A New Easy-change Gear
Page 49
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from the U.S.A.
ADEVICE named the Synchro-Shift for facilitating gear changing has'been developed in America by the Spicer Manufacturing Corporation for use in conjunction with the Brown-Lipe transmission on commercial vehicles, and is of interest, apart from the most ingenious mechanism it incorporates, by reason of its resemblance to a recently introduced British system, viz., the Dennis easy-change gear, which was fully described in the issue of The Commercial Motor dated October 28.
The principle is identical, insofar as the transmission is disconnected during the change by a dog clutch at the rear, and that the gearwheels are brought to a standstill by a clutch brake. A vacuum cylinder, too, is employed in the Spicer system, but only to operate the dog clutch, the disengagement of the friction clutch and the gear shift being accomplished by muscular effort, and the vacuum valve controlled mechanically.
The accompanying diagram of the general arrangement shows how the clutch and gear operation automatically actuate the valve, which causes movement of the piston (1) to the left. This movement disengages the dog clutch by sliding the inner member (2) out of mesh with the fixed member (3), the sliding member being retained in the disengaged position by the catch (4), but ell ready to mesh by pressure of the spring (5), so soon as the catch is released. The dogs, therefore, remain disengaged until after the gear change has been made and the friction clutch has taken up the drive again.
The novel feature of the mechanism is the system employed to make use of the synchronization of the speeds of the units of the dog clutch to bring them into re-engagement.
The complete dog-clutch mechanism is housed in a drum (6) which is splined to the outside of the dog (3). Splined internally to the drum and externally to the sliding dog is a simple over-running free wheel (7), the function of which is not to transmit the drive to the road wheels but simply to release the catch (4) at the crucial moment.
So long as the tailshaft is turning faster than the gearbox mainehaft, the free-wheel outer ring is over-running. So soon, however' as the engine accelerates and brings the speed of the mainshaft up to that of the tailshaft, the two parts of the free wheel turn as a unit. This point in the cycle of operations is immediately followed by a tendency for the free wheel to transmit the drive, which it attempts to do through the wedge-shaped dogs separating the'two parts of which its outer member is composed.
The natural consequence is for the two parts (marked 8 and 9 in the detail drawing) to move apart axially. In doing so, 8 exerts a thrust on the left end of the drum (6) against the springs (10). The ensuing motion of the drum to the left is transmitted by a lever (11) to the catch (4), which releases the sliding member and the dogs then mesh with one another.
To render the free wheel inoperative when using reverse gear, the fulcrum for the lever (12) which actuates tbe vacuum valve is brought, by the engagement of reverse, into such a position that the lever cannot depress the valve rod; this is accomplished by forming the fulcrum on a lug attached to the right end of the reverse-gear selector rod.
We are indebted to our American contemporary Automotive Industries, for the details of this interesting new gear.