Hydraulic circuit

A bypass line is provided to return an excess portion of working oil, whose pressure has been regulated by a regulator valve, to a suction side of an oil pump from a point between the regulator valve and a torque converter pressure regulator valve. An oil filter is inserted in the bypass line.

BACKGROUND OF THE INVENTION 
1) Field of the Invention 
This invention relates to a hydraulic circuit suitable for use with an 
automatic transmission. 
2) Description of the Related Art 
FIGS. 5 and 6 are circuit diagrams showing two examples of an 
oil-pump-containing section of a hydraulic circuit for an automatic 
transmission. FIG. 4 is a schematic side view illustrating one example of 
such an automatic transmission. 
According to the hydraulic circuit shown in FIG. 5, working oil in an oil 
pan 1 is sucked up by an oil pump 3 via an oil filter 2 arranged in a 
housing 11 of an automatic transmission 10. After the working oil is 
allowed to flow through a regulator valve 4, the working oil is divided 
into two portions, one being to be used as a line pressure and the other 
to be delivered to a torque converter and a lubricating circuit via a 
torque converter pressure regulator valve 5 which may hereinafter be 
called the "TC pressure regulator valve 5" as needed. Further, any excess 
portion of the working oil fed to the regulator valve 4 is returned to a 
suction side of the oil pump 3. 
Further, a further portion of the working oil, said further portion having 
been divided from a delivery side of the oil pump 3, is fed to various 
control valves of the automatic transmission by way of an external oil 
filter 6 mounted on an outer wall of the housing 11 of the automatic 
transmission 10 (see FIG. 4). 
According to the hydraulic circuit depicted in FIG. 6, working oil in the 
oil pan 1 is sucked up by the oil pump 3 via the oil filter 2 arranged in 
the housing 11 of the automatic transmission 10. After the working oil is 
allowed to flow through the regulator valve 4, the working oil is divided 
into two portions, one being to be used as a line pressure and the other 
to be delivered to the torque converter pressure regulator valve 5. The 
working oil which has flowed through the torque converter pressure 
regulator valve 5 is delivered via a cooler 7 to the external oil filter 6 
and further to a torque converter 8. 
The hydraulic circuit of FIG. 6 is also designed to have any excess portion 
of the working oil, which has been delivered to the regulator valve 4, 
returned to the suction side of the oil pump 3. 
The oil filter 2, which is arranged on the suction side of the oil pump 3 
and is accommodated within the housing 11 of the automatic transmission 
10, is to remove solid particles from the working oil in the hydraulic 
circuit. The oil filter 2 makes use of a filter medium having relatively 
large pores because unduly small pores lead to a greater loss of the oil 
pump 3. 
If the pores of the filter medium of the oil filter 2 are excessively 
large, however, the working oil delivered from the oil pump 3 still 
contains solid particles which may impair proper and smooth operation of 
each valve. As a result, it may become no longer possible to ensure the 
supply of the working oil in quantities as needed. 
To overcome such a potential problem, the external oil filter 6 is arranged 
in addition to the oil filter 2 disposed inside the automatic transmission 
10 so that the service life of the working oil of the automatic 
transmission can be prolonged and the maintenance of the automatic 
transmission can be facilitated. 
In the hydraulic circuit of FIG. 5, the external oil filter 6 provided 
internally with a filter medium having smaller pores than the filter 
medium of the oil filter 2 is arranged on a delivery side of the pump 3 in 
addition to the oil filter 2, whereby the working oil from which solid 
particles have been additionally filtered off is supplied to the 
individual control valves. 
According to the hydraulic circuit of FIG. 6, on the other hand, the 
working oil fed via the TC pressure regulator valve 5 is allowed to flow 
through the external oil filter 6 which is internally provided with a 
filter medium having smaller pores. Solid particles are hence filtered off 
further from the working oil. The working oil is then fed to the torque 
converter 8. 
Incidentally, the working oil fed from the oil pump 3 to the regulator 
valve 4 is released to the outside when the quantity of the working oil 
becomes greater than that required for the regulator valve 4. The working 
oil so released is returned to the suction side of the oil pump 3 in both 
the hydraulic circuits of FIGS. 5 and 6. 
