Fundamentals of Pneumatic Grippers for Industrial Applications

Pneumatic grippers are electromechanical devices used in industrial applications for grasping and lifting, holding, rotating, and placing objects into set locations. These grippers typically install on the furthest reaches of either workpiece-processing machines or six-axis, Cartesian, or selective compliance articulated robot arm (SCARA) robotic arms as end effectors to execute various material-handling tasks. Complemented by the last several decades’ worth of advancement in controls, sensors, and feedback connectivity, pneumatic gripper motions (primarily for grasping and releasing) typically coordinate with those of the machine axis or robotic arm to which they mount.

Pneumatic gripper operation

Fundamentals of Pneumatic Grippers for Industrial ApplicationsFigure 1: Shown here is a two-finger pneumatic gripper on the end of a robotic arm. The jaw fingers make physical contact with the object to be grasped and are what allow the gripper to hold and release objects. (Image source: Kazakov • Getty Images)

Fundamentals of Pneumatic Grippers for Industrial ApplicationsFigure 2: Parallel, three-finger, and angled grippers are the three most common gripper types in industrial applications. The three-finger pneumatic gripper shown here has fingers offset by 120° to gently stretch O-rings and mount them onto recipient shafts. (Image source: Schunk)

Pneumatic grippers are far and away the most common gripper type for industrial applications involving robotic pick-and-place, machine tools, workpiece machining, and assembly tasks. Though some pneumatic grippers take the form of bladder-type and suction-cup end effectors, pneumatic grippers with fingers or jaws are the most prevalent and those generally assumed when no other context is given.

Jaw pneumatic grippers rely on compressed air for their operation. Upon some command signal, valves allow air to travel through internal channels and activate mechanical linkages — which in turn open and close the gripper fingers. Supporting this primary set of subcomponents are pneumatic hoses, control subcomponents and wiring, mounting flanges for attachment to machines and robots, failsafe mechanisms, and a housing that encloses these components.

Though the released position (held by a mechanical compression spring) is usually the default, gripper designs that default to grasping are also available on the market. Where a closed (gripping) position is the default, a spring provides the gripping force … and allowing compressed air into the gripper serves to open the jaws. In fact, certain grippers rely on compressed air for both grasping and releasing force.

display: block; position: relative”>

Video 1: In one common variation, the pneumatic gripper connects via a specialty hose to a compressed air system. The force of compressed air displaces a piston that in turn (through some gear, toggle, or slide linkage) causes the external jaws to actuate through their range of stroke. (Video source: Schunk)

Control of air into a pneumatic gripper is often reliant on preprogrammed grasp-release cycles … or (in more sophisticated applications) feedback from sensors that detect held objects.

Types of pneumatic grippers

Fundamentals of Pneumatic Grippers for Industrial ApplicationsFigure 3: Two-finger parallel grippers in Schunk’s PGN-plus series deliver long jaw strokes and include seals, dirt-resistant round linear guides, and high-strength aluminum-alloy housings to survive dirty industrial environments. (Image source: Schunk)

Jaw and finger pneumatic grippers are classified by their:

  • Kinematic arrangement, number of fingers, action, and type of mounting
  • Physical size and maximum gripping force
  • Jaw and housing construction — including level of ingress protection
  • Connectivity to common industrial control networks

First commercially available in the 1970s, pneumatic two-finger grippers are the most widely applied today — accounting for more than half of all pneumatic gripper applications. The fingers in these designs slide or swing on pivot points to close like a gate or lobster claw around target objects. They can employ either parallel jaw action or angled finger action.

Pneumatic grippers with parallel jaw action: In parallel grippers, the two fingers slide inward and outward — in straight-line motion — on the same axis along tracks in the upper gripper body. Typically, the inward sliding action is what grasps the workpieces or other objects. However, applications abound in which the two fingers slide outward to secure hollow or open workpieces (such as O-rings or cylinders, for example) from their inner diameters. Benefits of these dead-simple grippers abound. The various subcomponents for such grippers are simpler to manufacture than others, making these grippers very cost-effective. In addition, there is one steady gripping force over the entire finger stroke — which simplifies the work associated with applications involving delicate or otherwise pressure-sensitive workpieces. Finally, parallel grippers can be designed to close and open quite wide — even to a couple feet or more.

Pneumatic grippers with angled finger action: In these grippers, the actuated ends of the fingers are pinned to a fixed pivot point. Upon application of pneumatic power, a piston action and mechanical wedge element cause the fingers to swing closed (or in other variations, open) like French doors. In the open position, the jaws wing outward beyond the gripper body or project straight out. In the closed (typically grasping) position, the tips of the gripper fingers tilt inward to close into a tapered grasping shape. One design caveat when using these grippers is that, unlike parallel-finger types, angled fingers have limited strokes and generate a gripping force that’s variable along the actuation stroke. That said, angled finger grippers under direct piston action can have exceptionally high gripping force — up to 2,300 N or higher.

