Field of invention;

The present invention relates to a method of reversibly converting a tandem drive axle into a two-speed axle system. The present invention also relates to a two-speed axle system converted from a tandem drive axle.

A rear axle provides right-angled drive from propeller shaft to the driven wheels. A rear axle is provided to a vehicle where the engine is mounted parallel to the frame. On rear-wheel drive vehicles, rear-axle ratio is an important part of a successful towing equation. It is expressed as a ratio between driveshaft revolutions per minute and the rear axle's wheels revolution per minute. Rear axle ratio plays a vital role in performance of a vehicle. Fuel consumption, acceleration, and gradability substantially depend on the rear axle ratio.

Background of the invention:

A rear axle provides right-angled drive from propeller shaft to the driven wheels. A rear axle is provided to a vehicle where the engine is mounted parallel to the frame. On rear-wheel drive vehicles, rear-axle ratio is an important part of a successful towing equation. It is expressed as a ratio between driveshaft revolutions per minute and the rear axle's wheels revolution per minute. Rear axle ratio plays a vital role in performance of a vehicle. Fuel consumption, acceleration, and gradability substantially depend on the rear axle ratio.

Most vehicles have only one rear axle ratio optimized for ladened condition of the vehicles. Generally, rear axle ratio is optimized for vehicle with full load to operate the engine near to its maximum torque speed band with peak efficiency. However, a vehicle does not operate in full load condition all the time, nearly half of the time it travels empty. Therefore, in trucks or other large vehicles meant for hauling heavy loads, it is advantageous to have two-speed rear axle. In which, low ratio is used for ladened condition and a comparatively high ratio for the unladened condition. The higher rear axle ratio increases the maximum achievable speed of the vehicle and advantageously reduces the vehicle turn around time.

Conventionally, two types of two-speed rear axles are known viz. outboard epicyclic bevel gear two-speed rear axle and helical two-speed rear axle. However, the conventional two-speed rear axles suffer due to its complexity in design, bulky and possessing less transmission efficiency due to addition of gear pairs. Besides, conventional two-speed rear axles are costly and their maintenance cost is also high.

US patent document US4774857 discloses a two speed axle assembly for automotive vehicles comprising a planetary gear set mounted within an axle carrier and including an input sun gear, an output sun gear, and a plurality of compound planet gears within a planet carrier. A pair of clutch packs selectively locks the planet carrier either to the input sun gear for rotation therewith as a unit for one mode of operation, or to the axle carrier case so that rotation of the planet carrier is prevented and the planet gears rotate about their axes to provide the other mode of operation. Compression springs are utilized to normally load one clutch pack and lock the planet carrier to the input sun gear for the direct drive mode. The axle assembly is provided with hydraulic bladder assembly for shifting from the direct to overdrive mode and vice versa. Disadvantageously, the two speed axle assembly as proposed by

US4774857 has a complex mechanism and cumbersome design. Moreover, the two speed axle assembly disclosed in US4774857 is for normal drive axle and is not meant for tandem drive axle. Further, US4774857 does not suggest the method by which a tandem drive axle is reversibly converted a into a two-speed axle system.

Therefore, it is needed to design and develop a low cost two-speed rear axle of simpler design. A most economic and feasible approach to achieve this objective, as proposed by the inventors, is to reversibly convert one of the axles in normal tandem drive axle, with both drive axle, into a two-speed axle.

Summary of the invention:

Accordingly, the invention provides a method of converting a tandem drive axle into two-speed axle comprising the steps of:

- introducing input drive torque through an input shaft into an interaxle differential for division of the input drive torque, said interaxle differential is provided with a pair of sun gears, a first sun gear and a second sun gear,

- transferring the power outputs from said interaxle differential for driving a pair of rear drive axles, a first rear drive axle and a second rear drive axle, through said pair of sun gears, wherein said first sun gear is connected to said first rear drive axle and said second sun gear is connected to said second rear drive axle, and each said rear drive axle having wheels rotatably mounted on either end thereof, and

- interrupting, when required, the transference of power output from one of the sun gears to its corresponding rear drive axle and simultaneously transferring the differential output power only through the other sun gear to the rear drive axle connected thereto, thereby achieving the desired conversion

Preferably, the power output from said first sun gear is transferred to said first rear drive axle by means of a gear mechanism and the power output from said second sun gears is transferred to said second rear drive axle by means of an intermediate propeller shaft.
According to a particular embodiment of the invented method, the transference of output power from one of the sun gears is interrupted by lifting its corresponding rear drive axle assembly and applying brakes on the wheels borne by said axle.

