1. Field of the Invention
This invention relates generally to automatic transmissions for trucks, and more specifically to automatic truck transmissions fitted with bypass valves for reducing dead time or delay in transmitting fluid control pressure signals when gears are changed manually.
2. Brief Description of the Prior Art
The Detroit Diesel Allision Division of General Motors Corporation is the largest producer of automatic transmissions for truck and other heavy vehicles. An important product has been the MT 650 transmission with five forward speeds. This transmission was introduced in model year 1974. The construction, operation, and repair of this transmission are described in detail in Service Manual, Allison Transmissions, Automatic Models MT 640, 643, MT 650, 653 (Detroit Diesel Allision, Indianapolis, 1983), incorporated herein by reference. The MT 653 transmission is a modified version of the 650 transmission.
In the early production models of the MT 650 and 653 transmissions an extended time delay of from about five to forty seconds of clutch-up or engagement for reverse gear operation occured in some transmission applications. The delay was a result of the priority design of the hydraulic control system and occured when a shift was made from low gear to reverse gear. In the MT 650 and 653 transmissions, low gear, the lowest forward gear, is a very low gear which must be engaged manually, and first gear is the lowest forward gear which is shifted automatically. There are always two clutches engaged in the drive train for each gear. In first gear these are the first clutch and the forward clutch; in reverse gear these are the first clutch and the fourth clutch. In neutral, there is only one clutch applied, the first clutch. Thus, the shift from low gear to reverse gear involve releasing the forward and low clutches and applying the first and fourth clutches.
In an attempt to remedy the delay, the transmissions were modified by installing a bypass fitted with a check valve in between the reverse pressure line (fourth clutch circuit) and the first clutch apply pressure line (first clutch circuit). The check valve included a plastic ball valve member and was positioned to seal when the pressure in the first clutch apply pressure line was greater than the pressure in the reverse pressure line, and to open when the pressure in the reverse pressure line was greater than the pressure in the first clutch apply pressure line.
The plastic ball of the check valve would tend to dissolve i the transmission fluid. After a period of about three to five years, in particular in transmissions equipped with power-take off ("PTO") fittings for supplying hydraulic pressure to hydraulic cylinders used for sundry purposes in certain types of heavy vehicles, the ball would fail to seal when the transmission fluid pressure was raised in the first clutch pressure line.
In these transmissions, a forward clutch and the low clutch are simultaneously applied in operation of the first forward gear, and the first clutch and a fourth clutch are simultaneously applied in operation of reverse gear. However, if, when first gear is engaged and the second clutch pressure line is then pressurized, the check valve fails to seal, the reverse pressure line is also pressured, and the forward, first and fourth clutches are all applied simultaneously, resulting in severe damage to the transmission. This requires a time-consuming and expensive rebuilding of the failed transmission. If the ball were to fail when the transmission was in neutral, the first and fourth clutches would be unintentionally applied, and the vehicle would begin to move backwards, a potentially very dangerous situation.
A number of different plastic materials for the ball, including nylon, were apparently substituted by the transmission manufacturer in attempts to solve this substantial problem, all without ultimate success.
Since the time that installation of the plastic check valve in new model transmissions was initiated by the manufacturer, the manufacturer also altered the transmissions by replacing the input oil pressure pump with a "wide gear" version so as to increase the rate at which input oil pressure increases when the engine is accelerated Previously, the drive and driven gears of the oil pump had a width specified to be from 0.6835 inch to 0.6855 inch while the "wide gear" pump which replaced the earlier model has drive and driven gears which have a width specified to be from about 0.8180 inch thick to 0.8205 inch thick Two different driven gear widths and four different drive gear widths have been provided by the manufacturer for replacement, as slightly wider gears must be fit when a transmission is rebuilt and the oil pump cavity has become worn through circulation of oil suspending abrasive particles. The width of the drive gear is specified to be within plus or minus two and one half ten thousands of an inch and the with of the driven gear is specified to be within plus or minus five ten thousands of an inch.
The transmission manufacturer provided instructions for eliminating the delay in units in field service in "S.I.L. 10-TR-80," Feb. 20, 1980, as well as in Instruction Sheet No. 230, used in connection with Service Kit Part Number 23041150, including a template and replacement separator plate for use in these transmissions. S.I.L. 10-TR-80 and Intruction Sheet No. 230 are each incorporated herein by reference.
Heavy vehicles equipped with Allison MT 650 transmissions are widely used, and the rebuilding required after about three to five years service is extremely costly for owners of fleets of such vehicles, including utility companies. The costs include not only the direct cost of rebuilding the transmissions, but also the cost of maintaining larger fleets than would otherwise be necessary, the costs to customers for whom service is delayed because of transmission failure, and the like. Despite apparently repeated efforts to remedy the problem, the manufacturer has apparently been unsuccessful. There is a significant need for an improved transmission which simultaneously overcomes the problem of delay encountered when shifting from first to reverse gear, and the premature failure encountered after only about three to five years of service of transmissions having plastic ball check valves in between the first clutch apply pressure line and the reverse pressure line.