Clean Oil - A Pre Requisite for any Hydraulic system

1. Importance of Filter
In hydraulic installation large volumetric flows pass through extremely small gaps and clearances at very high pressures. This means that such an installation is considerably more sensitive to dirt and contamination in the hydraulic oil, above all solid particles, than is the case with other types of machinery.

Experience has shown that more than 50% of the premature  breakdowns that occur in hydraulic installations are due to contaminated hydraulic fluid.

It is the task of the hydraulic filter to reduce this dirt and contamination to an acceptable as regards that size and concentration of the dirt particles concerned. This provides protection against premature wear of the components and assemblies.

2. Defining the degree of contaminations
The size and concentration of the dirt particles is quoted when defining the degree of contamination of hydraulic fluid. Counts have proved that the frequency of breakdown increases along with a reduction in the size of the dirt particles.  3 The effect of contamination The contaminant particles can be grains of sand, small pieces of metal or rust, dust etc. These aggravate the abrasion wear in the hydraulic components of those parts that move relative to one another and also in the seals and gaskets. Particularly affected by abrasion due to contamination are, for instance, the bearings, vanes, gears and pistons of hydraulic pumps and motors, as well as the pistons, piston rods and bushings of working cylinders. The abrasion wear on the sliding surfaces increases the clearances and results in higher internal leakage, qualities can also cause metal erosion at valves and on control edges, seating surfaces, restrictions and throttles. Solid, non-abrasive, foreign matter originating from seal wear, filter materials, textile fibers and flakes of paint etc. can cause breakdown and trouble by blocking passages, channels, gaps, lines and filters. Such non-abrasive foreign matter can also cause valves to jam.

Relatively large (>50 um) solid-matter particles often cause sudden machine breakdown shortly after the installation has been taken into operation for the first time. Contamination due to smaller particles (<10 um), on the other hand, generally causes gradual wear with the accompanying slow development and appearance of the resulting wear and damage.

The damage caused by solid contaminant particles depends upon their hardness, size and concentration, as well as upon the sensitivity to dirt of the component concerned. Particularly high levels of wear are caused by those particles of solid matter whose size approximates to the clearances between moving parts. Modern design & for high pressures are more sensitive to dirt than are their lower-powered counterparts. This is due to the tighter clearances and higher loading of the lubricating film on the high-pressure equipment.

It is possible to apply special constructive measures to reduce the harmful effects of contamination and therefore increase service life. For instance, self-adjusting gaps can be used, or specially paired materials, the overall result being a certain level of 'self-healing' at the damaged components.

4. Characteristic values for filters
4.1 Degree of filtration

In addition to the factors of contamination-retention capacity, rated through flow and resistance to through flow, it is above all the fineness of the filter pores that is of importance when specifying a suitable filter. This is because the filter pore size has considerable influence upon the efficiency and serviceable life of the filter.

Basically, it holds true that the smaller the pore size, the longer the service life that can be achieved by the installation. This applies more and more, the higher the pressures concerned generally, an average mesh size of 10-60 um is used nowadays in mobile and industrial hydraulics.

4.2 Miscellaneous characteristic values
In addition to figures on the degree of filtration, the following characteristic values can be derived from the effective filtration area:

A. Serviceable life: This specifies the length of time the filter can be left in operation until it has reached such a high degree of contamination that the permitted flow resistance is exceeded.

B. Nominal flow and pressure drop : These two values are directly related to each other and it had proved practical to present them in a P-Q diagram, Normally, a by-pass valve limits the maximum pressure drop, or flow resistance.

5. Location of the filter in hydraulic system
This criterion necessitates a number of different constructions and designs. The advantages and disadvantages of these will be dealt with in the following:

5.1 Return-line Filter: The location of the filter in the return lines is the most common arrangement. This solution is cheap, presents no problems and mans that full volumetric flow is circulated through the filter. A disadvantage though, is the fact that the contamination is retained in the filter after the hydraulic oil has left the circuit. When this location is chosen, it is assumed that dirt particles only represent a danger to the system after having passed through a number of times (this applies particularly to small particles), and that their removal in the return line therefore provides sufficient protection.

5.2 Suction filters: Many oil-hydraulic installations are also fitted with a wire sieve on the inlet side in addition to the return-line filter. This measure serves to protect the hydraulic pump against damage arising from large-sized dirt particles in the hydraulic fluid.

Fine-proed filters present more problems because the maximum partial vacuum of 0.7 bar (absolute) demanded by most pumps is very difficult to achieve when such filters are fitted. This applies particularly with cold oil and dirty suction filter.

The main purpose of the suction filter is to protect the pump at its inlet. In order not to jeopardize the success of this measure as a result of suction problems, and the resulting cavitations, It is advisable that the filter is equipped with an effective contamination-level display and that it is located in an easily accessible position for ease of maintenance.

5.3 High-pressure filters: These are located downstream of the pump, and usually have the task of protecting especially sensitive valves, e.g. servo-valves for instance. These filters require a pressure-proof housing and are relatively expensive.

5.4 Partial filtration: In order that relatively small filters can be used the principle of partial filtration is often used. Seen from the statistical viewpoint, all the oil in the system is cleaned after having circulated a number of times.

Partial filtration is achieved, for instance, by accumulation. The residual flow is then led off through a filter in  the secondary line.

Partial filtration is particularly effective in the booster line of hydrostatic transmissions.

