just touch a point.
- Payload drops to near zero at full extension (see below).
A practical rule: plan your task to use roughly 60–70% of stated reach. At that radius the arm still has meaningful payload and useful approach angles.
Payload: why it drops fast with distance
Payload is almost always stated at full extension — the worst case. The physics is a lever: a servo joint has to produce a torque equal to the weight × distance to hold the arm steady.
Required torque (kg·cm) = load (kg) × distance from joint (cm)A shoulder servo 20 cm from the wrist holding a 200 g payload:
0.2 kg × 20 cm = 4 kg·cm required at the shoulderNow add the weight of the forearm (say 80 g) at its centre of mass (10 cm out):
0.08 kg × 10 cm = 0.8 kg·cm additional
Total: 4.8 kg·cm at the shoulder servoAn MG996R servo is rated at ~10–13 kg·cm stall torque — so the shoulder can handle this, but only with ~2× headroom. Move the payload further out, or hang a heavier gripper, and that headroom disappears fast.
Rule of thumb: always target ≥ 2× torque headroom. Servos near stall run hot, draw more current, and fail faster.
Reading servo specs honestly
| Spec | What it means | Catch |
|---|---|---|
| Stall torque (kg·cm) | Torque at which the servo stops moving | Measured at rated voltage, at zero speed |
| Operating torque | Usable torque during motion | Usually 60–80% of stall |
| Speed (sec / 60°) | Degrees per second at no load | Speed drops substantially under load |
| Voltage range | Min–max operating voltage | Torque scales with voltage — run at the upper end |
The stall torque is what manufacturers put in the headline spec. For arm design, use 60–70% of stall torque as your working figure for sustained movement.
Comparing hobby arm specs
| Arm type | Typical reach | Rated payload | Servo class |
|---|---|---|---|
| SG90 4-DOF (MeArm-class) | 150–200 mm | 50–100 g | 9g micro |
| MG996R 4-DOF budget kit | 200–280 mm | 150–300 g | MG996R (10–13 kg·cm) |
| MG996R 6-DOF kit | 280–400 mm | 300–500 g | MG996R |
| Quality 6-DOF (metal frame) | 350–500 mm | 500 g–1 kg | High-torque digital |
The 6-DOF kits in our shop list reach and payload in their spec tables — use those numbers as maximums and plan your actual task at 60–70% of reach.
The link-weight problem
Every link beyond a joint adds to that joint’s load — even before you attach a payload. For a long arm with heavy servo motors throughout, the self-weight can consume most of the available torque at the base before any payload is added. Solutions:
- Lighter links — printed or aluminium tube instead of solid rod.
- Higher-torque servos at the base, smaller ones near the tip.
- Counterbalancing — a spring or weight on the opposite side of a joint reduces the holding torque needed. Common in professional arms.
- Keep payloads close to the base — task design matters as much as hardware choice.
Checking an arm before buying
- Find the stall torque of the shoulder servo (kg·cm).
- Multiply your intended payload (kg) by the stated reach (cm).
- If the result is more than 50% of the servo’s stall torque, pick a stronger servo or accept a reduced working radius.
Designing an arm around these numbers from the start is covered in robotic arm design. For a more capable first build, the best robotic arm kits guide notes which kits specify real payload figures versus marketing ones. And if you want to push past hobby torque limits, the stepper motor guide covers the step up to NEMA-class actuators.
Frequently asked questions
What does 'payload' mean for a robotic arm?
Payload is the maximum weight the arm can hold at its end effector (the tip) while still moving reliably. It is always stated at full extension — an arm rated for 500 g payload can hold 500 g when the arm is fully stretched out. The same arm might hold several kilograms when retracted close to its base.
What does 'reach' mean on a robotic arm?
Reach (sometimes 'working radius' or 'maximum reach') is the maximum horizontal distance from the arm's base centre to the tool-centre point at full extension. A 6-DOF arm with 300 mm reach can touch any point within roughly a 300 mm radius hemisphere in front of it — though the usable workspace is usually smaller because of joint limits.
How do I know if a servo is strong enough for my arm?
Calculate the torque needed at each joint: multiply the weight being carried (plus the weight of all links beyond that joint) by the distance from the joint to the centre of mass. Compare that to the servo's rated stall torque in kg·cm. Aim for at least 2× headroom — servos run hot and lose life at sustained high load.
Why does an arm feel weak even though the servo torque rating looks fine?
Two main reasons: the load is measured at full extension (worst case), and servo torque ratings are stall torque at rated voltage — the torque at normal operating speed is lower. Add the weight of the servo horn, wires and next link into the calculation, not just the payload.