Take a walk down the aisles of any hardware or housewares store, and you can't help but notice the growing popularity of thermoplastic elastomer (TPE) grips. TPE's tactile feel, matte appearance, moldability, and heat resistance have already made it the "soft-touch" material of choice for power and hand tools, appliances, kitchen tools, and other products. And thanks to the tactile characteristics of TPE, these soft-touch grips are invariably described as "ergonomic." Yet more often than not, there's no attempt to quantify this term--as if all ergonomic designs and materials are created equal.
| The lower the muscle activity—which is expressed in as a percent of the test subject’s maximum contraction capability or MVC—the more efficient a material from an ergonomic standpoint. One of the Santoprene TPE grades came in with the lowest mean muscle activity, while the bare metal knob chalked up the highest values. The muscles studied were the forearm extensor muscles, and two muscles in the hand—the adductor pollicis (AP) and the Interossei/lumbvricale (LUM).
Of course, competing designs are rarely equal, and the number of soft-touch materials available to the design engineer appears to grow almost daily. For these reasons, we believe that design engineers would benefit from an objective measurement system for grip ergonomics, one based on quantifiable measures such as 15% less muscle activity or 40% reduction in stressful wrist postures.
| “Feeling tone” ratings try to capture subjective notions of what makes a good grip material. Ratings were based on questionnaires filled out by study participants prior to testing, after the torque tests, and after the pinch tests. In each case a TPE, one of two Santoprene grades, scored the highest mean rating.
In what could serve as a model for future ergonomic assessments from a materials perspective, Ergonomic Technologies Corp. (Oyster Bay, NY) recently evaluated how well bare metal, polypropylene, and various Santoprene rubber TPE grades performed in two simulated grip designs: a knob and a sheet. The evaluation determined the effectiveness of each material in terms of the muscle effort exerted by 20 test subjects as they first twisted, or applied torque force, to the knob and later held the sheets in a pinch grip while counteracting a known weight. Our guiding principle: The more ergonomically effective the material, the lower the muscle effort required for a given operation.
Measuring ergonomics. For the torque test, the test subjects used a fist-like "power grip" to grasp the knobs while attempting to register a maximum torque on the tester. For the pinch test, the subjects held each sheet, fitted with5- and 10-lb weights, in a pinch grip with their arms hanging straight down for four seconds. All the tests were "randomized" both in the initial selection of the subjects and in the order in which the various grip samples were presented during testing.
To get a handle on muscle effort, the study relied on a technique called surface electromyography (EMG), which measures electrical activity on the skin surface as the exertion of an underlying muscle group triggers biochemical changes. Ergonomics Technology reassured those muscle sites responsible for pinch and power grips--the dominant arm extensor muscle, the adductor pollicis (AP) at the base of the thumb, and the dorsal interossei/lumbricale (LUM) area between the thumb and index finger.
Data from the EMG tests are expressed as a percent of maximum voluntary contraction (MVC)--that is, each subject's strongest "squeeze" as measured by grip and pinch dynamometers. When interpreting ergonomic results, bear in mind that a maximum value of 15% of MVC is typically considered an acceptable threshold for muscle activity levels during repetitive activities of at least six repetitions per minute.
Getting all this ergonomic data in hand did take some time. Each subject required about an hour to run through the physical tests and the battery of questionnaires. And all the data had to be statistically analyzed for significance and correlation. But as the marketplace continues to favor ergonomic designs and materials, tests such as these will come in handy, to say the least.
| Once again, less muscle activity is better. And here, too, Santoprene TPE outperformed polypropylene both in tests with 5-lb and 10-lb suspended weights. It’s worth noting that eight out of the twenty subjects could not hold the 10-lb suspended weight with the polypropylene sheet during the four-second testing period. Four additional subjects felt that the weight was slipping with the polypropylene sheet. Two out of twenty subjects could not hold the 10-lb weight with the Santoprene 8211-75 sheet, and one subject felt that the weight was slipping with it.
Goran Pesevski is a technical specialist for Advanced Elastomer Systems, which sells its Santoprene rubber in a variety of grip applications. Eugene Ylo is a territory manager for the same company.
Ask the experts
Materials selection is one of the most important aspects of a design engineer's job. And it is among the most complex. One way engineers learn more about materials is to find out how other design engineers have solved their materials problems. To help facilitate this dialogue, we invite readers to submit questions to our Design News Engineering Plastics e-mail newsletter.
We received this request for help from Rick Stevens, Project Engineer, Cooper Engineered Products Division:
"I am looking for a plastic material to replace a (relatively) expensive ceramic pad printer doctor ring. While I don't expect to find a plastic alternative with the same wear life as ceramic, the cost of a disposable ring may be able to offset the reduced life."
Readers came up with a variety of solutions, including the following:
Doug Garcia of Electro Scientific Industries Inc. notes: "You might try Kynar. We've been using it for the very reasons you mention, the chemical resistance approaches Teflon and the wear/bearing properties are similar to nylon."
John A. Nauman, of the Boeing Commercial Airplane Group, suggests, "Try Arlon (trade name for PEEK) made by Greene, Tweed and Co. It is wear resistant, has good machinability properties and is injection molded. Or Delrin (trade name) made by Dow Chemical Co."
Nigel F. Misso offers: "I would suggest PPS (polyphenylsulfide), injection molded, with no machining. A grade highly loaded with mineral and lightly loaded with some glass or carbon or arimid fiber."
"Without knowing the specifics of the application, material selection is very difficult. However, LNP Engineering Plastics Inc. in Exton, PA manufactures a line of plastic materials, LUBRICOMP, specifically designed for wear and low friction," suggests Alan Levin of Hayward Industrial Products.
Larry Dunham encourages Stevens to "...try phenolic! It's dimensionally stable, somewhat machinable, and great for cost. You may need to insert mold threaded inserts but look at automating this function."
Finally, Bill Mina of EGS notes, "Try the Ryton material family. Good chemical resistance/some compounds have a Teflon with very low friction coefficient, injection moldable, good machinability, and the price is reasonable."
For more help on plastics, go to www.designnews.com to the Plastics channel and look for the Ask the Expert section.