Selecting an air gripper: 8 questions to ask

 

Each gripper style has its own size, method of operation, operating atmospheres, and required level of human interaction. Take the time to consider how these functions meet the needs of the automation application or process.

Pneumatic or electric grippers perform three basic functions:

Part transfer for machine tending, part placement, load/unload, boxing, palletizing Part orientation for applying a label, preparing tray inserton, box packaging Hold part in place to withstand applied work forces due to drilling, stamping, marking processes To better understand gripper capabilitites, and to select the proper style, consider these 8 questions.

1. What are the operating requirements?

Users and system integrators must take a big-picture view of the facility’s operations to decide whether an electric or pneumatic-driven gripper is best for operating.

• Electric grippers are quieter than pneumatics and minimize contaminates in sensitive environments. They provide analytical feedback about operating performance or part size and weight. Installation is easy as units connect directly to a control system/programmable logic controller (PLC) with basic standard wiring.

• Pneumatic grippers are faster, smaller, and cost less than an electric grippers, when comparing grip force to size ratios. Very noisy compared to electrics, they provide limited feedback to control systems, including grip part, or open/close status. Initial costs are low, but they have significant hidden costs. Grippers require air lines, filters, fittings, valves, and compressors to connect to a control system or PLC. Pneumatically powered grippers, or air-powered grippers, have been the standard with more than 95% in use.

2. Is the environment clean?

Operators must identify and implement the correct type of gripper that can withstand the operating conditions.

• Clean environments require keeping grease or contaminants on the gripper from being released into the work environment to avoid contaminating parts or processes. Ensuring clean manufacturing environments is common where very minute amounts of airborne or surface contaminants are allowed. Look for a gripper that is cleanroom-certified. Scavenge ports, available on many models, prevent contaminants from the gripper from escaping into the environment.

• Contaminated environments still must combat the effects of dirt, debris, oil, and grease. Purge ports can prevent contaminants from entering grippers while providing lubrication. The purge port, located on the gripper body, has a channel to the system’s internal mechanism. During gripper operation, a small amount of low-pressure air is introduced to keep positive pressure within the gripper housing, preventing contaminants from being drawn into the internal mechanism. In harsh operating environments, grease fittings can be used to purge dirty grease and/or add new grease to the unit.

3. Is a sealed gripper or shielded gripper required?

Standard or custom-designed shields can deflect debris away from gripper mechanisms or keep grease and internal containments from escaping into clean environments. Gripper shielding can be simple formed-sheet metal components or covers, flexible boots, bellows, or lip-style wipers. Users may add their own shielding during system integration. Orienting the gripper in relation to the direction of contaminants striking the unit can minimize debris.

Pneumatic grippers can be made from various materials and specialized processes. Stainless steel, nickel-plating, and hard-coat anodizing keep surfaces from corroding and can prevent debris from sticking, a condition that eventually causes gripper jaws to bind. In cleanroom applications, coatings can prevent oxidation or bacteria buildup.

Lubrications can be high-temperature or water-resistant to better handle specific environments or wash-down maintenance requirements. Pneumatic seals that handle extreme temperatures or grit and debris play a role in shielding. Buna-N (nitrile) is the industry standard, with Viton and silicone selected for higher temperatures. Metal seals allow grippers to handle extreme heat and/or contamination.

4. What are key specifications?

A gripper consists a body (including power transmission), jaws, and fingers. Generally, gripper manufacturers only design and build gripper bodies and jaws – known as the actuation mode. The machine builder or user typically supplies custom fingers to grip or encapsulate the given part. When selecting a gripper, consider appropriate finger length; excess can cause a gripper to bind. When considering grip force, too much will damage the part and too little will drop parts. Gripper stroke is also important as too much stroke wastes operation time, and too little will incorrectly grip or release parts. Grippers have various actuation times that will impact throughput rates. Repeatability is more important than accuracy and becomes a key specification if trying to pick up very small objects (syringe needles), or working in a high-precision application where one object is being placed inside another for assembly. 5What type of jaw-support mechanisms should be considered? Various grippers may be the same size and perform the same function but can have completely different designs.

