Industrial Robot Payload-to-Weight Ratio Simulator

Parameters

Robot type

Each mechanism sets a typical TWR and repeatability

Rated payload m_payload

Maximum reach r

Tool (end-effector) mass

Tool COG offset r_COG

Distance from the flange face to the tool+workpiece COG

Target tip speed V

m/s

Motion type

Main application (display only; not factored into physics)

Results

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Robot mass (kg)

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Effective payload (kg)

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Wrist load torque (N·m)

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Wrist allowable torque (N·m)

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Inertia load (kg·m²)

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Cycle time (s)

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Robot arm schematic — motion arc and TWR gauge

Base → arm → wrist → payload links and an arc trajectory are animated. The gauge in the upper-right shows the typical TWR for the selected mechanism.

TWR comparison by mechanism

Payload vs reach — design map

Theory & Key Formulas

$$TWR = \frac{m_{payload}}{m_{robot}},\quad \tau_{wrist} = (m_{tool}+m_{payload})\,g\,r_{COG}$$

TWR (Payload-to-Weight Ratio) is the ratio of rated payload to robot mass. Wrist load torque τ_wrist is the tool+workpiece mass times the COG offset r_COG from the flange.

$$\tau_{allow}\;\approx\;m_{payload}\,r\,g\,\cdot\,0.1,\qquad J\,\approx\,(m_{tool}+m_{payload})\,r^{2}$$

Empirical wrist allowable torque (roughly payload·reach·g·0.1 for a 6-DOF) and inertia load J. A larger J inflates the tracking error during servo acceleration.

One-liner: 6-DOF TWR 0.05-0.10, SCARA 0.15-0.25, delta 0.05-0.10, cobot 0.08-0.12. Use wrist load to flag oversized tools.

Industrial Robot Payload vs Weight Ratio (TWR) — Payload Design

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I keep hearing that two robots rated for "10 kg payload" can have completely different body masses depending on the maker and mechanism. How do I actually pick one?

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Good observation. The key indicator is the payload-to-weight ratio, TWR. A 6-DOF articulated arm has to be stiff so it ends up around 125 kg just to lift 10 kg. A SCARA is mechanically simpler and pulls the same 10 kg with about 50 kg of body mass. A delta lightens the arms to 0.05-0.10, and a collaborative robot is 0.08-0.12 once you count the safety-rated reducers. Floor loading, base frame cost and electrical service all follow from this number, so TWR is the very first filter you apply when narrowing the field.

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OK, but we also attach tools — grippers, welding torches. Are those counted in the "payload"?

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Great question. The "rated payload" in the catalog is the total mass mounted on the flange. So a 3 kg vacuum gripper plus a 7 kg workpiece equals 10 kg, just at the limit of a 10 kg-rated robot. Then the COG offset r_COG matters: wrist torque is τ = m·g·r, so a workpiece sitting 100 mm out from the flange produces a much larger torque than its weight alone suggests. Always cross-check against the vendor's payload diagram.

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Longer reach lets us pick parts that are farther away, so longer should always be better, right? Any catch?

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Plenty of catches. As reach grows, the wrist allowable torque drops in relative terms (this tool approximates it as payload·reach·g·0.1), and the inertia load goes as J ∝ r², so doubling the reach quadruples J. The servos cannot accelerate hard enough, cycle time grows, path accuracy slips and vibration appears. The rule of thumb is "minimum required reach plus 10-15% margin". If you need 1.6 m for a body-weld job, pick a 1.8 m reach robot rather than a 2.5 m one.

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One last thing. Collaborative robots are everywhere lately — how do they really differ from a normal 6-DOF arm?

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The big difference is safety. A regular 6-DOF arm runs inside a fence at about 2 m/s, but a cobot has its tip force limited by ISO/TS 15066 to share space with people, so typical tip speeds are 0.25-1.0 m/s. Universal Robots, KUKA iiwa, FANUC CRX and TechMan TM live here. The trade-off is cycle time, so cobots are weak in high-volume pick-and-place but unbeatable in mixed-product cells where humans hand parts in.

Frequently Asked Questions

TWR (Payload-to-Weight Ratio) is the rated payload m_payload divided by the robot mass m_robot: TWR = m_payload / m_robot. 6-DOF articulated robots run 0.05-0.10 because they need stiff cast-iron links, SCARA reaches 0.15-0.25 thanks to its simpler kinematics, delta robots are 0.05-0.10 with lightened parallel arms, and collaborative robots sit at 0.08-0.12 due to safety-rated reducers. Carrying the same 10 kg, a 6-DOF arm weighs about 125 kg whereas a SCARA can be 50 kg. TWR affects floor loading, base cost and cycle time, so use it as the very first filter when picking a mechanism.

