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