Higher finger Counts: three and four-finger grippers

Where two-finger pneumatic grippers are inappropriate to handle an operation’s workpieces, three and four-finger grippers (and even five-finger grippers in specialty humanoid-type robotics applications) can provide better gripping support and stability. To be clear, though: all such grippers are far less common than two-finger grippers … and only three-finger grippers are common in industrial applications. Their higher level of applicability comes at a cost, but three-fingered grippers can grasp workpieces and other items with more complex or challenging geometry. So-called self-centering three-finger pneumatic grippers include a trio of fingers even spaced (120° apart on a machine chuck) that necessitate finger swap-outs for an operation change. These close inward to grip workpieces at a center point. In contrast, so-called adaptive three-finger pneumatic grippers set two fingers together and the third to oppose them like a thumb. Most common on mobile robotics, such grippers can pick up objects in several ways to accommodate variations on a given workpiece geometry.

Internal gripping and double acting

Though the majority of pneumatic grippers are used for grasping or cradling parts around their exterior (contacting outer object surfaces) internal gripping operations are essential to many assembly applications. Here, the gripper fingers open to grasp objects with hollow geometries from within. In some cases, grippers can be tasked with both external and internal gripping operations — though must be designed to have both capabilities.

Jaw and finger pneumatic grippers can take the form of single and double-action grip types as well. In single-acting grippers, the force of compressed air generates the gripping motion and force. Once the supply is shut off, the fingers return to and stay in their original position thanks to the action of a simple compression spring. In contrast, double-acting grippers require compressed air actuation for both the grip and release motions. In fact, double-acting grippers may be capable of both internal and external gripping as described above.

Common pneumatic-gripper applications

Fundamentals of Pneumatic Grippers for Industrial ApplicationsFigure 4: The Schunk PGN-plus gripper has an oval piston drive. (Image source: Schunk)

Pneumatic grippers are widely used in industrial settings — especially for automated workcells, assembly and production lines, machine tending associated with advanced manufacturing, hazardous plant areas, and logistics as well as automated warehousing operations. A small but growing array of commercial, recreational, and consumer robotics applications (including mobility bionics) also make use of pneumatic grippers.

Consider pneumatic grippers for material handling in food and beverage processing and packaging equipment. Here, the clean operation of pneumatics is an asset — and pneumatically actuated finger grippers complement the use of other air-powered bladder and suction gripper types to handle everything from boxes and wine bottles to eggs and bags of candy. In contrast, grippers in machine-tool applications are typically designed for just one workpiece type — and in some cases, are even tasked with holding those workpieces as machining or other processes are performed. Where pneumatic grippers are involved in assembly or sorting and selecting, they’re often supported by sensor or even machine-vision systems to direct their actions. Otherwise, Hall Effect and proximity sensors in the gripper can provide sufficient feedback.

Advantages and limitations of pneumatic grippers

One key benefit of pneumatic grippers over other gripper types is that they’re available in numerous sizes and grip forces, ranging from a few Newtons to several kilo-Newtons, and can be adapted for different applications — even those requiring thousands of repetitions per hour. Industrial pneumatic grippers also offer unrivaled repeatability for precision automation tasks. In addition, pneumatic grippers:

  • Are cost and power effective to run
  • Are lightweight and compact — especially when compared to certain motor and hydraulic-based options

Unlike their hydraulic and electric counterparts, pneumatic grippers are largely unaffected by their working environments. That’s in contrast with electrically actuated grippers with sensitive electronics than can malfunction in moist environments.

Of course, pneumatic grippers do have some drawbacks and limitations. These are primarily related to the operational cost and complexity of pneumatic designs and compressed-air systems in general. Initial setup of such systems can be costly and complicated. That said, there is economy of scale where an industrial operation already makes use of compressed-air systems elsewhere.

Pneumatic gripper selection criteria

Sizing and specifying pneumatic grippers for a given material-handling application should start with the clear definition of key design parameters.

Size and gripping force: Pneumatic grippers should open enough to accommodate the objects being handled. Required pneumatic-gripper finger force depends on the weight of objects being handled as well as the finger-to-object coefficient of friction, area of finger-to-object contact, and force to counteract that of the opposing fingers. Highly engineered gripper-finger materials and coatings can boost the finger-to-object coefficient of friction. Of course, the jaws of pneumatic grippers for use in food or pharmaceutical applications must be made from or coated with FDA-approved materials.

There is wide variability in the ratio of the handled part size and weight — with lightweight yet bulky items often posing the greatest gripper design challenges.

Part geometry: Handled objects with complex geometries can often necessitate pneumatic grippers with three instead of two fingers. That’s especially true where a series of workpieces may have slightly varying geometries. But where workpieces are consistent, two-finger grippers can incorporate customized surfaces and shapes to accommodate specific gripping points on these objects. The cost savings of two-finger grippers can often justify their use wherever this solution satisfies the operation’s requirements.

Operating environment: Pneumatic gripper bearings, internal mechanical elements, and housings abound to satisfy clean and contaminated operating environments. Especially important are pneumatic gripper temperature ratings (that stipulate ranges within which a gripper will optimally function) as well as IP ratings that define the level of particulate matter and moisture a given gripper can resist before ingress.

Conclusion

Pneumatic grippers are robotic end effectors that are essential to material handling on production lines. These grippers hold, orient, and place workpieces and other objects for processing, assembly with other parts, or rejection — as off a conveyor through a quality-control station. Despite the drawbacks of compressed-air systems necessary for pneumatic gripper operation, these are often the cleanest, fastest, and most suitable choice for parts handling.