The purpose of an unloading valve in a hydraulic system is to regulate and control pressure within the system, primarily to prevent overpressure conditions that could cause damage to hydraulic components.
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Key Characteristics of Unloading Valves:
Unloading valves are used in various applications across multiple industries due to their ability to manage pressure and protect hydraulic systems. Here are some common applications:
1. Construction Equipment
Excavators: Unloading valves help control pressure in hydraulic systems, preventing damage to the hydraulic components while lifting or digging.
Cranes: They ensure smooth operation and stability when lifting heavy loads by regulating the pressure within the hydraulic circuits.
2. Agricultural Machinery
Tractors: In tractors, unloading valves manage hydraulic pressure for attachments like loaders and tillers, ensuring safe and efficient operation.
Harvesters: They help control hydraulic systems used for cutting and collecting crops, preventing overpressure scenarios.
3. Manufacturing and Industrial Machinery
Hydraulic Presses: Used in pressing applications, unloading valves protect the press components by regulating pressure during operation.
4. Material Handling Equipment
Forklifts: Unloading valves help manage hydraulic pressure in lifting and lowering loads, providing safety and reliability.
Conveyor Systems: Used to control hydraulic actuators that move materials along conveyor belts.
While both unloading valves and pressure relief valves serve critical roles in hydraulic systems, they do so in different ways. Unloading valves are focused on managing specific flow and pressure within a circuit, whereas pressure relief valves are designed for overall system safety by preventing excessive pressure build-up.
Yes, unloading valves can be used in both open-loop and closed-loop hydraulic systems, though their applications may vary depending on the system design and operational requirements
Neglecting to use an unloading valve in a hydraulic system can result in overpressure, increased wear, inefficiency, heat build-up, reduced control, hydraulic shock, and ultimately higher operational costs. To maintain the reliability and safety of hydraulic systems, incorporating unloading valves is essential for proper pressure management and system performance.
Not using an unloading valve in a hydraulic system can lead to several significant consequences, including:
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Several types of pressure-control valves are found in fluid power circuits. Some keep the whole system from excess pressure while others only protect a portion of the system. Others allow flow to an isolated circuit after reaching a preset pressure. Some bypass fluid at low or no pressure when activated.
This chapter only covers relief valves and unloading valves because they are closely associated with hydraulic pumps. The other pressure-control valves are part of the control circuit and will be dealt with after directional control valves.
All fixed-volume pump circuits require a relief valve to protect the system from excess pressure. Fixed-volume pumps must move fluid when they turn. When a pump is unloading through an open-center circuit or actuators are in motion, fluid movement is not a problem. It is when the actuators stall with the directional valve still shifted that a relief valve is essential.
Pressure compensated pump circuits could run successfully without relief valves because they only move fluid when pressure drops below their compensator setting. (Most designers still use a relief valve in these circuits for reasons explained later.)
In either case, a relief valve is similar to a fuse in an electrical system. When circuit amperage stays below the fuse amperage, all is well. When circuit amperage tries to exceed fuse amperage, the fuse blows and disables the circuit. Both devices protect the system from excess pressure by keeping it below a preset level.
The difference is that when an electrical fuse blows it must be reset or replaced by maintenance personnel before the machine can cycle again. This requirement alerts the electricians to a possible problem and usually causes them to look for the reason before restarting the machine. Without the protection of a fuse, the electrical circuit would finally overheat and start a fire.
In a hydraulic circuit, a relief valve opens and bypasses fluid when pressure exceeds its setting. The valve then closes again when pressure falls. This means a relief valve can bypass fluid anytime . . . or all the time . . . without intervention by maintenance. (It also means the system can run hot even with a heat exchanger installed.)
Many fixed-volume pump circuits depend on this bypassing capability during the cycle, and some even bypass fluid during idle time. A well-designed circuit never bypasses fluid unless there is a malfunction, such as a control switch not closing or an operator overriding the controls. This eliminates most overheating problems and saves energy.
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There are two different designs of relief valves in use: direct acting and pilot operated. Both types have advantages and work better in certain applications.
Some terms relating to relief valves and their function are:
• Overshoot: The actual pressure reading when a relief valve first opens to bypass fluid. (It can be up to twice the actual pressure setting.)
• Hysteresis: The difference in pressure between when a relief valve starts letting some flow pass (cracking pressure) and when full flow is passing.
• Stability: The fluctuation of pressure as a relief valve is bypassing at set pressure.
• Reseat pressure: The pressure at which a relief valve closes after it has been bypassing.
• Pressure override: The difference in the pressure reading from the time a relief valve first opens (cracking pressure) until it is passing all pump flow to tank.
Figure 9-1 shows a cutaway view and the symbol for a direct-acting relief valve. The valve has a poppet that is pressed against its seat by an adjustable spring. An adjusting knob can be change the force on the spring to raise or lower maximum pressure. The poppet remains seated while pump flow goes to the circuit and pressure is lower than the relief valve setting. If pressure tries to go above spring setting, the poppet is forced off the seat just enough to pass excess pump flow to tank.
Direct-acting relief valves often are quite noisy due to the high velocity of the fluid bypass and the instability inherent in their design.
