How do I choose between different collaborative robots for payload and reach in assembly applications?

When choosing a collaborative robot for assembly, the main payload-and-reach question is not “what is the biggest robot?” but “what robot can reliably handle my part, tool, and motion envelope without being oversized.”

1) Start with the real payload

Use the total end-of-arm load, not just the part weight.

Include:

• Part or workpiece
• Gripper/tooling
• Fasteners, sensors, cabling, vacuum lines
• Any dynamic force from acceleration/deceleration

A simple rule:

• Payload capacity should exceed total tool + part weight with margin
• Common practice is to leave 20–30% headroom for robustness and future changes

Example:

• Part: 1.5 kg
• Gripper: 1.0 kg
• Camera/sensors/cables: 0.5 kg
• Total = 3.0 kg
  Choose a robot rated well above 3.0 kg, ideally with margin for acceleration and orientation effects.

2) Check reach against the actual work envelope

Reach must cover:

• Pick location
• Place location
• Any intermediate positions
• Clearance around fixtures, bins, conveyors, and operators
• Wrist articulation needs for angled insertion or screwdriving

Important:

• Robot reach is usually measured from the base to the center of the wrist at max extension
• Real usable reach is often less because of:
  • Mounting height
  • Base placement
  • Obstructions
  • Tool length

If the robot barely reaches, cycle time and layout flexibility suffer. Prefer a little extra reach rather than “just enough.”

3) Balance payload and reach together

Longer reach usually means:

• Lower stiffness
• Reduced speed at max extension
• Lower effective payload in some orientations

So for assembly:

• Short reach + low payload: compact benchtop assembly, tight spaces
• Medium reach + moderate payload: typical pick/place, screwdriving, machine tending
• Long reach + higher payload: larger workcells, multiple stations, palletizing-like reach needs

A robot that can lift the load may still be a poor choice if it must operate at full extension all the time.

4) Consider the motion type, not just static weight

Assembly often involves:

• Precise insertion
• Screwdriving
• Press-fit operations
• Handling small parts delicately

This means you should look at:

• Repeatability
• Stiffness
• Maximum allowable wrist torque
• Path accuracy
• Speed near the task

For example, a robot with sufficient payload may still struggle with insertion tasks if its reach is too long or the arm is too flexible.

5) Make sure the robot can handle tool orientation

If the part is held far from the wrist center, the effective load on the joints increases.

Also check:

• Maximum wrist torque
• Center of gravity limits
• Mounting orientation
• Whether the robot can hold the tool in the required angle during insertion

This matters a lot in assembly where the tool may be extended or tilted.

6) Match the robot to the assembly application

Typical guidance:

• Small electronics / light assembly: lower payload, short-to-medium reach, high precision
• General component assembly: medium payload, medium reach
• Large subassemblies: higher payload, longer reach, stronger wrist
• Multi-station cells: longer reach can reduce robot count, but may reduce speed/precision

7) Don’t forget cycle time and layout

A larger robot may cover more stations, but:

• It may take longer to move
• It may need more clearance
• It may be more expensive
• It may require a larger footprint

Sometimes two smaller cobots outperform one large one if the cell is split well.

8) Practical selection method

1. List the heaviest tool + part combination
2. Add margin for cabling and future changes
3. Map all pick/place points and required clearances
4. Determine the minimum reach needed with mounting constraints
5. Compare robots that exceed both payload and reach
6. Reject options that are only barely sufficient
7. Validate with simulation or a reach study if possible

9) Quick rule of thumb

For assembly:

• Choose the smallest robot that comfortably meets payload and reach
• Favor margin in payload and reach
• Prioritize stiffness and repeatability over maximum spec numbers

If you want, I can help you build a selection table or compare specific collaborative robot models for your assembly task.