幻灯片 1 - Cal Poly

幻灯片 1 - Cal Poly

Fluid Power Engineering Week 5/Lesson 2 Hydraulic valves Hydraulic valves In this lesson we shall Introduce the various kinds of hydraulic valves used to control pressure, flow, and flow path Look at how these three types of valves do their work in a hydraulic circuit Investigate some special-purpose circuits with these valves in them Introduce some of the calculations needed to analyze circuits with valves

Valves, switching elements At different times of a motion cycle we might want the fluid path to change We might also want to control pressure and also flow rate There are three types of valves 1. Pressure relief valves (PRV) 2. Flow control valves (FCV) 3. Directional control valves (DCV) Well cover these in that order Pressure relief valve Since hydraulic fluid is so incompressible, bottling it up and continuing to pump fluid into that volume isnt a good idea A pressure control valve (PRV) limits the pressure that can build up in a circuit When pump discharge

pressure reaches a certain value pilot pressure opens spring-loaded relief valve This arrow indicates that the maximum pressure setting is adjustable and flow is ported to reservoir Pressure relief valve Since hydraulic elements are rated for a certain pressure, it would be standard to

set pmax to a standard value 70 bar, 210 bar, 350 bar But a particular application might require that a certain p not be exceeded, for example to limit the amount of force with which a cylinder presses on a mechanical part = = Set pmax to this value pmax vs pcracking The pressure at which the relief valve starts to open is less than the maximum

pressure the circuit will see at full pump flow But at full pump flow through the valve p = pmax Valve starts to open at this pressure setting, pcracking But this is full pump flow, Qmax Thus the maximum pressure that the

circuit sees is pmax , not pcracking Pump pumps against this pressure Also noteworthy is that a pump pumps against this high pressure when it is so protected by a PRV This is a waste of energy to develop full pressure and then just dump it to the reservoir It also builds up heat to dump pressure through a valve in this manner Sequencing valve A sequencing valve is a little like a pressure relief valve pA rises to a certain

value, then valve opens Then pB rises until it reaches pA , then pA and pB rise together Flow can be either way once the sequence valve opens But note that output is not to tank Also, explicitly shows destination of pilot flow which can be elsewhere, other than to tank, as shown here

Pressure-reducing valve This valve, instead, limits downstream pressure Normal position of valve is as shown: it allows flow through If the pressure downstream gets too high, the pilot pressure closes the valve to more flow This line shows the flow of pilot line back to the tank

Thus this valve acts as a pressure regulator Unloading circuit This circuit has already been shown, but lets embellish it with an extra feature Its job is to unload the pump, so that it does not have to pump against full pressure once it has charged the system Once pump charges this Accumulator maintains system pressure during periods of inactivity The switch turns the pump off when p reaches pmax

and it turns the pump on when p drops below pmin section (and the accumulator) with pressure this opens relief valve, and pump does not pump against pressure Counterbalance valve The circuit at right is raising a weight But if shift DCV to right envelope, weight will fall Pump would be pushing it down, and it would fall

because cap side of cylinder connected directly to tank Counterbalance valve The answer to this is the counterbalance valve This pilot pressure is set high enough that weight itself does not provide enough pressure to open valve This allows us to have an open-center middle envelope, so pump

automatically unloaded To lower weight (right envelope), extra pressure here opens pressure relief to allow cylinder to retract To raise weight (left envelope), flow goes through this check valve In this center position, weight will stay up Flow control valve

A flow control valve (FCV) is used to limit Q and this limits the speed of an actuator Placing a restriction in the flow path limits Q = Arrow indicates restriction can be adjusted to limit flow to a specified value = So limiting Q

limits v Flow control valve A flow control valve (FCV) is an orifice The orifice equation is experimentally determined For a square-edged orifice, Cf = 0.6 For a sharp-edged orifice, Cf = 0.8 Use orifice to measure flow If we can measure the pressure difference, then we can measure the flow rate The orifice equation is experimentally determined