In larger-size systems, special pumps for the purposes of partial filtration, heating and cooling, drive secondary circuits.

6. Filter Mounting
Basically, one differentiates between in-line mounting and tank mounting. Particularly when the filter is located in  the return, the latter method facilitates easy maintenance because when cleaning, or when a cartridge is changed, no pollution or dirty surrounding result due to escaping oil.

If suction filters are fitted in the tank below the level of the oil surface, this is more problematical.

7. Maintenance
Filters used in hydraulic systems are components, which must be regularly serviced. Other wise they will not be able to function correctly.

Experience has shown that this fact is often neglected. The following measures are recommended in order to counteract this tendency:

A. The Clogging, replacement and/or cleaning intervals must be included in the operating instructions, or better still, displayed prominently on the machine itself. The most dirt appears shortly after taking the system into operation for the first time.

B. Fit clogging indicators. Depending upon the degree of importance (suction filter), these are to be combined with optical or acoustical warning devices, or even with system shut down.

C. Easy accessibility.

D. Lay in a stock of replacement parts (in the vicinity of the machine)

E. In order to achieve satisfactory service life, the size of the filter chosen is to be matched to the amount of dirt, which arises.

F. If it is required that machine is shut down during filter maintenance, and then it may be advisable to fit a dual filter.

4. Characteristic values for filters
4.1 Degree of filtration
In addition to the factors of contamination-retention capacity, rated through flow and resistance to through flow, it is above all the fineness of the filter pores that is of importance when specifying a suitable filter. This is because the filter pore size has considerable influence upon the efficiency and serviceable life of the filter.

Basically, it holds true that the smaller the pore size, the longer the service life that can be achieved by the installation. This applies more and more, the higher the pressures concerned generally, an average mesh size of 10-60 um is used nowadays in mobile and industrial hydraulics.

4.2 Miscellaneous characteristic values In addition to figures on the degree of filtration, the following characteristic values can be derived from the effective filtration area:

A. Serviceable life: This specifies the length of time the filter can be left in operation until it has reached such a high degree of contamination that the permitted flow resistance is exceeded.

B. Nominal flow and pressure drop : These two values are directly related to each other and it had proved practical to present them in a P-Q diagram, Normally, a by-pass valve limits the maximum pressure drop, or flow resistance.

5. Location of the filter in hydraulic system
This criterion necessitates a number of different constructions and designs. The advantages and disadvantages of these will be dealt with in the following:

5.1 Return-line Filter: The location of the filter in the return lines is the most common arrangement. This solution is cheap, presents no problems and mans that full volumetric flow is circulated through the filter. A disadvantage though, is the fact that the contamination is retained in the filter after the hydraulic oil has left the circuit. When this location is chosen, it is assumed that dirt particles only represent a danger to the system after having passed through a number of times (this applies particularly to small particles), and that their removal in the return line therefore provides sufficient protection.

5.2 Suction filters: Many oil-hydraulic installations are also fitted with a wire sieve on the inlet side in addition to the return-line filter. This measure serves to protect the hydraulic pump against damage arising from large-sized dirt particles in the hydraulic fluid.

Fine-proed filters present more problems because the maximum partial vacuum of 0.7 bar (absolute) demanded by most pumps is very difficult to achieve when such filters are fitted. This applies particularly with cold oil and dirty suction filter.

The main purpose of the suction filter is to protect the pump at its inlet. In order not to jeopardize the success of this measure as a result of suction problems, and the resulting cavitations, It is advisable that the filter is equipped with an effective contamination-level display and that it is located in an easily accessible position for ease of maintenance.

5.3 High-pressure filters: These are located downstream of the pump, and usually have the task of protecting especially sensitive valves, e.g. servo-valves for instance. These filters require a pressure-proof housing and are relatively expensive.

5.4 Partial filtration: In order that relatively small filters can be used the principle of partial filtration is often used. Seen from the statistical viewpoint, all the oil in the system is cleaned after having circulated a number of times.

Partial filtration is achieved, for instance, by accumulation. The residual flow is then led off through a filter in  the secondary line.

Partial filtration is particularly effective in the booster line of hydrostatic transmissions.

In larger-size systems, special pumps for the purposes of partial filtration, heating and cooling, drive secondary circuits.

6. Filter Mounting
Basically, one differentiates between in-line mounting and tank mounting. Particularly when the filter is located in  the return, the latter method facilitates easy maintenance because when cleaning, or when a cartridge is changed, no pollution or dirty surrounding result due to escaping oil.

If suction filters are fitted in the tank below the level of the oil surface, this is more problematical.

7. Maintenance
Filters used in hydraulic systems are components, which must be regularly serviced. Other wise they will not be able to function correctly.

Experience has shown that this fact is often neglected. The following measures are recommended in order to counteract this tendency:

A. The Clogging, replacement and/or cleaning intervals must be included in the operating instructions, or better still, displayed prominently on the machine itself. The most dirt appears shortly after taking the system into operation for the first time.

B. Fit clogging indicators. Depending upon the degree of importance (suction filter), these are to be combined with optical or acoustical warning devices, or even with system shut down.

C. Easy accessibility.

D. Lay in a stock of replacement parts (in the vicinity of the machine)

E. In order to achieve satisfactory service life, the size of the filter chosen is to be matched to the amount of dirt, which arises.

F. If it is required that machine is shut down during filter maintenance, and then it may be advisable to fit a dual filter.

Back to Technical paper