According to another embodiment of the invented method, the transference of power output from one of the sun gears is interrupted by disengaging intermediate propeller shaft connected thereto and simultaneously locking the sun gear.

When the vehicle is operating under laden condition one needs to lower the rear axle ratio. According to the invented method, the rear axle ratio of the two-speed axle system is lowered from its higher value by releasing the locked sun gear. Once the locked sun gear is released both the sun gears starts rotating again and the drive is transmitted to both the rear drive axles through their corresponding sun gears.

According to another aspect of the present invention, there is also provided a two-speed rear axle system for a vehicle converted from a tandem drive axle comprising:

- an input shaft to receive input drive torque from a power source;

- a pair of rear drive axles, a first rear drive axle and a second rear drive axle , having wheels rotatably mounted on either ends thereof,

- an interaxle differential to receive and divide the input torque from the input shaft, said interaxle differential comprises a pair of planetary gears, first planetary gear and second planetary gear, mounted on said input shaft for rotation therewith, and a pair of sun gears, a first sun gear and a second sun gear to mesh with and being driven by said planetary gears, wherein said first sun gear is linked with said first rear drive axle and said second sun gear is linked said second rear drive axle,

- a gear mechanism to transfer output power from said first sun gear to said first drive axle through a first pinion and crown wheel arrangement,

- an intermediate propeller shaft to transfer output power from said second sun gear to said second driving axle through a second pinion and crown wheel arrangement,

- an interrupting means to interrupt the transference of power output from one of the sun gears to its corresponding rear drive axle, and simultaneously transferring the differential output power only through the other sun gear to the rear drive axle connected thereto.

According to a preferred embodiment of the invented two-speed rear axle system, the intermediate propeller shaft has a bisectional construction and comprises of:

- a front intermediate propeller shaft in conjunction with the first sun gear, and

- a rear intermediate propeller shaft connected to said second pinion and crown wheel arrangement, and said interrupting means to interrupt transference of output power from one of the sun gears is a clutch-brake combination unit comprises of:

- a brake side unit having a bearing joint with the front intermediate propeller shaft

- a clutch side unit connected with the rear intermediate propeller shaft, and

- a sliding dog disposed between said brake side unit and clutch side unit;

such that when the vehicle is ladened said sliding dog is made to mesh with the clutch side unit so that differential output power is transferred from said second sun gear to said front intermediate propeller shaft and then on to said rear intermediate propeller shaft and finally onto the second rear axle ; and

and when the vehicle is unladened said sliding dog is disengaged from said clutch side unit and is made to mesh with said brake side unit so that when the second sun gear is locked, the rotation of the front intermediate propeller shaft is stopped thereby interrupting transmission of drive to the rear intermediate propeller shaft; the differential power output being thus transferred only through said first sun gear, which is free to rotate, whereby said first rear drive axle which derives power from the first sun gear starts rotating with a speed higher than the initial and a normal tandem drive axle is converted to a two-speed rear axle. The sliding dog is operable by a switch.

According to a further embodiment of the invention the interrupting means to interrupt transference of output power from one of the sun gears comprises a device for disengaging the inter axle propeller shaft and simultaneously locking its corresponding sun gear.

Brief Description of the Drawings;

For better understanding an illustrative embodiment of the invention will now be described with reference to the accompanying drawings. It will however be appreciated that the embodiments exemplified in the drawings is merely illustrative and not limitative to the scope of the invention, because it is quite possible, indeed often desirable, to introduce a number of variations in the particular embodiment that has been shown in the drawings.

Figure 1 shows Specific Fuel Consumption (SFC) curve of an engine when the vehicle is ladened.

