2 - TCA spa

ATTACHMENT VALVESIn order for users of mobile plant to operate auxiliaryequipment from the hydraulic system of the carrier soavoiding the need for an auxiliary power source accuratecontrol of the flow and pressure is vital. The requirements ofvarious attachments will differ both by type and manufacturerand the carriers operating system will also vary.Figure 1. 2FP priority flow regulatorNeedle ValveRadial Control HolesCompensating SpoolSpringThe following article charts the progress of several variantsthat solve the problem of differential pressure requirementswhilst at the same time offering additional benefits. Thestandard Priority Flow Regulator (2FP) is the basis for a rangeof valves designed to provide priority flow and a bypass flowwhich can be used at different pressures. The setting iscontrolled by a simple needle valve with a compensatingspool restricting the flow to the port working at the higherpressure thus maintaining the controlled flow from theregulated port. Before describing the variations in design andapplication it is necessary to understand the workings andperformance of the standard priority style flow regulator as thenormal operation of the variants (2FPH series) are the same.2FP series priority flow regulator: The flow from the pumpenters the inlet port and passes across the needle valve, thenon through the sleeve, past the compensating spool and outthrough the regulated port. The passage of oil across theneedle valve creates a pressure difference which is sensedacross the compensating spool. When the flow is sufficient tocreate a 7 bar pressure difference across the needle valve thecompensating spool will begin to move uncovering the radialholes in the sleeve and opening up a path to the bypass port.Oil will therefore begin to pass to the bypass line. If the flowtries to increase across the needle valve, and so to theregulated port, there will be an increase in pressure differencesensed by the compensating spool causing it to move furtheragainst the spring, open further the line to the bypass portand limiting the flow to the regulated line. If the inlet flow fallsbelow the setting of the valve the pressure difference acrossthe needle valve will drop below the 7 bar needed to keep thebypass line open therefore the priority line is always satisfiedbefore the bypass line opens. Changes in operating pressureon either of the two outlets will alter the inlet pressure to thehigher of the two pressures (plus the control pressure which is7 bar). If the working pressure in the regulated line is higherthan that in the bypass line then the tendency would be forthe flow to try to take the easy way out and flow down thebypass line. This would detract from the flow passing acrossthe needle valve lowering the pressure difference causing thecompensating spool to shift, increasing the restriction to theflow to the by-pass and reducing the restriction of the flow tothe regulated line. In this way the compensating spool willmaintain the regulated or priority flow at a constant level.InletIf the working pressure in the bypass line is higher than that inthe regulated line then there would be a tendency for the flowto increase through the regulated line, increasing the flowwould increase the pressure difference across the needlevalve and cause the compensating spool to metre theregulated line.During the normal operation of any system utilising this typeof valve both the regulated and the bypass pressures will beconstantly altering causing the compensating spool to metre.The valve will maintain the priority flow within +/- 10% of itssetting throughout its range. The largest movement in flow willoccur when pressure differential is transitional, the higherpressure varying between the bypass and regulated ports,this causes the compensating spool to move from meteringone ring of holes in the sleeve to the other. (See graph)Graph 1. Flow curve of the 2FPBypass higher thanRegulated pressure65Regulated Flow - L/min55453525150SleeveBypassRegulated FlowBypass FlowRegulatedRegulated higher thanBypass pressure200 150 100 50 0 50 100 150 200Load Pressure - bar2FP552FP25Spring Chamber2FPH55, 95 & 195: The 2FPH series of flow regulators arebased around the standard priority flow controls but with theaddition of a pressure control and a solenoid vent bothcausing all of the flow to pass to the bypass port.