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Volvo announces two transmission developments
Page 40
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• Two important developments in gearbox design were announced by Volvo of Sweden just before the opening of the Frankfurt Show last week. Although not featured inside the exhibition, details were given at a Press conference on the opening day, both developments being based on the existing Volvo R 60 eight-speed range-change synchromesh gearbox. The SR 61 is essentially the R 60 with the addition of splitter gearing at the front end to give 16 forward ratios while the MR 61 has a torque convertor built-in on the engine side of the conventional clutch to boost tractive effort.
In the basic R 60 gearbox the eight forward ratios are evenly spaced with around 40 per cent steps between each. The added splitter gearing gives an overdrive of 0.843 to 1 on each ratio making the steps about 20 per cent in all cases; bottom ratio remains at 11.22 to 1. But with the new gearbox a driver can keep engine speed at the best level and in an example illustrated by Volvo the range of engine speeds with the R 60 was between
1,500 and 2200, rpm while the SR 61 reduced the variation to between 1,800 and
2200, rpm.
With such multi-ratio transmissions a driver can have difficulty in remembering exactly which gear is engaged. Volvo caters for this by having the control for the splitter section mounted on the dash with a warning lamp to indicate when the overdrive gear is engaged; the range-change control is kept on the gear-change lever. Both controls can be pre-selected, the splitter change being completed by depressing the clutch and the rangechange when the main-change lever is in the neutral position.
As an example of the advantages obtainable with the SR 61, the R 60 used in conjunction with a final drive ratio of 4.9 to 1 gave a maximum tractive effort of 7.9 tons and with the same engine and SR 61, but coupled to a final drive ratio of 5.6 to 1 tractive effort increased to 9.7 tons while retaining the same maximum speed. Alternatively, maximum speed could have been increased for the same tractive effort.
Improved tractive effort is the main advantage of the MR 61 torque convertor/ synchromesh gearbox. In this case torque output from the gearbox is almost doubled in first gear and even in the higher gears 'Mere is a considerable improvement at low engine speeds.
The torque convertor has a lock-up clutch so that there can be a straight through drive when required. A switch is provided for the driver to do this and it allows improved fuel consumption; there would be no difference to a truck with a normal transmission.
But making use of the torque convertor in built-up areas reduces driving effort as far fewer ratio changes are needed and when the truck is lightly loaded a high ratio can be engaged all the time. On starting from rest a driver selects the appropriate gear as normal (using the clutch) but on releasing the clutch the torque convertor functions as an automatic clutch. Only after engine speed is increased and the brakes are released does the vehicle begin to move.
Starting from rest is smoother than with a conventional transmission and a vehicle is given a better performance on hills. Test figures given by Volvo show that the gradient ability without the torque convertor in use on a 42ton-gross F88 was 10 per cent and that it was increased to 19 per cent—in second gear as against first—with the torque convertor operative. Acceleration times were improved with the torque convertor and while fuel consumption was reduced by 12 per cent in city traffic there was an improvement of up to 8 per cent in hilly districts.