Figure 2 shows Specific Fuel Consumption (SFC) curve of an engine when the vehicle is unladened and the rear axle ratio of the vehicle is high.

Figure 3 shows Specific Fuel Consumption (SFC) curve of an engine when the vehicle is unladened and the rear axle ratio of the vehicle is low.

Figure 4 schematic shows the basic construction of conventional tandem drive axle.

Figure 5 schematic shows the basic construction of a tandem drive axle convertible into two speed axle system according to present invention.

Detailed Description Of The Drawings:

Generally the rear axle ratio of a vehicle is optimised for vehicle with full load so that the engine is operated near to its maximum torque speed bank with peak frequency. Figure 1 of the accompanying drawings illustrates, with the help of the Specific Fuel Consumption (SFC) curve, the performance of an engine operating at laden condition. It is seen that for most of the time, the engine is operating at its minimum SFC region, i.e. near to its maximum torque band and the efficiency of the engine is at its peak.

However, heavy vehicles, such a multi axle trucks, do not operate in full load condition all the time. It has been seen that for nearly half of the time such vehicles travels empty. Referring to figure 2 of the accompanying drawings the Specific Fuel Consumption (SFC) curve shows the performance of an engine operating in unladened condition. From the figure 2 one can see that the engine is not operating at its maximum torque band, i.e. not operating in least SFC zone. In other words, the engine is not operating with peak efficiency. This happens because the rear axle ratio of the vehicle is designed and optimised for full load condition of the vehicle, so with this optimum rear axle ratio the torque that the engine is desired to generate for an unladened vehicle is much lesser than the engine's maximum torque. However, if high rear axle ratio is used in unladened condition of the vehicle, the engine can be made to operate near to its maximum torque speed band with peak efficiency.

Referring to figure 3 of the accompanying drawings, the Specific Fuel Consumption (SFC) curve shows the performance of an engine operating in unladened condition with high rear axle ratio.

On comparing figures 1, 2 and 3 of the accompanying drawings, it is seen that low rear axle ratio is suitable for vehicle operating with full load, while high rear axle ratio is preferable for running the unladened vehicle. Typically, in the accompanying figures 1, 2 and 3, a ratio of 5.83 indicates low rear axle ratio, while a ratio of 2.91 indicates a high rear axle ratio.

Referring to figures 4 and 5 of the accompanying drawings, the tandem drive axle (A) comprises an input shaft (I) , a pair of rear drive axles , a first rear drive axle (AX1) and a second rear drive axle (AX2), an interaxle differential (D) to receive and divide the input torque from the input shaft (I), a gear mechanism Gl and G2 to transfer the differential power output to said first rear drive axle (AX1) , a intermediate propeller shaft (IS) for transferring the differential power output to second rear drive axle (AX2). The interaxle differential (D) further comprises a pair of planetary gears, a first planetary sun gear (PG1) and second planetary sun gear (PG2) mounted on the input shaft (I) for rotation therewith and a pair of sun gears, a first sun gear (SI) and a second sun gear (S2) to mesh with and being driven jointly by the planetary gears PG1 and PG2. There is no direct connection between the input shaft (I) with gear G1 or with the first sun gear (SI). The drive is transmitted to the sun gears (SI) and (S2) through the planetary gears PG1 and PG2. The differential output from the first sun gear (SI) is first transferred to the gear mechanism comprising of Gl and G2. Therefrom, the output drive is transferred to the first rear drive axle (AX1) through a first pinion and crown wheel arrangement (PI, CI). Finally, the drive is transferred to the wheels (Wl) and (W2) borne by the first rear axle (AX 1).Simultaneously, the output drive from the second sun gear (S2) is first transferred to an intermediate propeller shaft (IS); therefrom the drive is transferred to the second rear axle (AX2) through a second pinion and crown wheel arrangement (P2,C2), and finally on to the wheels (W3) and (W4) of the second rear axle (AX2).

