Working Principle of Hydraulic Filter

Fluid enters the inlet at the bottom of the tube, passes from the inside to the outside through the filter element and exits at the side outlet. When a new and recently cleaned filter element is fitted in the housing, the tell-tale indicator will indicate that it is ‘clean’.

As dirt deposits on the surface of the element, the pressure differential across the inlet and outlet of the element rises. The bypass piston senses this difference in pressure. The piston is held seated by a spring. When the element requires cleaning, the pressure differential is high enough to compress the spring, forcing the piston off its seat. The piston movement causes the tell-tale indicator to point towards the ‘needs cleaning’ position.

When the filter element is not cleaned when this signal comes on, the pressure differential continues to rise, causing the piston to uncover a bypass passage in the cover. This action limits the rise in the pressure differential to a value equal to the spring tension and the fluid bypasses the filter element. A cutaway view of a tell-tale filter is shown in Figure 7.9.


The following schematics show a few of the typical methods used for filtration. Figure 7.10(a) shows the location of a proportional flow filter. As the name implies, proportional flow filters are exposed to only a percentage of the total flow in the system. The primary disadvantage of this type of filtration arrangement is that, there is no positive protection of any specific component within the system and there is no way to know that the filter is dirty.


Figures 7.10(b)-(d) show full flow filtration in which all the flow from the pump is accepted.




Edge-type hydraulic filters

This is also referred to as a full flow filter, which means that all the oil in the system passes through it. Figure 7.7 illustrates an edge-type filter.


The edge-type filter consists of a stack of disks with holes in their center, like flat doughnuts, with very little separation between them. When entering the filter, fluid is guided from the bottom at the outer side of the stack. Before leaving the filter, it comes out of the center of the stack having passed through the disks. Impurities in the oil are left behind on the outer edge of the stack. A scraper blade moves over the outer surface of the stack, wiping off all the dirt collected. The blade is operated manually by means of a handle at the top of the filter housing.

Duplex-type hydraulic filters

Figure 7.6 shows a filter designed for either a suction line or a pressure line. This is a duplex filter.

A duplex filter, as the name suggests consists of two filters out of which only one is in use all the time. When the filter element gets clogged, the second filter is put to use. Dirty fluid comes into the middle section and passes down through the filter element. The filter element can be of fine gage nylon cloth or wire cloth or finely perforated stainless steel.

From the filter element, the fluid passes out of the unit and into the line. This unit has a ‘telltale’ indicator, which indicates when the filter element is excessively clogged and requires cleaning. If the filter is not cleaned after indication, the fluid bypasses the filter element and there is no filtering action. Such a bypass mechanism is important for a filter because, when the filter element is clogged heavily, the pump in line may get damaged due to starvation of hydraulic fluid.


Air-pressurized hydraulic reservoir

The required pressure in the reservoir is maintained by means of compressed air. Compressed air is generally introduced into the reservoir from the top at a pressure specified by the manufacturer. In order to control this pressure, a pressure control device such as a pressure regulator is provided in the airline entering the reservoir. The function of this pressure regulator is to maintain a constant pressure in the reservoir, irrespective of the level and temperature of fluid in the reservoir.

A pressurized reservoir will only have a single entry point for filling up the fluid in the tank. Since the reservoir is always maintained at a pressure, it becomes important to have a foolproof system with safety relief valves, for the filling of fluid in the reservoir. Sufficient guidelines are provided by all manufacturers of such pressurized reservoirs.

Non-pressurized hydraulic reservoir

As the name suggests this type of reservoir is not pressurized, which means, the pressure in the reservoir will at no point of time rise above that of atmospheric pressure. Very extensively used in hydraulic systems, these reservoirs are provided with a vent to ensure that the pressure within, does not rise above the atmospheric value.

Figure 7.1 shows the typical construction of such a reservoir conforming to industry standards.


