CONTROL SYSTEM COMPONENTS

CONTROL SYSTEM ,VALVES (UNIT NO-03)Dr.S.S.KhedkarDept. of Mechanical engineeringYCCE, Nagpur.POWER CONTROLLING DEVICES(VALVES)Vales are fluid power devices used for controlling & regulating working medium(liquid in hydraulic circuit & air in pneumatic circuit).Function of valves:-To start or stop the flow of fluidTo control & regulate the flow rate & pressure of working fluid.To control the direction of flow.CLASSIFICATION OF VALVESAccording to main function asDirectional control valvesFlow control valvesPressure control valvesAccording to construction asPoppet or seat type valves.Spool or sliding valves*According to their position asAn infinite position valve(e.g. Relief valves)Finite position valve (check valve)

DIRECTION CONTROL VALVESThe main function of direction control valve is to control the direction of fluid flow. In hydraulic circuit direction is controlled by check valve, shuttle valve, two way, three way, four way directional control valve. These valve may be manually, electrically or mechanically operated & they are specified by no. of ways & positions.CHECK VALVE (ONE WAY VALVE)Check valve allows free flow of fluid in one direction & fluid flow in reverse direction is blocked. Fluid can flow from P to A by upsetting the poppet or ball due to fluid pressure against spring load but flow in reverse direction i.e from A to P is completely blocked by ball or poppet. This valve is used to control the direction of fluid in hydraulic & pneumatic circuit.

APPA(Flow blocked)2-WAY,2-POSITION DIRECTIONAL CONTROL VALVE (ONE WAY VALVE)This valve has 2 positions & 2-way for fluid flow. The valve has two ports.(Pump port P & Cylinder port A). Fluid comes from port-P & flow out through port-A. In one position the port-P & A are connected as shown in fig. i.e fluid can flow from P to A. When spool is shifted to right side port- P & A are disconnected.

3-way valve has 3-ports & 3 internal passeges. One port is called as inlet port(P) through which fluid enters the valve & second port called as cylinder port (A) through which liquid leaves the valves. The third port is called as exhaust port (R) which exhaust out the fluid. Fig. shows 3-way valve(normally open position). In one position port (P) is connected to cylinder port (A) & exhaust port ( R) is blocked. As the spool is shifted to right side, the pump port (P) gets blocked & cylinder port (A) is connected to exhaust port (R ). These valve are used to actuate single acting cylinderThis valve has 4-ports & 4 internal passages. In one position as shown in fig. when spool are extreme right side, the pump port (P) is connected to cylinder port (A) & cylinder port (B) is connected to exhaust port (R).In other position (Not shown) when spool are shifted to left, the pump port (p) gets connected to cylinder port (B) & cylinder port (A) gets connected to exhaust port (R) . These valve are used in hydraulic & pneumatic system.

4-WAY 2-POSITION D.C. VALVEPressure CONTROL valvePressure control valves controls the pressure in fluid power system. If pressure in fluid power system increases greater than maximum value, it may damage the system or it may cause wastage of power or overheating hence fluid power system are provided with pressure control valve to protect the system from effects of over pressure. TYPES OF PRESSURE CONTROL VALVEPressure relief valves:-The pressure in a system is set and restricted by these valves. The control pressure is sensed at the input (P) of the valve.Pressure regulators:-These valves reduce the output pressure where there is a varying higher input pressure. The control pressure is sensed at the output of the valve.PRESSURE RELIEF VALVE

*Relief valve also known as Safety Valve is used to maintain desired pressure.*When force of fluid becomes greater than spring force, the poppet or ball is released from valve seat and relief valve open. The fluid is released to the reservoir at low pressure. The pressure at which valve start diverting flow to reservoir is called cracking pressure. Relief valve protect the pump and system from over pressure.FLOW CONTROL VALVEFlow control valves work by putting a variable restriction in the flow path. There are three basic types of flow control valves-Plug valveButterfly valveBall valvePlug valveThe plug valve controls flow by varying the vertical plug position, which alter the size of the Orifice between the tapered plug & valve seat. Normally the plug is guided & constrained from Sideways movement by a cage.

BUTTERFLY VALVEIt consists of a large disc which is rotated inside the pipe, the angle determining the restriction. Butterfly valves can be made to any size & are widely used for control of gas flow.

BALL VALVEThe ball has a hole which is rotated inside a machined seat.