Choosing an electric gripper: force, stroke and fingers

By Tolerance Labs engineering team · Updated 18 October 2026

The gripper is where a robot touches the work. Its force, stroke and fingers decide whether the part stays put through every move, and its behavior on a power loss decides what happens to the part when the robot stops.

Parallel, angular and adaptive grippers

A parallel gripper moves two or three jaws in straight lines toward a common center. The jaws stay parallel through the stroke, so the contact point does not shift with part size, and the jaws center the part on the closing axis. Most electric parallel grippers drive the jaws from a small brushless or stepper motor through a lead screw, a ball screw or a rack and pinion.

An angular gripper swings its jaws about pivots. It opens wide, but the contact point moves with part size. An adaptive gripper, also called underactuated or finger-following, has fingers with two or more phalanges driven by one motor through a linkage or tendon. When the inner phalanx touches the part, the outer one keeps closing and wraps around it. Tendon, linkage or joint motors explains how underactuated fingers split motion between joints.

Worked example: grip force

Most grippers hold by friction. The jaws press on the part, and friction at each contact face carries the part’s weight plus the inertial force of every acceleration. For a two-jaw gripper with the load acting along the jaw faces, the force each jaw must apply is F = m × (g + a) × S ÷ (2 × μ). Here m is the part mass, a the largest acceleration of the gripper, S a safety factor and μ the friction coefficient between jaw and part.

Example: a 0.5 kg aluminum part, moved vertically by an arm that accelerates at up to 5 m/s², with a safety factor of 2.

• Load: 0.5 × (9.81 + 5) = 0.5 × 14.81 ≈ 7.4 N.
• With the safety factor: 7.4 × 2 = 14.8 N.
• Bare steel jaws, with μ assumed at 0.2: 14.8 ÷ (2 × 0.2) = 14.8 ÷ 0.4 = 37 N per jaw.
• Urethane pads, with μ assumed at 0.6: 14.8 ÷ (2 × 0.6) = 14.8 ÷ 1.2 ≈ 12.3 N per jaw.

The friction coefficient changes the answer threefold, and oil, dust or a coated part lowers it further. Measure it on real parts: pull a gripped part out with a force gauge and divide the pull force by twice the jaw force. Size for the deceleration of an emergency stop, which is often higher than the planned acceleration.

Check whether the datasheet quotes force per jaw or for both jaws, and at what distance from the jaw face. Longer fingers lower the force at the tip and raise the moments on the jaw guides. A form-fit grip, where the finger shape carries the part instead of friction, needs far less force. A V-groove or a ledge under the part is often worth more than a stronger gripper.

Stroke and speed

Stroke is jaw travel, given either per jaw or in total. The open width must clear the largest part plus its position error plus a margin. The closed width must pass the smallest part, so the jaws can still apply force.

Example: parts are 40 ± 0.5 mm wide, part position varies by ±2 mm, and you want 3 mm of clearance on each side. Open width: 40.5 + 2 × (2 + 3) = 50.5 mm. The jaws must close below 39.5 mm, so total stroke must be at least 50.5 − 39.5 = 11 mm, or 5.5 mm per jaw. Choose more than that, because extra stroke lets one finger set cover a range of parts.

Stroke costs time, so compare closing time over the stroke you use, not the full stroke.

Finger design

• Material. Aluminum fingers are stiff and easy to machine. 3D-printed fingers are quick to change and suit low forces. Urethane or rubber pads raise friction and protect finished surfaces, but they wear and creep under steady load.
• Shape. Match the part: flat faces for boxes, a V-groove for round parts, pockets or steps for a form-fit grip. A V-groove also centers the part.
• Length and mass. Keep fingers short. Finger mass adds to the moving mass of the jaws and to the arm’s payload.
• Compliance. A little give in the pads tolerates position error. Too much lets the part rock when the arm accelerates.

Force control and grip detection

In an electric gripper, grip force comes from motor torque, so the driver sets it by limiting current. Friction in the screw or rack sits between the motor and the jaws, so force accuracy and repeatability are worse than the current control suggests. Ask for the force range, the force repeatability, and the jaw position at which they were measured.

Speed at contact matters as much as force. A fast close adds an impact spike on top of the set force. A soft grip mode closes fast until near the part, then slows for the last few millimeters.

Grip detection comes almost free from jaw position. After a close command, the gripper reports where the jaws stopped:

• Inside the expected width window: part held.
• Fully closed: part missed.
• Stopped early: wrong part, or two parts.
• Position changes while holding: part slipping or lost.

Check that the gripper reports position over its interface, not only a done signal. For contact detail on soft or irregular objects, see tactile sensing for robot hands.

Holding on power loss

Decide what the gripper must do when power drops or an emergency stop removes motor torque.

A gripper that keeps holding is usually safer for the part, but a person cannot free a trapped part or finger without power. Check for a manual release. If the robot stops with safe torque off, decide whether the gripper’s own motor is part of that stop.

Interfaces and mounting

• Digital I/O. Open and close signals with preset positions and forces, plus a done or part-held output. Simple, but each new part needs a new preset.
• Serial bus. RS485 with Modbus RTU is common, and some grippers use CAN, EtherCAT or IO-Link. A bus gives position, force and status for grip detection.
• Supply. 24 V DC is typical. Check the peak current at the start of a close.
• Tool flange. ISO 9409-1 defines the mechanical interface of robot tool flanges. Many collaborative arms use the pattern with a 50 mm pitch circle and four M6 screws. Check your arm’s flange drawing, and order an adapter plate if the patterns differ.
• Wiring. Match the pinout and current rating of the arm’s wrist tool connector, or route a cable along the arm.

The gripper, fingers and part all count against the arm’s payload, and their center of mass sets the moment on the wrist. Sizing robot arm joints shows how to check it.

Checklist

• Choose parallel, angular or adaptive from the part shapes and the position accuracy you need.
• Compute jaw force from mass, worst acceleration, safety factor and a measured friction coefficient, and confirm whether the datasheet quotes force per jaw or in total.
• Check force at your finger length and the allowed jaw moments.
• Size stroke from the part size range, position error and clearance.
• Confirm force setting, soft grip, and position feedback for grip detection.
• Decide power-loss behavior, and check for a manual release.
• Match the interface, supply and tool flange, and count gripper mass in the arm’s payload.

Browse electric grippers and dexterous hands, and compare force, stroke and interface side by side in Robot Parts.