Hydraulic Valves

– Fluid power is mainly managed using control devices known as valves.

– When selecting these valves, it’s important to consider not only the type but also the size, actuation method, and remote-control capabilities.

– There are three primary types of valves: (1) directional control valves, (2) pressure control valves, and (3) flow control valves.

– Automotive Lift Repair Florida Directional control valves determine the path of the fluid through a circuit, such as controlling the motion direction of a hydraulic cylinder or motor. This is achieved using check valves, shuttle valves, and two-way, three-way, and four-way directional control valves.

– Pressure control valves safeguard the system from overpressure, which may arise from excessive actuator loads or valve closure.

– Generally, pressure control is managed through pressure relief, pressure reducing, sequence, unloading, and counterbalance valves.

– Controlling the fluid flow rate is also crucial in various hydraulic circuit lines. For example, actuator speeds depend on flow rates.

– Flow control is managed using flow control valves.

– Non-compensated flow control valves are used where precise speed control is unnecessary, as flow rate varies with the pressure drop across the valve.

– Pressure-compensated flow control valves adjust automatically to changes in pressure drop, maintaining a constant flow rate.

Hydraulic Valves Usage Example:

– In a Automotive Lift Repair Florida welding machine application, a directional control valve, a check valve, and a sequence valve are used in a hydraulic circuit for positioning and holding parts during welding.

– This application requires a sequencing system for quick and precise holding of parts.

– A sequence valve is placed in the line to the second hydraulic cylinder.

– When the four-way directional control valve is activated, the first cylinder extends to complete the “positioning” phase.

– Oil pressure then builds up and surpasses the sequence valve setting, opening the sequence valve to allow oil flow to the second cylinder, completing the “hold” phase.

– The check valve allows the second cylinder to retract with the first cylinder when the four-way valve is shifted to allow oil to flow to both cylinders’ rod ends.

Directional Control Valves:

– Directional control valves manage the direction of flow in a hydraulic circuit.

– Each valve has ports, which are external openings for fluid entry and exit through connecting pipelines.

– The number of ports on a directional control valve is described by the term “way,” such as a four-way valve with four ports.

Check Valve:

– The simplest directional control valve is a check valve, which is a two-way valve with two ports.

– A Automotive Lift Repair Florida check valve allows free flow in one direction and prevents flow in the opposite direction.

– Fluid flows between the seat and poppet of the valve.

– A light spring keeps the poppet closed. In the free-flow direction, fluid pressure overcomes the spring force at around 5 psi.

– If flow is attempted in the reverse direction, fluid pressure pushes the poppet into the closed position, preventing flow.

– Higher pressure increases the force pushing the poppet against its seat, ensuring no flow in the no-flow direction.

 Pilot-Operated Check Valve

– The pilot-operated check valve is a specialized type of check valve.

– It allows free flow in one direction but blocks flow in the opposite direction unless pilot pressure is applied to the valve’s pilot pressure port.

– A light spring keeps the poppet seated in a no-flow state by pushing against the pilot piston. The separate drain port prevents oil from creating pressure buildup beneath the piston.

– The dashed line represents the pilot pressure line connected to the valve’s pilot pressure port.

– Pilot check valves are commonly used to lock hydraulic cylinders in place.

 Three-Way Valves

– Three-way directional control valves have three ports and are usually designed with a spool rather than a poppet.

– The spool is a cylindrical shaft with lands—large diameter sections machined to slide within a closely fitting bore in the valve body.

– The radial clearance between the land and bore is typically less than 0.001 inches.

– The grooves between the lands create flow paths between the Automotive Lift Repair Florida ports.

– These valves can have two or three unique spool positions and can be operated manually, mechanically, using pilot pressure, or electrically with solenoids.

– Such a valve is referred to as a three-way, two-position directional control valve.

– Flow paths are illustrated by two schematic diagrams (one for each spool position) and a graphic symbol with two side-by-side rectangles, often called “envelopes.”

– Spool Position 1: Flow can move from the pump port P (connected to the pump discharge pipe) to outlet port A, while tank port T (connected to the pipe leading to the oil tank) is blocked.

– Spool Position 2: Flow can move from port A to port T, with port P being blocked by the spool.

– Three-way valves are typically used to control flow directions to and from single-acting cylinders.

 Four-Way Valves

– A four-way, two-position Automotive Lift Repair Florida directional control valve directs fluid from the pump port to either outlet port A or B.

– Spool Position 1: Flow moves from P to A and from B to T.

– Spool Position 2: Flow moves from P to B and from A to T.

– The graphic symbol shows only one tank port T (totaling four ports), even though the actual valve may have two tank ports as depicted in schematic drawings.

– Each tank port serves the same function, resulting in four functional ports.

– The two internal flow-to-tank passageways can be combined within the valve to create a single tank port.

– The graphic symbol represents only the function of the Automotive Lift Repair Florida component, not its internal design.

– Four-way valves are commonly used to control the flow directions to and from double-acting cylinders, allowing the cylinder to extend (left envelope) and retract (right envelope) under hydraulic pressure.

 Manually Actuated Automotive Lift Repair Florida Valves

– In a spring-centered, three-position directional control valve, the spool is spring-loaded at both ends.

– When unactuated (no force applied to the lever), the valve returns to its center position due to the balancing spring forces.

– The graphic symbol includes labeled ports on the center envelope, representing the flow path configuration in the spring-centered position.

– The spring and lever actuation symbols at the ends of the right and left envelopes indicate a spring-centered, manually actuated valve.