Grippers are significant in a successful automation system, choosing the right one can lead to optimized performance, uptime and operator safety.
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.
Know Your Operating Environment
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.
Common Operating Environments
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.
- Clean - In clean environments, the focus is on preventing anything on or in the gripper from entering the work environment then contaminating the part or process. This is critical in the medical, pharmaceutical, electronics and food-production industries, where airborne or surface contaminants are detrimental. Many grippers carry a Clean Room rating for operation in a specified Clean Room Classified environment.
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.
Types of Coatings, Greases & Seals
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.
Gripper Design & Environmental Suitability
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
Common jaw-support mechanisms include:
- Plain Bearings (surface contact) - These include flat surface-to-surface bearings and cylindrical (bushing-type) bearings. These bearings are good for withstanding impact loading and provide excellent jaw support. They do not require adjustments over time and can maintain a high degree of accuracy when machined to tight tolerances.
- Roller Bearings (line contact) - These low-friction bearings include cross-roller bearings and Dual V bearings. They can be pre-loaded to achieve high accuracy and adjusted over time to maintain near zero side play. This low-friction design can allow ease of “dialing in” grip force by adjusting air pressure.
- Ball Bearings (point contact) - Very low-friction, which makes them good for precision applications and for operating at very low line pressures where a smooth consistent motion is critical.
Modes of Power Transmissions
The mode of power transmission, or general design of the gripper mechanism, should also be considered. Some popular designs include:
Popular Finger Designs & Gripping Methods
There are also numerous finger designs and gripping methods to consider, including:
- Friction - This is the most common gripping method, with contact surfaces that close and stop on the part, creating a frictional force to hold the work piece. If air pressure is lost, the part will drop. Avoid friction fingers if possible when handling oily or greasy parts. Higher grip forces are generally required (i.e., a larger gripper) when using this method, and special attention should be given to the gripping surface of the fingers. Carbide grips can be added to the face of the finger to improve overall gripping ability, though this may damage fragile surfaces. For fragile parts, urethane pads can be placed on the finger, increasing gripping friction without damaging the part.
- Cradled - Generally considered the most secure means of gripping, encapsulated fingers have a profile of the part, i.e. rectangle to rectangle. In this method, the fingers close and stop on or near the part and rely on the encapsulation to keep the part in position. This is typically the safest design, because if pressure is lost, the part will not drop unless acted on by an external force.
- Encapsulated - In clean environments, the focus is on preventing anything on or in the gripper from entering the work environment then contaminating the part or process. This is critical in the medical, pharmaceutical, electronics and food-production industries, where airborne or surface contaminants are detrimental. Many grippers carry a Clean Room rating for operation in a specified Clean Room Classified environment.
Prioritizing Safety First
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.