Control valves are required in
pneumatic and hydraulic systems to direct and regulate the flow of fluid from
compressor or pump to the various load devices. Relief valves are infinite
position valves. Finite position valves are used for passage or blocking of
fluid flow. There can be either two or four parts for control valves. The three
major types of control valves are poppet valves, rotary valves and spool valves.
Simple discs, balls and cones are used in poppet valves. The two varieties of
poppet valves are 2/2 simple valve and 3/2 valve. Poppet valves are economical
but require large operating force. Rotary valves are used for hand operation in
pneumatic systems. They are compact and have low operating forces. Spool valves
are constructed with a spool moving horizontally within the valve body. The
spool can be easily moved as there is no net force acting on the spool due to
system pressure. There are two types of spool valves. They are two-way spool
valve and four-way spool valve. Spools are shifted during the operation of
spool valves. For this, a button, lever or a solenoid is used.
