1.How does cracking pressure percentage change as the maximum pressure setting o
ID: 1849809 • Letter: 1
Question
1.How does cracking pressure percentage change as the maximum pressure setting of the relief valve increases? Why?
2.Explain the term pressure override as it relates to pressure control valves.
3.Why is an understanding of the concepts of cracking pressure and pressure override important when selecting and installing a relief valve in a hydraulic system?
4.How do the concepts of cracking pressure, pressure override and full-flow pressure relate to the operation of other control valves, such as sequence and counterbalance valves?
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Explanation / Answer
1:Most fluid power systems are designed to operate within a preset pressure range. This range is a function of the forces the actuators in the system must generate to do the required work. Without controlling or limiting these forces, the fluid power components (and expensive equipment) could be damaged. Relief valves avoid this hazard. They are the safeguards which limit pressure in a system by diverting excess oil when pressures get too high. 2:The pressure at which a relief valve first opens to allow fluid to flow through is known as cracking pressure. When the valve is bypassing its full rated flow, it is in a state of full-flow pressure. The difference between full-flow and cracking pressure is sometimes known as pressure differential, also known as pressure override. 3:Think of a relief valve in a hydraulic system as a fuse or circuit breaker in an electric circuit. An electric circuit never blows a fuse unless it overloads. When an electric circuit overloads, it is inoperable until reset. Usually the person responsible for resetting the fuse looks for the reason it blew and fixes the problem before restarting the machine. Many hydraulic circuits allow the relief valve to dump some or all pump flow to tank all or part of the time. The extra power to produce that unused flow is expensive. Also, heat generation from excess flow requires larger heat exchangers that are expensive to buy and operate. 4: Types of pressure relief valveinclude relief, reducing, sequence, counterbalance, and unloading. All of these are normally closed valves, except for reducing valves, which are normally open. For most of these valves, a restriction is necessary to produce the required pressure control. One exception is the externally piloted unloading valve, which depends on an external signal for its actuation.
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