The next transmission illustrated below has almost the same function and somewhat different structure as compared with the preceding one. As shown in the drawing, a hub on the shaft is provided with two grooves each of which can receive a key. As a rib projecting from the key surface is brought in engagement, by the spring force, with any of mating recesses in the sleeve, the movement of the sleeve is followed by the movement of the keys. Slided forward or rearward according to the movement o£ the sleeve, each key comes into contact with a balk-ring and pushes it against gear hub. The balk-ring has the same function as the cone we have studied in the previous pages, but it has a number of teeth around its circumference instead of a protuberance the cone is provided with. The balk-ring is also provided with two notches each of which is to receive the key, the correspondence to the transmission being allowed by taking up the notch in the hub with which the protuberance on the cone is brought in engagement.
When the sleeve is pushed toward a gear, conical surface on the balk-ring is forced to contact with that on the gear hub. Two surfaces in contact produce a friction force as a slip exists between the two rotating at different peripheral velocities. Under the effect of this friction force, the balk-ring tends to follow the rotation of the gear hub and this tendency makes the teeth of the sleeve out o£ alignment with that of the balk-ring, and thus causing the notch in the balk-ring to come into contact at its edge with the key.
As long as the said friction force exists, the sleeve cannot be allowed to reach the gear hub to engage, because the teeth of the balk-ring in misalignment with that of the sleeve restrain the sleeve from further movement.
When the equalization of peripheral velocities of the balk-ring and the gear hub is reached under the influence of the friction force abovementioned, the friction force ceases to exist as a matter of course. This makes the resistance of the balk-ring ineffective to protect the sleeve from engaging with the gear hub. Thereby, smooth engagement is achieved between the two.
This is a section view of a two-speed forward, two-speed reverse, synchromesh type transmission, widely in use for the forklift trucks.