How To Choose A Pneumatic Gripper?

Almost all the grippers used in automation today are pneumatically powered, although electric grippers are seeing some advances in design and operation. You’ll find them in such diverse industries as electronics, pharmaceutical, automotive and consumer goods – especially in pick-and-place automation systems. Despite this widespread use, it’s not always easy to select the most appropriate grippers for your system. You might be guided by familiarity, convenience, or an over-generalised end-user specification, but understanding how to choose a pneumatic gripper is much more complicated. 

Assessing The Operating Environment

Pneumatically controlled grippers are mostly used for these three basic tasks:

• To grip items while work is being carried out on them.

• To put items in the correct position for their next process.

• To grip and hold items being transferred between workstations, machines,  conveyors, etc.

These basic tasks can only be completed effectively if you choose the right type of gripper for the operating conditions. This means broadly determining whether your operating environment is clean or contaminated.

Clean Environments 

Common for handling parts or processes in the pharmaceutical, medical, food-production and electronics industries. Only very minute amounts of surface or airborne contaminants are permitted here, so it’s critical to prevent the release of anything in or on the grippers into the work environment. Clean Room Classified environments require grippers to comply.

Many gripper models feature scavenge ports, which prevent contaminants inside the gripper from escaping into the environment. The ports use a low-level vacuum to create negative pressure, which draws clean ambient air in through the gripper and disperses it outside the work cell.

Normal Environments

Even in less demanding working environments, contaminants like dirt, debris, oil and grease must always be kept out of the grippers. Extreme temperature variations can also affect their internal workings and operation. Many models feature a purge port – a port on the body of the gripper with a channel to its internal mechanism. This brings low-pressure air into the gripper housing to keep up its positive pressure and prevent contaminants from reaching the mechanism. 

Supplemental Safeguards

Additional measures for either type of environment include specialist seals, greases, surface materials and coatings like stainless steel. You can also increase gripper protection with standard or custom-designed shields that deflect debris away or keep contaminants contained. These include lip-style wipers, flexible boots and bellows, or simple sheet metal shields or covers. Shielding may be integral to the gripper or added on as part of machine integration. 

Design And Suitability

Grippers have three basic parts: the body, jaws and fingers. Manufacturers usually only produce the body and jaws, while those building or using the machine acquire the fingers. You need to choose an appropriate finger length for your application, along with the correct grip force, stroke, accuracy, actuation time, etc. These specifications are usually supplied by manufacturers for each gripper model. Grippers may look and act the same but will be designed differently for specific operating environments. Their function may be impacted by their internal method of transmitting power to the jaws, or the type of bearing used to support the jaws. 

Common Jaw-Support Mechanisms

• Ball bearings (point contact) are good for precision applications as they’re very low-friction. They can operate at very low line pressures where a consistently smooth motion is critical.    

• Roller bearings (line contact) are low-friction bearings that achieve high accuracy. They’re also adjustable so that near-zero side play can be maintained during the gripper’s lifespan.   

• Plain bearings (surface contact) can be flat or cylindrical, provide excellent jaw support and withstand impact loading well. They don’t need adjusting and when machined to tight tolerances can maintain a high degree of accuracy.  

Mechanism And Mode Of Power Transmission 

• Cam drives offer line contact for direct, synchronised power transmission to gripper jaws. This method produces a high level of grip force in a relatively small unit and is typically chosen for angular jaw motions.

• Rack-and-pinion synchronised drives transmit the piston force through a rack, so the drive parts suffer virtually no wear. They’re designed for use in high-precision, clean environments.

• Direct drives couple a piston directly to the jaw with a pin or rod. These simple, cost-effective drives are easy to shield. They are usually designed with twin pistons, with a linkage to synchronise the jaws.  

• Double-sided wedge drives provide a large surface area that transmits synchronised, equal power to the jaws. They’re typically very rugged single-piston units with a high grip force-to-size ratio, to withstand higher impact loads.

Finger Designs And Gripping Methods

• Encapsulated fingers offer the most secure grip. They’re profiled to match the shape of an object. They won’t drop it even if they lose air pressure. The fingers close around the object and this encapsulation keeps it in place. 

• Cradled fingers are also profiled to match the object, with their force and shape generating the grip force. They may open under gravity if you lose air pressure, and drop the object.

• Friction fingers are the most commonly used but aren’t suitable for handling oily or greasy objects. Their contact surfaces close to the object and stop, creating a frictional force that holds it in place. But if you lose air pressure, they’ll drop it. If you choose this type, you’ll need a larger gripper to get a higher grip force. You may also need to augment the fingers to improve their gripping ability.   

When choosing the finger design for a pneumatic gripper, safety must come first. This means having backup for potential air pressure losses, to prevent the grippers from accidentally dropping objects and causing potential damage or injury. Options include external fail-safe valves, internal springs or rod locks.

Get A Grip

A pneumatic automation system may easily be compromised by the wrong choice of grippers. To avoid operational failures, make sure you choose the most appropriate gripper design for its operating environment. Carefully assess the options available, including the various design specs and any available custom solutions. Only when you optimise these areas can you truly be certain that you’ve chosen the best gripper for the application.