Get a Grip
Grippers are very flexible. They can be applied to such activities as part orientation, machine loading and assembly.
Because of this flexibility, you’ve decided that grippers will be a part of your assembly system. However, there are various gripper types and styles, and gripping methods. Therefore, it is imperative that the correct gripper is chosen for the application. What criteria should be used when making this decision? This handbook will review the fundamentals of grippers, describe their features and operating characteristics, and discuss how grippers are used in assembly applications.
What Is a Gripper?A gripper is a device that grasps, retains and eventually releases a workpiece. It can hold, tighten, handle and release an object just like a human hand. It can be attached to a robot or be part of a fixed automation system. Because this component handles the workpiece, the gripper must be selected and applied carefully.
The right gripper can provide design flexibility and optimizes the sizing of other system components. Good grippers are easily installed, require low maintenance, and provide many trouble-free cycles. However, if an incorrect gripper has been applied, the user will find that other system components must be sized to accommodate the gripper. Installation and setup will be troublesome, and constant maintenance will be required. Low cycles will be common, and more system downtime will ensue.
Grippers can be the most design-intensive component of an automation device. Building a special gripper to handle a specific workpiece is expensive and time-consuming. However, many applications can use a standard gripper. And a standard gripper, when properly tooled, can perform as well as a special gripper.
Standard grippers comprise many styles and sizes. Their drive systems are either pneumatic, hydraulic or electric. The drive system powers the gripper and forces the jaws open and closed. This opening and closing motion allows the gripper to pick up and release parts.
Gripper TypesThere are three primary types of standard grippers: parallel, angular and concentric. The types vary according to the motion of the gripper jaw in relation to the gripper body.
With a parallel gripper, the gripper jaws move parallel to the gripper body. The jaws come together at the same point when closed, and have external and internal gripping applications. Parallel grippers can pick up numerous part sizes with one set of jaws. However, parts to be gripped must be located accurately in the same place each time. Parallel grippers are typically more accurate than angular or concentric grippers and are the most commonly used grippers.
When part location varies, an asynchronous parallel gripper is a better choice for use. The jaws on this gripper move independently of each other, making it suitable for applications where parts shift position, such as on a conveyor. These grippers do not shift parts to center, as do synchronous parallel grippers. Instead, the fingers conform to the part’s diameter. Parts picked up off-center will be released off-center.
Parallel grippers come in two basic technologies—direct-acting piston and piston-wedge—and offer external and internal gripping actions.
The jaws of an angular gripper open and close around a central pivot point. They move in a sweeping or arcing motion and have external gripping capabilities. Angular grippers are often used when space is limited or when the jaws need to move out of the way. They are usually dedicated to picking up only one part type. Therefore, angular grippers are not normally used for positioning or placement of many different parts. As the most simple of the gripper designs, angular grippers are usually the least expen-sive. Angular grippers come in two basic technologies—single-acting spring return and double-acting spring assist—and offer external gripping actions.
Concentric grippers are best suited for round parts. This gripper has external and internal gripping actions, and the force remains constant throughout the stroke. It has a high grip-force-to-size ratio and a higher moment capacity compared with other designs. A center bore can be added to increase the flexibility of the gripper. The basic technology for this type is piston-wedge and direct-acting piston.
Gripper StylesCustom jaws are needed for each application, because workpieces come in various shapes and sizes—flat, round, convex and concave. Jaws are what actually make contact with the part. Careful consideration when designing these jaws can greatly reduce the size and grip force needed for the application. The key to using standard grippers is to consider the part geometry and to tool the jaws to correctly grasp the part.
There are two primary methods that gripper jaws can hold a workpiece. The two-jaw gripper is the most popular style. This gripper—either angular or parallel—provides two mounting locations for the fingers to contact the part. The jaws move in a synchronous motion, opening and closing toward the central axis of the gripper body.
The three-jaw gripper is a more specialized style. These include parallel and concentric grippers and provide three mounting locations for the fingers that contact the part. The jaws move in a synchronous motion, opening and closing toward the central axis of the gripper body. Three jaws provide more contact and accurate centering with the part than two-jaw grippers do.
Gripping the PartThere are two actions that two- and three-jaw grippers use to hold workpieces. One method involves squeezing the part. This is called a friction grip. With a friction grip, the jaws rely on the force of the gripper to hold the part. The gripper’s squeeze does all the work.
