We are all familiar with using hydraulic tools and repairing hydraulic tools. They are a great combination of simplicity and directed force. But are you aware of the science that explains how hydraulic pressure generates the force the equipment outputs? In this article, we will explain some of the science behind hydraulics to help you understand what is really happening in your hydraulic equipment.
Hydraulic power is one of the simplest and most powerful forms of producing considerable amounts of force within a confined space, using hydraulic fluid pressure to generate that force. From the early inventions of low pressure and heavy hydraulic lifting jacks through to the latest state of the art high-pressure hydraulic systems of today, hydraulic power has remained an extensive power assist device for many industries.
Hydraulics works because of a simple principle: you can’t compress a liquid. Gases are easy to compress, as you do when you squeeze a balloon or fill a tire. Solids are very difficult to compress a block of metal or wood with your fingers. Liquids, however, are a bit like solids in some ways, and a bit like gas in others. Since liquids easily flow from place to place (like a gas), you might think they can be squeezed (pressurized), but that is not the case. Liquids are virtually incompressible. So when squeezing a liquid (hydraulic fluid) that pressure has to go somewhere else.
Pascal’s law states that pressure applied at any point upon a confirmed fluid is transmitted undiminished in all directions within the fluid. This means that by using hydraulic pressure as a medium a small force can be converted into an appreciable multiple of itself. Essentially, because the liquid in the pipe is incompressible, the pressure must stay constant all the way through it, even when you’re pushing it hard at one end or the other. The actual fluid pressure involved plays a very important role in this ‘Multiplication of Force’ and in this context, there are some features of hydraulic pressure which need to be considered for our purposes:
1. Hydraulic pressure is measured as a force per unit of area e.g. Bar (kg/cm2) or PSI (Pounds per square inch).
2. The hydraulic pressure at any point within the fluid is the same in all directions (provided of course that the fluid is static).
The accepted international standard for maximum working pressure in high-pressure hydraulic tools is 700 Bar (10,000 PSI) and the majority of the tools in the market are in line with this standard. For example, a 10-tonne capacity cylinder (such as in a bottle jack) has a 10 tonne capacity at the maximum working pressure of 700 Bar (10,000) PSI.
The criteria for establishing the maximum output force of a hydraulic cylinder at 700 Bar pressure is the size of the effective area of the cylinder bore i.e. the area to which the hydraulic fluid at a pressure of 700 Bar is being applied. The hydraulic pressure is provided by a hydraulic pump that pumps the hydraulic fluid into the cylinder bore via flexible hydraulic hoses connected to the cylinder quick connect inlet coupling.
Hand-operated pumps are the simplest form of pump and consist of a pumping piston, release valve, and suction and delivery check valves. The pump is operated by closing the valve and then raising and lowering the handle to pump fluid from the reservoir to the pump outlet connection. This action produces a steadily increasing fluid pressure generated by the downward leverage of the pump handle in conjunction with the opening and closing of the suction and delivery valves.
Of course, power pumps replace hand leverage with a motor-driven rotational force that can far exceed hand pump generated pressure much faster. Motor hydraulic pumps enable a wide range of applications as a result, such as positioning buckets, driving breaker points repeatedly, or pumping other fluids.
As the hydraulic fluid enters the bore of the cylinder it forces the cylinder piston to move upwards. Any resistance to the upward movement of the piston (the load), results in the fluid pressure increases as the operator continues to actuate the pump (switch, handle, whatever mechanism the hydraulic tool has for controlling the pump pressure). The fluid pressure will continue to increase until:
- the piston overcomes the resistance (load),
- moves upwards until it reaches the end of its designed stroke length,
- the fluid pressure reaches the end of its designed stroke length,
- the fluid pressure reaches the maximum permissible pressure of 700 Bar and the pump safety pressure relief valve is activated preventing over pressurisation above 700 Bar.
The usable result is great force in a relatively small area, such as at the end of a breaker bit. This pressure can also be used to drive a motor for rotational forces in a small engine, such as a hydraulic chainsaw. In such an application the motor can deliver a constant, steady, high force that far exceeds electric motors or gas motors.