According to a particular embodiment of the invented method, a tandem drive axle is converted into a two-speed axle by lifting one of the drive axles and simultaneously applying brakes on the wheels of the lifted axle. Referring to figure 4 of the accompanying drawings, when the second rear axle (AX2) is lifted and simultaneously brake is applied on the wheels (W3), (W4), the lifted wheels (W3) and (W4) become jammed, which in turns jam the second crown wheel (C2) and the second pinion (P2).

The intermediate propeller shaft (IS) is also jammed and the second sun gear (S2) stops rotating. Thus, the power output from the second sun gear is interrupted and the input torque, being delivered to the planet gears PG1 and PG2 by the input shaft (I), is transferred only through the other sun gear i.e. the first sun gear (SI) which is free to rotate. Although the second sun gear (S2) gets locked and stops rotating, both the planet gears (PG1 and PG2) continue to rotate about their own axis. Owing to this continued rotation of the both sun gears (PG1 and PG2) the first sun gear (SI) will start rotating at a speed higher than that of the initial by the action of the third differential. The first sun gear (SI) is indirectly connected to the wheels Wl and W2 of the first rear wheel axle (AX1), and so the wheels Wl and W2 are made to rotate with a speed higher than that of the initial. Thereby, a normal tandem drive axle is converted to a two-speed rear axle. By adopting this method one gets the advantages of both lift axle and two-speed axle without incurring much cost.

A mechanism of lifting a rear drive axle has been disclosed in our co¬pending Indian patent application 2604/CHE/2009 entitled "Lift Axle Suspension System".

According to another embodiment of the invented method, a tandem drive axle is converted into a two-speed axle by disengaging the intermediate propeller shaft (IS) and simultaneously locking its corresponding sun gear. Referring to a particular embodiment shown in figure 5, the intermediate propeller shaft (IS) is a bisectional construction comprising a front intermediate propeller shaft (II) and a rear intermediate propeller shaft (12). The front intermediate propeller shaft (II) is in conjunction with the first sun gear (SI) while the rear intermediate propeller shaft (12) is connected to the second pinion and crown wheel arrangement (P2, C2). The front intermediate propeller shaft (II) and a rear intermediate propeller shaft (12) are provided with a clutch-brake combination unit (CBU). The clutch-brake combination unit (CBU) comprises a brake side unit (Ul), a clutch side unit (U2) and a sliding dog (SG) disposed between the brake side unit (Ul) and the clutch side unit (U2). The brake side unit (Ul) has a bearing joint with the front intermediate propeller shaft (II), while the clutch side unit (U2) is connected with the rear intermediate propeller shaft (12). The sliding dog (SG) is operable by a switch (not shown).

When the vehicle is laden, the sliding dog (SG) is made to mesh with the clutch side unit (U2) and the differential output power is transferred from the second sun gear (S2) to the front intermediate propeller shaft (II) and then on to the rear intermediate propeller shaft (12). Finally, the drive is transmitted to the second rear axle (AX2).

When the vehicle is unladen, the sliding dog (SG) is disengaged from the clutch side unit (U2) and is made to mesh with the brake side unit (Ul). Once the sliding dog (SG) is meshed with the brake side unit (Ul) the sliding dog (SG) stops rotating, which in turn stops rotation of the front intermediate propeller shaft (II). When the front intermediate propeller shaft (II) becomes stationary, the rotation of the second sun gear (S2) is also arrested. Since the second sun gear (S2) is locked, no drive can be transmitted to the rear intermediate propeller shaft (12); and, by the action of the third differential, the differential power output is only transferred through the other sun gear, the first sun gear (SI) which is free to rotate. Thereby, the first rear drive axle (AX1), which derives power from the first sun gear (SI), starts rotating with a speed which is higher than the initial and a normal tandem drive axle is converted to a two-speed rear axle.

As already mentioned, the foregoing description is illustrative of the invention and not limitative to its scope, because it will be apparent to persons skilled in the art to device other alternatives embodiments without departing from the broad ambit of the disclosures made herein.