The other method is an encompassing grip, which captures parts between the jaws with little or no squeezing. The jaws bear the weight of the workpiece—adding stability and power, and reducing the necessary grip force. However, the additional jaw travel required to encompass or retain the workpiece must be considered when choosing this type.
An encompassing grip also provides a major advantage, because the jaws must be driven open for a workpiece to be dropped. In the case of the friction grip, the jaws don’t have to be moved significantly for the workpiece to fall. The rule of thumb is that a friction grip requires four Arial the force to handle the same workpiece as an encompassing grip.
Grippers also have two different holding options—external and internal. The geometry of the workpiece, the process to be performed, the orientation of the workpieces and the physical space available determine the option used.
External gripping is the most common way to hold workpieces. The closing force of the gripper is used to hold the part.
Internal gripping is used when the workpiece geometry is appropriate and when the process to be performed needs access to the workpiece’s outside surface. The opening force of the gripper holds the workpiece. Certain jobs require grippers to grasp the inner diameter of a workpiece. Many Arial, these parts have painted or plated surface finishes that cannot be compromised. Also, inner diameter gripping works for handing workpieces to another gripper or lathe chuck that is gripping the outer diameter.
Force and Torque ConsiderationsEngineers typically select grippers based on the force they must apply to the whole workpiece. Jaw style plays a major role in determining the force required in a gripper application. Another critical factor in determining the gripper force is the weight of the workpiece that the gripper experiences from gravity and acceleration. Often, particularly in robot applications, the acceleration that the robot imparts can be three or four Arial that which gravity imparts. Thus, both weights must be considered when determining the gripper force.
Speed means nothing to a gripper. Acceleration and deceleration are important. The faster you start and stop, the greater the force.
With grippers, 10 pounds of force from each jaw deliver a total of 10 pounds of force to the workpiece. Essentially, half the force provided by each jaw is used to counteract the other jaw.
Another rule of thumb is to keep tooling fingers as short as possible. Gripping force decreases when jaw length increases. This is caused by friction on the bearing surfaces resulting from deflection of the tooling fingers.
While force is a major consideration, the torque that is experienced by the gripper is equally important. There are two sources of torque. The gripper can generate torque on itself, or it can be generated by the part’s acceleration and weight. This means that long jaws are often required. Either the part is bulky, or it must be held at a distance to fit in a machine. In either case, the longer the jaw, the greater the torque the gripper imposes on itself.
The next task is to determine the torque the gripper will experience from the workpiece. Workpiece torque is essentially acceleration Arial workpiece weight Arial jaw length. The total torque that the gripper will see is the addition of the jaw torque and the workpiece torque.
Designing and Maintaining the Gripper Design criteria for grippers are demanding. Grippers must be strong and durable. They are susceptible to damage or distortion due to robot programming errors, stuck parts or crashes. They must be as light as possible, because every pound of gripper weight is a pound less of payload that can be handled by the robot. They must have dimensional stability and be able to hold the workpiece orientation under gravitational forces. The inherent repeatability of the robot is meaningless if positional accuracy is lost in the gripper. They must often have some built-in compliance or automatic alignment capability to accommodate positioning tolerances. They must be fast-acting. Clamping and unclamping motions always add to the work cycle and directly affect production rate.
To successfully handle long workpieces, a gripper must have superior bearing support to accommodate larger movements and cantilevers. Basically, the contact surface of each jaw is extended to stabilize the workpiece in motion and balance the long jaws.
Reduction of jaw weight is an important objective in gripper design, particularly for high-speed applications. One way to minimize jaw weight without sacrificing strength is by using a lightweight material, such as aluminum. Another option is to narrow the gripper jaws. This reduces weight and helps avoid interference each time the jaws open to release a part.
For multiple workpiece handling and applications that require specific grip forces, jaws that adjust to a gripper’s stroke provide the most flexibility.
Jaw alignment is critical. If they do not meet accurately, the part’s position and gripper-holding power are greatly compromised. To help ensure proper alignment, dowel holes and/or a surface to key on are a must.
Preventive maintenance is essential but simple. Gripping surfaces wear. Sliding bearing surfaces are subject to foreign material buildup and damage. Linkages loosen up. Regular lubrication may be required, and worn workpieces should be replaced. This is particularly true of the gripping surfaces. They should be designed for easy replacement.