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Machine Design

Electric brake relies on tuned transformer for long life and reliability

Electric brakes on cranes are routinely jogged from forward to reverse, and they must work despite hard use and dirty environments. But the heat they generate and how they're operated causes frequent breakdowns and maintenance stand downs. To solve this problem, Bob Snyder, a crane repairman for 40 years, invented and patented a dc brake that eliminates most of the flaws in ac and dc brake designs.

Like other electric brakes, his mounts on the motor shaft and uses from one to three friction discs compressed by springs against a steel plate to set the brake. But for durability, the toothed-wheel mounted on the motor shaft is steel rather than cast iron and it is fitted to the shaft by a tapered hub rather than a spline. This eliminates the excessive backlash splines tend to develop after many reversing cycles.

The electrical portion of the brake, what Snyder calls a tuned transformer, consists of eight magnet coils in two parallel sets of four, each supplied from a rectifier's 110 Vdc. Larger brakes are also possible. The largest to date uses 24 coils and a 16-in. friction disc. The high-impedance coils draw only 1 A from a relatively low-rated control transformer. Miniature plug-in double-pole relay switches connect the rectifier and brake coils. The rectifier's coil connects to the transformer secondary.

The low-rated transformer gives the circuit less residual flux (or stored energy) when the brake is de-energized. The low level of flux, combined with the small relays, reduces the usual delays in setting the brake. The low current levels reduce coil heating, and extend the life of the relays. Aluminum heat sinks between the coils keep temperatures below 180F. A cooling fan pushes filtered air through the brake, further cooling the coils and friction discs.

The brakes have been field tested on cranes and are proven to be less noisy thanks to the use of dc voltage. The brake generates less heat because of the heat sinks, fan, and the fact it uses less electricity (less than 1 A to release). There is also no delay when closing the brake, a safety factor on cranes that carry tons of steel. And compared to ac brakes, there is no chattering between armature plate and coil-plate irons.The brake comes in two standard sizes, a 72-lb version that handles 5 to 15-hp motors and loads of 5 to 15 tons, and a 26-lb version that handles motors up to 5 hp and loads to 5 tons. But the brakes can be made to handle any sized motor and tonnage.

TAGS: Technologies
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