To understand the principle of the hydraulic press, we recall the rule of communicating vessels. Its author Blaise Pascal established that if they are filled with a homogeneous liquid, then its level in all vessels is the same. At the same time, the configuration of the containers and their sizes do not matter. The article will describe several experiments with communicating containers that will help us understand the device and the principle of operation of a hydraulic press.
Experiment
Suppose we have communicating vessels with different cross-sectional areas. The smaller area is denoted by s, the larger - S. We fill the containers with liquid. According to the law of communicating vessels, the surfaces of liquids are at the same height.
Close the vessels from above with pistons. We can assume that s and S are the area of the pistons. Press the smaller force f. It will go down, the fluid will flow into the larger cylinder, and the piston on the left will begin to rise. To prevent him from rising, we will also apply force to him. Denote it F.
To get closer to understanding the operating principle of a hydraulic press, we will try to find a connection between these two forces. We will proceed from the equilibrium condition. Before we covered the vessels with pistons, the liquids were in equilibrium. The pressure in the tanks was the same (p = P). Push both pistons so that the fluid remains in balance. The pressures p and P, of course, increase. However, they will remain the same, because they will increase by the same additional value. This is the amount of pressure created by the pistons. It is transmitted by law of Pascal everywhere.
Here is the equilibrium condition: p = P. You can consider the pressure created by the pistons, or the pressure of a liquid column. The result will be the same. Note that the pressure created by the pistons is a thousand times greater than the hydrostatic pressure of the liquid column. A column of water several centimeters high creates a pressure of hundreds of pascals. And the pressure of the piston is hundreds of kilopascals, and sometimes megapascals. Therefore, in the future, we will neglect the pressure of the liquid column and assume that the pressures p and P are created exclusively by the forces f and F.
The dependence of the pressure force of the pistons on their area
We derive the formula, the principle of the hydraulic press without it will be incomprehensible. p = f / s and similarly P = F / S. We make a substitution in the equilibrium condition. f / s = F / S. Now compare the forces of f and F. For this, we multiply the left and right sides of the expression by S and divide by f. We get f * S / s * f = F * S / S * f. Reduce f and S in both sides. The result is the equality F / f = S / s.
The concept of gain in power
If S> s, then the pressure on the piston in the large vessel will be so many times greater than the force that presses on the small piston, how many times the area of the larger piston exceeds the area of the small. In other words, applying a small force to a small piston, in a large vessel we will get a force much higher than that with which we press on a small piston. This is an effect called gain in strength. It shows how many times the forces differ, that is, what is the ratio of F to f. If we take the vessels, the cross-sectional areas of which are very different, we can get a gain in strength of ten or a thousand times. The analysis of forces makes it clear: the gain in force is equal to the ratio of the areas of the large and small pistons.
Piston movement of a hydraulic machine
In many industries, the principle of the hydraulic press is used: in physics, construction, materials processing, agriculture, automotive industry, etc. Examples of the use of hydraulic machines are presented in the figure.
Consider all the same two communicating vessels with pistons, but now we will not pay attention to the force, but to the distance that the pistons travel when moving. Imagine that their initial position is different. A piston of area S is located below the piston of area s. Move the smaller piston at a distance h. Water from a smaller vessel passed into a larger one and pressed on the piston. He moved to a height of H.
Knowing the ratio between the areas, we find the ratio between the heights. The volume that went under pressure from the left cylinder to the right is denoted by v. A liquid of volume V has entered the right cylinder. The liquid is incompressible. How can this be written mathematically? v = V. Express volume through area and height. v = s * h and V = S * H. Therefore, s * h = S * H. S / s = h / H. Therefore, the gain in force is F / f = h / H. This ratio gives us an understanding of how the hydraulic press works. We conclude: since F is greater than f, then H is less than h, and by the same amount.
Let’s say a hydraulic machine gives a hundredfold gain in strength. This means that if we lower the smaller piston by 100 mm, the other piston will rise by only 1 mm. And there are cars that give a gain in power a thousand times. But what about when the car is on the piston and needs to be raised to a height of several meters?
The device and principle of operation of the hydraulic press
In the small area piston there is a valve that closes the tube leading to the machine oil reservoir. Water in hydraulic presses is usually not used, because it causes corrosion, moreover, it has a relatively low boiling point. The piston drives the handle. Fluid is transferred from a smaller cylinder to a larger cylinder through a tube.
The large vessel also has a valve and a piston. When we raise the lever, the oil is sucked into the smaller cylinder by atmospheric pressure. When we lower the piston, the valve closes, the oil has nowhere to go, so it goes into a larger vessel. It lifts the valve in it, the oil volume increases, because of this the piston rises. When we again lift the small piston, the valve in the large vessel closes, so the oil does not go anywhere and the piston remains in place.
The principle of operation of the hydraulic press is such that any vibrations of the small piston always lead to the movement of the large piston up. The device has a mechanism that allows the large piston to lower. This is a hose with a tap in a larger vessel. When we close the tap, we seal the large cylinder, and when we open it, we bring the hydraulic press to its original position, the oil is drained. It returns to the reservoir, which allows the piston to be lowered.