Choosing a micro linear servo actuator for fingers, hands and wrists

By Tolerance Labs engineering team · Updated 13 October 2026

Micro linear servo actuators drive the fingers of many robot hands, as well as wrists, grippers and small linkages. This guide covers the specs that matter at this size and sizes one for a finger.

Stroke, force and speed

Stroke is the rod travel. Strokes of roughly 10–50 mm are common in this class. Size it from the linkage geometry, with margin so the rod never reaches its end stop under load.

Force is quoted for push and pull, which can differ. Check if a figure is continuous (sustained without overheating) or peak (short bursts or stall). A grasp held for several seconds is a continuous load unless the screw self-locks.

Speed trades against force. A lead screw converts motor torque T, after any gearbox, into rod force F = 2π · η · T / L, where L is the lead (travel per turn) and η the screw efficiency. Rod speed is v = n · L, with n in revolutions per second.

Halving the lead doubles force and halves speed. Output power F · v is capped by the motor, so one body sold with several ratios gives points on one power curve.

Inline and side-motor layouts

Inline units put motor, gearbox and screw on one axis. They are slim but long: the body is at least motor length plus screw length. They suit narrow spaces such as a forearm.

Side-motor units place the motor beside the screw, joined by a gear stage or belt. They are shorter and wider, and fit where length is limited, such as inside a palm. The extra stage can add backlash and noise.

Brushless and coreless brushed motors

• Coreless brushed: low inertia, smooth at low speed, and runs from a simple H-bridge. Brush wear sets the life, and high current and frequent reversals, which grasping produces, speed it up. The rotor has little thermal mass, so it overheats quickly at stall.
• Brushless: no brushes, so life is set by bearings, gears and the screw. Heat comes from stator windings that sit against the housing, so it escapes more easily. Commutation is handled by the built-in driver.

For either, check the rated duty cycle against how long the mechanism holds force. Brushless micro servos make sense when the actuator cycles all day.

Built-in force sensing

Some micro servos include a force sensor, usually a strain-gauge element in line with the rod. Motor current is a poor substitute through a lead screw: friction absorbs much of the torque and varies with load, direction and temperature. A self-locking screw can hold a large force at zero current.

A direct force reading lets the controller:

• Stop at contact and hold a set grip force instead of a set position.
• Grasp soft or fragile objects without crushing them.
• Detect slip or a dropped object from a sudden change in force.

Position feedback and control interfaces

Position feedback is usually a potentiometer on the rod, which is absolute, or a motor encoder, which is relative and needs homing at startup.

• RS485 with Modbus RTU: a differential multi-drop bus. Each actuator has an address, and position, force and status are read as registers. Suits a hand with several actuators on one cable.
• TTL serial: half-duplex UART at logic levels. Easy to wire to a microcontroller, but meant for short runs.
• PWM: hobby-servo style, where pulse width sets position, commonly 1–2 ms in a 50 Hz frame. Nothing comes back, so you cannot read force or position.

To integrate a finished unit instead, see dexterous hands.

Self-locking screws and power-off holding

A lead screw self-locks when friction stops the load from back-driving it. A common rule of thumb: below about 50% forward efficiency, a static load will not back-drive the screw. Fine-pitch sliding lead screws usually qualify; ball screws do not.

Self-locking lets a hand hold an object at zero current, which cuts heat and battery draw. The costs: more motor power per newton, and a finger that cannot be back-driven by hand or by an impact, which matters for compliance and work near people. Vibration can still cause creep. Check the rated holding force separately from push force.

Sizing a finger actuator: worked example

Assume a finger joint driven by a rod on a 10 mm lever arm, with the fingertip 80 mm from the joint. Target: 10 N at the fingertip, closing through 90° in 0.5 s.

• Joint torque: 10 N × 0.080 m = 0.80 N·m.
• Rod force with the lever perpendicular to the rod: 0.80 N·m / 0.010 m = 80 N. The 8:1 lever ratio works against you.
• The lever swings ±45°. At the ends the effective arm is 10 mm × cos 45° ≈ 7.1 mm, so rod force rises to 0.80 / 0.0071 ≈ 113 N.
• Linkage losses at an assumed 80% efficiency: 113 N / 0.8 ≈ 141 N.
• Stroke: 2 × 10 mm × sin 45° ≈ 14.1 mm.
• Speed: 14.1 mm / 0.5 s ≈ 28 mm/s.
• Mechanical power at the rod: 141 N × 0.028 m/s ≈ 4 W.

So you need about 140 N at the worst angle, at least 15 mm of stroke, and about 28 mm/s at that force. If nothing in your envelope meets force and speed together, lengthen the lever arm (force drops and stroke rises in proportion) or accept a slower close. With a self-locking screw, the 140 N is needed only while closing, not while holding.

Checklist

• Stroke from linkage geometry, with margin before the end stops.
• Force at the worst lever angle, after linkage losses, for push and pull.
• Speed read from the force-speed curve at working force.
• Inline for long, narrow spaces; side-motor where length is short.
• Brushless for high cycle counts; coreless brushed for simple drive at lower duty.
• Force sensor if you need grip force control.
• RS485 or TTL bus for several actuators; PWM only for open-loop position.
• Self-locking for power-off holding; back-drivable for compliance.