Servo Motor Torque

Variables

Common mistakes

• Selecting a servo motor only by power: Servo motors should be selected using torque, speed, inertia, duty cycle, and motion profile as well as power.
• Ignoring acceleration torque: Fast acceleration can require substantially more torque than the steady-state load.
• Using peak torque as continuous torque: Peak torque is normally available only for a limited duration. Continuous loading must remain within the motor and drive thermal limits.
• Ignoring RMS torque: Repetitive acceleration and deceleration can thermally overload a motor even when individual movements appear acceptable.
• Ignoring reflected inertia: Load inertia reflected through a gearbox can strongly affect servo response and tuning.
• Using the wrong gear-ratio direction: For a reduction gearbox, reflected load inertia is commonly calculated as Jload/G² when G represents motor speed divided by load speed.
• Ignoring transmission efficiency: Gearboxes, belts, screws, bearings, and couplings introduce losses that increase the required motor torque.
• Ignoring gravity torque: Vertical axes require torque to support and move the load against gravity.
• Confusing mass and force: Kilograms represent mass. Force in newtons is calculated using F = mg when gravity is involved.
• Using pulley diameter instead of radius: Torque calculations using T = Fr require the radius, not the full diameter.
• Mixing millimetres and metres: Convert dimensions to consistent units before performing calculations.
• Mixing RPM and rad/s: Equations such as P = Tω and T = Jα require angular quantities in compatible SI units.
• Ignoring deceleration torque: Rapid stopping can require torque comparable to or greater than acceleration torque.
• Ignoring regenerative energy: High-inertia or vertical systems may return energy to the servo drive during braking.
• Ignoring the torque-speed curve: A servo's available torque can depend on operating speed. Always verify the required torque at the actual machine speed.
• Assuming one inertia-ratio limit applies to every servo: Acceptable inertia ratios depend on the servo system, tuning, mechanical rigidity, gearbox, and manufacturer recommendations.
• Ignoring a holding brake on vertical axes: A motor brake or suitable mechanical safety system may be required to prevent uncontrolled movement when power is removed.

Applications

Servo motor torque calculations are used whenever accurate control of position, speed, acceleration, or mechanical load is required. Correct torque sizing helps ensure that the servo system can accelerate the machine, maintain the required speed, overcome friction and gravity, and complete repetitive motion without excessive thermal loading.

CNC Machines

Servo torque calculations are used for CNC X, Y, and Z axes to determine the torque required to accelerate tables, ball screws, workpieces, spindles, and other moving components.

Industrial Robots

Robot joints require torque calculations that account for link mass, payload, center of gravity, angular acceleration, gearbox ratio, and changing gravitational loads.

Packaging Machines

Servo motors used in indexing, sealing, cutting, feeding, filling, and labeling systems frequently operate with rapid acceleration and deceleration. Peak and RMS torque calculations are therefore important.

Ball Screw and Linear Motion Systems

Servo torque calculations are used to convert the required linear force and acceleration into screw torque while considering screw lead, efficiency, inertia, and operating speed.

Conveyor Systems

Servo-driven conveyors require torque to overcome load inertia, belt friction, product mass, roller resistance, acceleration, and any conveyor inclination.

Rotary Indexing Tables

The torque required to rotate and accurately position an indexing table depends strongly on table inertia, payload position, acceleration time, gear ratio, and cycle frequency.

Vertical Lifting Axes

Vertical servo axes must overcome gravitational force in addition to acceleration and friction. Brake requirements and regenerative operation must also be considered.

Automated Assembly Machines

Servo motors are commonly used for pressing, positioning, feeding, rotating, and synchronized assembly operations where accurate torque and motion control are required.

Robotic Arms and Pick-and-Place Systems

Torque calculations help determine whether each joint servo can support the arm, payload, tooling, and required acceleration throughout its operating range.

Gearbox-Driven Machinery

A gearbox changes the relationship between motor torque, load torque, speed, and reflected inertia. Servo sizing calculations help determine an appropriate motor and reduction ratio.