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Carbon Brushes (May 2012): Rubber Hard Tops & Metal Plates

May 29, 2012
What is the difference between a

metal hammer plate and a

rubber hard top?

One of the more common pieces of hardware found on brushes is the metal plate or rubber top. The purpose of this ‘hammer plate’ is to provide a mechanical contact point for the brush holder spring or arm that holds the brush in place during commutation.
Metal clips are attached to the carbon by rivets. Depending upon the application design, the shape can be square, rectangular, offset and sometimes with raised edges. Some extend over the front face on the brush while others may have holes drilled through them into the top of the brush. Still others have lifting clips attached.
Rubber or mica hard tops may or may not be recessed into a depression that has been machined into the top of the brush. Rubber tops are glued to the carbon(s). These contact tops have been in use for over 30 years. The common shapes are square, rectangular, convex, concave and channeled (raised edges).
Some manufacturers prefer the rubber hard tops because they believe it’s a better shock absorber than the metal plate. Still others claim it provides better spring insulation. However, many motor manufacturers continue to use the time-tested metal plates. The bottom line is that they are industry interchangeable as long as they conform to manufacturer specifications.
For more examples please click on the Carbon Brushes link above. Choose the Carbon Brush Reference link and then, scroll down to Metal Hammer Plates & Mica and Rubber Hard Tops. If you have any application questions please call and ask for Scott Tussey.

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