Grasping the fundamentals of gripper design

[ Penton Business Media • 2011-09-01 ] 

Michael Guelker, product manager, pneumatic actuators, Festo USA, Hauppauge, NY

Identifying the best gripper for the application can be a daunting task given the thousands of different combinations, part numbers and variation in gripper names. Yet there are only three basic types of grippers. Knowing the fundamentals of each can help engineers narrow their choices and speed the design process. 

Paging through a gripper supplier's parts catalog or searching for parts on the Internet can lead a design engineer to an overwhelming selection of components. To find the right gripper for the application as quickly and accurately as possible, this article provides some helpful tips.

Start with the basics
The majority of grippers are variations of three fundamental designs: Parallel, three-finger, and angled grippers. Parallel grippers are just what their name implies—two slides either closing parallel to the workpiece to grip its outside edges, or opening out to put pressure on inside walls. Parallel grippers are by far the most commonly applied design.

Three-finger grippers are used to center the workpiece between the fingers, which are offset by 120-deg. The three fingers slide the workpiece to the gripper's center, holding the object snugly.

Another option is angled grippers, which approach the sides of the workpiece from various angles, for example, 30-deg, 40-deg or 80-deg—visualize mechanical fingers coming out at an angle to hold a basketball. Many standard angled grippers can be adjusted to various angles according to the workpiece.

Angled grippers are suitable for holding larger workpieces and those with odd shapes. They're also used where space is too tight for a parallel gripper to open wide enough to hold the workpiece. A variation of the angled gripper is the radial gripper: The fingers open a full 180-deg—useful where vertical space is limited. Fully open radial fingers can rest just above or below the workpiece and close when it is in position.

Parallel, three-finger, and angled grippers come in a host of sizes for varying degrees of gripping force, which can range from 20 N to more than 6,300 N for high-force models. Closing times can vary from 0.1 to less than 0.001 sec for standard parts with repetition accuracy an average of ± 0.01 to ± 0.05 mm. What's more, suppliers often work with design engineers on custom solutions not accommodated by standard parts.

Specialty grippers
Beyond the traditional parallel, three-finger, and angular types, other styles of grippers exist. For example, bellows grippers are used for internal gripping on the inside walls of fragile workpieces, such as bottles or jugs. The flexible bellows expand, pressing against the workpiece.

A new generation of adaptive grippers is also being developed, such as one gripper based on the mechanics of a fish fin. This robotic gripper's fingers effectively wrap around and adapt to different shapes with flexibility up to 360°. The end effector has an exceptional range of motion and may not need to be changed when different products come down the line. That said, these specialized grippers have fairly limited applications compared to the traditional gripper designs.

Pneumatic versus electric designs
All three of the basic gripper styles are available in either pneumatic or electric versions. Pneumatic grippers are generally less expensive upfront, simpler, lighter, and offer higher gripping forces than electric versions, but electric grippers are somewhat less expensive to operate than their pneumatic counterparts. Even so, today's pneumatic systems offer diagnostic features that help maintenance personnel and machine operators identify air pressure loss, which contributes to higher energy costs in pneumatic systems.

The motion system design process starts with the shape and size of the workpiece plus the available space, and then works up through the gripper and the rest of the mechanical assembly. For example, a lighter gripper requires a lighter handling system, while a heavier gripper requires heftier machine parts to support it.

A cost/benefit analysis can help determine whether a pneumatic or electric device is more advantageous over the machine's lifecycle.

Common variations
Pneumatic grippers are typically designed as double action - either air open or air closed. Variations include springs to ensure that the workpiece remains gripped should air flow stop. Grippers can also come in sealed configurations suitable for harsh conditions, such as protection against drilling emulsion or dust. Other options include long-stroke grippers with strokes from 20 to 150 mm; for long centering strokes, electric grippers are available with strokes to 1.5 m. Sensing and feedback options abound as well. In short, a host of gripping solutions exists for everything from baked goods to automotive parts. The three basic designs - parallel, three-finger, and angled grippers - accommodate most workpiece sizes, available machine space, torque and repetition requirements, and environmental constraints.

SCENARIOS: ROBOTICS FOCUS

New robot safety standard debuts
Did you know that a new robot safety standard is in the pipeline for North America? Much of the work on harmonizing North American standards with those of ISO is already published in the Robotic Industries Assn. (RIA) Draft Standard R15.06-201X Robots and Robotic Devices—Safety Requirements, which clarifies what's in store for the industry.

"This (publication of ISO 10218) means the basic technical requirements are now set for the revised R15.06," says Jeff Fryman, RIA's director of standards development. "Since the ISO standard only looks forward from the date of publication, the issues to be resolved by the R15.06 drafting committee have to do with any guidance for existing robots and robot systems, particularly those that do not meet the technical requirements of the standard's 1999 edition. Unique requirements for the U.S., mostly directed to the end user, must be added, and the annexes updated."

Once ratified, the ANSI/RIA R15.06 National Robot Safety Standard will enable worldwide compliance from one document on robot safety, and also provides new risk-assessment guidance. Find out more at the National Robot Safety Conference, Sept. 19-21, in Knoxville, TN.

Robotic layer-picking offers gripping alternative
Today's beverage suppliers need to be as automated and centralized as possible to offer competitive prices and streamlined retail distribution. Much of this efficiency can be gained by automating tasks such as palletizing and layer picking. Case in point: KUKA Systems Corp. North America, Sterling Heights, MI, recently designed and installed an integrated pallet-building line for a major beverage company. The result? The newly automated line offers a major increase in throughput over manual operations traditionally conducted by forklift operators moving pallets around warehouses.

The KUKA Systems layer palletizing and case-picking line is now in use, helping reduce overall supply chain costs. Such a setup lets the soft drink maker move completed order pallets from a large regional distribution center directly to major retail outlets, shortening the distribution pathway. Previously, these pallets had been assembled manually at small local warehouses, which was both time and labor intensive.

The newly deployed system can process 5,000 cases per hour. It features KUKA's automated, high-speed layer-picking system, as well as its layer-forming and mixed palletizing cell. What's more, handling of the highest demand products is fully automated: This particular cell features an advanced layer-pick head on a robot arm that lifts cases from underneath, reducing the risk of product damage associated with traditional grippers that use clamps or vacuum technology to grab cases from the top or sides. By combining layer-picking and mixed layer-building technology with order sequencing logic, the system handles peak volume while strategically building and staging pallets for loading onto trucks and delivery to stores. 

Source: Festo

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