BOOK IN SERIES
(8) facilitates piston maintenance and disassembly as well as
oil change. These pumps are usually directly connected to a
small tank matched with the pump itself.
Figure 4.4
Figure 4.3
Since manual operated systems cannot deliver a substantial
flow, lever pumps are employed only in simple applications
with single-acting short-stroke cylinders (return results from
the load thrust); yet, this is for sure the most widespread
pump worldwide because it is used in countless fields of
application. A pressure of 700 bar can be generated with a
lever carefully dimensioned vis-à-vis the fulcrum and it can
be even as much as almost 2800 bar in some special systems.
In double-acting manual pumps (Figure 4.4), when the lever
is operated, the fluid continuously flows out from the outlet
due to the recurrence of the inlet/outlet phases of the two
pumping parts.
By acting on the by-pass valve grip, the outlet (which is double
in this case due to a possible second actuator) connects to
the inlet, putting the actuator on stand-by. The pistons suck
and deliver the fluid alternately when the by-pass is closed
and the lever is operated.
90
Two-stage versions with two coaxial pistons are available for
the applications characterised by a long phase of approach
to the load condition without resistance (low pressure and
maximum flow) and a major stress during the final stage
(maximum pressure and low flow): in the first stage the larger
piston ensures the maximum flow whereas, when a small
lever stops it, the second piston (whose bore is dramatically
smaller) helps reaching the maximum pressure. Special
stainless steel systems for water applications are available.
v
The following list includes only few applications of manual
pumps:
v
Jacks for general lifting purposes (short stroke)
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Pullers
v
Bolt cutters
v
Parallelogram platforms
v
Emergency pump in series with automatic pump (for
instance, if there is no tension the machine must be
handled for safety reasons)
v
Tensioning of steel ropes
v
Trolley jacks (essential in repair shops)
v
Portable cranes (for short-range movements of engine
test benches,containers,etc.)
v
Small presses for occasional deep drawing or bending
v
Flexible hose fittings
v
Benchtop pipe cutters, bending machines and tube
closures
v
Wine torques (for small-sized businesses because large-
sized companies use complex pneumatic systems)
v
Laboratory tests on pressures up to 2000 bar
External gear pumps
Apart from manual pumps, which are used (albeit widely)
only for simple flying applications, external gear pumps are
the simplest and most common oil hydraulic pumps operated
by a motor. Their success is accounted for by a number
of advantages, like their extreme lightness, mechanical
simplicity, wide-range viscosity tolerance, optimum suction,
their wide range of flow rates, adaptability to any position
and space and, last but not least, their cost, which makes
them one of the cheapest types on the market.
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