book in series
the piston is covered with a material adequately tempered and shaped into a hemisphere in order to reduce the contact surface . Likewise , the positioning of the return springs on the pistons depends on the design and suction and delivery valves often have no springs .
In the version of Figure 4.57 , pistons are connected to the plate through a ball joint / sliding block and a roller thrust bearing , while the return spring placed in the front part of the pump casing pushes on the base of the ball joint . When the shaft is revolving , the angle of the plate makes pistons move . Oil flows through the space between the plate and the pistons and fills the sucking pistons , whose front end has some clearances connected to the internal space ; in the following phase , the ball of the suction valve clogs the internal cavity of the front part , thus allowing the fluid to flow into the outlet through the delivery valve .
A different version of this pump has rounded front piston ends ( less friction ) connected to the swash plate through a spring placed inside the piston and back end of the pump casing ( Figure 4.58 ).
to guarantee a solid contact between the sliding blocks and the plate , is usually twisted around the drive shaft ( in the back part inside the pump ) and it is held by a washer on the pump casing and another washer over the swash plate connected with some pins to a ring placed on the transmission shaft . The force on the sliding blocks is transmitted by the inclined ring ( Figure 4.59 ).
The hydraulic principle of these pumps and fixed cylinder block pumps is not the same because when the pistons rotate suction and delivery are sent to their back end through the fixed intermediate bush ( also known as distributor plate or wear plate ) provided with suction and delivery slots ending in correspondence with the dead ends ( maximum and minimum piston stroke ). The openings of the head , the same as the bush at the beginning , end with the inlet and outlet ; the bush is not found in many versions because the friction of the cylinder block is largely limited by the fluid film .
Figure 4.59
Figure 4.58
Suction / delivery valves ( which are not found in bent axis versions or versions with rotary cylinder block ), the long path of the fluid in the space inside the pump casing ( this ensures abundant lubrication and excellent heat exchange ) result in high performances with pressures up to 500 bar , revolutions between 1000 and 3000 rev / min , g = 0.9 ÷ 0.95 and suction pressures of 0.9 absolute bar . These generators are not much affected by dirty oil and require a viscosity range between 30 and 70 cSt .
Swash plate pumps with rotary cylinder block , whose mechanical operation is similar to the previous ones , differ in the cylinder block because it is rotary and rigidly connected to the transmission shaft . The swash plate is fixed on the pump casing and the slipping with the pistons is made possible by the ball joint / sliding block . The preload spring , whose task is
The fluid for the hydrostatic sustenance of the sliding blocks and internal lubrication ( Figure 4.60 ) is pushed into a clearance in the front piston head , the joint and the sliding block ; tiny drains in the sliding blockdrive the fluid into the pump casing .
Figure 4.60
Bent axis piston pumps have a plate perpendicular to the drive shaft ; the piston unit usually has an inclination of 25 ° and it is revolved by a cardan shaft , a system of connecting rods or a pair of bevel gears ( Figure 4.61 ).
JuNe-july 2018 | Global MDA Journal | 65