BOOK IN SERIES
without the relief valve because once the maximum working pressure is exceeded ( for instance when the directional valve is on the closed centres ), it takes rather a long time to obtain the zero displacement position .
Figure 4.43
The versions equipped with a spring mechanical controller can sustain a working pressure of 80 - 100 bar with flow rates up to 150 l / min at 1500 rpm ( displacement 100 cm3 ) while an hydraulic servo-control ( see next chapter ) or an electrohydraulic controller ( Figure 4.44 ) replacing the spring ensures working pressures of 150 bar . The recommended viscosity at 50 ° C ranges between 25 and 45 mm2 / s , the fluid temperature must be between – 10 ° C and 70 ° C , the primer ( effect of the centrifugal force on the vane operation ) occurs at 800 rpm and the maximum speed is 1800 rpm . Fluid contamination sensitivity demands decontaminations with filter meshes not exceeding 20 micrometres .
Figure 4.44
Large vane pump Variable displacement
displacement = 100 cm 3 maximum working pressure = 150 bar electrohydraulic controller
It is recommended that the pump should not be installed
Figure 4.45
Special variable displacement pumps made of special materials and with more accurate balances , albeit with a low displacement ( 8 ÷ 16 cm3 ), can sustain nominal pressures of 210 bar and peak pressures up to 250 bar ( Figure 4.45 ).
The stator ring ( 3 ) can move and slide diagonally vis-à-vis the transmission shaft , but it is held back by both the force the controller spring exerts ( 9 ) and the control pin ( 6 ) connected to the outlet via a small channel ( Figure 4.46 ).
The sliding block ( 2 ) can be adjusted so as to balance the pump hydraulically ; this must be carried out when the system is operated by comparing the pressures displayed by the manometers positioned according to the manufacturer instructions ( for example , with a hydraulic controller a manometer is connected to the outlet and the other manometer to the controller clearance ). By acting on the sliding block balancing adjustment screw ( 2 ), there is an ideal balance as long as the differential pressure is equivalent to the pressure printed in a dedicated table in the use and maintenance instructions provided by the manufacturer .
Flow rate and pressure are respectively maximum and minimum when the stator ring is in the right position marked as x ( Figure 4.47 ); the stator is held back by the controller spring ( 9 ) that opposes the force exerted by the spool ( 6 ). An increase in pressure inside the circuit results in an opposite and higher force than the spring force in the pin ( 6 ), with the ensuing left translation of the stator . The higher the system pressure is , the more the ring shifts . may 2018 Global MDA Journal 67