Direct answer: Robot payload is not the part weight. It is the mass of the end effector plus the mass of the part, held at the distance the tooling puts between them and the wrist flange. In a loading cycle the same tooling carries the part in and carries the finished part out, so the tooling mass is present for the whole cycle. Size the arm on the sum, at the real offset, and keep margin.
Who this is for: Written for engineers and buyers sizing a robot for machine loading and unloading, where a purpose-built end effector carries the part into and out of a machine.
Scope: This EVST guide covers what a payload check has to include, why the distance to the centre of mass matters as much as the mass, and what to supply before a supplier can size an arm. It does not cover gripper design, machine interfacing, or cycle-time modelling.
What a robot payload figure has to cover
The first question in most enquiries is how heavy the part is. It is the right question and only half of the answer, because what the robot carries is never the part on its own.
In the reference footage a six-axis arm holds a flat ring workpiece in a large purpose-built end effector and lifts it clear of a machine. The tooling plate and the ring hang on the wrist together. A payload table filled in from the part alone leaves out a mass that is present in every second of the cycle.
According to ISO 10218-2:2025 it is the robot application rather than the robot alone that has to be assessed, and payload behaves the same way commercially: the arm, the end effector and the workpiece are one selection, not three.
The cheapest correction on a project like this is made on paper. A robot payload figure that omits the tooling is usually discovered on the floor, when the arm is already bolted down.
The loading cycle carries tooling both ways
The footage shows two moves that matter: the part going into the machine, and the finished part coming out. Both are done by the same end effector.
That means the tooling mass is constant across the cycle and only the part comes and goes. The worst case for the wrist is not a single instant but the loaded state, and it occurs twice per cycle rather than once.
Where a cell loads and unloads with the same tooling, sizing against the loaded state is the honest baseline. Where separate tooling is used for each direction, each has to be checked on its own.
Distance is part of the load
Rated payload is quoted at a stated centre-of-mass offset. The same mass mounted further from the flange raises the moment the wrist has to hold, and a large end effector usually pushes the workpiece further out than a simple gripper would.
This is why two cells with identical part weights can need different arms. The difference is geometry, not mass. According to ISO 9283:1998, performance criteria for industrial robots are stated under defined test conditions, which is a reminder that a catalogue figure describes a test rather than your cell.
When you send figures to a supplier, send the offset as well: the approximate centre of mass of the tooling, and of the part once it is held. Two masses without a distance cannot be checked against a payload diagram.
What the footage proves and what it does not
The footage proves a visible sequence: a robot with a purpose-built end effector grips a ring workpiece, places it into a machine, and lifts the finished part out again, with tooling and part on the wrist together.
It does not prove any mass, any offset, any cycle time or any positioning capability. Nothing in this article should be read as a measurement of the cell shown.
Keeping the visible evidence and the engineering conclusion apart is what makes the conclusion portable to your own part. According to ISO 12100:2010, risk assessment begins by determining the limits of the machinery, and a robot payload limit is one of those. Robot payload, in short, is a selection input rather than an acceptance result.
A short decision table
If the tooling is light relative to the part and sits close to the flange, a payload check on the sum is usually enough.
If the tooling is heavy or long, the offset governs and the wrist moment has to be checked against the manufacturer’s diagram at the real distance, not at the nominal point.
If loading and unloading use different tooling, treat them as two load cases. If a single tool does both, size on the loaded state and confirm the unloaded moves are within the same envelope.
| Situation | What governs | What to check |
|---|---|---|
| Light tooling close to the flange | Total mass | Sum of tooling and part against rated payload |
| Heavy or long tooling | Centre-of-mass offset | Wrist moment at the real distance, not the nominal point |
| Different tooling each direction | Two load cases | Each direction sized on its own worst state |
| Shared tooling for several parts | Heaviest variant | Robot payload margin against the worst variant |
What to prepare before asking for a quotation
Part weight and outline dimensions, with an approximate centre of mass. Gripper concept: how it holds, roughly what it weighs, how far it projects.
The geometry between pick and place: distance, height difference, and whether the arm has to reach inside a machine enclosure.
Cycle requirement and batch size, and whether more than one part variant shares the tooling.
Where this fits in a wider cell
Payload sizing sits alongside reach and pose checking; an arm that carries the load may still fail to reach the pose the machine opening requires.
For machine loading applications the interface with the machine, the door interlock and the part presentation usually shape the tooling as much as the part does.
Treat payload, reach and interfacing as one iteration rather than three sequential decisions.
Frequently asked questions
Does rated payload include the gripper?
Yes. Rated payload is the total mass on the flange, which means the end effector plus the workpiece, quoted at a stated centre-of-mass offset.
Why does the same weight need a bigger robot on one cell than another?
Because the offset differs. A mass held further from the flange raises the wrist moment even though the mass is unchanged.
What if the part is only carried one way?
Then the loaded and unloaded moves are separate load cases. Size on whichever is heavier at its own offset.
How much margin should be left?
That depends on the manufacturer’s diagram, the motion profile and any future part variants; it is a project decision rather than a fixed percentage.
Project inputs for an application review
If you are sizing a robot for machine loading, the fastest way to a useful answer is to send the load case rather than a single weight. EVST works through payload questions against a real part rather than a catalogue figure.
- part weight, outline and approximate centre of mass
- gripper concept, approximate mass and projection
- pick and place geometry, including any reach into a machine
- cycle requirement, batch size and part variants
Send those four and EVST will check payload and wrist moment against your actual part and tooling. Related reading: robot machine tending, reach and pose checking.