Customers care whether the robot can reach, hold and place accurately, which map to reach, payload and accuracy. You don't need kinematics, but you must read an English datasheet and refuse to say “it's enough” while conditions are unclear.

• Sketch a robot system
• Explain J1–J6
• Read a load diagram
• Tell repeatability from accuracy

What a robot system is made of

A working robot system has at least a controller driving the arm, end-of-arm tooling (EOAT) doing the gripping or process work, and a teach pendant for programming. Vision is optional; safeguarding is not. Only its form varies: fences, light curtains, safety scanners, or force and speed limits in collaborative applications.

Degrees of freedom: more axes isn't automatically better

Degrees of freedom (DOF) is the number of joints that can move independently. 4-axis robots mostly do planar pick-and-place (SCARA, palletisers). 6-axis robots can bring the tool to any position and orientation in space, covering most welding, machine tending and painting work. 7-axis robots add a redundant joint to work around obstacles or in tight spaces. More axes mean more flexibility, but also more cost, interference risk and maintenance. What matters is whether the work envelope fits the task, not the number of axes.

The six axes

A six-axis industrial robot has joints J1–J6 from the base to the flange. In a first meeting, “base, shoulder, elbow, three wrist axes” helps a customer picture the motion. Payload and accuracy are both rated at the flange.

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8:20

Payload: rated payload, maximum payload and load diagrams

Rated payload is the mass the robot can handle reliably at the flange in a specified pose and centre-of-gravity position. Count both the tool (gripper, torch) and the part. Maximum payload is a limit, often with pose or speed restrictions; never size a robot on it.

Datasheets usually include a payload diagram: the further the load's centre of gravity is from the flange (the larger the offset), the less mass is allowed. The allowable moment (N·m) and allowable inertia (kg·m²) figures express the same limit in numbers.

Worked example: why off-centre loads exceed the limit

The idea behind a load diagram is the lever from school physics. The weight of the load creates a moment (torque) on the wrist joint:

Moment M (N·m) = mass m (kg) × gravity g (≈ 9.8 m/s²) × distance L from the centre of gravity to the joint axis (m)

An example. A customer needs to move an 8 kg sheet-metal part with a 4 kg gripper, 12 kg in total. A salesperson sees a robot rated at 20 kg and thinks it has plenty of headroom. But the part is long and wide, the gripper holds it at one end, and the combined centre of gravity sits about 0.2 m from the flange, giving a wrist moment of about 23.5 N·m. If that robot's datasheet lists an allowable J5 moment of, say, 22 N·m, the moment limit is exceeded even though only 60% of the mass rating is used. The options are a larger robot, or a gripper redesign that brings the grip point closer to the centre of gravity.

This is why earlier chapters keep insisting: weight alone is not enough; always ask for the centre of gravity and dimensions. You don't need to calculate to the decimal point, but you need to understand the principle to see why engineers ask for this information, and to explain it to the customer.

Reach and work envelope

Reach describes how far the arm extends; the working envelope is the actual shape of the space the tool can reach, not just a radius. There are often dead zones close to the base, ceiling or wall mounting changes the usable area, and nearby equipment can interfere. When a customer says “1.4 m reach is enough”, ask where the pick and place points are and how the machine door opens and at what height.

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2:26

Repeatability vs. absolute accuracy

Repeatability (measured to ISO 9283) is how closely the robot returns to the same taught point, the figure most discussed on the shop floor. Absolute accuracy is how closely it reaches a point given in theoretical coordinates; it is typically an order of magnitude worse than repeatability and can be improved by calibration. When a customer says “it must be accurate”, read it as repeatability first, then check whether absolute accuracy is needed.

Ballpark figures (vary widely by model; always check the datasheet): SCARA about ±0.01–0.02 mm; small six-axis about ±0.02–0.03 mm; large heavy-duty six-axis about ±0.05–0.1 mm or more.

How cycle time is estimated

The number customers care about most is often cycle time: how many seconds per part. It is not a robot specification but the sum of a whole work cycle. For a machine-tending cell, one cycle typically looks like this:

These times are only examples; real cycle times come from simulation or tests on a sample cell. Remember three things. First, the “max speed” on the datasheet is rarely reached, because short moves spend most of their time accelerating and decelerating. Second, the robot's cycle must be seen alongside the machine's; the bottleneck isn't always the robot. Third, cycle times in demo videos are filmed under ideal conditions and never go straight into a contract.

Joint reducers (what sales needs to know)

Joint motors spin fast with little torque; a reducer converts that into slow, high-torque motion. Harmonic drives (strain wave gears) are compact with high torque density, common in light arms and cobot joints. RV reducers are stiff and shock-resistant, common in the big joints (J1–J3) of industrial arms. Two things matter in sales: backlash affects how consistently the robot returns to a point, and service life depends on the duty cycle (continuous heavy loads, shocks). Engineering selects the parts; your job is to ask about the operating conditions.

Key datasheet fields

Five steps to read an English datasheet

1. Confirm the model and seriesIndustrial arm or cobot, and payload class. Compare across the wrong product line and the comparison is meaningless.
2. Mark the core fieldsPayload, reach, repeatability and IP go onto your one-page selection sheet.
3. Check poses and footnotesRead the load diagram and the conditions in the footnotes (at flange, with wrist down, temperature range, whether J6 can rotate endlessly).
4. Match mounting and environmentFloor/ceiling/wall, cleanroom, explosion-proof, food-grade lubricant and similar options.
5. Flag the unknownsEOAT mass, eccentricity, interference and cycle time go into your notes for pre-sales.