End Effectors & Grippers

What an end effector does

The robot arm gets the tool to the right pose; the end effector does the actual work. End effectors fall into two large categories: tool-type (welding torches, paint sprayers, drills, deburring spindles, screwdrivers) and gripper-type (everything that grasps an object). This article focuses on grippers, since they're where most application engineering goes — a welding torch is a hot wire and a gas tube; a gripper has to make and break a physical bond with an object whose properties may not be known until grasping.

Every gripper imposes three trade-offs:

• Versatility vs precision. A vacuum cup grips many shapes; a custom-machined V-groove jaw grips one part perfectly. Adaptive tendon-driven fingers are between.
• Grip force vs gentleness. A robust grasp wants high force (low slip, fast acceleration). A fragile object wants low force (no crushing). Position-controlled grippers can't easily do both.
• Speed vs reliability. Pneumatic grippers close in 50–100 ms; servo-electric grippers in 100–300 ms with controlled force; vacuum needs a few hundred ms to evacuate; jamming grippers a full second.

Worked example: parallel-jaw friction grip

You're picking a 2 kg machined aluminum part with a smooth surface, accelerating at 2g during transport, with rubberized jaw pads (μ ≈ 0.5). What grip force do you need?

Maximum tensile load on the grasp during motion: F_load = m·(g + a) = 2·(9.81 + 2·9.81) ≈ 59 N.

For a two-jaw friction grip with coefficient μ on each side, slip is prevented when 2·μ·F_grip ≥ F_load. Solving:

F_grip,min = F_load / (2μ) = 59 / 1.0 = 59 N.

Apply a safety factor of 2× to absorb modeling error, slip onset, and impulse loads from collisions: target ≈ 120 N grip force. A typical pneumatic 50-mm-stroke parallel gripper at 6 bar supply provides 200–400 N — comfortably above target. A small servo-electric gripper (Robotiq 2F-85) tops out at ≈235 N — also adequate but with less headroom.

Now drop μ to 0.15 (steel jaws, no rubber) and the required grip force jumps to ≈400 N. The difference between rubber pads and bare jaws is the difference between buying a basic gripper and buying a heavy industrial one. Friction coefficient drives gripper sizing more than payload does.

Gripper types compared

Gripper type	Grip mechanism	Strength	Weakness	Typical use
Parallel-jaw (rigid)	Two opposing jaws driven by motor or piston	High force, simple, repeatable; cheap	One geometry per jaw set; can't conform to shape	Factory pick-and-place, machining cells
Adaptive (tendon, multi-finger)	Underactuated fingers wrap around object	One mechanism handles many shapes	Lower force; complex maintenance	Warehouse picking (Robotiq Adaptive 2F/3F)
Vacuum (suction cup)	Negative pressure against flat surface	Gentle on fragile flat parts; fast cycle	Needs air source; surface must be smooth and porous-free	Glass, sheet metal, packaging, displays
Magnetic (electro- or permanent)	Magnetic flux through ferromagnetic part	No mechanical contact needed; very fast	Only ferrous metals; residual magnetism on part	Steel sheet handling, foundry work
Festo fingertip / soft pneumatic	Silicone fingers bend when pressurized	Conforms gently to delicate or irregular shape	Limited grip force; air supply; finger fatigue over millions of cycles	Produce, packaging, soft goods
Shape-deposit / jamming gripper	Granule-filled membrane forms a mold under vacuum	Picks any shape with no per-object planning	Bulk; one-side access only; slow cycle	Research, mixed-SKU handling demonstrations

Real-world specs

• Robotiq 2F-85 (servo-electric two-finger) — 85 mm stroke, 5–235 N adjustable grip force, 5 kg payload, position-controlled with current sensing. Standard end effector on UR collaborative arms.
• Schunk PGN-plus (pneumatic parallel) — multiple sizes from 100 N to 5 kN grip force, 10 mm to 30 mm stroke, sub-100 ms close time. Workhorse industrial gripper.
• Piab piSAVE / PIAB BX vacuum cups — silicone or polyurethane, 10–200 mm cup diameter, holding force scales as ≈π/4·d²·Δp. A 100 mm cup at 60 kPa vacuum holds about 470 N.
• Festo MultiChoiceGripper / BionicSoftHand — pneumatic soft-finger grippers, FinRay-effect bending, 12-DOF research version with biomimetic anatomy.
• Boston Dynamics Atlas claw / hand — three-finger custom design with strain-gauge force feedback, ≈14 joints across all fingers, capable of picking and placing 25-pound boxes during dynamic whole-body motion.
• Empire Robotics VERSABALL — first commercial jamming gripper; granular-filled latex bladder, picks irregular parts with no per-SKU programming. Discontinued but the technology continues in research labs.

Variants and special cases

• Soft pneumatic grippers — silicone fingers with internal channels; pressurization curves the fingers around an object. The dominant approach in research labs for fruit and produce handling. Soft Robotics Inc. spun out from Harvard's Whitesides lab specifically to commercialize this.
• Shape-deposit grippers — descended from MIT/Cornell research; replaced by jamming grippers in most modern demos but still used where the deposit material doubles as a thermal interface (cryo handling, food).
• Electroadhesion grippers — high-voltage electrodes induce charge on a target surface; weak grip but works on dust, fabric, and dielectric materials where vacuum and magnets fail. Research stage.
• Geckskin / dry-adhesion grippers — bio-inspired van der Waals fibrils, like a gecko foot. Holds without contamination on smooth surfaces; sensitive to dust and surface quality.
• Cryogenic grippers — freeze a thin layer of water on a porous surface to bond the gripper to the part. Used for fragile electronics and biological samples; releases cleanly when warmed.
• Tool-changer end effectors — a passive flange that lets the arm dock to any of several tools stored on a rack. Robotiq, Schunk SWS, and ATI all sell standard tool-changer interfaces.

Common failure modes

• Over-force crushing fragile parts. A position-controlled gripper that closes until it hits its commanded position will deliver whatever force the actuator can produce against a soft object — cracking eggshells, denting electronics, or shattering glass. Always use force-feedback grippers (Robotiq, OnRobot RG6) for fragile work, or pre-tune jaw stops to hard-limit travel.
• Slip from low friction or dust contamination. Rubber pads degrade with oil and abrasion; smooth jaws lose grip on lubricated surfaces. A 2-kg part that slips during a 2g acceleration generates ≈4 kg of impulse load on a pinch point if it falls back into a fixture. Inspect and replace pads on a maintenance schedule, not on failure.
• Vacuum leak from porous parts. A vacuum cup on a porous surface (cardboard, sintered metal, perforated sheet) leaks faster than the pump can re-evacuate. Either use closed-cell foam interface pads, switch to a Bernoulli (high-flow) gripper that tolerates leaks, or use a different gripping principle.
• Magnetic residual on parts. Electromagnetic grippers leave a magnetic field in the part after release, which can attract chips, swarf, and other ferrous debris on the next cycle. Demag (degauss) cycles after each release add 100+ ms of cycle time.
• Jamming gripper losing granules. A torn membrane spills granules into the work cell — coffee grounds across the factory floor. Jamming grippers need regular membrane inspection and replacement; some use pleated fabric covers to protect the membrane from sharp objects.
• Grasp planning failure. Even a perfect gripper fails if the planner tries to grip in the wrong place. Boxes with tape seams, parts with featureless surfaces, and parts at workspace limits all break grasp planners. Modern systems use camera feedback and learned grasp networks (Dex-Net and successors) to score candidate grasps before commanding.