Shifter Fork Stiffness Correlation to Gear Shift Quality 2013-01-2447
Shift quality of a manual transmission is a critical characteristic that requires utmost care while structuring the transmission. Shift quality is affected by many factors viz. synchronizer design, shifter design, gear design, transmission oil selection etc. This paper presents a correlation between stiffness of the shift fork in manual transmission with the gear shift quality using a gear shift quality assessment setup. Stiffness of shifter fork is optimized using contact pattern analysis and stiffness analysis on MSC Nastran. All the subsystem (i.e. synchronizer and the shift system component) are constrained to optimize the shift fork stiffness. A-5-speed manual transmission is used as an example to illustrate the same. A direct correlation of gear shift fork stiffness with the shift force experienced by the driver is established. The shift system was modeled in the UG NX 6.0 software to collate the synchronization force, shift system gap etc with the constraint on the shift fork. The shift fork is constrained by the synchronizer sleeve and the fork mounting on the gear shift rail. The synchronizer force is then applied on the gear shift fork pads which are translated to the synchronizer sleeve. It has a number of pads which come into contact at different occasion of the synchronization because of the varying stiffness of the fork. The contact is distributed to optimize the deflection of the fork in the synchronizer for abuse load.
The synchronization force is distributed over the pads which are in contact during the synchronization phase. The fork tends to deflect with the synchronizer sleeve during synchronization thus acting as a damper and storing energy. In the free flight zone the energy is released to provide a positive drop (The derivative function of the detent ramp profile should turn negative) and reducing the double bump i.e. (the force generated when the synchronizer sleeve hits the clutch body ring) during synchronization. It also reduces the fork preloading as the fork is retrieved after synchronization from the sleeve contact. The gear shift lever vibration and gear rattle is also reduces as the shift fork contact from the gear shift sleeve is relieved.
The optimized stiffness was tested in a vehicle and force impact was closely measured using GSQA. Using this approach the impact of stiffness was directly studied and results were verified.