The jaw guide, not the gripping force. Every gripper carries a maximum bending moment and a maximum lateral load for its guide, and a long finger multiplies the part's weight into both. Manufacturers quote gripping force at a set distance from the jaw face and publish a derating curve beyond it. Exceeding the moment rating wears the guide oval, after which repeatability drifts long before the gripper fails outright.

External gripping suits solid work and anything whose outside diameter is the machined reference. Internal gripping - jaws opening inside a bore or a ring - frees the whole outer surface for machining or inspection and is often the only option for thin-walled parts that would deform under an external squeeze. Most parallel grippers do both, but the force ratings differ between opening and closing because the piston areas differ.

Three jaws at 120 degrees converge on one axis, so the part self-centres regardless of its starting offset within the stroke. Two jaws define a line, not a point, so a round part is free to sit anywhere along that line. Where a downstream operation needs the part on a known axis - a lathe load, a bore inspection, a cap application - the three-jaw arrangement removes an alignment step.

Guide wear from contamination and from moment loading. Fine abrasive dust works into the dovetail or rail and laps it, and every over-length finger accelerates the process. Symptoms appear as a growing scatter in where the part ends up rather than as a loss of force, which is why force testing alone misses it. Sealed or booted variants exist precisely for dusty cells.

Not by itself. A standard double-acting gripper releases as pressure decays. Where a dropped part is unacceptable, the options are a spring-closed single-acting unit that grips without air, a pilot-operated check valve mounted at the port to trap pressure in the closing chamber, or a toggle mechanism that locks over centre. Each has a different failure behaviour and the risk assessment decides which applies.

Take the largest part diameter plus the clearance needed to enter and leave, then confirm the smallest part still sits inside the jaw travel with force to spare. Running a gripper at the very end of its stroke is where force falls away and where the jaws can bottom on their stops. A stroke with headroom at both ends also absorbs future part changes without a new gripper.

A cover or boot over the jaw slot, positive internal pressure bled through the body, and material choice at the exposed surfaces. Grippers offered for foundry, machining and food duties differ mainly in how the guide is sealed. Retrofitting protection to an open gripper rarely works because the boot has to move with the jaws without adding drag.