 

• High-impact loading application require a plain or wedge-type bearing design – a large sliding surface contact bearing, such as flat surface-to-surface bearings and cylindrical bushing-type bearings. A wide bearing surface area can withstand continuous high-impact loading and can maintain accuracy when machined to tight tolerances. Typically, there is zero-to-limited adjustability to compensate for gripper wear throughout time.
• Low-friction high-accuracy applications benefit from line-contact roller-bearing designs such as cross-roller bearings and Dual V bearings. Bearing supporting jaws can be pre-loaded for high accuracy. This system is easy to maintain with external pre-load adjustability. Use in applications that require zero side play of the gripper fingers during the gripper’s lifetime. This low-friction design also allows an easy method of dialing in grip force by adjusting air pressure.
• Low air pressure precision applications Can use a point-contact ball bearing design. This mechanism can operate at very low air pressures where smooth, consistent motion is critical. There is also reduced grease splatter from the gripper during operation The type of bearing used and the amount of surface contact employed in the bearing design will determine if the gripper can operate at very low air pressure, impact resistance, repeatability, wear patterns, and if gripper wear can be compensated for through bearing pre-load adjustability. 6What mode of power transmission is needed? Power transmission mode refers to the linkage and transfer of power from the internal air piston(s) to the gripper jaws that open and close.
• High force-to-size applications require high grip force, but are constrained by the system’s working space. Double-sided wedge gripper designs with large surface areas for power transmission are often required. This style usually features a single-piston design that is capable of a high ratio of grip force-to-size. The gripper jaw/finger motion is synchronized without requiring additional components. The double-sided wedge can withstand high impact loads that may be imparted back onto the mechanism.
• Parallel-gripper solutions use direct drive design with a pin or rod to couple the piston to the jaw. These are normally twin-piston designs and require a jaw-synchronizing linkage. The design is simple, cost effective, and easy to shield.
• Angular-gripper solutions typically use a cam-driven design with direct, synchronized power transmission and bearing-line contact. This has one pivot point per jaw with a minimal number of moving parts. The cam can generate mechanical advantage, resulting in a gripper with high grip force in a relatively small overall package. The cam is commonly used in angular-jaw-motion grippers but can be found in other gripper types.
• High-precision high-repeatability applications benefit from the smooth, synchronous operation of a rack-and-pinion drive systems that produce minimal wear for long durability. A rack and pinion, machined to tight tolerances, will produce zero to very little jaw play when installed and make it very easy to build a non-synchronous jaw gripper.

 

7. What are the best finger designs, gripping methods?

Gripper fingers design should prevent dropping a part under loss of air pressure whenever possible. A safety analysis can minimize risk of injury or system damage from a dropped part. Pay attention to the material used for gripper fingers and the gripping surface of the product. To avoid grip marks, use nylon, delrin, plastics, and other soft materials for gripper fingers instead of aluminum and steel. For fragile parts, urethane pads can be placed on the finger, which will increase gripping friction without imparting undue force that may cause damage.

Popular gripping methods include:

• Friction grip, the most common gripping method, has contact surfaces that close and stop on the part, creating a frictional force that holds the workpiece. If air pressure is lost, the part will drop unless the gripper has built-in safety mechanisms. Friction fingers should be avoided when handling oily or greasy parts.

• Cradled grip allows the fingers to profile the part being handled, (round to round). The fingers close and apply force on the part like a cradle. If air pressure is lost, the part is typically held in place. If the weight of the part is significant enough to offset the backdrive force required to open the gripper, then fingers may cam open due to gravity, allowing the part to drop.

• Encapsulated, generally considered the most secure grip, allows fingers to have a profile of the part. Close and stop on or near the part, and the encapsulation can keep the part in position. If air pressure is interrupted, the part will not drop unless acted on by an external force.

8. What other safety features should be considered?

During a power failure that causes an operational air pressure loss, there are other means of preventing a part from accidentally releasing from the gripper and causing injury or part damage.

An internal spring can bias the piston and maintain finger/jaw position on or around the part. Care must be taken to ensure adequate spring force. A second option is using external fail/safe valves that are added to the ports to check air to the gripper in the open or closed position. A third option is the use of rod locks that automatically clamp on the guide rods of the jaws when air pressure is lost. Some, but not all, gripper styles can support rod locks.

Conclusion

Several pneumatic grippers perform the same function, but it is their unique features and capabilities that determine if they will operate long term in various applications.

The right size and right type of gripper can only be specified after all options are considered. Should there be any question to the suitability of a gripper to an application, contact the manufacturer to validate the gripper performance requirements to the allocated budget.

Destaco
https://www.destaco.com

About the author: Gary Labadie is the global product director, Engineering for Destaco, Auburn Hills, Michigan. He can be reached at glabadie@destaco.com.