Published On: Thu, Mar 28th, 2013

# ✔ Basic hydraulic circuit for construction equipment – Tutorial.

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Video Animation – Explaining basic hydraulic circuit.

This topic will give you a good idea about how a basic hydraulic circuit works, you might be aware of the fact that hydraulics has spread its wings all over the engineering industry. The reliability of the hydraulics machines and equipments are tremendously in demand. There is a vast carrier opportunity in this industry.

In mechanical engineering, fluid mechanics is the part which deals with hydraulics.

In this topic we will be explaining about the basic hydraulic circuit of a construction equipment.

Before learning hydraulics, you should first learn, what is fluid mechanics?

Fluid mechanics is the branch of physics that studies of fluids. A fluid can be liquids, gases or plasmas.

Here liquids are our point of interest, as the study of mechanical properties of liquids is called as Hydraulics.

How hydraulics comes into play in construction machines?

Hydraulics is an integral part of a construction machines. In backhoe loader machines like JCB, L&T, Caterpillar, Hyundai, Volvo, Doosan, Hitachi, Mahindra, Leyland, Ace, New Holland etc there are two major technologies used:

1. Hydraulic technology (can be compared to human hands).
2. Automobile Technology (can be compared to human legs).

We will be talking only about hydraulic technology in this topic:

As humans we use our hands to do various works, same as that, construction equipment have loaders and excavators to do the various jobs. Human hands are powered by blood which is similar to oil in construction equipments; the blood vessels are similar to hoses and our human heart similar to hydraulic pump.

Now the question arises, why do we use liquids (oil) to power our machines and why don’t we use gas (pneumatic) to power it.

Hydraulic system consists of liquids e.g.: oil. It is incompressible, were as pneumatic system e.g.: air is compressible. So hydraulics is preferred over pneumatic system, as it is incompressible and no powers loss will happen due to this property.

All the hydraulic system works on the basis of Pascal’s law, what is Pascal’s law?

Pascal’s law, simply states that pressure applied on a confined fluid is transmitted undiminished in all directions, and acts with equal force on equal area and at right angle to them.

Now as you are clear that Hydraulic is the best method to be adopted for construction machine, let’s see how a basic hydraulic circuit works?

This video explains the working of a basic hydraulic system. The basic hydraulic circuit consists of a hydraulic tank (reservoir), hydraulic pump, main relief valve (MRV), valve block, hydraulic ram etc.

Basic Hydraulic Circuit

The reservoir or the hydraulic tank stores the fluid or the hydraulic oil. The hydraulic oil is the basic medium used in the circuit.

As shown in the figure of hydraulic circuit, there will be a hose connected to the bottom of the hydraulic tank through a strainer; this hose will suck the hydraulic oil from the hydraulic tank through this strainer. The function of the strainer is to strain out or avoid the sediments or foreign material been sucked into the circuit, it does a job of primary filtration before the oil is be sucked by the hydraulic pump. Next the hose is connect to the hydraulic pump. Construction equipment mainly uses two types of pumps for this specific application.

1. Gear pump (commonly used pump in BHL- back hoe loader machines)
2. Piston pump (commonly used on tracked excavator machines)

The hydraulic pump gets its drive from the engine. The function of the hydraulic pump is to create a flow. Hydraulic pump supplied the oil to the valve block through the main relief valve (basic function of main relief valve is to protect the hydraulic pump from back pressure). The valve block works as a gate. It channels the flow to the hydraulic ram. Hydraulic ram converts the hydraulic energy to mechanical energy. That’s how the work happens.

Video explaining – Schematic representation of basic hydraulic circuit.

Why hydraulic tank is used?

It’s the reservoir for the hydraulic oil.

Why hydraulic pump is used in our machines?

Hydraulic pumps are used in our machine to create a flow. This can be measured in LMP (litres per minute), GPM (gallons per minute) etc. As the flow increases the speed of the system increases. So if we need a system with better speed we need to have a hydraulic pump with a higher flow rate. The hydraulic pump used in a JCB back hoe loader machine is GEAR type pump (fixed displacement pump) and in tracked excavator machine is PISTON pump (variable displacement pump).

Why Main relief valve (MRV) used?

The function of this valve is to protect the hydraulic pump, if a back pressure is occurring.

Why valve block is used?

Valve block routs the flow of oil to different hydraulic rams (actuators) as we move the controls.

What is the function of Hydraulic ram?

Hydraulic ram converts the hydraulic energy to mechanical work on our machine.

How work is happening on a Hydraulic machine?

As you have gone through the hydraulic circuit you will be now aware that the movement of the hydraulic ram is creating work. But do you really understand how the hydraulic ram moves. It moves as pressure is created inside the ram.

How pressure is created?

As you know the hydraulic pump created flow, but flow will not create pressure. Then what is that one thing that creates pressure. “Restriction” to the flow of oil created pressure inside the ram. So restriction is that one thing that creates pressure. So when restriction increased pressure increases and when restriction reduces pressure reduces. The unit of pressure is Bar, Psi etc (1 bar = 14.5 psi)