Despite the gripper’s importance, engineers who design pick-and-place automation systems for automotive, pharmaceutical, electronics and consumer goods fail to choose the right gripper. There are important considerations when choosing a gripper. Among these are the effects of dirt, oil/grease, cutting fluid, temperature, and the level of human interaction required.
This article highlights the operational characteristics to consider before a successful gripper choice can be made.
More than 95% of grippers are pneumatically powered. Although there have been advances in electric grippers, pneumatic grippers have remained the standard for many years and for the foreseeable future.
Pneumatically controlled grippers are generally used for three basic tasks. The first is to grip and hold a product or component while it is being transferred - for example, from or to a conveyor, workstation, or machine - such as picking up an aspirin bottle off a conveyor belt and placing it into a box.
The second is part orientation, or putting the part or product in the correct position in preparation for the next task, such as inverting the box of aspirin so a label may be applied.
The third is gripping a part during work, such as a robotically mounted gripper holding the box of aspirin while it is being sealed or a label is being applied.
While some applications appear straightforward, effective operation is only assured if the correct type of gripper is chosen for your operating conditions. There are two common classes of operating environments that may require special attention:
Contaminated - In contaminated environments, it is important to keep your gripper protected. Dirt, debris, oil, high temperatures and grease may be present in automotive, foundry, machining and general industrial applications.
Purge ports are available on many grippers. A purge port is an air port on the gripper body that has a channel to the internal mechanisms. Low-pressure air is introduced to keep positive pressure within the gripper housing and prevent contaminants from being drawn into the internal workings. Grease fittings may also be needed as part of a preventative maintenance program in harsh environments to purge dirty grease and/or add new grease.
Scavenge ports are available on many grippers. These ports are often dual purposed like the purge ports mentioned above. The difference is that when the port is used, it can prevent contaminants that may be inside the gripper from escaping into the environment. To accomplish this, a low-level vacuum is applied to the port to create negative pressure and draw the clean air from inside the work atmosphere through the gripper and out of the work cell.
Whether operating in a clean or dirty environment, shielding can increase reliability. Standard or custom-designed shields can deflect debris away from the gripper or help to keep grease and internal containments contained in a clean environment. The shield can be formed-sheet metal shields,covers, flexible boots and bellows, or lip-style wipers. These accessories may be offered with your gripper - either standard, optional, as a special offering, or you can add them as a part of your machine integration. Orientation of the gripper in relation to the direction of contaminants striking the gripper should be considered to help minimize the amount of debris that may contact moving surfaces or openings.
Gripper materials and coatings - such as stainless steel, nickel-plating and hard-coat anodizing can keep surfaces from corroding or debris from sticking and causing gripper binding. In clean-room or food-processing applications, these measures can prevent oxidation or bacteria buildup in the work environment.
Greases can be high-temperature, food-grade or water-resistant - to better handle the environment or any wash-down maintenance requirements. Pneumatic seals help handle extreme temperatures or grit and debris. Buna-N (nitrile) is normally standard, with Viton® and silicone selected for higher temperatures. Metal seals may be available on models for extreme heat and/or contamination.
Basic gripper design and construction can impact the performance in any operating environment. A gripper consists of three basic parts: body (including means of power transmission), jaws and fingers.
Generally, the gripper manufacturer designs and builds the gripper’s body and jaws - known as the “mode of actuation” - with the machine builder or end user supplying the custom fingers to grip or encapsulate the part. Consider the appropriate finger length, grip force, stroke, actuation time, and accuracy specifications that the gripper manufacturer provides.
Operating environment plays a significant role in determining the right gripper design. The jaw-support mechanism (bearing type) can have an impact on function. The internal design (means of power transmission from piston to jaw) can also have an impact. Grippers may be the same size and perform the same function, yet have different designs, with some being better depending on the environment.
Common jaw-support mechanisms include:
The mode of power transmission, or general design of the gripper mechanism, should also be considered. Some popular designs include:
Rack-and-Pinion Drive- Designed for high-precision, clean environments. The 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, including:
When considering gripper finger designs, safety is the most important aspect. In the event of power failure (loss of air pressure), there are other means of preventing a part from accidentally releasing from the gripper and causing bodily injury or machine damage. An internal spring may be an option to bias the piston and maintain finger/jaw position on or around the part, but take care 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.
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 your grippers, pay attention to the operating environment and the gripping options available, including possible custom solutions from manufacturers.
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