Using hydraulic fluid pressure to generate a force
22 May 2016
Hydraulic power provides one of the simplest and most powerful forms of producing considerable amounts of force within a confined space, using hydraulic fluid pressure to generate a force. Since 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 extensively used and widely respected assistant to mankind’s drive for even greater power and knowledge.
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.
The actual fluid pressure involved plays a very important role in this ‘Multiplication of Force’ and in this context there are two features of hydraulic pressure which is essential to remember:
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 the high pressure hydraulic tools industry is 700 Bar (10,000 PSI) and the majority of the products offered by Hi-Force are in line with this standard. As such, a 10 tonne capacity cylinder from Hi-Force such as a HLS101 single acting low height cylinder, 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. Because of these simple criteria it is possible to manufacture cylinders in the Hi-Force range from 4.5 tonnes up to an excess of 1000 tonnes capacity.
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. Power pumps replace hand leverage with a motive driven rotational force i.e. electric, air or petrol engine driven motor.
As the hydraulic fluid enters into the bore of the cylinder it forces the cylinder piston to move upwards. Any resistance to the upward movement of the piston e.g. a load, will result in the fluid pressure increasing as the operator continues to actuate the pump lever up and down. The fluid pressure will continue to increase either until the piston overcomes the resistance (load) or moves upwards until it reaches the end of its designed stroke length or the fluid pressure reaches the end of its designed stroke length or 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.