Relay valve

Relay valve

A relay valve is an air-operated valve. It is most-commonly used in air brake systems to control the brakes at the rear of a trailer in a tractor-trailer truck.

It would take too long for sufficient air to travel from the brake pedal valve to the rear of the trailer to operate the brakes in conjunction with the other brakes. Therefore an air supply is stored at the rear of the trailer whenever the emergency brakes are released. When the driver presses the brake pedal a small amount of air opens the relay valve which directs air from the local source directly to the rear brakes.

An emergency relay valve works on the same principle as a standard relay valve. The emergency function in it acts if a major drop in air pressure occurs from the coupling. In this scenario the air passes from the park system to the service line thus allowing the vehicle to stop safely, instead of all the air being dumped and the trailer park springs applying.

A relay valve is analogous to a pneumatic transistor.


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  • relay valve — See ABS Relay Valve …   Dictionary of automotive terms

  • ABS relay valve — An electrically controlled valve which modulates the air pressure in the ABS …   Dictionary of automotive terms

  • valve — A device used to either open or close an opening to allow or prevent the flow of a liquid or gas from one place to another. There are many different types. See ABS relay valve accumultor valve air valve carburetor air control valve air gulp valve …   Dictionary of automotive terms

  • valve relay — lempinė relė statusas T sritis fizika atitikmenys: angl. vacuum tube relay; valve relay vok. Röhrenrelais, n rus. ламповое реле, n pranc. relais à tubes électroniques, m …   Fizikos terminų žodynas

  • Relay cylinder — In a variable expansion central valve engine, a small auxiliary engine for automatically adjusting the steam distribution to the load on the main engine. [Webster 1913 Suppl.] …   The Collaborative International Dictionary of English

  • relay — [1] An electro magnetically operated switch used to make and break the flow of current in a circuit. As low amperage current is passed through one circuit of the relay, it switches and permits a greater current through another line. For example,… …   Dictionary of automotive terms

  • vacuum-tube relay — lempinė relė statusas T sritis fizika atitikmenys: angl. vacuum tube relay; valve relay vok. Röhrenrelais, n rus. ламповое реле, n pranc. relais à tubes électroniques, m …   Fizikos terminų žodynas

  • pilot valve — noun : a relay valve that controls the operation of another valve …   Useful english dictionary

  • Relief valve — Relief Re*lief (r? l?f ), n. [OE. relef, F. relief, properly, a lifting up, a standing out. See {Relieve}, and cf. {Basrelief}, {Rilievi}.] 1. The act of relieving, or the state of being relieved; the removal, or partial removal, of any evil, or… …   The Collaborative International Dictionary of English

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