Figure 1-20.-Magnetic brake assembly.

The lowest leaf of brush contact 6 should just barely

contact. The wiping action keeps the surface bright and

touch contact 5. If the lower leaf hits the plate too soon,

clean, and thus maintains a low contact resistance.

bend the entire brush assembly upward slightly.

The contactor is operated by connecting the coil (2)

The contact dimensions should be measured with

directly across a source of dc voltage. When the coil is

the contactor in the OPEN position.

energized the movable armature (3) is pulled toward the

Refer to the manufacturer's instruction book when

stationary magnet core (4). This action causes the

making these adjustments.

contacts that carry current (5, 6, 7, and 1) to close with

a sliding action.

The main contacts (5 and 6), called brush contacts,

ELECTRIC BRAKES

are made of thin leaves of copper, which are backed by

several layers of phosphor bronze spring metal. A silver

An electric brake is an electromagnetic device used

brush arcing tip (7) is attached to the copper leaves and

to bring a load to rest mechanically and hold it at rest.

makes contact slightly before the leaf contact closes.

Aboard ship, electric brakes are used on motor-driven

The stationary contact (5) consists of a brass plate,

hoisting and lowering equipment where it is important

which has a silver-plated surface. Since the plating

to stop the motor quickly. The type of electric brakes

lowers the surface resistance, the contact surfaces

used depends on whether the motor is ac or dc and

should never be filed or oiled. If excessive current

whether a dc motor is series or shunt wound.

causes high spots on the contact, the high places maybe

AC SOLENOID BRAKE

smoothed down by careful use of a fine ignition-type

The magnetic brake assembly shown in figure 1-20

file.

is the main component of this electric brake. When the

You can check the operation and contact spacing by

coil is energized, two armatures are pulled horizontally

manually closing the contactor (be sure the power is off).

Figure 1-20.-Magnetic brake assembly.

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