Patent Application
20080296123
The present invention relates generally to a transmission clutch system in a moving vehicle, and more importantly to an apparatus for maintaining the optimized running clearance of the clutch plate(s) to minimize windage loss, and to improve transmission shift quality.
Powershift transmission clutches are commonly used in a variety of work vehicles. Powershift transmission clutches generally include a clutch drum (driving or driven member) having an expandable piston operating fluid chamber or piston cavity, a piston axially slidably positioned against the piston-operating piston cavity, a clutch hub (driven member) coaxially disposed in the clutch drum, and a clutch plate pack interposed between the clutch drum and the clutch hub having one end directed to die piston. The clutch plate pack includes first and second groups of plates which are alternately juxtaposed. When the piston cavity is fed with pressurized operating fluid, the piston is forced to press the clutch plate pack thereby to engage the first and second groups of clutch plates and the corresponding clutch drum and hub.
To shift from one forward gear speed ratio to another gear speed ratio, one or more clutches associated with the current speed ratio is disengaged or released (off-going), substantially simultaneously with the engagement of one or more additional and different clutches (on-coming) by introducing fluid into the clutches (also known as clutch fill time) being engaged at the same time fluid is depressurized from the clutches being disengaged. Under this engaged condition, the clutch drum and the clutch hub are united and thus can rotate together. When the pressurized operating fluid is depressurized from the piston cavity, the piston is pushed back, which releases the clutch plate pack thereby to cancel the engagement between the first and second groups of clutch plates.
Under this disengaged condition, the clutch drum and the clutch hub can rotate separately or individually. The amount of time it takes to move the clutch from the disengaged to the engaged position is programmed into the control logic to optimize the shift quality that a vehicle will have. The optimum situation is to begin engaging the oncoming clutch(es) while the off-going clutch(es) are disengaged. Thus, as torque applied, by the off-going clutch(es) decreases, torque applied by the oncoming clutch(es) increases. This overlap of torques minimizes the sudden pressure or spike in the clutch(es) and provides a smoother transition between gears. The difficulty however, lies in the vehicle's ability to properly overlap the torques due to inherent and changeable time delays during clutch engagement and disengagement.
One danger of the shifting process is that of wear or damage to the gears and clutch plates. As one set of clutch plates are disengaged while another set is engaged, slippage tends to occur and the plates become worn. Specifically, if the two are both engaged simultaneously, this can cause serious damage to the transmission as gear teeth break, or extreme wear as the clutches are forced to slip excessively with respect to each other. Alternatively, if neither of the gears break or the clutches slip, simultaneous engagement in two gear ratios can bring the vehicle to a sudden and precipitous stop.
For this reason, the timing and synchronizing of clutch engagement and disengagement is of critical importance when shifting. To accurately coordinate the engagement and disengagement of the clutches, it is necessary to determine the amount of time it takes for the clutches to engage or disengage. One known method is to perform the coordination by a control valve software recalibration process. This recalibration process, which is performed periodically as required, enables the control software to adjust the timing based on changed clutch fill time due to the wear of the clutch plates, which increases the piston travel time to engagement.
Generally, over a period of time, as the thickness of the clutch plates decrease or plates begin to wear, the running clearance tends to increase. Running clearance also known as the disengaging travel distance is essentially the gap between the clutch plates when the system is de-energized. Some running clearance between clutch plates during its de-energized phase is desired to minimize: 1) the rubbing of clutch plates, 2) brake disk wear, and 3) heat generation and inefficiency due to disk drag. In addition, insufficient running clearance increases windage loss mainly due to hydraulic fluid escaping through small gaps between the clutch plates.
Alternatively, too much running clearance is not desirable for the following reasons: 1) delayed time for hydraulic clutch piston to travel for full clutch engagement, thus affecting transmission shift quality; 2) more axial space is required to accommodate parts in worst-case condition (tolerance stackup); and 3) more hydraulic fluid is needed to move the hydraulic clutch piston this additional distance.
For the above reasons, including the ability to optimize transmission shift quality, it has been the desire of the industry to control the total running clearance of a clutch-plate pack, particularly after the clutch assembly has been in service, and clutch plates have worn due to usage. It is therefore highly desirable to have a clutch piston retractor system that could automatically maintain an “ideal” running clearance regardless of how long the system has been in service. This would enable the designer to optimize the transmission shift quality without having to periodically recalibrate the control as is required in most of today's powershift transmissions.
The present invention provides a simplified and improved clutch piston retractor system for controlling the running clearance of clutch plate(s) to minimize windage loss and to improve transmission shift quality. This improved shift quality is desirable to improve operator's comfort, reduce shock loads on power transmission components, and to reduce the energy input to the clutches when changing gears during the acceleration and deceleration of the vehicle. Usually, high loads on the clutches can result in the clutch disks slipping with respect to each other, even when the clutch is engaged. The clutch piston retractor system solves this problem by self adjusting itself to accommodate for the wear to the disks and optimizing the shift quality, while compensating for the additional travel distance. Thus, the need to periodically recalibrate the entire system is eliminated.
Several embodiments of the invention are described in greater detail in the following drawings,
Reference is made first to
Within the clutch housing 16 is a sleeve 3 that is press fitted onto the bore of the hydraulic clutch piston 2 and both are configured to move together during normal operation, and slip relative to each other to make adjustment for maintaining optimized total running clearance 8. Where total running clearance 8 is the sum of individual running clearance 8a through 8n between separator plates 14 and friction plates 15. When clutch 1 is energized,
When clutch 1 is de-energized,
In an alternative embodiment, the difference between spring's installed height
Having described the preferred embodiment, it will become apparent that various modifications can be made without departing from the scope of the invention as defined in the accompanying claims.
