MAGM 262

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MAGM 262
Directional Control Valves
Mr. Conrado
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Directional Control Valves

• Used to direct the supply of oil to the actuator
  in a hydraulic system.
• The valve body is drilled, honed and heat
  treated. The inlet and outlet ports are drilled
  and threaded.
• The valve spools are heat treated, ground to size
  and polished, some chrome plated.
• The body and spool are then mated in assembly.
• After assembly the spool is the only moving part.

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Valve Spool

• The spool consists of lands and grooves.
• The lands block oil flow through the valve body.
  
• The grooves allow oil to flow around the spool
  and through the valve body.
• The position of the spool when not activated is
  called the normal position.
• When an open center valve is in the normal
  position, the supply of oil flows through the
  valve and back to the tank.
• When a closed center valve is in the normal
  position the oil is blocked by the valves pool.

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Open Center Hold Position

• In the hold position the pump oil flows into the
  valve body, around the spool and return to the
  tank.
• The oil also flows to the load check valve. The
  passage behind the load check is filled with
  blocked oil. The blocked oil and the load check
  valve spring keep the load check valve closed.
• The spool also blocks the oil in the line to the
  rod end and the head end of the cylinder.

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Open Center Raise Position

• When the spool is moved to the raise position,
  the valve spool blocks the oil flow to the tank.
• The oil is also open to the load check valve.
  The spool in addition connects the cylinder head
  end to the oil behind the load check valve and
  the rod end to the tank return. The load check
  valve prevents the oil in the head end of the
  cylinder from flowing into the pump passage. The
  blocked pump oil flows causes an increase in oil
  pressure.

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Open Center Raise Position

• This increase in pump oil pressure overcomes the
  pressure behind the load check valve and unseats
  it. The pump oil then flows past the load check,
  passed the spool and to the head end of the
  cylinder.
• The oil in the rod end of the cylinder flows past
  the spool and to the tank.

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ISO Symbols

• The basic valve symbol consists of one or more
  envelopes. The number of envelopes used
  represents the number of positions that the valve
  can be shifted.

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Valve Ports

• Here we show the valve ports for attaching the
  working lines. A valve with two ports is
  commonly referred to as a two-way valve. Dont
  confuse this with a two-position valve.
• Valves can have as many position and ports as
  needed. But most commonly valves have one to
  three positions and ports from two to six.
• For example you can have a valve that has two
  positions and but controls three cylinders and
  such three ports.

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Flow Path

• The lines and arrows inside the envelopes are
  used to represent the flow paths and directions
  between ports.

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Three Position Valve

• Here we show thee ISO symbols of the three
  position valve. In the three position valve, the
  center position is the neutral or hold position.
  When the valve is not doing work, the valve is
  placed in the hold position.
• The valve on the top is a closed center valve.
  When in the hold position the center spool blocks
  all oil flow.
• The middle valve is a tandem center valve. When
  in the hold position ports A and B are blocked
  but the valve connects the pump (P) to the Tank
  (T).
• The bottom valve is an open center valve. In
  hold the spool connects all ports to the tank.

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Three Position, Six Way, Open Center, Manual
Control Valve

• This valve is now in the hold position.
• The pump oil flows around the spool to the tank.
  The oil in the cylinder is blocked at the valve
  spool.

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ISO Valve Actuator Symbols
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Check Valve

• The purpose of the check valve is to readily
  permit oil flow in one direction but prevent oil
  flow in the opposite direction. The check valve
  is often referred as a one-way check valve.
• Most consist of a spring and a tapered seat, but
  some are free floating.
• For the valve on the left, when the pump oil
  overcomes the spring tension the seat opens and
  allows oil to flow.
• The valve on the right, when the pressure on the
  implement side becomes less than the pump side
  the seat will open allowing oil flow.

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Check Valve ISO Symbols
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