by Paul J. Heney, Editorial Director
Grippers grasp or enclose parts for transfer, insertion, or assembly in a variety of manufacturing situations.
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.
The parallel style of grippers employs a mechanism to convert the piston or its rod motion to a parallel jaw travel. Different mechanisms 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, and
• A 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.
Following is a selection of grippers currently available from manufacturers.
1.Suitable for big jobs
LPG grippers are suitable for handling oversize parts and heavy loads up to 300 lb. An adaptation of the company’s EZ Series linear slides, its jaws are a pair of tool bars. Integral, double acting air cylinder drives shorter jaw bar and inboard pair of guide shafts. Double rack-and-pinion arrangement transfers force to outboard guide shafts holding wider jaw bar. Jaw bars can be cantilevered or wide stance, where the jaw bars are positioned on opposite sides of the gripper body. Jaw bars move with absolute parallelism and precise synchronous motion. Centering is accurate to within 0.002 in. repeatability, providing virtually “play free” gripping. End caps are machined on top and bottoms surfaces, allowing gripper to be mounted with its port side up or down. Options include magnetic position sensing, adjustable stops, and bumpers.
Fabco-Air
Fabco-Air.com
2.O-ring grippers
O-ring assembly tools come in two sizes: the AGP-6-OR (which has six jaws; shown) and the AGP-8-OR (which has eight). These pneumatic grippers can accommodate placement applications from 0.125- to 12-in. I.D. Both are designed to facilitate part ejection and part seating and are suitable for installations where an ejector is needed after the part is positioned, or when a seal needs to be placed down a long shaft. The O-rings, as a result of the octagon–shape they take on, undergo significantly less deformation than they would if applied through alternative means of assembly. Additionally, because the strokes are adjustable to precise measurements, there is little risk of over-expansion. The technology can be employed in semi-automatic single-station hand assists and fully automated assembly line, and can be implemented in a range of manufacturing industries.
American Grippers Inc.
www.agi-automation.com
3.Low-cost, high value
Robohand line of low-cost, high-value G110 Series parallel grippers provide full array of solutions for part-handling applications. Available in four bore sizes with five stroke lengths per bore, grippers are rated to provide up to 560 N, lengths from 22 to 150 mm, and strokes from 25 and 50 mm. Key features include precision rack and pinion synchronizing mechanisms, non-corrosive exterior components, shielded-drive mechanism, piston magnets for four sensing positions, precision dowel holes, and counter-bores on jaws for mounting customer-supplied fingers with optional locating sleeves. Low-profile design provides superior jaw support, which allows for longer fingers and stable support for rugged, high-cycle, and heavy-duty applications. Modular nature with company’s DirectConnectmounting eliminates need for custom-adaptor plates that can add weight, height, design time, and cost.
DE-STA-CO
www.destaco.com
4.End effectors
AirPicker and AirGripper lines of end effectors combine pneumatic pressure control with physical attributes of rubber. The result can give robotics or mechanical handling systems a degree of touch and allow products to be handled securely and gently. Elastomeric bladder construction conforms to any shape. By varying the air pressure in the picker or gripper, user can conform the end-effectors to the diameter that is being handled and the force being applied to the work piece. This is especially critical in applications with sensitive, thin-walled or fragile objects. The object being picked up does not have to have a circular configuration. Each picker and gripper can work with a range of diameters and work piece weights. There is a wide range of standard configurations and sizes to meet most industrial applications.
Firestone Industrial Products
www.firestoneip.com
5.Compact and rugged
Series GRV Angular Pneumatic Micro Grippers are compact, rugged precision grippers. Available in four sizes, the flexible design provides large moment capacities and long tooling lengths. Tested to five million cycles, they provide flexible mounting capability with precision dowel pin holes that ensure accurate alignment of tooling and gripper mounting. Slim design allows ample space for stacking grippers in a confined area, making this gripper suitable for a wide variety of automation applications. Can be used in many industries and process including: medical device manufacturing, semiconductor manufacturing, laboratory processing applications, clamping and fixturing during assembly operations, and space-restricted processing and manufacturing equipment. Grippers are supplied switch-ready for easy integration of available magnetic position sensing switches. Teachable two-position switch allows for sensing of two jaw positions while using single switch slot.
PHD Inc.
www.phdinc.com
14 tips for maximizing gripper life
As with any device, proper attention to design criteria is critical for the performance and product performance. Here are some tips to help maximize the life of a gripper:
1. Besides the obvious additional lubrication at regular intervals, correct sizing is critical.
2. Keep the tooling jaws short as possible.
3. Keep the center of mass of the gripped part as close to the gripper as possible.
4. Ensure that the X, Y and Z axes to which the gripper is attached have smooth acceleration and deceleration.
5. Maintain application temperatures within the rated guidelines of the gripper.
6. Many times, applications are overloaded due to changes in the product or process requirements. This can lead to breakdowns because of material stress, bearing failures, wear on pivots, heat stress, and so on.
7. Put some compliance in the tooling or gripper base to reduce stress on bearings.
8. The surface finish of the work piece is another critical variable in affecting the life of a picker. The smoother the finish, the longer the life.
9. The inflatable bladder may be damaged if the surface of the hardware is extremely rough or sharp. If the application requires unrestricted inflation, a protective rubber sleeve or a casing surrounding the bladder is recommended. The sleeve or casing will reduce the amount of strain on the rubber bladder.
10. Sometimes, angular grippers are overloaded due to an increase in length of grip point to pivot point beyond the manufacturer’s recommendations.
11. Ensure that both the compressed air and the application is as clean as possible.
12. Pay attention to the inflation pressure and diameter requirements. Manufacturer catalog has graphs that demonstrate the limits. The gripper will last longer if it is used properly.
13. Never use WD-40 on any gripper!
14. If contamination exists in the application, such as machining chips or grinding dust, use a sealed gripper or cover the gripper.
Thanks to the following for supplying tips: American Grippers Inc.; PHD Inc.; Don Foulke, Global Industrial Channel Manager, Firestone Industrial Products; and George Edwards, Chief Engineer, Fabco-Air Inc.
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