Considering all operational and design factors can optimize performance, uptime, and operational safety when selecting devices.

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.

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.

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.

• 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.

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.