These reservoirs are constructed with welded steel plates. The inside surfaces are painted with a sealer, to prevent the formation of rust which might in turn occur due to the presence of condensed moisture. The bottom plate is sloping and contains a drain plug at its lowest point, to allow complete draining of the tank when required. In order to access all the internals for maintenance, removable covers are provided. A level indicator which is an important part of the reservoir, is also incorporated. This allows one to see the actual level of the fluid in the reservoir, while the system is in operation. A vented breather cap with an air filter screen helps in venting the entrapped air easily. The breather cap allows the tank to breathe when the fluid level undergoes changes in tune with the system demand.

The baffle plate in the reservoir extends lengthwise across the center of the tank. Figure 7.2 shows a cross-sectional view of the reservoir depicting the baffle plate function.


The height of the baffle plate in the reservoir is about 70% of the maximum fluid height. The purpose of the baffle plate is to separate the pump inlet line from the return line. This is done to prevent the same fluid from circulating continuously within the tank. In this way it is ensured that all the fluid is uniformly used by the system.

Essentially the baffle plate performs the following functions:
• It permits foreign substances to settle at the bottom
• It allows entrained air to escape from the fluid
• It prevents localized turbulence in the reservoir
• It promotes heat dissipation from the reservoir walls.

The reservoir is designed and constructed to facilitate the installation of a pump and motor on its top surface. A smooth machined surface of adequate strength is provided to support and maintain the alignment of the two units.

The return line enters the reservoir from the side of the baffle plate, which is opposite to the pump suction line. It should be below the fluid surface level all the time, in order to prevent foaming of the fluid. Similarly, the strainer or the foot valve should be located well below the normal fluid level in the reservoir and at least 1 in. or 2.5 cms above the bottom of the reservoir. If the strainer is located too high, it will lead to the formation of a vortex or crater that will permit ingress of air into the pump suction line.

The sizing of the reservoir is based on the following criteria:
• It should have sufficient volume and space to allow the dirt and metal chips to settle and the air to escape freely.
• It should be capable of holding all the fluid that might be drained from the system.
• It should be able to maintain the fluid level high enough to prevent air escaping into the pump suction line.
• The surface area of the reservoir should be large enough to dissipate the heat generated by the system.
• It should have sufficient free board over the fluid surface to allow thermal expansion of the fluid.

For most hydraulic systems, a reservoir having a capacity of three times the volumetric flow rate of the pump has been found to be adequate.

Hydraulic shock absorbers

A shock absorber is a device, which brings a moving load to a gentle rest through the use of metered hydraulic fluid. Figure 6.48 shows the cut away section of a common type of shock absorber.


These shock absorbers are mounted, complete with oil. Therefore, they may be mounted in any position or angle. The spring return units are entirely self-contained units and extremely compact. A built-in cellular accumulator accommodates the oil displaced by the piston rod as the rod moves inwards. Since it is always filled with oil, there are no air pockets to cause spongy and erratic action.

Shock absorbers are multiple orifice hydraulic devices. When a moving load strikes the bumper of the shock absorber, it sets the rod and piston in motion. The moving piston pushes oil through a series of holes from an inner high-pressure chamber to an outer lowpressure chamber.

The resistance to the oil flow caused by the restrictions, creates a pressure, that acts against the piston to oppose the moving load. Holes are spaced geometrically according to a proven formula which in turn produces a constant pressure on the side of the piston opposite the load. The piston progressively shuts off these orifices as the piston and rod move inward. Therefore, the total area decreases continually while the load decelerates uniformly. At the end of the stroke, the load comes to a rest and the pressure drops to zero. This results in a uniform deceleration and gentle stopping with no bounce back. In bringing a moving load to a stop, the shock absorber converts work and kinetic energy into heat, which is dissipated to the surroundings.

One application of shock absorbers is in the energy dissipation of moving cranes. Here shock absorbers prevent bounce back of the bridge or trolley. The most common applications of shock absorbers are the suspension systems of automobiles.