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. |
