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 TM 9-8000
c. As the oil is forced into cylinder B, it will push piston
Section III.20-8.Construction.A simple hydraulic jack is
B upward, lifting the load through the ram.
constructed using four basic components; each is
discussed below.
d. Piston B will rise each time the jack handle is moved
up and down.
a. Reservoir. The reservoir (fig. 20-8) is a storage
location for the liquid medium. As the piston height is
20-10. Mechanical Advantage in the Jack. Because the
increased, fluid is transferred from the reservoir to the
hydraulic jack is capable of lifting heavy loads with
cylinder under piston B. The lowering of the piston
relatively small amounts of physical force, the jack is
displaces the fluid back to the reservoir.
known to have mechanical advantage. The mechanical
b. Hand Pump. The hand pump (fig. 20-8) is a small-
advantage of the hydraulic jack is obtained from the
following areas:
diameter piston and cylinder combination used to
generate fluid pressure to raise the large piston.
a. The mechanical advantage from the jack handle,
c. Lifting Cylinder. The lifting cylinder (fig. 20-8) is
which is in reality a lever (para 20-3). The hydraulic jack
shown in figure 20-8 has a distance of 10 inches from the
composed of a large-bore piston and cylinder, which
end of the jack handle to the piston and a distance of 1
receives fluid pressure from the hand pump. The fluid
inch from the piston to the fulcrum. The mechanical
pressure acts on the bottom of the large piston to raise it.
advantage of the lever is equal to:
The increased area of the piston allows heavy loads to
be moved with relatively small pressure applied to the
10 + 1 = 10.
jack handle.
b. The mechanical advantage in the hydraulic system,
d. Check Valves. Two check valves (fig. 20-8) are
which is equal to the quotient of the areas of the lift piston
placed in the system, one before the small piston, and
divided by the pump piston, is equal to:
one after. The check valves control the flow of the fluid to
raise and lower the ram.
10 + 1 = 10.
20-9. Operation (Fig. 20-8).
The total mechanical advantage of the jack then is
calculated by multiplying the two mechanical advantages
a. As the jack handle is pulled upward, piston A rises in
in a and b above as follows:
cylinder A, pulling in oil from the reservoir through check
valve 1.
10X10 = 100.
b. As the jack handle is lowered, piston A forces oil
It is thus seen that the hydraulic jack shown in figure 20-8
from cylinder A through check valve 2 and into cylinder B.
produces a total mechanical advantage of 100.
20-6
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