A baulk-ring type of synchromesh unit is commonly used to synchronize the speeds of two gears before engagement.
Three metal inserts, with a ridge, fit into slots in the hub. A radial spring at each end holds them out, so that the ridges locate in a groove inside the sleeve, and hold it centrally on the hub.
When the synchronizer is assembled, the hub is splined to the main shaft, and the engagement sleeve is splined to the hub.
A bronze baulk ring, with recesses to accommodate the ends of the inserts, is located in each end of the hub, and a conical inner surface faces a matching steel cone on the gear.
Fine grooves are machined on the conical surface, and the teeth on the outer edge are the same size as the dog teeth on the gear, and the spline on the sleeve.
The recesses are wider than the inserts, to allow the baulk ring to move radially. Then the teeth on the baulk ring can be out of register with the teeth on the sleeve.
When the sleeve is moved to select a gear, the spring-loaded inserts move the baulk ring into contact with the conical face of the gear.
The grooves in the face of the baulk ring help to break through the oil film, and the difference in speed of the two components causes the baulk ring to be dragged around with the gear to the limit of the recesses, where it is held by the inserts.
Since the teeth of the baulk ring are now out of alignment with the teeth of the sleeve, this baulks, or prevents, the sleeve from moving over the ring into engagement.
The force exerted by the driver now presses the sleeve against the teeth of the baulk ring, and forces the conical face against the cone of the selected gear, until the friction created, causes the two components to rotate at the same speed.
The baulk ring teeth can now come into alignment and the sleeve slides over them, and into engagement. This is assisted by a chamfer on the teeth, which helps to guide the sleeve into position.
Smooth and rapid gear changes are thus ensured.