Split! Spicer's SST 10 is positively convenient
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It may be over designed, but it is reliable, says Bill Brock
IN ORDER to make the best use of available engine power, a gearbox must be provided with a spread of ratios giving the desired cruising speed at one extreme and the required gradeability at the other.
The choice of ratios and the differences between them must be based upon the engine characteristics, the type of operation and the gross vehicle weight. Ideally, the number of steps will allow progressive acceleration without the engine having to lug.
Spicer's 10-speed transmission designated SST-10 (Spicer split-torque transmission) has been designed for medium and heavy duty on and off-road applications with vehicles of 27,200kg gvw and above.
Physical dimensions of the unit as a whole are determined by the size and number of gears included. In the case of the SST-10 the deep, low ratio of 8.86 to 1 ffts in to an overall width of 615mm (241/4in) and the ten ratios are accommodated in a length of 713mm (28.1 in) from the flywheel housing. The compact twin counter-shaft design divides the engine torque allowing a reduction in the face width of each straight-cut constantmesh gear. In effect, the SST-10 is two five-speed boxes united by two sets of primary gears so that the gear ratios overlap. This provides full or half-gear selection with either a mechanical or air-shift change. All full gear changes are made through the mechanical shift, and intermediate ratios are engaged with the air-operated splitter unit mounted on the gearbox and controlled by a switch on the shift lever.
Division of the input torque provides the box with a high torque capacity of 1,630Nm (1,200lb ft) and a transmission efficiency of about 97 per cent.
Spicer is the only gearbox manufacturer to employ curvic coupling, which locks with the taper under load.
The driver is able to select either full or half gears and is assured of a positive change even though no pre-select is fitted as standard. The traditional H-pattern gate allows split ratios to be missed out, and the gearbox used as conventional five-speed transmission in light laden conditions.
Each of the ratios gives a geometric progression through the range in incremented steps of around 30 per cent.
Among British commercial vehicle manufacturers only Ford does not include the box as an option. Leyland adopted it for its Roadtrain and Constructor models as standard. At this weight the choice of gearbox type and manufacture is very limited.
The SST-10's twin countershaft design is, however, smal ler and lighter than singlelayshaft designs. Many thousands of this type of box in use all over the world are evidence of its reliability.
Influenced by Continental practice, Leyland developed its own pre-select set to operate at a predetermined clutch hydraulic pressure. In practice, no two clutches are identical. Inevitably ratchetting results on occasions as the four dogs of the curvic coupling take up the load. It is, however, built to withstand this type of treatment.
To meet the requirements of bulker and tipper vehicles, two pt-o openings are provided underneath and are at the rear, from which drives may be taken to power hydraulic tipping gear and blowers.
Many operators looking for ways in which to increase vehicle payload potential specify the aluminium casing in place Of cast iron to save 34.4kg (80Ib) and a further 18.2kg (40Ib) by specifying an alloy clutch housing also.
Initially designed to match the large American engines, the unit has ample reserve capacity for European operation. However, this can only improve its high reliability record.
Spicer advocates its own brand of range change model for heavy off-road operation where the number of ratios in the gearbox may be as high as 14, but with just ten ratios the splitter concept is just so much more convenient to drive.