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Motion System Design

Clever cable keeps servomotor clean and dry

It's no secret that motors and moisture don't mix. That said, in certain extreme environments, spraying water, high humidity, and excessive moisture are unavoidable. One such application is a sheet-metal forming method known as deep drawing — a multi-die technique of forcing flat sheet metal into tubular shapes, for use as doorknob trim pieces and one-piece electrical enclosures, for example.

In one recent application involving the forming of gas tank filler necks, servomotors were used in a deep drawing setup with coolant flooding the dies and everything else. Motors typically survived for two or three weeks and would then short internally from coolant seeping inside. Although the motors were equipped with a shaft seal kit, when the motor was shut down, the subsequent cooling created a negative pressure inside the motor case that drew in moisture.

Then the servomotor manufacturer's factory support team visited, and proposed drilling a hole in the motor case and installing an air fitting to allow application of positive pressure to the motor's interior. This appeared to be a workable solution, but drilling a hole into the motor case voided the manufacturer's warranty.

Coincidentally, Flex-Cable Inc., Howard City, Mich., was visiting the plant on an unrelated issue and was told in passing about the problem and the end user's dislike of the proposed solution. It was then that the Flex-Cable engineering team began brainstorming for other solutions.

Because most of the servomotor cables that Flex-Cable produces are overmolded, the assembly process involves putting a metal molding adapter onto the back of the connectors used. Most connectors have some clearance through which air is allowed to pass.

Now, a modification to Flex-Cable's adapter allows introduction of dry air into the back of the encoder connector, which makes its way through the cable connector, into the motor connector, and finally inside the motor itself. In this way, dry air or nitrogen under positive pressure is introduced into the motor without voiding its warranty. The initial prototype was installed in early December 2011 and is still running without failure. For more information, visit

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