Filter

This invention pertains to a filter in general and particularly to a filter for use in filtering hydraulic oil circulating in a hydraulic system provided for a hydraulic machine. The filter is formed in a loop and includes at least two filtering materials having different filtration porosities. Those filtering materials are arranged in the circumferential direction of the loop. Since filtering materials are arranged in the circumferential direction, the filter can be manufactured at the same size as the existing filters so that it can be accommodated in the existing filter cases. Also, since the filter has more than one filtration accuracies, it is capable of catching impurities of various sizes. In operation, most of the liquid to be filtered tends to pass through the less fine filtration martial. Thus, the filter creates a small pressure loss.

FIELD OF THE INVENTION 
This invention pertains to a filter in general and particularly to a filter 
for use in filtering hydraulic oil circulating in a hydraulic system 
provided for a hydraulic machine. 
BACKGROUND OF THE INVENTION 
A filtration device for use in a hydraulic system has a filter of a 
cylindrical shape. The filter is made of a surface-filtering material, 
such as filtration paper, and catches solid impurities in hydraulic oil in 
its surface when the oil passes through the filter. 
Conventionally, such a filter is made of a single filtering material (prior 
art 1). Generally, the performance of a filtering material is evaluated 
based on three factors: filtration capability; pressure loss; and pressure 
drop. Filtration capability indicates how fine impurities a filtering 
material can filter out and is defined by the size of pores in the 
material. Thus, the filtration capability of a filtering material denotes 
the minimum size of impurities that the material can catch. In other 
words, a filtering material with a finer filtration capability, i.e., a 
finer filtration material, can filter out smaller impurities. Pressure 
drop indicates a difference in oil pressure between the upstream and 
downstream of the filtering material. Useful life indicates how long a 
filter material can be used and is usually measured by an increase in 
pressure drop across the filtering material. These three factors are not 
independent but are related to one another. For instance, a finer 
filtration material gives rise to a greater pressure drop and has a 
shorter useful life. Since the conventional filter is made of a single 
filtering material, it is difficult to balance those competing factors. In 
addition, it is expensive to construct the entire part of a filter with a 
fine filtration material. Also, when a filtration capability needs to be 
changed, the filter element must be replaced with another filter element 
having a desired filtration capability. This replacement work is time 
consuming. 
A filtration device of another conventional type (prior art 2) is used in 
the full-flow filtration method. Such a conventional filtration device has 
a main filter formed of a filtering material with a medium filtration 
capability and a bypass filter formed of a filtering material with a fine 
filtration capability. In this filtration device, most of the oil flows 
through the main filter. The remaining oil (normally, less than 1/100 of 
the total filtration flow) branches from the hydraulic circuit and flows 
through the bypass filter for fine filtration. Since most of the oil flows 
though the medium filtration filter, the overall pressure loss is small. 
In addition, fine impurities are eventually caught by the bypass filter 
while circulating several times in the hydraulic system. However, the 
installation of the bypass filter requires a special filter case and 
piping, which make the initial cost very high. In hydraulic systems for 
motor vehicles, an installation space for the bypass filter is sometimes 
not available. 
This filtration device has a cylindrical filter consisting of two 
cylindrical filtering materials arranged coaxially. One cylinder is 
shorter and has a finer filtration capability than the other. In one 
embodiment, the shorter cylinder is larger in diameter and fit around the 
outer surface of the longer one. Thus, those two cylinders are laid to 
overlap each other in the radial direction at one end of the longer 
cylinder. In another embodiment, those two cylinders are equal in diameter 
and connected side by side along their axis. However, overlapping one 
cylinder with the other or connecting the two cylinders requires a special 
technique. Since the filter is larger in diameter or longer than the 
ordinary filters, it cannot be accommodated in the existing filter cases. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a filter of a simple 
construction while capable of removing fine impurities without causing a 
large pressure loss. It is a further object of this invention to provide a 
filter of the same size as the existing filters so that it can be 
accommodated in the existing filter cases. 
To achieve the above objects, the filter of the present invention is formed 
in a loop and includes at least two filtering materials having different 
filtration porosities. Those filtering materials are arranged in the 
circumferential direction of the loop. Since filtering materials are 
arranged in the circumferential direction, the filter can be manufactured 
at the same size as the existing filters so that it can be accommodated in 
the existing filter cases. Also, since the filter has more than one 
filtration porosity, it is capable of catching impurities of various 
sizes. 
The filtration porosity of each filtering material and the ratio of the 
area it occupies with respect to the entire filtration area are determined 
based on how the filter will be used. In a particular embodiment for 
hydraulic systems, the filter consists of two filtering materials. One 
material has a filtration porosity of more than 20 .mu.m; the other 
material has a filtration porosity of less than 6 .mu.m and occupies 
10%-30% of the entire filtration area. In operation, most of the oil flows 
through the less fine filtration. Therefore, the filter creates a low 
pressure loss. Fine impurities which escape the less fine filtration 
material are eventually caught by the finer filtration material while 
circulating several time in the system. 
