To get an idea of the exigencies involved in robotic handling of large, heavy parts, run out to the grocery store and buy a half-gallon of milk.
Got it? Now, put it on the counter and then, with your arms straight out in front of you, squeeze the carton from opposite sides and try to pick it up. Sorry, you can't use the handle. Not too easy, is it? Especially if the carton is slippery with condensation.
"If you can get your fingers underneath the carton or through the handle, it's a whole lot easier," says Daniel Peretz, director of sales at De-Sta-Co Industries (Madison Heights, MI).
The same principal holds true for a six-axis robot trying to lift and position a large, heavy object, such as an automobile wheel, with a pneumatic gripper. If the gripper can encircle the part or capture some feature, less force is needed to hold the part and it's much safer for nearby people and equipment.
Gripping big, heavy parts is not like gripping small, lightweight parts. The most obvious difference is that grippers for large parts are, well, bigger.
Consider the IPW-200 parallel gripper from IPR Automation (Weston, CT). It weighs 154 pounds, it has a 7.87-inch stroke, and it produces a maximum gripping force of 1,410 pounds. The gripper can be equipped with long fingers for handling wide, heavy parts, such as engine castings. The jaws are actuated by a pair of large-bore pneumatic cylinders. Each cylinder is supported by four stainless steel shafts, which enable the gripper to tolerate high moment loads. The jaws can be configured for nonsynchronous operation, which allows the fingers to center on the part instead of the centerline of the gripper.
And that's not even the biggest gripper IPR Automation makes. The RA55 angular gripper, which holds parts during sawing and deburring operations, produces a gripping force of 6,000 pounds. "It's a beast!" declares Nicky Borcea, applications engineer with IPR Automation. "It's like a boat anchor."
Grippers for large parts are not simply scaled-up versions of small grippers, says Jesse Hayes, product manager for Schunk Inc. (Morrisville, NC). Whereas the jaws of small grippers are usually actuated by a single piston through a cam or wedge mechanism, the jaws on a big gripper are more often actuated directly by individual pistons.
"The direct-acting piston design provides a long stroke for capturing wide parts," says Hayes. "It also reduces the overall weight of the gripper. With big parts, the total payload of the robot is an issue. We try to keep the weight of the gripper as light as possible, to reduce the size of the robot needed for the application."
Another difference between big grippers and small ones is that big grippers need much more support for their jaws. Ordinarily, gripper designers like to keep the grip point of the fingers as close to the gripper face as possible. However, that's not always possible when handling large parts. As a result, the jaws on large grippers must be supported by heavy-duty bearings. "Think of a bolt cutter," explains Peretz. "The handles are very easy to move, but they generate an enormous amount of force at the cutting point. The fingers of the gripper are like those long handles, so you must have very strong support where those fingers are attached to the gripper."
Safety is a major factor in the design of grippers for big parts. If a small gripper drops a little plastic part, the worst that could happen is that the robot or machine might skip a cycle. But, if a robot drops a 40-pound aluminum casting, someone or something is going to get hurt.
As a result, large grippers are equipped with various safety devices to keep the jaws locked on the part in the event of a sudden loss in air pressure. One such device is a check valve, which traps air in the cylinder if air pressure fails. Another option is a spring-loaded brake, which prevents the piston rod from moving after a pressure loss.
"One concern with check valves is that they can leak, so they're not ideal for long-term safety," says Peretz. "However, no safety device is foolproof."
Unlike grippers for small parts, large grippers cannot rely on friction to hold heavy parts, says Hayes. The fingers must maintain a positive grip on the part. Fingers can be curved to encircle the part, or they can be notched or grooved to capture specific features on the part. In some cases, fingers can be inserted directly through holes in the part.
Fingers can be coated with plastic or rubber to prevent damage to finely machined surfaces.
The additional "real estate" afforded by large grippers gives engineers room to add helpful technology. For example, the XRAY grippers from Zaytran Inc. (Elyria, OH) are equipped with sensors that measure the size of the part with an accuracy of ±0.125 millimeter. This enables the gripper to provide data for rejecting parts or for statistical quality control. The gripper's control system includes two set-point relays that allow the gripper to generate emergency stops or flags at two user-definable positions. Standard models provide a grip force of 280 or 560 pounds. Stroke length ranges from 100 to 500 millimeters.
On the other hand, when designing robotic handling systems for large parts, the trade-offs between cost, power and capability can be significant, warns Peretz. For example, a gripper can be designed to handle wheels ranging from 13 to 32 inches in diameter. "However, that's a huge stroke differential, and that's going to affect cycle time," he says.
Pneumatic grippers are not always the best end-effector for gripping large parts. For applications requiring very high gripping force, hydraulic grippers may be necessary. Vacuum cups are an alternative to grippers for parts that are bulky, but not too heavy, such as windshields and sheet-metal stampings. "For some applications, we've used multiple gripping techniques: vacuum cups, power clamps, and locating pins," says Borcea.
Similarly, a six-axis robot may not be the best choice for handling the parts. For very heavy loads, a custom-built gantry system may be a better option.