A gripper force rating is not proof of a secure grasp

A force number describes the gripper, not the finished grasp

A gripper can deliver its published force and still fail to retain a particular part. The finished grasp also depends on where the fingers contact, the part and fingertip surfaces, whether the geometry creates friction or form fit, the direction of gravity, the robot's motion and the external loads applied during the task.

The buyer should therefore ask for a grasp envelope: the permitted part population, configured tool, loading cases, damage limits, failure behaviour and evidence that the exact setup completes the required trajectory. A force rating is one input to that record, not proof of secure handling.

Define the part-to-finger interface

ISO 14539:2000 provides vocabulary and a way to present characteristics for grasp-type grippers; ISO records that edition as confirmed in 2021 and currently under review. That public scope helps teams name gripper characteristics consistently, but it does not verify a product or establish that a proposed grasp will hold a buyer's parts.

• Minimum and maximum mass, dimensions, geometry, centre of mass, stiffness and permitted deformation or marking.
• Surface material and relevant dry, oily, wet, dusty, coated, hot, cold or worn states.
• External or internal grip, friction or form fit, contact locations, available contact area and insertion depth.
• Part presentation, tolerance, seams, flash, labels, flexible features and other variation that can move the contact.
• Fingertip material, texture, compliance, wear limit, cleaning method and replacement rule.

Connect force to the actual gripping point

Do not mix commanded force, effective contact force, force per finger and the arithmetic total across jaws. Preserve the manufacturer's definition, measurement conditions, operating pressure or settings, tolerance and the point along the finger at which the value applies.

SMC's current LEHR selection document uses workpiece mass, friction, finger count and a margin in its manufacturer calculation. It also tells users to check effective force at the actual gripping point, external loads and finger moments. Festo's current HPPF datasheet likewise plots force against gripping-point distance and says external fingers, workpiece loads and acceleration contribute to jaw loading. These are maker-specific sizing instructions, not independently verified results or universal design factors.

Build the complete motion and load case

Robotiq's April 2024 Hand-E manual tells users to consider the complete trajectory, robot acceleration and stopping deceleration, gravity direction, eccentricity and forces applied while placing the object. This is first-party guidance for the documented Robotiq configuration; it does not independently prove a proposed cycle and should not be generalized to other tools.

• Robot pose, gravity direction, path, speed, acceleration, planned deceleration and relevant stop cases.
• Part centre-of-mass offset, gripping-point distance, finger overhang and loads on jaws, bearings and adapters.
• Vibration, impact and forces from insertion, extraction, machining, dispensing, inspection or contact with fixtures.
• Normal motion, rejected picks, retries, recovery and foreseeable utility or communication loss.
• A minimum retention requirement and a maximum allowed contact force, stress or deformation for damage-sensitive parts.

Check stroke, fingers and sensing as separate gates

Robotiq documents different recommended payloads for friction and form-fit grasps on the same Hand-E product and provides object-contact and contact-loss states. Those product features illustrate why grasp type and sensing belong in the requirement, but they remain manufacturer claims until the complete configured operation is tested.

• Usable opening and closing range for every material part state, including approach and release clearance.
• Finger geometry, stiffness, fasteners and permissible forces and moments at the actual offsets.
• Part-present, position, force, current, pressure, vacuum, slip or contact-loss signals and their update timing.
• The exact evidence each signal supplies: contact is not automatically correct seating, sufficient retention or an undamaged part.
• Logic for contradictory signals, marginal readings, retries, rejected parts and operator intervention.

Specify power-loss and recovery behaviour

Record what the exact tool does when electrical power, pneumatic pressure, vacuum, robot motion or communications are lost. A tool may open, remain mechanically locked, retain force temporarily or enter another state. Then define the consequence of a dropped, shifted or trapped part and the approved recovery method.

Robotiq says Hand-E has a power-off brake that blocks finger motion. That is a first-party feature claim for that product, not proof that every held part remains retained in every orientation or fault. ISO/TR 20218-1:2018 provides safety-design guidance for the design and integration of end effectors; it does not certify a configured cell or remove the need for application risk assessment.

Validate the representative grasp envelope

RobotAtom can record the grasp requirements, source boundaries, unknowns, manufacturer calculations and test evidence as distinct items. It does not certify gripper suitability, part retention, machinery safety or regulatory compliance. The responsible buyer, integrator, tool specialist and safety owner must decide what applies and whether the evidence is sufficient for the actual application.

• Freeze the robot, gripper, fingers, adapters, software, settings, part set, environment and trajectory.
• Exercise representative dimensional and surface variation, edge cases, starts, stops and agreed utility-loss behaviour.
• Predefine retained, slipped, damaged, dropped, mis-seated, recovered, intervened and invalid-run outcomes.
• Preserve raw results, configuration identifiers, measurement uncertainty, deviations, reviewer and decision.
• Reopen affected evidence after changes to the part, surface, fingers, settings, motion, load or failure controls.