There are various operational characteristics that must be considered before an educated—and successful—gripper choice can be made. Article by Gary Labadie, Destaco.
In the world of manufacturing, the ability to consistently get—and maintain—a good, reliable grip can be the difference between operational success and failure. However, the engineers who design pick-and-place automation systems used in such diverse industries as automotive, electronics and consumer goods, often give inadequate attention to the most suitable type of gripper to use with their system. There’s a vast array of gripper styles available, and engineers are designing systems that can have thousands of parts. Often, convenience, familiarity or a generalised end-user specification contribute to a less-than-optimal decision.
There are many considerations that should be addressed when choosing a gripper. Among these are the effects that dirt, grit, oil, grease, cutting fluid, temperature variation, cleanliness and the level of human interaction can have on the operation of an automation system. It is not enough to arbitrarily choose a gripper from off the shelf or from the pages of a catalogue.
Know Your Operating Environment
Although there have been some advances made in the design and operation of electric grippers, pneumatic grippers have been the standard for many years and will continue to be the majority for the foreseeable future. In fact, more than 95 percent of the grippers in use in today’s automated manufacturing environment are pneumatically powered.
Pneumatically controlled grippers are generally used for three basic tasks: for gripping and holding a product or component while it is being transferred, for example, from or to a conveyor, workstation, machine; for part orientation, or putting the part or product in the correct position in preparation for the next process; and for gripping a part while work is actually being done. While these tasks would appear to be straightforward, their effective operation is only assured if the correct type of gripper is chosen for the operating conditions.
There are two common classes of operating environments that may require special attention:
Contaminated: Characterised by an environment with high levels of dirt, debris, oil, grease, or higher temperature variations. These environments are common in automotive, foundry, machining and general industrial applications.
Clean: In this type of environment, the focus is on keeping anything on or in the gripper from being released into the work environment and contaminating the part or process. This is common in the medical, pharmaceutical, electronics and food-production industries.
Whether operating in a clean or dirty arena, shielding can be an effective means of increasing reliability. Standard or custom-designed shields can deflect debris away from the internal workings in a dirty environment, or help to keep grease and internal containments contained in a clean one. Gripper materials and coatings such as stainless steel, nickel-plating and hard-coat anodizing can also keep surfaces from corroding or debris from sticking, which can eventually cause binding.
Gripper Design and Environmental Suitability
Basic gripper design and construction can also have an effect on the performance in any given operating environment. A gripper consists of three basic parts: body, jaws and fingers. Generally, the gripper manufacturer only designs and builds the gripper’s body and jaws, with the machine builder or end user supplying the custom fingers to grip or encapsulate the given part. When selecting a gripper, considerations for any application should include appropriate finger length, grip force, stroke, actuation time, and accuracy. The manufacturer normally publishes these specifications for any given gripper model and need to be followed.
Again, specific operating environments will play a significant role in determining which type of gripper design should be considered. The jaw-support mechanism (bearing type) can have an impact on function. The internal design (means of power transmission from piston to jaw) can have an impact, as well. Simply put, various grippers may be the same size and perform the same function, but can have completely different designs, with some being better than others for differing operating environments.
The mode of power transmission, or general design of the gripper mechanism, should also be contemplated. Some examples are double-sided wedge, direct drive, cam driven, and rack-and-pinion drive. There are also numerous finger designs and gripping methods to consider: friction, cradled, and encapsulated.
When considering finger design, safety should always be paramount. In the event of power failure (loss of air pressure), there are other means of preventing a part from accidentally being released from the gripper and potentially causing bodily injury or damage to part or machine. An internal spring may be an option to bias the piston and maintain finger/jaw position on or around the part, but care must be taken to ensure the spring force is adequate. External fail-safe valves can be added to the ports to check air to the gripper in the open or closed position. Some gripper styles allow for rod locks that automatically clamp on the guide rods of the jaws when air pressure is lost.
Designers and engineers who don’t give proper attention to gripper selection may eventually need to be told to ‘get a grip’ when considering their choices. This demand can rise when the performance of an automation system is compromised because the proper grippers were not chosen and unsatisfactory operation ensues.
The performance of any automated manufacturing system is only as strong and reliable as the performance of its weakest link. To ensure that the weak link is not the gripper, strict attention must be paid to the operating environment and a suitable gripper specified based on gripper design and the array of options available, including possible custom solutions the manufacturer may be willing to offer. Only when these areas are optimized will the operator truly know that the best gripper for the application has been selected.
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