BOOK IN SERIES
HYDRAULICS
in Industrial and Mobile Applications
CHAPTER 4
OIL HYDRAULIC PUMPS
Get your copy of this book :
http://www.assofluid.it/en/publications-requestt
Oil hydraulic pumps are classified first as fixed or
variable displacement pumps.
E
ven if pressure reaches the maximum limit and the prime
mover can generate the power needed, flow in fixed
displacement pumps is constant as long as the rotational
speed of the mechanical generator stays the same. Flow rate
changes when the number of revolutions on the transmission
shaft declines: flow diminishes as revolutions per minute
decrease and vice versa; pressure does not change (under
specific conditions and if the load is constant).
Special mechanisms inside the pump casing in variable
displacement pumps decrease displacement from the
maximum limit to zero flow (see chapter 5). The external
intervention can be carried out automatically or manually
by means of a hand wheel. In the first case, the dedicated
controllers can be controlled with a mechanical spring, load
sensing, electromechanical or electroproportional system.
Figure 4.1
Like any other piston pump, their operating principle is based
on the reciprocation of suction and delivery (Figure 4.2).
Variable flow pumps (not all types are suitable) that can
reverse the flow are available: their outlet becomes the inlet
while their prime mover has a constant direction, which is
the prerequisite for closed circuit applications like wheel and
track drive or hoist and winch handling.
FIXED DISPLACEMENT PUMPS
Manual lever pumps
Manual pumps (Figure 4.1) consist of a piston sliding inside
its cylinder and operated by a lever that can be moved by
hand or with a spring return pedal during suction depending
on the applications.
www.ghmediabusiness.com
Figure 4.2
By pulling the lever (10) upward, the rod (6) moves the piston
(1) during suction: the inlet valve (2) opens allowing the oil to
flow into the cylinder; the intermediate valve (3) prevents the
delivered fluid from flowing back into the pump casing (9).
At the upper dead centre, delivery is started by pushing the
lever downward: the inlet valve (2) closes preventing the
fluid to flow back into the tank, the intermediate valve (3)
allows the fluid to flow during the delivery. Seals (5) block
leakages between the pump casing (9) and the rod; the plug
89
The hydraulic pumps described in this paragraph can have
only a fixed displacement for a number of mechanical
reasons.