For a square-edged orifice, Cf = 0.6 For a sharp-edged orifice, Cf = 0.8 Such a device is called an orifice-plate flow meter Note that these units make it so that the constant,

0.0851 has units of Needle valve variable restriction A needle valve is a variable restriction Small orifice area Large orifice area Obviously the flow area has the shape of an annulus Needle valve CQ Valves are rated by their capacity coefficient, CQ

= This will give the Qmax through the valve for a given Dp We use CQ in lieu of CfA in the orifice equation If Cf is unitness, then CQ has units mm2 Needle valve Example We have a 5 kN load supported by a piston with Dcap = 50 mm and Drod = 25 mm We want to limit the descent rate to 25 cm/sec

The oil has a specific gravity of 0.90 What capacity coefficient CQ do we need for the valve? Solution: The pressure in the cap side of the cylinder is Needle valve Example In an application, we shall want a particular actuator speed, which gives Q We need to select a needle valve that will handle this specified Q = We need Q:

0.0851 Needle valve Example Thus, 2 2.945 4 0.9 2 =

=6507 0.0851 2546 Single-acting cylinder, adjustable retraction The graphic at left shows how a flow control valve can be used to limit the speed of retraction of a single-acting cylinder The needle valve can be adjusted closed to limit the flow and slow the speed of

retraction of the cylinder Hydraulic pump for automobile A simple hydraulic pump for an automobile consists of a hand pump that pumps the car up But if you open the valve too fast, the car comes down rapidly and bounces One closes this valve during lifting operation

To lower the car, one opens the needle valve (usually using the end of the lever to turn the valve stem) Hydraulic linkage Remember, fluid is just a malleable linkage It carries the metal components that it is embedded in along with it That is, the pistons, the motor gears or vanes If it moves fast, so do those metal components So speeding up or slowing down flow changes the velocity of the part it is driving Pressure-compensated FCV In general for an orifice/valve, Q is proportional to

But with a flow control valve we would like the flow to be the same regardless of the pressure drop across the valve Thus we need pressure compensation This arrow indicates can adjust flow This arrow indicates valve is pressurecompensated In reverse direction, flow is normalnot restricted, not pressurecompensated

Pressure-compensated FCV In a circuit, if the load changes, pressure in cylinder will change But pressure compensation will make flow the same, regardless of pressure drop across valve Because of pressurecompensated FCV, even if load changes, cylinder will extend at same rate During retraction, flow is normal not restricted, not

pressurecompensated Directional control valve Weve seen a standard directional control valve (DCV) Spring centered Extend envelope This is a 4-port, 3-envelope (4/3), hand-lever-operated, springcentered DCV Retract envelope

Spring centered Closed (locked) center Usually there are detents to keep it in each position (envelope) A 4/2 valve wouldnt have the hold position in the center Check valve, a type of DCV Weve seen already a check valve This is okay; ball leaves seat and allows flow in

this direction A check valve is the analog of an electrical diode This is not okay; flow pushes ball onto seat; this prevents flow flow in this direction Pilot check valve A pilot check valve is like a normal check valve, in that it blocks flow in one direction But it takes a pilot signal too Pilot signal

If a pilot pressure is sent to the valve, this opens it so that reverse flow is allowed Shuttle valve A shuttle valve is made of two check valves It is used to direct flow from two separate hydraulic sources Shuttle valve The shuttle valve is a check valve with two seats The valve has two input ports but just one outlet port The valve blocks the input port with the lesser pressure In this circuit, if the main pump fails or is shut off, the emergency pump is

activated. The shuttle valve prevents backflow through the main pump discharge. Aircraft ram air turbine An airliner has a ram air turbine, which can be deployed if hydraulic power from the pumps is lost Turbine powers a hydraulic pump or a generator to provide emergency power for landing if

hydraulic power lost Outside learning To better understand this subject matter, view the following videos on the Big Bad Tech Channel Dont forget to turn the closed-captioning on to be able to understand better the details of the lectures Watch: Pressure relief valves Pressure reducing valves Flow control valves Check valves Directional control valves Valve Solenoid Basics End of Week 5/Lesson

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