JUNE-JULY 2018 Digital copy | Page 67

book in series Bevel gear pumps, which are widely employed for commercial vehicles, usually have displacements between 40 and 80 cm3/rev, working pressures of 350 bar, peak pressures of 400 bar and a maximum speed of 2000 ÷ 2500 rpm. former inlet becomes the outlet. Axial piston pumps - Variable displacement Figure 4.66 Displacement variation in axial piston pumps results from the adjustment of the inclination of the axis between the plate and the cylinder block. The variation must be possible in every moment of the operational phases and with displacements ranging from zero (pistons forming a right angle with the plate = maximum pressure, minimum flow) to the maximum angle allowed. In addition, it must always be possible (closed circuits) to reverse the flow (the inlet becomes the outlet and vice versa), always under the same conditions of variability without reverting the revolution direction of the prime mover. Although every type of axial piston pump can undergo the displacement variation, it is usually performed in in-line or bent axis rotary cylinder block piston pumps. Figure 4.67 and Figure 4.68 briefly show the shifting device of the plate or the c ylinder block; controllers are dealt with more in detail in the next chapter . Figure 4.67 Assuming 25° is the maximum angle to which pump maximum displacement cmax corresponds, during the variation of the angle , the displacement of the pump cx can be determined by means of the following formula: cx = cmax • 25sinsin Given a bent axis pump whose maximum displacement cmax at 25° is 150 cm3/rev, nominal pressure is 180 bar, volumetric efficiency v = 0.9, overall efficiency g = 0.85 and rotational speed of 1750 rpm, if the inclination is changed to 12°, its displacement c12° is: The design of variable displacement bent axis pumps differs from fixed displacement pumps only in the larger space of the mobile cylinder block. In Figure 4.67 flow variation is performed manually through a hand wheel/worm screw. When the angle is maximum, flow too is maximum: the less the angle between the cylinder block and the axis of the transmission shaft is, the less the flow rate is; flow is zero as long as the two axis are in line. Flow direction can be reversed by reversing the angle: the JuNe-july 2018 | Global MDA Journal | 67