Step-up step-down overdrive gearbox
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• Three overdrive ratios in front of a four-speed constant-mesh gearbox and a wide-ratio final drive are features of a seven-speed transmission fitted to a Foden 24-ton-gross tractive unit and a Foden eight-wheeled tipper by Reg Knowles and Sons Ltd, Tansley, Matlock; Derbyshire, that reduce the torque throughput .of the gearbox in all the ratios and provide a top speed of 70 mph plus. The transmission is of outstanding technical interest, writes Paul Brockington, as it is an application of the step-Up/step-down principle to a road vehicle combined with a range of overdrive ratios that provide for greatly reduced engine rpm at high cruising speeds.
Possibly of greater importance, the principle enables a transmission with a relatively low-torque rating to be used for a variety of applications in which the torque input is up to twice that of the torque throughput. And this obviously increases the scope of the transmission, the highest step-up ratio that can be employed being dependent on the availability of a back axle with a suitable final drive ratio.
The axles of the two Fodens are of the worm-drive type with a ratio of 7.5 to 1 which replace axles having a ratio of 5.75 to I. The seventh gear has a ratio of 0.40 to 1, which increases the rpm of the propeller shaft by 2.5 times compared with direct drive, but this is cited as an advantage by Reg Knowles—who evolved the gearbox —given that the centre distance of the shaft does not exceed about 4ft. He claims that the low-torque loading of the gearbox and propeller shaft promotes long life Of the gears and bearings of the box and of the splines and yokes of the propeller shaft. The
Foden tractive unit covered around 100,000 miles in the Esso fleet with the Knowles gearbox fitted (the unit has since been purchased by Reg Knowles for demonstration purposes) and inspection after the side cover had been removed revealed that the gears were in an as-new condition.
A 30-mile motorway run in the vehicle operating solo indicated the probability that it was capable of at least 70 mph. At a cruising speed in 7th ratio of around 55 mph, the Leyland 0.680 diesel (derated to 150 bhp) is operating at about 1200 rpm. Its rated maximum rpm is 2100 rpm; equipped with a standard five-speed direct-top gearbox the road speed of the vehicle was less than 40 mph.
The Foden eight-wheeled tipper fitted with the box is operated by Hullard Products Ltd, of Derby. It is powered by a Gardner 6LX diesel developing 150 bhp at 1700 rpm and was originally equipped with a Foden 12-speed box giving an overdrive ratio of 0.77 to 1. Conversions of the transmissions of 40/50-ton low-loaders powered by Gardner 6LXb diesels are to be made for two companies in the near future and firm orders have been received from a number of operators of standard types of goods vehicle. An eight-speed version of the box will be built shortly.
Reg Knowles is a service and spares agent of the Foden company and use is made where possible of Foden gears, the main four-speed section of the Knowles seven-speed box being based on the Foden four-speed unit of the company's 12-speed transmission. The casting of the casing and a number of components, including gears and shafts, are however produced by local specialists. The transmission is marketed at a very competitive price.
The ratios of the 5th and 6th speeds are 0.66 to 1 and 0.53 to 1, (compared with the 7th gear overdrive ratio of 0.40 to 1) and the relatively wide steps provided do not conform to the stated requirements of some vehicle makers and operators. If, however, the power available is sufficient to propel a vehicle at over 70 mph and normal cruising speed in 55/60 mph, the 6th overdrive gear ratio could be near ideal in typical applications. Down changes of overdrive ratios can be made without using the clutch, given that they are matched to the appropriate road speed.
Of special importance, in Reg Knowles's view, the gearbox is controlled with a single lever. He contends that dual control in any form is undesirable on the score that it can, and is; misused by the driver—and that mis-use is costly in terms of maintenance and premature failure.
While the three overdrive ratios could be used (in theory) to split the ratios of the main section to give 12 speeds, this is of academic interest only because splitting would not give acceptable steps and would necessitate the use of dual control. Variations of the gear ratios of the overdrive section could readily be provided to suit operator requirements. Although worm gearing is the only type of final drive that can give the necessary wide ratio, in the case of a single-reduction axle, the gearbox could be employed with a conventional type of double-reduction axle.