Direct-acting relief valves are not normally used on industrial hydraulic systems, except for those with flows under 3 gpm, and as pilot control devices. Most industrial designs use long springs that gain little force per compression increment to keep pressure override low.
When a direct-acting relief valve is specified as preset, non-adjustable, always specify whether the valve is to be set for cracking pressure or full flow. If full flow is desired, a flow must be specified also.
Figure 9-4 shows cutaway views of two common types of pilot-operated relief valves. There are many variations of these designs but the function and symbol are the same. The pilot section on each valve is a low-flow direct-acting relief valve that sets maximum system pressure. Because the valve is small and passes very little flow, it has less than 50-psi pressure override as it operates.
The control orifice in the balanced piston or poppet usually has a diameter around 0.040 in. This size gives good relief-flow stability and is not prone to becoming blocked with contamination. If the orifice is plugged, the balanced piston or poppet will open at approximately 20 psi and dump all pump flow to tank.
A flow path from the outlet of the control orifice . . . on top of the balanced piston or poppet . . . leads up to the pilot section, which contains a spring-loaded poppet. Adjusting the tension on the spring-loaded poppet sets the pressure in the circuit. Fluid used by the pilot section returns to tank through the tank port. The balanced-piston type has a hole through it that lets control fluid flow to tank. The vent port in the pilot section is normally plugged. (Removing the plug allows this valve to perform other functions.)
Many inline-mounted valves have two inlet ports as a piping convenience. Pump flow comes in one inlet and exits through the opposite one. This eliminates the need for a tee in the pump line plumbing.
Pump flow enters the inlet port and flows to the circuit and through the control orifice to the top side of the balanced piston or poppet. It also travels up to the pilot section’s spring-loaded poppet, where it is blocked. When pressure is too low to unseat the spring-loaded poppet, pressure is the same on either side of the balanced piston or poppet. Because hydraulic forces are equal on both sides of the balanced piston or poppet, the light spring holds them in their normally closed position. This condition continues until pressure reaches approximately 25 to 50 psi below the pressure set at the relief valve pressure-adjusting knob.
For example, if pressure was set at psi, at around 950 psi the spring-loaded poppet in the pilot section will crack open and allow a small amount of fluid to pass to tank. At this point the amount of fluid passing the spring-loaded poppet can easily flow through the control orifice so pump flow to tank is blocked. As pressure continues to increase, it finally forces the spring-loaded poppet in the pilot section to open far enough so that flow through it is greater than flow through the control orifice. When flow through the spring-loaded poppet is more than flow through the control orifice, pressure on top of the balanced piston or poppet decreases. When the pressure imbalance is great enough, the balanced piston or poppet moves toward the decreased pressure and opens a flow path to tank. Flow to tank is just enough to bypass any excess fluid the system is not using. As a relief function, this valve never opens more than enough to bypass excess flow.
When system pressure decreases, the spring-loaded poppet in the pilot section reseats. Fluid trapped on top of the balanced piston or poppet forces it to close and block pump flow to tank.
A pilot-operated relief valve allows all pump flow to go to the actuators almost to its final setting. This means the valve can operate at a lower maximum pressure and it will not slow actuator speed when forces increase.
(This valve design is also used as an unloading relief valve in accumulator circuits. Chapter 16 on Accumulators will have a circuit using this valve.)
A pilot-operated unloading relief valve is the same as a pilot-operated relief valve with the addition of an unloading spool. Without the unloading spool, this valve would function just like any pilot-operated relief valve. Pressure buildup in the pilot section would open some flow to tank and unbalance the poppet, allowing it to open and relieve excess pump flow.
In a pilot-operated unloading valve, the unloading spool receives a signal through the remote-pilot port when pressure in the working circuit goes above its setting. At the same time, pressure on the spring-loaded ball in the pilot section starts to open it. Pressure drop on the front side of the unloading spool lowers back force and pilot pressure from the high-pressure circuit forces the spring-loaded ball completely off its seat. Now there is more flow going to tank than the control orifice can keep up with. The main poppet opens at approximately 20 psi. Now, all high-volume pump flow can go to tank at little or no pressure drop and all horsepower can go to the low volume pump to do the work. When pressure falls approximately 15% below the pressure set in the pilot section, the spring-loaded ball closes and pushes the unloading spool back for the next cycle.
An unloading valve requires no electric signals. This eliminates the need for extra persons when troubleshooting. These valves are very reliable and seldom require maintenance, adjustment or replacement.
Often a cylinder needs very little force to stroke to and from the work -- and only a short high-force stroke to perform the work. When this is the case, the hi-lo circuit in Figure 9-10 works well and costs less.
For example: if a single-pump circuit needs 60 gpm to make the required cycle time and psi to perform the operation, the circuit would require a 110-hp electric motor to drive it (60 × × 0. = 105 hp).
The circuit in Figure 9-10 is a typical hi-lo pump circuit that consumes less horsepower while maintaining fast cycle times. It uses a 25-hp motor and supporting equipment for less expense up front, as well as during its useful life. The motor drives a 50-gpm low-pressure pump and a 15-gpm high-pressure pump — for a total of 65 gpm. The extra flow is required to maintain cycle time because the work stroke is slower. The tank, valves, and line sizes are still rated for 65-gpm flow and psi, but the electric motor and controls are much smaller.
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