Wrist load torque is τ_wrist = (m_tool + m_payload) × g × r_COG, the product of the tool plus workpiece mass and the COG offset r_COG from the flange. When it exceeds the allowable wrist torque (a rule of thumb is payload × reach × g × 0.1 for a 6-DOF), you see (1) wrist servo overload, (2) accelerated reducer wear and (3) a sharp loss of path accuracy. The fix is a lighter tool, a redesign that pulls the COG back toward the flange face, or stepping up to a larger model. Always read the manufacturer's payload diagram (FANUC, ABB, Yaskawa).

Each robot joint has a maximum angular speed, typically around 200 deg/s ≒ 3.5 rad/s. End-effector speed v = ω × r is proportional to reach r, but the acceleration distance grows with r². Doubling the reach doubles peak speed yet quadruples the distance to stop, so one pick-and-place cycle ends up 1.4-1.8 times longer. This tool uses cycle ≈ 4·reach / V as a first-order estimate; for accurate motion timing use the vendor simulators such as ABB RobotStudio, FANUC RoboGuide or Yaskawa MotoSim.

Repeatability is the scatter when the robot returns to the same taught point — the RP value defined by ISO 9283. Typical figures are ±0.05 mm for 6-DOF, ±0.01 mm for SCARA, ±0.02 mm for delta and ±0.10 mm for cobots. Absolute accuracy is the gap between the commanded coordinate and the actual reached coordinate; with link tolerances, joint compliance and calibration residuals it is usually 0.05-0.5 mm. For offline-taught programs without vision correction, accuracy dominates: pick-and-place and palletizing care about repeatability, while sealing, assembly and bonding care about absolute accuracy.

Real-World Applications

Selecting robots for automotive spot-welding lines: Body-in-white welding lines need 165-210 kg payload arms to swing 2.5-3.5 t servo guns, a niche dominated by FANUC R-2000iC, ABB IRB 6700 and Yaskawa GP110/180/210. With TWR around 0.08, the robot itself weighs 1.2-2.6 t, so floor loading and anchor design become first-priority gates: not only "can it carry the weight" but "can the floor carry the robot".

Delta robots for high-rate electronic-parts pick-and-place: Confectionery packing and small-part transfer commonly demand 100-200 picks/min, which selects SCARA or delta. The ABB FlexPicker IRB 360 and Adept Quattro use a parallel-link layout to slash arm inertia. TWR sits at 0.05-0.10, but tip inertia is tiny and tip speeds of 3-10 m/s become reachable, which is why this tool widens its top-speed window when delta is selected.

Parts feed and assembly with collaborative robots: Universal Robots UR10e, KUKA iiwa, FANUC CRX, TechMan TM12 use ISO/TS 15066-compliant force limiting to run without fences, perfect for mixed-product lines and cells where humans hand parts in. The catch is the safety-rated slowdown that 1.5-2× the cycle time, so for volumes above 10 k units/year the conventional 6-DOF-plus-fence often wins on ROI; the selection hinges on that calculation.

Palletizing robots — payload and stacking speed: Stacking 20 kg bags at 1200 bags/h across 15 layers calls for Yaskawa MPL160, FANUC M-410iC, ABB IRB 660 and other 160-310 kg purpose-built palletizers. The selection cube is "payload × stack height × reach": payload alone is not enough; 3.1-3.5 m reach, high-torque 3-axis wrists and Z-axis motion optimization all combine in a three-dimensional evaluation.

Common Misconceptions and Pitfalls

The first trap is treating "catalog rated payload" as "workpiece capacity". A "Payload 10 kg" line in the catalog is the cap on the total mass mounted on the flange, tool included. Add a 3 kg vacuum gripper and the workpiece limit is 7 kg. Worse, without reading the vendor's payload diagram (load chart), COG offset and inertia can cut the usable payload to half the rated value. FANUC, ABB and Yaskawa data sheets always include a Load Capacity Diagram — overlay your real tool and workpiece on it before committing.

The second misconception is "higher TWR equals a better robot". SCARA and delta have high TWR but their workspace, degrees of freedom and reachable orientations are restricted, so they cannot compete with a 6-DOF for general use. For three-dimensional sealing or assembly, a TWR-0.07 6-DOF crushes a TWR-0.20 SCARA in capability. TWR compares mechanisms within a single use case, not across use cases.

The third pitfall is confusing repeatability with absolute accuracy. Catalogs love to quote ±0.05 mm repeatability, but that is the scatter at a single taught point, not the deviation when you hand the robot a CAD coordinate. Absolute accuracy is typically 5-10× the repeatability figure — 0.2-0.5 mm is the norm. Offline teaching, vision-less assembly and precision laser machining all live in the accuracy-dominated regime, so discussing required precision in terms of repeatability is a classic field surprise. Look at calibration options such as ABB Absolute Accuracy or Yaskawa AC Calibration.

Industrial Robot Payload vs Weight Ratio (TWR) — Payload Design

Frequently Asked Questions

Real-World Applications

Common Misconceptions and Pitfalls

How to Use

Worked Example

Practical Notes