Pneumatically operated industrial grippers have been commercially available for only a few decades. Prior to this, most gripping mechanisms were designed and built by machine builders employing a combination of pneumatic cylinders and external tooling for each individual application. Their most common purpose is to grasp or enclose parts for transfer, insertion, or assembly in automated manufacturing and processing systems. Additional uses may include operating in environments that are hazardous for human presence.
Pneumatic grippers provide their motion and actuation by introducing compressed air into a chamber of the device and powering a piston or a rolling diaphragm attached to a rod. The resultant motion of that rod is then converted mechanically to some form of gripping motion. Grippers are typically available in either an angular or parallel jaw version. The term parallel implies that the gripping surfaces of the jaws remain parallel to one another throughout the gripper’s travel.
The simplest of the angular design is a basic linkage or toggle mechanism attached to a piston rod, which is then connected to one of the gripper’s jaws. A directly opposing linkage is attached to another jaw. These two opposing jaws then make up the simple angular gripping device. With the appropriately designed and attached tooling, this type of gripper can provide a fairly large force in an angular fashion. The force is generally directly proportional to both the pressure applied to the piston and, usually being a simple lever, the length of the linkage or toggle and the length of tooling attached to the jaws. One of the limitations of this angular style of gripper jaw motion is that lacking sophisticated tooling, it can only efficiently grip a limited size range of parts. A variation of this angular gripper could include a third, or even a fourth jaw, defining a center gripping axis or region.
Another design or style of the pneumatic gripper is the parallel version, which employs one of several different types of mechanisms that convert the piston or its rod motion to a parallel jaw travel.
Different mechanisms could include:
- A lateral direct coupling of a pair of pistons to guided opposing jaws moving opposite each other.
- A design where opposing jaws are driven by a component of the piston rod riding on a cam surface portion of the jaws.
- A rack, powered by pistons, driving a pinion that in turn employs a scotch yoke cam device to drive opposing jaws.
- A rather complicated mechanism where the piston or one of its members drives a scroll mechanism similar to a machine lathe-chucking device.
All of these parallel mechanisms may also be designed to incorporate three or more jaws for gripping round or unusual shaped parts. Similar to angular grippers, typical parallel gripper forces are proportional to the applied pressure. Due to inherent friction within their jaw mechanisms, parallel grippers may also require de-rating factors based on the length of tooling attached to the jaws. While the parallel gripper may cost slightly more than a comparable angular version, it is considered by some to be advantageous over the angular style because it can often be applied to grip a wider range of part sizes without changing the attached tooling.
Additional features of both angular and parallel pneumatic grippers include the ability to operate as dual acting cylinders. This allows them to be used for gripping on either external or internal features of the part. Also possible are single acting versions with spring return. Another variation employs springs to assist with the gripping actuation for even greater force capability in a given size.
Material construction of most commercially available pneumatic grippers is aluminum bodies with various surface treatments where additional wear characteristics are required. Other materials could include stainless steel or even engineered plastics where corrosion resistance and wash down service is desired.
Pneumatic grippers are available in a wide range of sizes developing grip forces of just a few ounces to several hundred pounds. Gripper force development does not always translate directly into the capacity to carry a specific load. Additional consideration must also be given to the actuator’s ability to withstand moments that are reflected back into the jaws of the gripper from forces developed while in motion. Most manufacturers of today’s grippers provide sizing assistance through their technical sizing manuals, sizing software, or both.