GEARING-UP THE TOP SPEED OF THE F.W.D.
Page 14
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Details of a Simple and Practicable Type of Auxiliary Gearbox.
TN AN editorial which apPeared in _Lour issue of June 30th, we pointed out some of the good points of the geared-up top speed which could be utilized in advantageous circumstances, whereas the direct drive could be employed in hilly country, traffic driving, etc.
Following this, we have received from the Four Wheel Drive Lorry Co., Ltd., 46, Charing Cross, London, S.W. 1, details of the auxiliary gearbox which they designed some two or three years ago for the F.W.D. chassis, and which has proved to give many benefits, including a considerable saving of petrol and a higher speed if required.
The gearbox, of which we reproduce drawings, is designed to be placed between the clutch and the main gearbox in the standard model F.W.D., of which many thousands are used in this country. This gives an indirect top gear, the increase in the actual model illustrated being 37i per cent.
The auxiliary box fits into the chassis in a very simple manner, as it registers into the front end of the main gearbox casing in place of the original front housing, and as it embodies an alignment joint which replaces the original joint, the engine has to be moved forward in the chassis only 1i ins.
The engagement of this gear is by a separate hand lever, and no complications in driving have been found to result from the addition of the device, as
it is a perfectly simple matter to operate the original change-speed lever until top gear is reached, and then merely to pull over the auxiliary lever in the ordinary manner.
Reference to our illustrations will show that the auxiliary gearbox is designed in a thoroughly straightforward manner. Driven from the clutchshaft through the medium of a universal joint is the primary shaft of the auxiliary gearbox. On this is carried, by a roller bearing, a large spur wheel meshing with a smaller pinion on the short countershaft.
At the rear end the primary shaft is splined to carry a double dog clutch ; it is also recessed for the roller spigot bearing• of the primary shaft of the main gearbox, the forward end of which carries, on a square just behind the spigot, a spur wheel meshing with a second pinion on the conntershaft.
It will thus be seen that a free-engine position can be obtained by placing the clutch midway between the two spurwheels. If moved towards the rear, however, it gives direct drive from engine to main gearbox, whereas, if moved forward, it locks the primary shaft to the spurwheel, which, in the ordinary way, runs freely upon it and conveys the drive from the engine through the countershaft and back to the primary shaft of the gearbox.
All the shafts run on large ball bearings, and a ball thrust bearing is carried between the recess in the primary shaft of the auxiliary gearbox and the spigot of the primary shaft of the main gearbox.
Locking of the sliding dog clutch is effected by a spring plunger which forces a steel ball into recesses formed in a special arm on the dog-clutch-operating sleeve.
It will be realized that this overdrive can be employed with all the other gears as well as the top speed, so that it actually doubles the number of speeds.