The hydraulic circuit shown in each of FIGS. 5 and 6 must however be 
designed in such a way that, if the external filter is clogged, a relief 
valve provided in combination with the external oil filter is operated to 
feed the working oil to a downstream side. Further, the hydraulic circuit 
of FIG. 5 is accompanied by the problem that load to the oil pump is 
increased by the flow resistance of the external oil filter. 
SUMMARY OF THE INVENTION 
With the foregoing in view, the present invention has as a primary object 
the provision of such a hydraulic circuit that working oil having a low 
degree of contamination can be fed to various portions without increasing 
load to an oil pump by arranging a second oil filter in a bypass circuit 
through which any excess portion of the working oil regulated in pressure 
by pressure regulator means is allowed to flow back to a suction side of 
the oil pump. 
In one aspect of the present invention, there is thus provided a hydraulic 
circuit suitable for use with an automatic transmission of a vehicle, said 
hydraulic circuit having an oil pump, a first oil filter arranged on a 
suction side of said oil pump, a second oil filter provided with a filter 
medium having smaller pores than said first oil filter, an oil reservoir 
for storing working oil therein, regulator means for regulating the 
pressure of the working oil sucked up by said oil pump from said oil 
reservoir via said first oil filter, said regulator means being arranged 
on a delivery side of said oil pump, and an excess oil line connected to 
the regulator means for guiding excess working oil from said regulator 
means to a hydraulically actuated device, characterized in that said 
hydraulic circuit further comprises a bypass oil line connected to an 
intermediate point of said excess oil line to return at least a portion of 
the excess oil to a point between said oil pump and said first oil filter; 
and said second oil filter is arranged at an intermediate point of said 
bypass oil line. 
In the hydraulic circuit according to the one aspect of the present 
invention, the second oil filter is arranged in the bypass line of the oil 
pump, through which bypass line any excess portion of the oil regulated in 
pressure by the regulator means is returned to the suction side of the oil 
pump. It is therefore possible to feed oil with a low degree of 
contamination to various portions without applying extra load to the oil 
pump despite the arrangement of the second oil filter. 
In another aspect of the present invention, there is also provided a 
hydraulic circuit having an oil pump, a first oil filter arranged on a 
suction side of said oil pump, an oil reservoir for storing oil therein, 
regulator means for regulating the pressure of the oil sucked up by said 
oil pump from said oil reservoir via said first oil filter, said regulator 
means being arranged on a delivery side of said oil pump, and an oil 
return line for returning oil from said regulator means to a point between 
said oil pump and said first oil filter, characterized in that a second 
oil filter is arranged in said oil return line at an intermediate portion 
thereof. 
In the hydraulic circuit according to the another aspect of the present 
invention, a continued operation can promote decontamination of the oil so 
that high-speed sliding portions lubricated by the oil are protected from 
damages.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS 
The hydraulic circuit according to the first embodiment of the present 
invention will hereinafter be described with reference to FIGS. 1 and 2, 
in which the numerals commonly used in FIGS. 3 through 5 indicate 
substantially the same elements. 
In FIGS. 1 and 2, working oil in an oil pan 1 is sucked up by an oil pump 
3, which is driven by an output of an engine 21, via a first oil filter 2 
arranged in a housing 11 of an automatic transmission 10 (see FIG. 4). 
After the working oil is allowed to flow through a regulator valve 4, the 
working oil is divided into two portions, one being to be delivered as a 
line pressure to a control circuit through a line pressure delivery line 
22 and the other to be delivered to a torque converter and a lubricating 
circuit via a torque converter pressure regulator valve 5 which may 
hereinafter referred to as the "TC pressure regulator valve 5". The 
construction of the above circuit is the same as that illustrated in FIG. 
5. 
Between the regulator valve 4 and the TC pressure regulator valve 5, an 
excess oil line 18 is disposed. A portion of excess working oil from the 
regulator valve 4 is branched out through a bypass line 9 connected to the 
excess oil line 18. This excess working oil is divided, as needed, into 
two portions, one being to be fed to the torque converter and the other to 
be guided to a suction-side line of the oil pump 3 at a point downstream 
of the first oil filter 2. In addition to the first oil filter 2 arranged 
inside the automatic transmission 10, a second oil filter 6 provided 
internally with a filter medium having smaller pores than a filter medium 
of the first oil filter 2 is inserted in the bypass line 9. The second oil 
filter 2 is replaceably mounted on an outer wall of the housing 11 at an 
appropriate position thereof (see FIG. 4). 
