We see cars all day. When we apply a slight brake on the pedestal, the car starts to deaccelerate. If pushed hard enough, the car stops altogether. How is a push by our foot able to stop a vehicle as big as a car? The answer to this question lies in the understanding of Hydraulic Machines. Hydraulic machines work by using liquid fluid power.

A common example is heavy construction equipment. Hydraulic fluid is pumped to numerous hydraulic motors and hydraulic cylinders throughout the machine in this type of machine, and it becomes pressurised based on the resistance present. Let us learn about the working of various hydraulic machines and the principle that governs them in detail. Continue reading to know more.

Pascal’s Law

Pascal’s Law states that the external pressure exerted on a fluid contained in a container is transmitted uniformly and equally throughout the entire fluid in all its directions. In simple words, when pressure is applied on a fluid contained in a vessel, at rest, then this pressure will be felt, equally, at all points inside that fluid and even at the walls of the container, in contact with the fluid. All hydraulic machines are based on Pascal’s Law.

Also, we know the relation between the pressure (\(P\)), force (\(F\)) and area (\(A\)) of the surface as:
\(P = \frac {F}{A}.\)

What are Hydraulic Machines?

Hydraulic machines are machines that employ the power of liquid fluid. They use the power of the liquid stored in a vessel to perform work. Governed by pascal’s law, these machines display amazing strength and agility. Most of the big construction sites around us are hydraulically operated, from bulldozers, backhoes, shovels, loaders, forklifts and even cranes.

The control surfaces in any aeroplane, modern car parking places, car service centres, elevators and even the backyard log splitters employ the same techniques and work on the hydraulics of the fluid. While applications of hydraulics are increasing by the day, some significant hydraulic machines are:
a. Hydraulic lift
b. Hydraulic brake

Hydraulic Machine Mechanics

The most basic of hydraulic machines involves at least a pair of cylinders (input and output), each of which is fitted with a suitable moveable piston.
The input piston: The piston on which the user applies pressure.
The output piston: The piston from which we derive work.
The entire system is filled with a suitable fluid, which is generally oil. As one piston is pushed slightly in, the other one moves accordingly. Pascal’s law suggests that pressure applied at one point is transmitted equally throughout the entire hydraulic fluid and is equal to force upon the area. Thus, if the areas of the two pistons are the same, they will experience the same amount of force. But if the two pistons do not have the same area, then:

1. If the area of the output piston is greater than the area of the input piston, greater will be the force acting on it.
2. If the area of the output piston is smaller than the area of the input piston, a smaller force will be acting on it.

Clearly, just by having a large area of the output cylinder (piston), we can produce a large force using a hydraulic machine.

Designing a Hydraulic Machine

The basic design of a hydraulic machine involves two or more pistons fitted suitably in a cross-section. Thus, designing hydraulic machines involves choosing the relative areas of the pistons in only an equivalent way. From the pivot point, the relative length of the line of action along which the force acts determines the design of the lever.
Being able to tap into the pressure within the fluid at any point makes hydraulic machines quite simple to engineer. You’ll easily design a machine that exerts an output force of some fixed size, compared to the input force, at any angle you select.
You can also easily have an equivalent effect in two different places by creating two pistons of equal cross-sectional area. This will be used, for instance, to exert equal braking forces on all four wheels of a car. Producing an enormous force by providing an outsized area of output piston seems to be too good to be true. There must be a catch! The downside is that the output piston, which exerts massive force, cannot move very far (compared to the input piston). It’s a bit like a lever.
For example:
Thus, if the area of the output piston is eight times the area of the input piston, the force produced at the output piston will be eight times. But the outer piston will only be able to move half a quarter distance of the input piston.

Hydraulic Lift

The most common hydraulic machine is perhaps the hydraulic lift. It is used to lift heavy objects to a certain height. The figure below shows the basic structure of such a lift:

