Twin speed rear axle – functions explained.

The Twin speed rear axle is used in vehicles with a single drive axle. The operator can select the range or speed of this axle with a button/lever. The button/lever actuates the required gear and changes the drive ratio accordingly.

The Twin speed rear axle doubles the number of gear ratios available for driving the vehicle under various load and road conditions. For example, a vehicle with a two-speed unit and a five-speed transmission, ten different forward speeds are available. This unit provides a gear ratio high enough to permit pulling a heavy load up steep grades and a low ratio to permit the vehicle to run at high speeds with a light load or no load.
Normally, city driving and highways need different torque and gear ratios. This means a single  drive line cant be suited for varying duty and load cycle. Which means a drive line which is not specifically suites for an application may have significant influence in the fuel efficiency. Identifying this requirement, the twin-speed rear axle technology has been developed. Ideal for an open highway condition, the technology allows the driver to shift to the fuel-economy mode while cruising, just by pressing the ‘Twin-Speed’ button.

The conventional spiral bevel pinion and ring gear drives the two-speed unit, but a planetary gear train is placed between the differential drive ring gear and the differential case. The internal gear of the planetary gear train is bolted rigidly to the bevel drive gear. A ring on which the planetary gears are pivoted is bolted to the differential case. A member, consisting of the sun gear and a dog clutch, slides on one of the axle shafts and is controlled through a button or lever accessible to the operator.

When in high range, the sun gear meshes with the internal teeth on the ring carrying the planetary gears and disengages the dog clutch from the left bearing adjusting ring, which is rigidly held in the differential carrier. In this position, the planetary gear train is locked together. There is no relative motion between the differential case and the gears in the planetary drive train. The differential case is driven directly by the differential ring gear, the same as in the conventional single fixed gear final drive.
When shifted into low range, the sun gear is slid out of mesh with the ring carrying the planetary gears. The dog clutch makes a rigid connection with the left bearing adjusting ring. Because the sun gear is integral with the dog clutch, it is also locked to the bearing adjusting rings and remains stationary. The internal gear rotates the planetary gears around the stationary sun gear, and the differential case is driven by the ring on which the planetary gears are pivoted. This action produces the gear reduction, or low speed, of the axle.
Meritor offers this product and is available in various ratios and a manufacturer can choose depending on his requirement. MS 13 240 was one product developed specifically for India and is available in nine ratio sets ranging from 4.10/5.59 to 7.17/9.77. This means there is enough options available for every single duty cycle.
Ashokleyland 2615
The rear wheels of a vehicle do not always turn at the same speed. When the vehicle is turning or when tire diameters differ slightly, the rear wheels must rotate at different speeds.
If there were a solid connection between each axle and the differential case, the tires would tend to slide, squeal, and wear whenever the operator turned the steering wheel of the vehicle. A differential is designed to prevent this problem.
Merits:
By using a lower or shallow ratio during cruising, the engine will be running in a relatively lower rpm and which essentially takes less fuel and there by improving fuel performance upto 6% to 8%.
Demerits:
The usage of lower ratio is purely a tricky  subject and driver should know exactly when to use it. Otherwise he may start to burn more fuel than desired, as without enough torque,the engine may stall.
Product availability:
Ashokleyland offered this for the first time in 2012 in all their haulage models including 2516IL, 2516XL, 3116IL and 3118IL.
Currently Tata Motors is offering in their haulage models that includes LPT3118 56wb

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