Split the torque with a belt
Page 64
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HOW THE DAF SYSTEM MIGHT BE ADAPT TO TRANSMIT HIGHER POWERS ACCORDING to Van Doorne's Autoniobielfabriek NV, Eindhoven, Holland, the company's Variomatic fully automatic infinitely-variable belt-drive transmission operates at an overall efficiency of 96 per cent in top gear, the power loss between the engine and driving wheels being 4 per cent. This is comparable to the efficiency of a conventional gearbox (an improved relative efficiency is claimed for the lower ratios), and when considering the potential of stepless belt-drive transmission for applications to larger vehicles, the designer would be concerned in the main with evolving a system that would transmit higher powers without increasing the bulk of the transmission unduly, and would operate for an acceptable period between belt replacements. Providing adequate efficiency would not be a problem.
In the event, a transmission could probably be produced that would be cheaper as well as more efficient than any other current form of infinitely-variable drive, with the added advantage that it would be shock proof.
The air-cooled engine of the DAF car, light van, four-wheel-drive Pony (developed for the United Slates Army) and various special vehicles develops only 30 b.h.p. and, in the case of the car, the Variomatic transmission on average runs over 20,000 miles before belt replacement is required. It is notable that 'first, the car was designed round the transmission and secondly, that the transmission has been successfully applied to a Formula 3 racing car, the power unit of which develops over 90 b.h.p. Special vehicles fitted with the transmission include a mail van for the Swedish post office, a 5-cwt. truck for an American gas company and motor sleighs for use in Norway, France and Italy.
A big increase
Although the favourable record of the transmission and its application to a racing car give promise of wider acceptability in a limited field, the possibility of developing a successful belt drive for standard vehicles in the 50 h.p.-plus category would appear to be remote unless it were incorporated in a split-torque system in which a major proportion of the torque were transmitted by epicyclic gearing. This could undoubtedly be done and might enable the torque rating to be increased by say 300 per cent and belt life to be greatly extended. Transmission specialists in this country have confirmed that the project is feasible. Taking an example of an advanced type of hydrostatic-mechanical split-torque transmission, the system may be cited that is being developed by Dr. J. G. Giles, exdeputy director of the Motor Industry Research Association, which was described in COMMERCIAL MOTOR some two years ago (March 13, 1964).
In this and comparable systems, splitting the torque enables efficiency to be increased by reducing the proportion of power transmitted by the pump and motor of the hydrostatic section to a maximum of 45 per cent and a minimum of zero per cent, the units being locked hydraulically when the transmission is operating in the highest and lowest ratios, thus eliminating pumping losses.
Mechanical brake
In the proposed system, an infinitelyvariable belt drive would be used in place of hydrostatic units and thereby it might be possible to reduce the maximum proportion of the power relayed by the belts by a comparable or greater amount (a well-known gearbox manufacturer considers that a reduction to 30 per cent might well be achieved) and the average proportion to 1015 per cent. Locking the belt pulleys would be performed with the aid of a mechanical brake; in an alternative system the pulleys would operate at a 1 to 1 ratio in the highest gear, which would be favourable to long life compared with transmitting the power differentially.
It is appropriate to re-emphasize that the cost (and possibly weight) of such a system should compare very favourably with that of other forms of stepless transmission. designed for higher powers and tile designer could afford to specify the best belt that money could buy. If a low-bulk belt system were available that could transmit up to 50 b.h.p. continuously and it were incorporated in a split-torque unit, the drive should be capable of handling powers of over 150 b.h.p. and of performing reliably for 50,000 miles before belt replacement was required. A proprietary type of load-speed governor could be used to give automatic control, and a manual over-riding control could be offered.
Long-life application
A different type of application, the replacement of the hydraulic convertor of a Voith , Diwabus type of transmission by a variablepulley drive, also represents an interesting design exercise. In this case, the belts would transmit a high torque for a relatively short proportion of the total operating time and zero torque at all times that the lock-up brake were applied to give the highest ratio. lithe vehicle were used mainly for long-distance operations, the life of the belts would be increased proportionately.
The possibility of applying the Variomatic system in its existing form to larger vehicles is solely dependent upon developing a rubber belt (or a chain type that can operate in a grooved pulley) that was man enough for the job. In the unlikely event that belt quality could be improved to match the progress made since the days of the old Rudge-Multi motorcycle having a manually-controlled variable-pulley system, a 100 b.h.p. (plus) drive could be developed without the complication of splitting the torque. The standard Variomatic offers a bonus advantage in providing differential action (in effect) without a differential and a distribution of torque between the two driving wheels that automatically varies according to the load on the wheels.