We claim:

1. A method of converting a tandem drive axle into two-speed axle
comprising the steps of:

- introducing input drive torque through an input shaft into an interaxle differential for division of the input drive torque, said interaxle differential is provided with a pair of sun gears, a first sun gear and a second sun gear,

- transferring the power outputs from said interaxle differential for driving a pair of rear drive axles, a first rear drive axle and a second rear drive axle, through said pair of sun gears, wherein said first sun gear is connected to said first rear drive axle and said second sun gear is connected to said second rear drive axle, and each said rear drive axle having wheels rotatably mounted on either end thereof, and

- interrupting, when required, the transference of power output from one of the sun gears to its corresponding rear drive axle and simultaneously transferring the differential output power only through the other sun gear to the rear drive axle connected thereto, thereby achieving the desired conversion.

2. The method of converting a tandem drive axle into two-speed axle as claimed in claim 1, wherein the power output from said first sun gear is transferred to said first rear drive axle by means of a gear mechanism and the power output from said second sun gears is transferred to said second rear drive axle by means of an intermediate propeller shaft.

3. The method of converting a tandem drive axle into two-speed axle as claimed in any of claims 1 and 2, wherein the transference of output power from one of the sun gears is interrupted by lifting its corresponding rear drive axle assembly and applying brakes on the wheels borne by said axle.

4. The method of converting a tandem drive axle into two-speed axle as claimed in any of claims 1 and 2, wherein the transference of power output from one of the sun gears is interrupted by disengaging intermediate propeller shaft connected thereto and simultaneously locking the sun gear.

5. A two-speed rear axle system for a vehicle converted from a tandem drive axle comprising:

- an input shaft to receive input drive torque from a power source;

- a pair of rear drive axles, a first rear drive axle and a second rear drive axle , having wheels rotatably mounted on either ends thereof,

- an interaxle differential to receive and divide the input torque from the input shaft, said interaxle differential comprises a pair of planetary gears, first planetary gear and second planetary gear, mounted on said input shaft for rotation therewith, and a pair of sun gears, a first sun gear a second sun gear to mesh with and being driven by said planetary gears, wherein said first sun gear is linked with said first rear drive axle and said second sun gear is linked with said second rear drive axle,

- a gear mechanism to transfer output power from said first sun gear to said first drive axle through a first pinion and crown wheel arrangement,

- an intermediate propeller shaft to transfer output power from said second sun gear to said second driving axle through a second pinion and crown wheel arrangement,

- an interrupting means to interrupt the transference of power output from one of the sun gears to its corresponding rear drive axle, and simultaneously transferring the differential output power only through the other sun gear to the rear drive axle connected thereto.

6. The two-speed rear axle system for a vehicle converted from a tandem drive axle as claimed in claim 5, wherein said intermediate propeller shaft has a bisectional construction and comprises of:

- a front intermediate propeller shaft in conjunction with the first sun gear, and

- a rear intermediate propeller shaft connected to the second pinion and crown wheel arrangement,
and wherein said interrupting means to interrupt transference of output power from one of the sun gears is a clutch-brake combination unit comprisies of:

- a brake side unit having a bearing joint with the front intermediate propeller shaft

- a clutch side unit connected with the rear intermediate propeller shaft, and

- a sliding dog disposed between said brake side unit and said clutch side unit;
such that when the vehicle is ladened said sliding dog is made to mesh with the clutch side unit so that differential output power is transferred from said second sun gear to said front intermediate propeller shaft and then on to said rear intermediate propeller shaft and finally onto the second rear axle ;

and when the vehicle is unladehed said sliding dog is disengaged from said clutch side unit and is made to mesh with said brake side unit so that when the second sun gear is locked, the rotation of the front intermediate propeller shaft is stopped thereby interrupting transmission of drive to the rear intermediate propeller shaft; the differential power output being thus transferred only through said first sun gear, which is free to rotate, whereby said first rear drive axle which derives power from the first sun gear starts rotating with a speed higher than the initial and a normal tandem drive axle is converted to a two-speed rear axle.

7.The two-speed rear axle system for a vehicle converted from a tandem drive axle as claimed in claim 6, wherein said sliding dog is operable by a switch.

8.The two-speed rear axle system for a vehicle converted from a tandem drive axle as claimed in claim 5, wherein said interrupting means to interrupt transference of output power from one of the sun gears comprises a device for lifting its corresponding rear drive axle.