The filter of this invention may be made of a sheet material and folded in 
pleats to increase the filtration area of the filter. The material may be 
filtration paper, synthetic resin fibers or cotton. 
The invention, together with further objects and advantages, will best be 
understood by reference to the following detailed description of the 
preferred embodiment of the invention, taken in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the attached drawings, FIG. 2 shows a filtration device 10 
having a filter case 11, a filter storage 12 provided inside the filter 
case 11 and a filter element 13 placed in the filter storage 12. This 
filter device 10 is used in a hydraulic system provided for a hydraulic 
machine (not shown) for filtering hydraulic oil (e.g. mineral oil). 
The filter case 11 has an inlet pipe 15 into which oil enters and an outlet 
pipe 16 from which the oil exits. The inlet pipe 15 communicates with the 
filter storage 12 via a flow-in-port 17. The filter storage 12 
communicates with the outlet pipe 16 via a flow-out port 18. A seal 19 is 
provided between the filter case 11 and a base member 20 to close them in 
a liquid-tight manner. 
A relief valve that opens when the filter element 13 is clogged may be 
provided between the inlet pipe 15 and the outlet pipe 16 for bypassing 
the liquid between the inlet pipe 15 and the outlet pipe 16. 
The filter element 13 is composed of a cylindrical frame 32 with end plates 
30 and 31 and a folded-sheet filter 33. A number of holes 34 (only a part 
of such holes are shown) are provided for the frame 32 so that the oil 
introduced to the filter storage 12 can flow into the outlet pipe 16 via 
the flow-out port 18 after passing through the filter 33. 
As shown in FIG. 1, the filter 33 is formed of a sheet-like filtering 
material, such as filter paper. The filtering material is folded in pleats 
and wound in a cylindrical shape. This filter 33 is composed of two 
filtering materials 41 and 42 having different filtration. The filtration 
porosity of the first filtering material 41 is less fine than that of the 
second filtering material 42. Those two material 41 and 42 are arranged in 
the circumferential direction of the filter 33. The letter "L" in FIG. 1 
indicates the range across which the second filter material 42 is 
provided. As shown in FIG. 4, the filtering materials 41 and 42 are bound 
together at ends 41a and 42a by binding sheets 43 made of filter paper. 
The filtration porosity of the first filtering material 41 is approximately 
30 .mu.m. The filtration porosity of the second filtering material 42 is 
approximately 6 .mu.m or 3 .mu.m or 1 .mu.m. The area ratio of the second 
filtering material 42 is approximately 20%. Thus, the second filtering 
material 42 occupies approximately 20% of the entire filtration area of 
the filter 33. 
Experiments using a hydraulic system (return hydraulic system) have shown 
that the filter 33 exhibits preferable results in terms of both filtration 
porosity and pressure loss when the first filtering material has a 
filtration porosity of between about 20 .mu.m and 50 .mu.m and the second 
filtering material has an area ratio of between about 10% and 30%. 
The filtering materials 41 and 42 are made mainly of paper pulp, but other 
materials, such as synthetic resin fibers and cotton, may also be used. 
The filtration porosities of the filtering materials 41 and 42 can be 
varied by changing the base material of the filtering materials (e.g. by 
reducing the fiber diameter of the filtering materials). The filter 33 may 
include three or more filtering materials having different filtration 
porosities arranged in the circumferential direction in such a manner as 
described above. 
When the filter device 10 is functioning normally, the oil enters from the 
inlet pipe 15 into the filter case 11 as shown by the arrow in FIG. 2, 
then passes through the filter element 13 and exits from the outer pipe 16 
via the flow-out port 18. In operation, the oil passes in part through the 
finer filtration material 42 remove fine solid impurities. The remaining 
oil passes through the less fine filtration material 41 to remove solid 
impurities of sizes larger than that indicated by its filtration porosity. 
Since most of the oil passes through the less fine filtration material 41, 
a pressure loss is small. The fine solid impurities which escape the less 
fine filtration material 41 are eventually caught by the finer filtration 
material 42 after the oil circulates a few times through this element 13. 
Since the filter 33 is composed of two filtering materials 41 and 42, it 
has two binding sections. Thus, the filter 33 has only one extra binding 
section compared to the conventional filters composed of one filtering 
material. Therefore, it can be manufactured much more inexpensively than 
the filter element disclosed by the prior art 3. 
Also, even when used in a full-flow-type hydraulic circulation system, the 
filter 33 of this embodiment can greatly reduce contamination without 
increasing pass-resistance by properly adjusting the area ratio of the 
filtering materials 41 and 42. 
Although the present invention has been described in detail by way of 
illustration and example, it should be understood that a wide range of 
changes and modifications can be made to the preferred embodiment 
described above without departing in any way from the scope and spirit of 
the invention.