End effectors and grippers

Types of end effectors and grippers, how to choose one, and how to size friction grip force, vacuum cup capacity and gripper cylinder force.

Drafted with Aria, reviewed by the AiCanCode.org team. Spotted an error? Use Give Feedback at the bottom of the page.

Why it matters

The arm only brings the tool to the right place; the end effector is what actually touches the part. Most failed robot cells fail at the gripper — parts slip during fast moves, delicate parts get crushed, or suction cups lose vacuum — so sizing grip force and choosing the right gripper type is a core design skill.

Key ideas

End effector is the general name for whatever is mounted on the robot's tool flange. Two families:

  • Grippers — hold a part: mechanical (finger), vacuum (suction cup), magnetic, adhesive, soft/compliant, needle grippers for textiles.
  • Tools — do a process: spot-welding gun, arc torch, spray gun, screwdriver, deburring spindle, laser head, dispensing nozzle.

Mechanical grippers use two or more fingers driven by a pneumatic cylinder, electric motor or hydraulic actuator through a linkage, gear, cam or screw.

  • Parallel grippers — jaws translate towards each other, so the jaw faces stay parallel and the grip point does not shift with part size.
  • Angular grippers — jaws pivot about a pin; cheaper and compact, open wide for clearance, but the jaw angle changes with part size.
  • Grip can be by friction (flat pads squeezing the part — force-limited by μ) or by form/physical constraint (fingers shaped to the part so it cannot move — much more secure, needs less force).

Vacuum grippers hold by the pressure difference between atmosphere and the evacuated cup. Best for flat, smooth, non-porous parts (sheet metal, glass, cartons). Vacuum comes from a venturi ejector (compressed air) or a vacuum pump. Porous or rough surfaces leak; use foam cups or higher flow.

Magnetic grippers — electromagnets (switch off to release) or permanent magnets (need a stripper mechanism). Only for ferrous parts; residual magnetism and picking two sheets at once are known issues.

Soft and adaptive grippers conform to irregular or delicate objects (fruit, food, medical items) using elastomer fingers inflated by air, or underactuated fingers.

Selection factors: part mass, size, shape and material; surface finish and porosity; required accuracy; accelerations during the move; temperature and cleanliness; cycle time; tool changing needs; whether grip loss must be fail-safe (e.g. spring-closing jaws on air loss).

Compliance. A Remote Centre Compliance (RCC) device between wrist and gripper lets the part shift sideways and tilt about a point near the tip, so a peg self-aligns into a hole despite small position errors.

Gripper sensing: finger position switches, force sensors to limit squeeze, vacuum switches to confirm pick, tactile or slip sensors.

Formulas

n_f · μ · F_g ≥ m · (g + a) · SF — friction grip holding a part against gravity while accelerating upward

  • n_f — number of contact surfaces (fingers) providing friction (2 for a two-finger gripper); μ — coefficient of friction (dimensionless); F_g — normal grip force per finger (N); m — part mass (kg); g = 9.81 m/s²; a — upward acceleration of the part (m/s²; use 0 for a static hold); SF — safety factor (typically 1.5–3).
  • Applies when gravity acts parallel to the finger faces so only friction resists slip.

F_g = m · (g + a) · SF / (n_f · μ) — required grip force per finger

F_v = Δp · A = Δp · (π·d²/4) — holding force of one vacuum cup

  • Δp — pressure difference between atmosphere and cup (Pa); A — effective cup area (m²); d — cup diameter (m). Divide by SF to get usable capacity.

F_cyl = p · (π·D²/4) — force of a pneumatic gripper cylinder (push stroke)

  • p — supply gauge pressure (Pa); D — piston diameter (m). Finger force follows from the linkage's mechanical advantage.

Worked examples

Example 1 (standard). A two-finger parallel gripper holds a 5 kg part statically, with the finger faces vertical. μ = 0.4 and no safety factor. Find the minimum grip force per finger.

  1. Formula: F_g = m·g / (n_f·μ).
  2. Weight: m·g = 5 · 9.81 = 49.05 N.
  3. F_g = 49.05 / (2 · 0.4) = 49.05 / 0.8 = 61.3 N.

Answer: 61.3 N per finger. (If you forget n_f = 2 you get 122.6 N — twice the true requirement.)

Example 2 (GATE level). The same type of gripper lifts an 8 kg casting with an upward acceleration of 6 m/s². μ = 0.35, safety factor 1.5. Find the grip force per finger.

  1. Formula: F_g = m·(g + a)·SF / (n_f·μ).
  2. m·(g + a) = 8 · (9.81 + 6) = 8 · 15.81 = 126.48 N.
  3. With SF: 126.48 · 1.5 = 189.72 N.
  4. F_g = 189.72 / (2 · 0.35) = 189.72 / 0.70 = 271.0 N.

Answer: 271 N per finger.

Example 3 (vacuum). Two suction cups of 40 mm diameter work at a vacuum of 60 kPa below atmosphere. With a safety factor of 2, what is the largest mass they can lift vertically?

  1. Cup area: A = π · 0.040² / 4 = 1.257 × 10⁻³ m².
  2. Force per cup: F_v = 60 000 · 1.257 × 10⁻³ = 75.4 N.
  3. Two cups: 150.8 N; usable after SF: 150.8 / 2 = 75.4 N.
  4. Mass: m = 75.4 / 9.81 = 7.69 kg.

Answer: 7.69 kg.

