In various types of work machines, in order to transmit power in the form of rotational motion generated by a prime mover such as an internal combustion engine to a driven element, which may be a rotatable wheel or other propulsion device associated with the machine, a powertrain operatively links the engine and driven element. The powertrain itself may include components such as a transmission to adjust and change the torque and/or speed characteristics of the transmitted power output. Transmissions include a plurality of gears that can be selectively engaged in different ratios to increase or decrease the rotational speed and, in an inverse relation, the torque transmitted through the powertrain. The gear ratios may include a forward-neutral-reverse gear as well as a plurality of fixed gear ratios that provide different ranges of speed and torque for the machine. Transmissions may be manual or automatic depending on the level of operator control over the selective shifting between gear ratios.
To switch between gear ratios, some transmissions utilize a hydraulic circuit configured to selectively operate clutches that are associated with the various gears. A standard clutch is a mechanical device in which adjacent rotatable elements coupled to different parts of the powertrain are moved into frictional engagement so that their relative rotational speeds synchronize with each other. In particular, when shifting between gear ratios, an “oncoming clutch” may engage a first pair of gears while an “off-going” clutch may disengage a second pair of gears. The driven gear associated with the oncoming clutch speeds up or down to match the speed of the driving gear. This engagement and disengagement of gears occurs simultaneously to continue power transfer through the transmission without interruption while the transmission attempts to smoothly change the speed and torque ratios. The hydraulic circuit directs hydraulic fluid to and from the oncoming and off-going clutches to move the elements into and out of frictional engagement.
Transmissions are sometimes calibrated to accommodate the initial speed difference between engaging gears and the inherent time delay in filling and draining hydraulic fluid from the oncoming and off-going clutches. Still, some degree of disruption often occurs during gear shifts, part of which may be caused by improper hydraulic engagement of the clutches. For example, if the oncoming clutch experiences an early fill event, filling too quickly with hydraulic fluid, the transmitted torque may suddenly spike causing the machine to jerk or lurch. Likewise, if the oncoming clutch experiences a late fill event such that the oncoming clutch is unable accepted the full torque transmission before the off-going clutch disengages, the machine may lug or temporarily drag before full torque transmission is restored. Besides being unpleasant for the operator of the machine, the jarring motions may dislodge or spill a load being carried by the machine. The jarring also subjects the components of the transmission to excessive wear and friction.
Machine manufacturers have developed various systems and methodologies to reduce or mitigate the effects of disrupted gear shifts. For example, U.S. Pat. No. 6,640,950 (“the '950 patent”), assigned to the assignee of the present disclosure, describes a method of engaging a clutch associated with a gear by directing hydraulic fluid to the clutch. A control system monitors the hydraulic pressure of the hydraulic fluid flowing to the clutch to determine when the clutch fills with fluid. The control system can thereafter operate the hydraulic circuit in various ways to gradually and smoothly move the rotatable elements of the clutch into full engagement. The present disclosure is directed to similar considerations regarding clutch engagement in a machine.