Notes on Derby Bentley final drives

Uprated Final Drive

The final drive gears – crown-wheel and pinion, ring-gear and pinion, or just CWP – fitted to the early Derby Bentley chassis were similar to those designed for the Rolls-Royce Peregrine chassis. In the fullness of time it was found that these were not strong enough to cope with all driving conditions or styles of driving. This was highlighted after the introduction of the 4 1/4L car, which offered more power and torque. The design of the CWP was altered in detail, and any gears that were fitted or supplied subsequently would have been the later stronger design.

The changes that were made are as follows:
1. The taper on the pinion to which carries the driving flange was made more shallow. This made it more difficult for the tapers to separate.
2. The Woodruff keys between the pinion and driving flange were increased in thickness, from 5/32” to 3/16”.
3. The thread on the pinion that carried the nut retaining the flange was increased in diameter to 7/8” from 3/4”

These enhancements were intended to strengthen the connection between the driving flange and the pinion. On occasions the flange had been known to spin on the pinion, shearing the Woodruff keys.

4. The final change to the pinion concerned the mounting of the out-rigger roller bearing supporting the nose of the pinion. In the original design the inner race of the bearing was carried on an extension of the pinion, and retained by a castle nut and washer. Because of the relatively small diameter of the extension carrying the bearing, and only a small radius in the corner between the extension and the nose of the gear, after a period of time metal fatigue would set in and the extension break away from the pinion. In the later design, the separate inner race of the bearing was deleted, and the diameter of the nose extension of the gear increased such that the rollers ran directly on the extension.

The two features that most readily identify which generation of gear you have is the diameter of the thread for the pinion flange nut and the style of nose bearing – does it have an inner race or not?

5. As well as changes made to the pinion, the mounting of the crown-wheel was also changed. This is mounted on the differential box or carrier assembly, and is retained by a ring of fifteen studs. On the original design the holes in the crown-wheel to which the studs were fitted were tapped 1/4” BSF. At the time the pinion changes were made the threads in the crown-wheel were increased to 5/16” BSF and fitted with larger studs.

If a new CWP gear-set is fitted to replace the early style a number of associated components will also have to be changed as a consequence of the design changes. These are as follows:

Pinion driving flange DA240
Pinion flange nut QSN114
7/8” lock tab washer for the pinion flange nut QTW125
Woodruff keys TKY020
Pinion nose bearing DA025

If a conversion to a modern style of pinion oil seal using DA016 is also being considered we would recommend the use of the driving flange DA241 as this has a smooth surface where the seal is intended to run.

Any new crown-wheel supplied by us will already have the larger stronger studs fitted.