Hydraulic Jack

In this system (Figure 2-1), a reservoir and a system of valves has been added to Pascal’s hydraulic lever to stroke a small cylinder or pump continuously and raise a large piston or an actuator a notch with each stroke. Diagram A shows an intake stroke. An outlet check valve closes by pressure under a load, and an inlet check valve opens so that liquid from the reservoir fills the pumping chamber. Diagram B shows the pump stroking downward. An inlet check valve closes by pressure and an outlet valve opens. More liquid is pumped under a large piston to raise it. To lower a load, a third valve (needle valve) opens, which opens an area under a large piston to the reservoir. The load then pushes the piston down and forces the liquid into the reservoir.

Hydraulic Jack Principle

This system uses a piston-type hand pump to power a single acting hydraulic cylinder as illustrated in Figure 2.6.

A hand force is applied at point ‘A’ of handle ‘ABC, which pivots about point ‘ C The piston rod of the hand pump is pinned to the input handle at point ‘B’. The hand pump contains a cylinder for aiding the up and down movement. When the handle is pulled, the piston moves up, thereby creating a vacuum in the space below it. As a result of this, the atmospheric pressure forces the oil to leave the oil tank and flow through check valve 1. This is the suction process.

When the handle is pushed down, oil is ejected from the hand pump and flows through the check valve 2. Oil now enters the bottom of the load cylinder. The load cylinder is similar in construction to the pump cylinder. Pressure builds up below the load piston as oil is ejected from the pump. From Pascal’s law, we know that the pressure acting on the load piston is equal to the pressure developed by the pump below its piston. Thus each time the handle is operated up and down, a specific volume of oil is ejected from the pump to lift the load cylinder to a given distance against its load resistance. The bleed valve is a hand-operated valve which when opened, allows the load to be lowered by bleeding oil from the load cylinder back to the oil tank. This cylinder is referred to as single acting because it is hydraulically powered in one direction only.

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