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The term solenoid brake refers to a group of braking mechanisms which rely on an electric solenoid for their actuation. The brake mechanism usually consists of two or more brake shoes which are pushed against the rotating element by solenoid actuated linkages. Solenoid brakes are typically used to slow and stop rotating equipment including winch drums, domestic washer tubs, and vehicle wheels. There are two basic types of solenoid brake installations — one that exerts braking pressure when power is applied and the other when the power is interrupted. Solenoid brakes may be used on their own or in combination with mechanical brakes.
An electric solenoid consists of a wire coil wound around a hollow core with a spring loaded metal plunger inside. When an electric current is passed through the coil, it creates an electromagnetic field that draws the plunger into the core. When the power is cut to the coil, the spring pushes the plunger out again. This conversion of electromagnetic force into linear movement is the basis of the operation of a solenoid brake. Typically the plunger acts through a series of linkages which move a set of brake shoes against whatever needs to be slowed or stopped.
These sets of shoes vary considerably in design according to the application. Drum winches such as those used in underground mining operations use a set of half round brake shoes which are pressed against the braking collar on the drum. Other solenoid brakes pull a flat disc lined with brake material against a similar flange to stop rotation. The linkage systems employed also differ with some solenoid brake mechanisms using no physical linkages. These systems relying instead on a spring loaded collar being pulled against a brake lining on the face of the solenoid.
The overall operation of solenoid brakes falls into two basic categories: power on and power off. Power-on brakes rely on the actuation of the solenoid to exert braking forces. Power-off systems exert braking force when the power is cut and only release when power is applied to the solenoid. Power-off systems are frequently used on vertical mining hoists transporting personal as a safety measure against power failures.
Solenoid brakes are often included as a safety back up to mechanical braking systems. One such application is the reverse brake lock-out on trailers. These systems use the back-up light signal to activate a solenoid that disengages the trailers brakes while reversing. Many vehicles also feature solenoid brake systems which lock their brakes until such time as the motor is running. This prevents stationary, unattended heavy vehicles from starting to roll inadvertently.