# 3.2 How valves function in hydraulic systems

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## How valves function in hydraulic systems

ACTIVITY 1:

To be able to describe the functioning of valves in hydraulic systems

[LO 2.3]

The simplest valve is the one-way valve. It is used to keep the piston that moved up as a result of pressure on the hydraulic liquid, in position.

The valve is necessary because unlike gases, liquid cannot be compressed. The functioning of a valve can be explained as follows:

1. In the figure a force is exerted on the piston of cylinder A and is transmitted through the liquid as pressure. This causes cylinder B to rise and to experience a force.

Should the force be removed, piston B will fall back again.

2. This can be prevented by using a valve that moves to the right when the force is exerted on piston A and moves to the left when the force is removed.

Pressure is distributed to the right through liquid and then the valve also moves to the right.

3. When pressure is no longer being exerted, the liquid recedes and the spring then forces the valve to move to the left.

The valve closes as a result of the liquid that recedes when the force is removed and this allows the spring to push the valve to the left.

Questions

Study figures 1 and 2 and answer the questions:

A single hydraulic jack consists of a small input piston, A, and a larger output piston, B. The oil is stored in the reservoir and is let into the system at point C.

A small force is exerted on the small area of the input piston, A, through the action of the lever. This causes a small amount of oil to move from the reservoir into the main cylinder, D. The one-way valve prevents the piston, B from moving back after the first pumping action, and in this way a great force is lifted little by little gradually lifting the car.

1. What is the function of the valves in both figure 1 and figure 2?

2. Why must the lever move back and forth in order to raise piston, B to the top?

3. Why is the diameter of the input cylinder smaller than that of the output cylinder?

## Assessment

 LO 2 TECHNOLOGICAL KNOWLEDGE AND UNDERSTANDINGThe learner will be able to understand and apply relevant technological knowledge ethically and responsibly. We know this when the learner: systems and control:2.3 demonstrates knowledge and understanding of interacting mechanical systems and sub-systems by practical analysis and represents them using systems diagrams:gear systemsbelt drive or pulley systems with more than one stage;mechanical control mechanisms (e.g. ratchet and pawl, cleats);pneumatic or hydraulic systems that use restrictorsone-way valves;systems where mechanical, electrical, or pneumatic or hydraulic systems are combined.

## Memorandum

ACTIVITY 1

1. Prevents the reverse flow of the liquid/fluid so that the piston/plunger remains in position and the pressure is maintained.

2. When the lever moves upwards, a small amount of oil is released from the reservoir to the small cylinder.

When the lever moves downwards and input force is brought to bear on the small cylinder, the oil flows through the one-way valve to the large cylinder. The pressure is propagated through the liquid according to Pascal's principle and provides a great output force on the large cylinder.

When the lever is raised again, the one-way valve shuts and the oil is unable to flow back. This maintains the power advantage. The process is repeated from the beginning.

3. A small force on a small cylinder provides great force on a large cylinder.

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