Hydraulic systems are everywhere in mechanical systems and made of a variety of standard components. With properly located and perfectly designed components, the hydraulic system should generate a minimum of heat (waste energy) and operate with minimum maintenance. In this article we will look over some of the primary systems that make up a hydraulic system.
Primary Components of Hydraulic Systems
The hydraulic power pack is the heart of any hydraulic system. It has a motor, hydraulic pump, oil reservoir, air breather and various other components. The function of each
sub-component is as follows…
- Hydraulic power pack (featuring Reservoir, Primary energy source, Hydraulic pump, Pressure relief valve and pressure indicator, Filters, Fluid level indicator, Air breather)
- Direction control valve
- Other control valves
- Hoses (fixed and flexible)
The reservoir stores hydraulic fluid and protects it from contamination. The primary energy source is the unit which will supply mechanical energy to the hydraulic pump. The primary energy source can be either an internal combustion engine (for mobile applications), a DC motor powered by DC batteries (mostly used on ships) or a 3-phase AC motor (for industrial application). The primary energy source converts input energy into mechanical energy. This mechanical energy goes into the hydraulic pump to generate hydraulic energy (pressure and flow). The selection of the primary energy source doesn’t affect the performance of the hydraulic system, even though it may affect the weight, vibration, noise and size of the system.
The hydraulic pump develops hydraulic energy. It can be one of many different types depending upon the application. The most frequently used pumps are reciprocating piston pumps, vane pumps, and gear pumps. This pump-motor unit together decides the power of the system and its selection limits the maximum pressure and maximum flow rate that can be generated from the system.
The pressure relief valve is an NC (normally closed) safety valve for the hydraulic system. As the pressure increases beyond a certain prescribed limit, the relief valve will bypass the fluid into the tank and maintain the system pressure below the maximum level.
Filters are another important component in any hydraulic system. A filter prevents
contaminants from entering the hydraulic system and ensures satisfactory working with minimum maintenance. A suction line filter is located at the suction side of the pump and prevents the contaminants in the reservoir fluid from entering the hydraulic system. Reservoir fluid can get contaminated from small atmospheric dust particles coming from the air breather. The return line filter is located at the end of the return line. Small particles enter the fluid due to wear of the system and the return line filter takes care of them.
There are various sources of contamination:
- Reservoir vent ports.
- Ingression during maintenance and topping of new liquid.
- Degradation of hydraulic fluid.
- Wear and abrasion by existing particles.
If a filter is not used in a hydraulic system, the following problems can occur:
- Increase in viscosity of fluid.
- Abrasive particles can wear critical components, seals, and valves.
- logging of pipes and hence decrease in flow rate.i
- Wear of hydraulic pump (in the case of vane pump or gear pump).
However, zero is not a good number when it comes to the purity of the hydraulic fluid. A very small amount of impurities of size (<5-10μm) can clog minor holes and prevent leakage in the system! In addition, extremely pure fluid increases the cost of the system.
The fluid level indicator indicates the amount of fluid in the system reservoir. An empirical way is to store double the amount (by volume) of fluid a system demands at steady state operation.
The air breather is a non-return type valve. As the hydraulic pump sucks in liquid, a vacuum will be generated in the reservoir. This adversely affects the pump operation (decreases the head) and can also lead to cavitation during continuous operation (as it reaches vapor pressure) in the case of water as the hydraulic fluid. Hydraulic fluid has very low vapor pressure (0.01kPa) compared to water (2.3kPa) at room temperature and so is less prone to cavitation.
The air breather is a NC (normally closed) valve and as the pressure inside the reservoir decreases below atmospheric pressure, it opens and allows air to enter the system. The most advanced and sensitive hydraulic systems use air breathers that can stop the contaminants in the air (mostly >3 μm) and also prevent atmospheric water (moisture) from entering the system.
In Part 2, we will focus on omnipresent control valves in hydraulic systems and why they are required.