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Limited-slip Differential to be
Page 49
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Made in Britain
American Powr-Lok Differential Suitable for Commercial Vehicles to be Produced by Salisbury Transmission
ikLTHOUGH the application of differential gearing to the driving axles of road vehicles was an allimportant development, it created the bogey of lost traction when the coefficient of friction between the tyre and road surface on one side was substantially lower than the friction provided on the opposite side.
If one of the wheels of a conventional back axle slips because of reduced friction, the amount of power that can be transmitted to the opposite wheel is reduced correspondingly, despite a higher coefficient of friction between the tyre and road surface. Various designs of controlled-slip differential have been produced over the years to overcome this difficulty without introducing tyre scrub on curves, but until recently their use has been Strictly limited.
Reduced Wear and Tear
Whilst an improvement in traction by transferring some of the torque from the slipping wheel to the one on the opposite side is the main advantage provided by slip control in normal road and site operations during wintry weather, it offers many extra advantages to the typical commercial-vehicle user. These include the elimination or reduction of shock stresses in the transmission and lower tyre wear in the case of a vehicle running empty on the return journey. It can also prevent skidding when accelerating.
Many transmission failures can he directly attributed to a wheel which spins freely on an icy or greasy surface and then moves to harder or rougher ground giving a much higher coefficient of friction. The kinetic energy of the rapidly decelerated wheel assembly must then be absorbed by the transmission system and this may impose a load . which will fracture .a component of the system by imposing, an overload in excess of any normal safety margin.
An important claim for constantperiodicity suspension systems is that they maintain improved 'contact between the tyre and the road when the vehicle is lightly loaded and thus reduce tyre wear. The control-slip differential offers much the same advantage, in that it prevents all the driving torque being transferred to one wheel during the period of bounce and momentary scrub of the tyre when the accelerated wheel again makes contact with the road. Its use also eliminates transmission shock stresses resulting from inertia loading.
Some two years ago full-scale production of the Powr-Lok differential was started by the Dana Corporation of America, in collaboration with the patentees, the Thornton Axle Co. After successful application to cars and light commercial vehicles up to a capacity of 3 tons, this patented unit is to be produced by Salisbury Transmission, Ltd., Birmingham, 6. The range will include units for heavier commercial vehicles if the demand warrants their manufacture in quantity.
Commercial-vehicle manufacturers who offer the Powr-Lok differential as an optional extra in America include Studebaker, Ford and Dodge, as well as seven makers of private cars.
Torque " Proportioned "
• Known as a " torque-proportioning " unit, the Powr-Lok basically comprises a conventional differential, with a modified form of pinion and cage assembly, and two small multi-plate clutches. The diameter of the unit is similar to that of a standard differential but its overall length is somewhat increased, although it may normally be installed in the space available for the standard differential.
When a torque is applied through the crown wheel to the pinion cage a thrust is imparted to both clutches, which tend to lock the half-shafts to the cage. When the torque loading is reduced on the outer wheel because of a curve in the road, the thrust on the clutch is reduced compared with the thrust on the opposite clutch, and this allows the outer wheel to move through a larger circle without causing tyre scrub. At the same time, an increased torque is applied to the inner wheel.
In practice, the maximum " torqueproportioning " ratio is 4 to 1, which applies to, the two wheels of a vehicle travelling on a curve and to a comparison between a slipping and non-slipping wheel.
If the coefficient of friction on one side approaches zero, no torque can be transmitted to the other wheel, but, in the event of one wheel being virtually free to rotate, driving torque can be applied to the opposite shaft by light application of the brakes. Although the braking force will be the same on both sides, torque multiplication of 4 to 1 enables the wheel on the better surface to provide up to three times the traction of the slipping wheel.
A design detail of particular interest is the method of assembling the snider gear, which comprises four pinions mounted on separate shafts, between which a certain relative movement is allowed both axially and rotationally. Each shaft is mounted in -its respective half-cage in V-slots, the Vs of one shaft being on the opposite side to those of the second shaft.
Shafts Separate When torque is -applied to the cage the two shafts tend to separate. In both cases the separating force is transmitted through cylindrical extensions of the pinions to a thrust ring splined to the half-shaft, which also carries a set of clutch plates on external splines cut in the outer periphery of the main bore section.
The mating plates are formed with peripheral tongues which fit into slots in the cage. Steel clutch plates are employed and full engagement is provided with little axial movement of the thrust ring.
The angle of the V-slot may be 30° or 45", according to the clutch action required for a given driving torque. A special type of pearlitic malleable iron is employed for the two half-cages.
Each clutch comprises a total of either four or five plates, which can be arranged in different positions to vary the friction characteristics. For example, two ringmounted plates may be located in adjacent positions in the centre or to mate on both sides with the cage-mounted plates.
If severe conditions of service frequently impose heavy loading on the clutch units, renewal may be necessary before replacement of the main differential components is required, Normal differential action is, however, provided in the event of plate failure, After dismantling the axle, replacement plates can be fitted in a few minutes. The only special precaution that must be taken is to fill the differential with a special type of lubricant for efficient operation of the clutches.