This invention relates to a synchro gear and synchro ring for an automotive synchromesh transmission.
The transmission system in a vehicle multiplies the engine torque in accordance with the gear ratio and supplies it to the wheels. A typical; 5 speed synchromesh transmission has five forward gear pairs and one reverse gear pair. The drive and driven gears are in constant mesh for all the gear pairs, one of the two rotating along with the shaft on which it is mounted while the other is freely rotating over its shaft. To select a particular speed gear, the freely rotating gear is made to rotate with its shaft using a synchropack. A gear shift is actuated by the driver through external gear shift mechanism and proceeds via internal gear shift mechanism to the synchro pack. The shifter shafts and the shifter forks are the parts of the internal gear shift mechanism.
During manufacture of the synchro ring and the synchro gear, care should be taken so as not to heat treat the spline surface where the synchro ring will be press fitted. Therefore an anti carburising paste is applied on the spline portion of both gear and synchro ring. This process prevents carbon from entering the surface of the metal thus preventing hardening of the components. One more operation involved in this type of design is the pressfitting of synchro ring over the synchro gear. This requires additional time in the assembly procedure as the press fitting operation has to be performed.

A preventive measure for the above problem is to propose a synchro ring which need not be press fitted to the gear. This can be achieved by designing a synchro ring with controlled tolerances for linear and conical dimensions with respect to the center axis. This synchro ring will be having a location with the synchro gear. Thus the synchro ring will be butting against the synchro gear.
According to this invention, the synchro gear and synchro ring for an automotive synchromesh transmission system the gear and ring are case hardened, without the application of anti carburising agent on the splines thereof; the gear and ring being, however, machined to predetermined tolerances enabling the splines of the gear and ring to
form a location fit, instead of a press fit, between them.
This invention will now be described with reference to the
accompanying drawings which illustrate in
Fig. 1 ~' i the known synchro gear SGI and synchro ring SRI

assembled on the shaft
Fig. 2 illustrates, by way of example and not by way of limitation, one
of possible embodiments of this invention, namely, the synchro gear
SG2 and the synchro ring SR2.
Fig. 3 illustrates the synchro gear and synchro ring SG2 and SR2
assembled together as a subassembly and this subassembly assembled
on the shaft.
In the known art (Fig. 1) the synchro ring SR is press fitted to the synchro gear SG. The synchro pack SP is assembled next to the synchro gear. The conical surface of the synchro cone rotates freely on the mating conical surface of the synchro gear. When the shifter sleeve is moved linearly to engage a gear, the synchro cone presses itself against the mating cone surface and thus synchronises the speeds of the gear and the shaft. The shifter sleeve engages with the teeth of the synchro gear and transfers the torque from the synchro gear to the shifter sleeve and thus to the shaft through the synchro pack. The purpose of the synchro ring is to provide a conical surface for mating with the synchro ring and peripheral teeth to engage with the shifter sleeve.
Fig. 2 illustrates one of possible embodiments of the synchro gear SG2 and the synchro ring SR2, according to this invention. Unlike the known art, the synchro ring and the synchro gear will be case hardened without using any anti carburising paste. However, the said gear and ring will be machined to predetermined tolerances such that

they form a location fit at the splines after the heat treatment. Then the synchro gear and synchro ring will be assembled together as a subassembly as shown in Fig. 3 and this subassembly will be assembled on to the shaft.
In the known art, the manufacturing process involves softening of the splines of the synchro gear and the synchro ring for the purpose of press fitting. In the present invention, however, this procedure is eliminated and replaced by a transition fit between the two components through controlled tolerancing on both synchro ring and synchro gear. Due to this, two operations are eliminated, one being the application of an and carburising agent during heat treatment for avoiding heat treatment of the splines and the other being the press fitting of the synchro ring to the synchro gear.
The terms and expressions herein are of description and not of limitation, since this invention is not confined to the embodiment herein described and illustrated but various other embodiments thereof are possible without departing from the scope and ambit of this invention.

We Claim:
1. A synchro gear and synchro ring for an automotive
synchromesh transmission system wherein the gear and ring are case hardened, without the application of anti carburising agent on the splines thereof the gear and ring being, however, machined to predetermined tolerances enabling the splines of the gear and nng to form a location fit, instead of a press fit, between them.
2, A synchro gear and synchro ring for an automotive synchromesr. transmission system substantially as herein described with reference to, and as illustrated in, the accompanying drawings.