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Novel materials and purge keep gripper running
Consideration of the customers real needs allows Ted Zajac to extend this gripper
By Brian J. Hogan, Managing Editor -- Design News, March 3, 1997
Elyria, OH--Meetings with customers convinced Ted Zajac Jr., chief operating officer at Zaytran, Inc., that many of his company's customers needed a gripper capable of withstanding harsh industrial environments. Others needed units approved for food contact, or tolerant of deionized (DI) water. In every case, they wanted a gripper that exhibited little or no play in its slide system to permit the use of long jaws.
Although a gripper made from 316 stainless steel could satisfy these requirements, it would be heavy and too expensive. Consequently, Zajac and his colleagues decided that two different grippers made from different materials would be necessary.
In one version of the new gripper, called the 130 Series Magnum®, the Zaytran team designed a one-piece, hard-coated aluminum gripper body. Instead of box slides, rollers or conventional linear bearings, Zaytran machines a rail into the body. Made with proprietary tools to ±0.0001 inch, the hard-coated, PTFE-impregnated rail enhances the gripper's moment-load capabilities. The gripper slides travel on bearing rods trapped between the rail and slide. Most self-lubricated polymers proved unsuitable for the bearing application because of coefficient of thermal expansion (CTE) problems, as well as difficulties encountered in machining or forming the materials.
As it turns out, however, the CTE of polyamide-imide (specifically Amoco's Torlon® 4301) is close to that of aluminum. And it forms well. "Torlon® machines like metal," says Ted Zajac, "and it grinds like glass. It's a beautiful thing when it's done." Parts made from the material can be produced to a tolerance within the ±0.0001 inch Magnum design requirement.
Zaytran made the pistons that drive the gripper's jaws from a partly crystalline thermoplastic polyester based on polyethylene terephthalate (PET-P). Light and easily machined, the material allows broaching of a female helix used in the gripper's jaw synchronization system. That patented system employs a stainless steel helix which rotates to synchronize piston movement, without bearing loads. Gripper force results solely frompneumatically generated piston movement. Quad seals used on the pistons accommodate the difference in CTE between aluminum and PET and prevent either interference or blow-by.
Zaytran developed a polymer version of the Magnum because a requirement for contact with DI water made PTFE-impregnated aluminum unacceptable as a body material. Holidays in the coating always exist. When DI water finds those openings, it immediately begins grabbing ions. "It takes a while," says Zajac, "but invariably the aluminum turns to mush."
Instead, Zaytran decided to use PET as a body material. While PET is not suitable for use at temperatures above 200F, the material machines well, and PET can run as a bearing on itself. Furthermore, the 200F limit meets the known customer requirements, and using an essentially one-material design eliminates CTE as a problem. In this version of the Magnum gripper, PET slides ride on PET bearing rods captured by a rail machined into the PET body.
Finally, in both the aluminum-body and PET-body designs, a purge system allows evacuation or pressurization of the gripper's interior to either draw out debris or push it out of the gripper. Purge air enters through a hole drilled along the full length of the bearing rail. To prevent leakage through the openings that connect the two slide drive bars to the pistons, Zaytran uses PTFE O-rings trapped between the slides and gripper body.
In the Zaytran design, the PTFE O-rings cold-flow to take out tolerances, and also present self-lubricating surfaces to the gripper body when the slides travel. Seals made from more conventional materials would act like brakes when the slides moved. "If the customer runs the purge system at 10 psi, it will work, but they often run it at factory system pressure," says Zajac. The purge system has enabled the Magnum gripper to achieve Class One cleanroom certification.
First marketed in March, 1996, the Magnum gripper comes in versions weighing from 0.28 to 0.56 lb and produces grip-ping forces of 130N (29 lbs) at 100 psi supply pressure. Stroke can vary from 13 to 26 mm. Zaytran intends to offer both smaller and larger versions of the new gripper. In tests under load, the aluminum and polymer versions of the 130 Series demonstrated a life exceeding 10 million cycles.
Additional details…Contact Ted Zajac, Jr., Zaytran Inc., 41535 Schaden Road, P.O. Box 1660, Elyria, OH 44036, (216) 324-2814
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