What To Consider When Choosing Grippers
Article From: April 2019 Manufacturing Engineering, Dan Campbell, DESTACO
Automated manufacturing operations are finely tuned ecosystems in which all components must function in complete harmony. Grippers used to pick and place, orient and hold components or end products at various points along the production chain are key to this process.
Grippers come in many sizes and styles, and several considerations should be addressed before the best gripper can be chosen. Among these are the effects that dirt, grit, oil, grease, cutting fluid, temperature variation, cleanliness, and human interaction have on automation systems.
Pneumatically controlled grippers are used in a high percentage of applications and perform three basic tasks: gripping and holding a product or component while it is being transferred; part orientation; and gripping a part while work is being done. These tasks cannot be completed until the correct gripper is chosen for two general classes of operating environments:
• Contaminated: Contaminants must be kept out of the gripper to ensure trouble-free functioning. High levels of dirt, debris, oil and grease, along with temperature variations, can affect the gripper’s internal workings.
• Clean: The focus is on keeping anything on or in the gripper from being released into the work environment. This is common in industries where only minute amounts of airborne or surface contaminants are allowed.
The use of standard or custom shields can deflect debris from the internal workings in a dirty environment, or help to keep internal containments and grease contained in a clean one.
Considerations for any gripper application should include appropriate finger length, grip force, stroke, actuation time and accuracy. In this area, common jaw-support mechanisms include:
• Plain Bearings (surface contact): Typically flat surface-to-surface bearings and cylindrical (bushing-type) bearings that can withstand impact loading while holding high accuracy.
• Roller Bearings (line contact): Low-friction cross-roller bearings and Dual V bearings that are pre-loaded to achieve high accuracy and adjusted over time to minimize side play.
• Ball Bearings (point contact): Very low friction, making them good for precision applications and operating at low line pressures.
The mode of power transmission should also be contemplated. Some examples are:
• Double-Sided Wedge: Have a large surface area for transmitting power to the jaws with the power equally divided between them.
• Direct Drive: A pin or rod is used to direct couple the piston to the jaw.
• Cam-Driven: Direct, synchronized power transmission and line contact for sending power to the jaws.
• Rack-and-Pinion Drive: A synchronized drive transmits piston force through a rack, and there is virtually no wear on the drive parts.
There are also numerous finger designs and gripping methods to consider:
• Friction: Contact surfaces that close and stop on the part, creating a frictional force that is utilized to hold the component.
• Cradled: The fingers have a profile of the part, i.e., round to round. The finger closes and stops on the part with the force and shape of the finger generating the grip force.
• Encapsulated: Said to be the most secure means of gripping. The fingers have a profile of the part, i.e. rectangle to rectangle.
The performance of any automated manufacturing system is only as strong and reliable as its weakest link. To ensure that the weak link is not the gripper, a suitable gripper must be specified based on its design and the array of available options. Only when these areas are optimized will the operator know the best gripper for the application has been selected.