Pressure-Reducing Valves
Automotive Lift Repair Florida Pressure-reducing valves are the most practical components for maintaining lower pressure in a hydraulic system. Typically, these valves are normally open, 2-way valves that close when sufficient downstream pressure is reached. There are two main types: direct acting and pilot-operated.
Direct Acting:
A Automotive Lift Repair Florida direct-acting pressure-reducing valve limits the maximum pressure in the secondary circuit, regardless of changes in the main circuit pressure. This assumes there is no backflow into the reducing valve port, which would cause the valve to close. The valve senses pressure from the downstream side (secondary circuit), operating inversely to a relief valve, which senses pressure from the inlet and is normally closed.
A direct-acting, pressure-reducing valve is held open by a spring. As the outlet port pressure increases, the spool shifts to the right, closing the valve.
As pressure in the secondary circuit rises, hydraulic force acts on the valve area, partially closing it. The spring force opposes the hydraulic force, allowing just enough oil to flow past the valve to maintain the desired pressure in the secondary circuit. The spring setting is adjustable.
When the outlet pressure reaches the valve setting, the valve closes except for a small amount of oil that bleeds from the low-pressure side, usually through an orifice in the spool, and flows to the reservoir.
If the valve closes completely, leakage past the spool could cause pressure buildup in the secondary circuit. Some valves have built-in relieving capabilities to eliminate the need for this orifice.
Constant and Fixed Pressure Reduction:
These Automotive Lift Repair Florida valves balance the secondary-circuit pressure against an adjustable spring force that attempts to open the valve.
Fixed pressure-reducing valves provide a fixed pressure reduction regardless of the main circuit pressure. For example, if a valve is set for a 250 psi reduction, and the main circuit pressure is 2,750 psi, the reduced pressure will be 2,500 psi; if the main pressure is 2,000 psi, the reduced pressure will be 1,750 psi.
This valve operates by balancing the main circuit pressure against the combined forces of the secondary circuit pressure and the spring. Since the pressurized areas on both sides of the poppet are equal, the reduction is determined by the spring force.
Automotive Lift Repair Florida Pilot-Operated Pressure-Reducing Valves:
In pilot-operated pressure-reducing valves, the spool is hydraulically balanced by downstream pressure on both ends). A light spring keeps the valve open. A small pilot relief valve, typically built into the main valve body, relieves fluid to the tank when the reduced pressure reaches the pilot valve’s spring setting, causing a pressure drop across the spool. This pressure differential shifts the spool toward its closed position against the light spring force.
A pilot-operated pressure-reducing valve is held open by a light spring, with reduced pressure acting on both ends of the spool.
The pilot valve relieves just enough fluid to position the main valve spool so that the flow through the main valve matches the flow requirements of the reduced-pressure circuit. Leakage of high-pressure fluid into the reduced-pressure section then returns to the reservoir through the pilot-operated relief valve.
Automotive Lift Repair Florida Sequence Valves
In hydraulic circuits with multiple actuators, it’s often necessary to operate them in a specific order. Sequence valves can be used to control this order, especially when space constraints and force requirements dictate the cylinder size. Sequence valves are normally closed, 2-way valves that regulate the sequence of operations in a circuit. They resemble direct-acting relief valves but with their spring chambers usually drained externally to the reservoir rather than internally to the outlet port, as in a relief valve.
A sequence valve is held closed by an adjustable spring and opened by pressure at the inlet port acting on the left side of the spool.
A sequence valve allows pressurized fluid to flow to a secondary function only after the primary function is completed. When the primary function is satisfied, the pressure in the primary circuit rises and is sensed by the valve, compressing the spring and shifting the valve spool to allow fluid flow to the secondary circuit.
Automotive Lift Repair Florida Counterbalance Valves
Counterbalance valves are normally closed and are used to maintain a set pressure in part of a circuit, typically to counterbalance a weight or external force, such as preventing a load from free-falling. The pressure setting is slightly higher than the pressure needed to prevent free-fall.
When pressurized fluid flows to the cylinder’s cap end, the cylinder extends, increasing pressure in the rod end and shifting the main spool in the counterbalance valve, creating a path for fluid to flow to the directional control valve and then to the reservoir. As the load is raised, the check valve opens to allow the cylinder to retract freely.
If backpressure needs to be relieved at the cylinder or force increased at the bottom of the stroke, the counterbalance valve can be operated remotely. These valves are usually internally drained. When the cylinder extends, the Automotive Lift Repair Florida valve opens, and the secondary port connects to the reservoir. When the cylinder retracts, the load pressure in the drain passage is bypassed by the check valve.
Unloading Valves
Unloading Automotive Lift Repair Florida valves are typically used to unload pumps by directing pump output flow to the reservoir at low pressure after system pressure has been reached. The spring keeps the valve closed. When an external pilot signal acting on the opposite end of the valve spool exerts enough force to exceed the spring force, the valve opens, diverting pump output to the reservoir at low pressure.
An unloading valve is spring-loaded to the closed position and opens when system pressure overcomes the adjustable spring force.
In high-low circuits that use two pumps for traverse and speed or clamping, unloading valves improve efficiency. The output from both pumps is needed only for fast traverse; during feed or clamping, the larger pump’s output is unloaded to the tank at low pressure.
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