The Payload-Reach Trade-off: Why You Can't Have It All

In collaborative robot (cobot) selection, payload and reach are the two most critical specifications—and they are inversely related. As reach increases, payload capacity typically decreases due to the physics of lever arms and structural stiffness. Integrators must balance these parameters against application requirements, cycle times, and safety factors. This article provides a decision framework for specifying cobots, with practical tips for sourcing from Chinese manufacturers.

Understanding Payload Specifications

Payload is the maximum mass a cobot can carry at the tool flange, including the end effector and workpiece. However, the rated payload is often specified at a specific center of gravity (CoG) offset and speed. Key considerations:

• Derating with distance: Payload capacity drops as the CoG moves away from the flange. Always check the payload diagram (payload vs. CoG offset) provided by the manufacturer.
• Dynamic loads: Acceleration and deceleration create inertial forces that can exceed static payload. For high-speed applications, apply a safety factor of 1.5 to 2.
• Tooling weight: The end effector, cables, and fixtures count toward payload. A 5 kg payload cobot with a 2 kg gripper leaves only 3 kg for the workpiece.

When specifying, define the actual payload (workpiece + tooling) and the CoG offset. Request the payload curve from the supplier to verify capability at your operating conditions.

Reach: More Than Just Distance

Reach is the maximum distance from the robot base to the tool flange. But usable reach is affected by:

• Joint configuration: Cobots with different kinematic structures (e.g., 6-axis vs. 7-axis) have varying dexterity at full extension.
• Mounting orientation: Floor, ceiling, or wall mounting changes the effective workspace. Gravity affects payload capacity differently in each orientation.
• Singularities: Near maximum reach, the arm may approach singular configurations where control becomes unstable. Avoid operating at the extreme edge of the workspace.

For a given application, map the required workspace and identify the most demanding point (farthest from base, highest acceleration). Then select a cobot whose reach exceeds that point by at least 10-15% to allow for approach and retreat motions.

Application-Driven Selection Criteria

Different applications prioritize payload or reach differently:

• Machine tending: Often requires moderate reach (700-1300 mm) to access machine doors, with payload determined by workpiece weight (typically 3-10 kg).
• Palletizing: Requires long reach (1300-2000 mm) to cover pallet dimensions, with payload up to 20 kg or more. Consider the payload at full reach—it may be significantly lower than the rated payload.
• Assembly: Prioritizes precision and dexterity over reach; payload needs are usually low (1-5 kg).
• Welding: Requires reach to access joints and payload to carry the torch and wire feeder (often 5-15 kg).

Create a requirements matrix with payload, reach, repeatability, speed, and safety certifications. Use this to filter suppliers and avoid over-specifying (which increases cost) or under-specifying (which risks failure).

What to Specify Before Ordering

When requesting quotes from Chinese cobot manufacturers, provide:

• Maximum payload including tooling and workpiece, with CoG offset.
• Required workspace dimensions and mounting orientation.
• Cycle time and acceleration requirements.
• Repeatability and accuracy tolerances.
• Safety requirements (e.g., ISO/TS 15066 compliance, force limits).
• Communication interfaces (EtherCAT, PROFINET, etc.) and I/O needs.
• Environmental conditions (temperature, dust, washdown).

Ask for payload-reach curves, dimensional drawings, and safety certification documents. Verify that the supplier can provide a sample or demo unit for testing if the application is critical.

Cost Considerations

Payload and reach directly impact cost. Longer reach often requires larger motors and stiffer structures, increasing price. Higher payload similarly adds cost. However, Chinese manufacturers offer competitive pricing—often 20-40% lower than established Western brands for comparable specifications. Consider total cost of ownership:

• Integration cost: A cobot that exactly meets requirements may be cheaper to integrate than an oversized model.
• Safety measures: Cobots with higher payloads may require additional safety scanners or fencing if they exceed force limits.
• Maintenance and spares: Check the availability and cost of spare parts, and whether the supplier provides local support.

Request detailed quotes that include necessary accessories (controller, cables, software licenses) and clarify warranty terms.

Sourcing Tips from Chinese Factories

China is home to numerous cobot manufacturers, ranging from established players to emerging brands. To ensure quality:

• Verify certifications: Look for CE, ISO 10218, and ISO/TS 15066 compliance. Request test reports.
• Request references: Ask for case studies in your industry and contact existing customers.
• Audit the factory: If possible, visit or use a third-party inspection service to assess production quality and testing procedures.
• Start with a pilot: Order one unit for evaluation before committing to volume. Test it under real conditions.
• Clarify after-sales support: Ensure the supplier offers technical support, training, and spare parts availability. Language barriers can be mitigated with local distributors.

Platforms like Robotebuy can help connect you with vetted Chinese suppliers and streamline the sourcing process.

Bottom Line

Payload and reach are not independent—they trade off against each other and against cost. Integrators should define the application's true requirements, including tooling weight, CoG offset, workspace, and cycle time, and then select a cobot with a 10-15% margin on reach and a verified payload curve. When sourcing from Chinese manufacturers, demand transparency on specifications and certifications, and validate performance with a pilot before scaling. The right cobot is not the one with the highest payload or longest reach, but the one that reliably meets your application's demands at the lowest total cost of ownership.