Common mistakes

  • Writing μ·F = W for a two-finger gripper — each finger contributes friction, so it is n_f·μ·F_g.
  • Equating grip force with weight (N = m·g): the grip (normal) force is perpendicular to the weight; only friction opposes it.
  • Ignoring acceleration: during a fast upward move the inertia force m·a adds to the weight.
  • Using gauge supply pressure for vacuum force — the useful pressure difference can never exceed atmospheric (≈101 kPa).
  • Over-squeezing delicate parts; a form-closure finger design is often better than more force.
  • Choosing vacuum for porous or oily parts, or magnets for aluminium and stainless steel.

For GATE ME

Questions are mostly conceptual MCQs on gripper types, actuation and selection (vacuum for flat smooth parts, magnetic for ferrous, RCC for assembly). Numericals involve friction grip force with a given number of fingers, acceleration and safety factor, vacuum cup capacity, and pneumatic cylinder force. Practise drawing the free-body diagram of the part between the fingers first.

Quick check

  1. A two-finger gripper with μ = 0.5 must hold a 3 kg part statically. Minimum force per finger?
  2. Which gripper type suits flat glass sheets?
  3. Why is a form-closure grip safer than a friction grip?
  4. What does an RCC device do?
  5. One cup of area 2 × 10⁻³ m² at 50 kPa vacuum — holding force?

Answers: 1. 29.4 N; 2. Vacuum gripper; 3. The finger shape blocks motion, so holding does not depend on μ; 4. Provides lateral and angular compliance about a point near the tool tip so parts self-align during insertion; 5. 100 N.

Interview questions

All Robotics interview questions

Try answering each one aloud before you open it.

  1. 1.What is an end effector in robotics?Concept

    An end effector is a device or tool that's connected to the end of a robotic arm, designed to interact with the environment. It can perform various tasks such as gripping, welding, painting, or assembling. The design and functionality of an end effector depend on the specific application it is intended for.

  2. 2.Explain the difference between a gripper and an end effector.Concept

    A gripper is a type of end effector specifically designed to grasp and hold objects. While all grippers are end effectors, not all end effectors are grippers. End effectors can include a wide range of tools such as welding torches, suction cups, or sensors, depending on the task they are meant to perform.

  3. 3.Why are vacuum grippers commonly used in the packaging industry?Application

    Vacuum grippers are commonly used in the packaging industry because they can easily handle flat, smooth, and non-porous surfaces, such as cardboard boxes or plastic packages. They provide a gentle yet firm grip, reducing the risk of damaging delicate items. Additionally, vacuum grippers can quickly pick and place items, increasing the efficiency of packaging operations.

  4. 4.What factors should be considered when selecting an end effector for a robotic application?Application

    When selecting an end effector, consider the type of task, the weight and size of the objects to be handled, the required precision, the environment (e.g., temperature, humidity), and the compatibility with the robotic arm. Additionally, consider the material of the objects, as this may affect the choice of gripping mechanism, such as suction, magnetic, or mechanical.

  5. 5.How does a parallel gripper differ from an angular gripper?Concept

    In a parallel gripper the jaws translate towards each other, so the jaw faces stay parallel and the contact point does not shift with part size; it suits parts of varying width and precise centring. In an angular gripper the jaws pivot about a pin, so they swing open through an angle. Angular grippers are simpler, cheaper and give wide jaw clearance for approaching a part, but the jaw angle and grip point change with part size, so they are best when the part size is fixed.

  6. 6.What happens if a gripper is not properly calibrated for its task?Application

    If a gripper is not properly calibrated, it may apply too much or too little force, leading to potential damage to the object or failure to securely hold it. This can result in dropped items, reduced efficiency, and increased wear on the gripper itself. Proper calibration ensures optimal performance and longevity of the gripper.

  7. 7.Explain the role of sensors in robotic grippers.Concept

    Sensors in robotic grippers provide feedback on the position, force, and condition of the object being handled. They help in adjusting the grip force, detecting slippage, and ensuring precise placement. Sensors enhance the adaptability and reliability of grippers, especially in dynamic or unpredictable environments.

  8. 8.Why might a soft gripper be preferred over a rigid gripper in certain applications?Application

    A soft gripper is preferred in applications where the objects are delicate, irregularly shaped, or easily damaged. Soft grippers can conform to the shape of the object, providing a gentle yet secure grip. They are ideal for handling items like fruits, textiles, or fragile components, where a rigid gripper might cause damage.

  9. 9.Calculate the gripping force required for a parallel gripper to hold a 2 kg object with a coefficient of friction of 0.3.Numerical

    To calculate the gripping force (F), use the formula: F = (m * g) / (2 * μ), where m is the mass of the object, g is the acceleration due to gravity (9.81 m/s²), and μ is the coefficient of friction. F = (2 kg * 9.81 m/s²) / (2 * 0.3) = 32.7 N. Therefore, each jaw of the gripper must exert a force of 32.7 N to hold the object securely.

  10. 10.A two-finger gripper holds a 5 kg object with each finger pressing 100 N on it (finger faces vertical). What is the minimum coefficient of friction to prevent slipping in a static hold?Numerical

    Both finger contacts supply friction, so the condition is n_f·μ·F_g ≥ m·g. The weight is 5 × 9.81 = 49.05 N, so μ ≥ 49.05 / (2 × 100) = 0.245. In practice a safety factor and any upward acceleration would raise the required μ or grip force.

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