GEAR SELECTION INTERLOCK MECHANISM FOR COMMERCIAL VEHICLE TRANSMISSION 2014-01-1718

In a transmission for Automobiles, the driver's comfort of smooth shifting and selection of gears is a major concern for the transmission designer. To achieve this comfort, lot of work and improvement has been done in the past few years and still some more improvements are in continuation by the automobile manufacturer. Apart from the smooth shifting and selection of gears while driving, the safety of the driver is also a major concern for the vehicle manufacturer. This paper relates to a safety Interlock mechanism of vehicle gear shifter selector lever to a drive position, until a predetermined condition is satisfied. There are various gear shift pattern exists in the vehicles, which depends upon the number of gears in the gearbox. Generally, two types of gear shift pattern are commonly used in commercial vehicles. 1^st-2^nd, 3^rd- 4^th, 5^th-6^th, Rev and Rev-1^st, 2^nd-3^rd, 4^th-5^th, 6^th. In the first type of gear shift pattern, neutral gear is placed in between 3^rd-4^th gear and reverse gear is separate. In this case the gear select position is determined by moving the gear selector lever to left and right and a suitable interlock mechanism is provided for the reverse gear selection. This gear select pattern is commonly used in automotive and commercial vehicles. But in the second type of gear shift pattern, there is a chance of wrong gear selection, which causes a personal and property damage. To avoid this, a gear selector Interlock Mechanism is designed, which ensure the movement of gear selector lever to a required predetermined position and safe driving. In this paper a six speed manual transmission with second type of gear shift pattern is considered for interlock mechanism design.

For example 5MT350 gearbox denotes 5 speed Manual Transmission designed for 350Nm of Torque capacity. The gear shift pattern of 5MT350 gearbox is shown in Figure 1. In this pattern the neutral is in between the 2^nd and 3^rd gear. 1^st -Rev and 4^th-5^th are on adjacent side of neutral. If the same gearbox converts to 6 speeds, the additional gear is to be place inside the gearbox and gear shift pattern changed as shown in Figure 2. This gear pattern is also similar to 5 speed, the only difference is that 6^th gear is comes adjacent to 4^th-5^th on the upper side.

In the 6 speed gear shift pattern as shown in Figure 2, as long as the vehicle is running up to 5 speeds, the vehicle is running smoothly and safely at predetermined condition of vehicle, but as the gear is shifted from 5^th to 6^th the risk factor increase on the vehicle. While shifting from 5^th to 6^th speed, the force on select spring inside the gearbox increases. While returning or downshifting, when the driver shifts the gear shifter selector lever, due to high force on the select spring, the lever immediately comes to its neutral position and instead of 5^th gear the 3^rd gets engaged as shown in Figure 3. This behavior of gear shift pattern can cause the engine seizure, which results in property damage. Same problem arises, if we shift neutral position from 2^nd-3^rd to 4^th-5^th gear as shown in Figure 4. As the driver shifts the gear shifter selector lever from 1^st to 2^nd gear, due to high force on select spring, lever tends to move towards neutral position and instead of 2^nd gear, 4^th gear gets engaged. This behavior of gear shift pattern increase the load on engine, which causes vehicle stop and loss of fuel economy.

To ensure safe gear shifting while driving, one way is to design the gear shift pattern as shown in Figure 5, so that the reverse gear is at extreme end side of pattern and all forward gears are placed around the neutral gear. But if we need to convert the 5 speed shift pattern as shown in Figure 1 to 6 speed shift pattern, then designer must ensure about the safety of driver. This paper describes the design of gear select Interlock Mechanism, to ensure the safety concern and avoid any kind of damage, mentioned above while driving. A cam profile and stopper mechanism is designed and placed in between 1^st - Rev and 2^nd-3^rd, which locks the movement of gear shifter selector lever from 1^st gear to 4^th gear and allows to shift either 2^nd or 3^rd gear only. As the gear shits to 2^nd or 3^rd gear shift position, the Interlock mechanism releases and comes to its original position as shown in Figure 6.