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BASIC PRINCIPLES OF HYDRAULICS

Introduction



The basic principles of hydraulics are not difficult to understand, knowing how and why hydraulic tools work will help the user to best select the most suitable Hi-Force tools for the job, ensuring maximum performance at the most economical cost
If the "Basic Principles of Hydraulics" detailed in this section of the catalogue are really of assistance to the reader, then its purpose of helping with the selection of the correct Hi-Force tool for the job has been achieved.

 

Using hydraulic fluid pressure to generate a force


 

a) Hydraulic Pressure
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, heavy hydraulic lifting jacks through to the latest state of the art high pressure hydraulic systems of today, hydraulic power remains 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 confined fluid (liquid) is transmitted undiminished in all directions within the fluid (see figure 1). 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 are important 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 (non moving) -see figure 1.

b) The Industry Standard

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 detailed in this catalogue have a maximum working pressure of 700 Bar (10.000 PSI). Therefore where a particular cylinder is specified in this catalogue as having a 10 tonnes maximum capacity, it must be noted that the maximum capacity is calculated at the maximum working pressure.



 

c) Pressure and Force
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 this simple criteria it is possible to manufacture cylinders in the Hi-Force range from 4.5 tonnes up to in excess of 500 tonnes capacity.
The equation for calculating the output force of a hydraulic cylinder, given that the effective area and design maximum working pressure are known, is simply : -
Effective area (cm2) x Pressure (bar)
981
= Output Force (Tonnes)
For example Hi-Force model reference HLS502 (page 11) has an effective area of 71.3 cm2 and a maximum working pressure of 700 bar therefore,
71.3 (cm2) x 700 (bar) 981
= 50.88 Tonnes (see figure 2)




d) The Pump
Hydraulic pressure is provided by a hydraulic pump (manual or powered operation) that pumps the hydraulic fluid into the cylinder bore via a flexible hydraulic hose 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 check valves. Power pumps replace hand leverage with a motive driven ratational 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) and moves upwards until it 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.

 

REMEMBER
If you need any help with selecting the correct Hi-Force tool for your application, do not hesitate to ask us!







 

 

 

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