It contains fluid in a large container. There are two openings in the container, one small and the other big. In the two openings, suitable cylinder-piston systems are installed. Let the areas of the two pistons be \(A_1\) and \(A_2\) where, \(A_2 > A_1\). Let the small piston exert a downward force \(F_1\) on the liquid. Thus the pressure experienced by the liquid surface in contact with the piston is:
\(P = \frac {F_1}{A_1}\) ……..(1)
According to Pascal’s law, this pressure will be uniformly transmitted through the liquid. Thus, the fluid just below the larger piston will exert the same pressure on the piston in contact with it. Thus, the upward force experienced by the larger piston is:
\(F_2 = P. A_2\)
Using equation (1), we get:
\(F_2 = \frac{F_1}{A_1} . A_2\)
Or, \(\frac{F_2}{F_1} = \frac {A_2}{A_1}\)
This means that the output force is \(\frac {A_2}{A_1}\) times the input force. This factor is called the mechanical advantage of the given device.
Also, by changing the force on the smaller piston, we can vary the force on the larger piston. Thus, if an object is to be lifted, pressure is applied at the smaller piston, and the force generated at the larger piston lifts the object to a height. This pressure is removed when the piston is restored to its initial position. A hydraulic lift can generally be seen in car service stations and parking lots.
Mechanical advantage is the ratio of output force to the input force in a machine system or a lever or pulley. It basically determines the level of amplification of force achieved by a mechanical device. In simple terms, it is the ratio of Load to Effort for a given system.

Hydraulic Brakes

The working of a hydraulic brake is quite similar to the working of a hydraulic lift. As we apply some force on the brake pedal with our foot, this force is transmitted to the master piston placed in the master cylinder. As the piston moves in the cylinder, the pressure generated by it is transmitted throughout the brake oil. Thus, it is equally felt at the piston of a larger surface area present in the middle. A large force acts on this piston, and it moves down. The downward motion of the piston pushes the brake shoe apart against the brake lining. Thus, by applying a small force on the brake pedal inside our car, a large retarding force is generated on the wheel. What is quite interesting here is that this pressure created at the pedal by our feet is transmitted across the four cylinders attached to the wheels in our car. Thus, the braking effect generated on each wheel is the same, thus, helping us prevent the danger of skidding due to non-uniform retardation.

Hydraulic Machines Application

Hydraulic machines do the heavy lifting for us, giving us a major superhero feel. From helping us lift cars and trucks to ensuring proper braking mechanisms in all such vehicles, hydraulic based systems have made our jobs lighter and easier. Here are some more uses of hydraulic machines:

1. Hydraulic machines are used to power various rides and swings in amusement parks.
2. Hydraulic machines are extensively used at construction sites. Excavators, bulldozers and the latest shovels are all based on the same mechanism.
3. Hydraulic motors are used to power various machines like tracks and loaders.
4. Hydraulic lifts are being installed at all the latest housing societies.

Summary

Hydraulic machines are machines that employ the power of liquid fluid. They use the power of the liquid stored in a vessel to perform work. Governed by Pascal’s law, these machines display amazing strength and agility. Pascal’s law states that the external pressure exerted on a fluid contained in a container is transmitted uniformly and equally throughout the entire fluid in all its directions. The basic design of a hydraulic machine involves two or more pistons fitted suitably in a cross-section.

Let the areas of the two pistons be \(A_1\) and \(A_2\) where, \(A_2 > A_1\). Let the small piston exert a downward force \(F_1\) on the liquid. Then by pascal’s law, the pressure transmitted throughout the fluid will be same, ie., \(\frac {F_2}{A_2} = \frac {F_1}{A_1}\) or \(\frac {F_2}{F_1} = \frac {A_2}{A_1}.\)

This means that the output force is \(\frac {A_2}{A_1}\) times the input force. This factor is called the mechanical advantage of the given device. Hydraulic brakes installed in our automobiles and hydraulic lifts installed in car parks and garages are two important hydraulic machines that we use in our daily lives.

Frequently Asked Questions

Q.1. What is a hydraulic machine?
Ans: Hydraulic machines are mechanical devices that use the power of liquid to perform work.

Q.2. What is the principle behind the working of a hydraulic machine?
Ans: Pascal’s law governs all hydraulic machines.

Q.3. Describe the working of a hydraulic lift?
Ans: In a hydraulic lift, the liquid is contained in a vessel in which two cylinders of unequal areas are installed. Force is applied on the piston of a smaller area, and the pressure generated by this force is transmitted throughout the fluid and is experienced by the larger piston. According to Pascal’s law, the pressure at the larger piston will be the same; thus, the force generated at the outer piston will be more too because of its greater area. This force helps to lift heavy objects in a hydraulic lift.

Q.4. Give a few uses of hydraulic machines.
Ans:
1. Hydraulic lifts are used in car service garages.
2. Hydraulic brakes are used in cars and trucks.
3. Hydraulic motors are used in loaders and tracks at a construction site.

Q.5. Why is the area of the output cylinder larger than the area of the input cylinder in a hydraulic machine?
Ans: To increase the mechanical advantage of a hydraulic machine so that a larger force can be generated in the output, the area of the output cylinder is greater than the area of the input cylinder.