In the illustrated first embodiment, the regulator valve 4 is designed to 
release, as excess oil, an extra portion of working oil beyond necessity 
when the rotation of an engine exceeds, for example, 1,000 rpm. 
By guiding the excess oil to the bypass line 9 and returning it to the 
suction side of the oil pump 3 via the second oil filter 6 provided with 
the filter medium having the smaller pores, more excess oil is 
recirculated from the regulator valve 4 to the main line as the rotation 
of the engine increases, while solid particles are additionally removed 
therefrom. As the operation continues, working oil having a progressively 
lowering degree of contamination is supplied to various portions. 
Incidentally, designated at numeral 12 in FIG. 2 is an oil strainer 
arranged in the line pressure delivery line 22 on a line pressure delivery 
side of the regulator valve 4. This oil strainer 12 will hereinafter be 
described in brief. 
Production of chips is unavoidable when the oil pump 3 is assembled and 
accommodated inside the housing 11. When the engine is started with such 
chips remaining on the delivery side of the oil pump 3, solid particles 
mixed in the working oil inside the oil pan 1 are removed by the oil 
filters 2,6 but the chips produced upon the assembly are caused to enter a 
valve body 19 in which the regulator valve 4 and the respective lines are 
accommodated or formed. These chips may therefore become a cause for valve 
sticking. 
To prevent such inconvenience, the oil strainer 12 is arranged on the line 
pressure delivery side of the regulator valve 4. The strainer 12 is 
provided with a filter medium having greater pores than the first oil 
filter 2. Despite its ability to remove such chips, the strainer 12 is 
therefore resistant to clogging so that the strainer 12 can sufficiently 
transmit the line pressure therethrough. 
Numeral 13 designates a relief valve arranged on the delivery side of the 
oil pump 3, while numerals 14 to 17 indicate orifices, respectively. The 
orifice 14 is arranged on an upstream side of the second oil filter 6 to 
ensure the delivery of working oil in sufficient quantities to the torque 
converter and the lubrication system. 
Owing to the construction described above, the hydraulic circuit according 
to the present invention makes it possible to recirculate and feed the 
working oil having a lower degree of contamination to the second oil 
filter 6 as the operation of the engine continues. Moreover, the second 
oil filter 6 provided with the filter medium having the smaller pores is 
disposed in the bypass line 9 and filters an excess portion of the working 
oil subsequent to its pressure regulation by the regulator valve 4. The 
load to the oil pump 3 is therefore not increased. 
The second oil filter 6 has the potential problem that it may be clogged, 
resulting in an increase in flow resistance and hence in a decrease in 
flow rate. Nevertheless, it is unnecessary to provide any additional 
relief valve because the second oil filter 6 is arranged in the bypass 
line 9 for any excess oil. 
It should be noted that the present invention is not limited to hydraulic 
circuits for automatic transmissions. The present invention can also be 
applied, in a similar manner to the above-described first embodiment, to 
the engine lubrication circuit shown as the second embodiment in FIG. 3, 
in which elements similar to those described above in connection with the 
first embodiment are indicated by like numerals. Description of such 
similar elements are omitted herein. 
Referring now to FIG. 3, the engine lubrication circuit will be described 
in brief. Designated at numeral 20 is a relief valve. When the delivery 
pressure of the oil pump 3, namely, the hydraulic pressure supplied to a 
lubricated portion has increased to at least a predetermined value, the 
relief valve 20 opens. Accordingly the relief valve 20 has pressure 
regulating function to stabilize a hydraulic pressure to be supplied. A 
delivery pressure from the relief valve 20 is returned through an oil 
return line to a point between a first oil filter 2 and an oil pump 3. A 
second oil filter 6 is inserted at an intermediate point of the oil return 
line. The second oil filter 6 is provided with a filter medium having 
smaller pores than the first oil filter 2. According to this construction, 
oil is filtered further whenever the relief value 20 opens so that a 
continued operation makes it possible to lubricate with more purified oil. 
As a consequence, the hydraulic circuit according to the second embodiment 
can achieve more efficient protection of high-speed sliding portions such 
as a cam shaft, pistons and liners (not shown) from damages.