My invention comprises improvements in automatic power transmission mechanisms of the type shown, for example, in U.S. Pat. Nos. 4,978,328 and 5,081,886. Both of these patents disclose automotive vehicle transaxles having a hydrokinetic torque converter mounted on an engine axis and multiple ratio gearing mounted on an axis that is in parallel disposition with respect to the engine axis. Each transaxle has structural features that are similar to the other, although the transaxle of the '328 patent has five forward driving ratios, whereas the transaxle of the '886 patent has four forward driving ratios.
Each transaxle has a positive displacement pump that is connected drivably to the impeller shaft for a hydrokinetic torque converter situated on the engine crankshaft axis. A control valve body is secured to the transmission housing adjacent the positive displacement pump. The control valve body controls distribution of control pressure to pressure-operated servos for the clutches and brakes of the multiple ratio gearing.
An example of a control valve system capable of controlling the servos of the planetary gearing described in the '886 and '328 patents can be seen by referring to U.S. Pat. Nos. 4,637,281 and 4,665,770. U.S. Pat. No. 5,083,481 shows a control system capable of performing the functions of the hydraulic valve system of the '770 and '281 patents, but it includes microprocessor based electronic solenoid valve operators for triggering the operation of the hydraulic control valves in establishing ratio changes in the planetary gearing.
All of the patents referred to in the preceding discussion are assigned to the assignee of my present invention.
A transaxle of the type described above includes a housing and a transaxle sump in which is positioned an oil filter located on the suction side of the positive displacement pump. In a typical commercial embodiment of a transaxle of the kind described above, the filter on the suction side of the positive displacement pump is capable of filtering particles in the 60 micron range. It has been found, however, that particles that are of a size less than 60 microns will pass through a typical prior art filter system in the sump of the transaxle.
In a typical automotive-type transmission valve body having aluminum alloy valve elements in an aluminum alloy valve body, the manufacturing clearances for the valve elements and the bores in which they fit range from 0.0003 inch to 0.0013 inch. Therefore, it is possible for contaminant particles of sizes between 3 microns to 33 microns to enter the clearances and cause valve element sticking. This particle size range is not within the filtering capability of a typical prior art filter system which cannot capture particles smaller than about 60 microns.
Such valve element sticking may result in high warranty costs for the vehicle manufacturer. If an attempt were made to use finer filter medium in a filter system in the fluid supply line for a transaxle positive displacement pump, and if the filter medium were designed to entrap and to hold particles with sizes less than approximately 60 microns, inadequate fluid flow might occur when the filter medium becomes contaminated. Thus, the effective pump capacity might be reduced so that inadequate flow for the transaxle clutch and brake servos would be developed when high pump capacity is required.