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Biswas et al. 2011. Int. J. Vehicle Structures & Systems, 3(4), 241-246 ISSN: 0975-3060 (Print), 0975-3540 (Online) doi: 10.4273/ijvss.3.4.05 © 2011. MechAero Foundation for Technical Research & Education Excellence

Inter nati onal Jour nal of Vehicle Structures & Systems Available online at www.ijvss.maftree.org

Validation of Rear Axle with Differential Lock for Off-Road Vehicles Sanjoy Biswasa, Saswata Ranjan Dasb, Goutam Mandalc, and Sanjay Sharmad Engineering Research Centre (ERCJ), Tata Motors Limited, Jamshedpur, India. a Corresponding Author, Email: [email protected] b Email: [email protected] c Email: [email protected] d Email: [email protected]

ABSTRACT: In today’s scenario, rear axle with differential lock arrangement is one of the key required parameter for military vehicles. As per the current central motor vehicles rules an all wheel drive army vehicle can be considered as an offroad vehicle if the vehicle at least has an arrangement of differential lock at its rear axle. This paper details the development of such rear axle with differential lock and validation of the design in-line with the durability test on heavy duty chassis dynamometer. A comparison of recommended design standards and test results is also presented. KEYWORDS: Differential Lock; Design Validation Plan; Tractive Force; Chassis Dynamometer; Central Motor Vehicles Rules CITATION: S. Biswas, S.R. Das, G. Mandal, and S. Sharma. 2011. Validation of Rear Axle with Diffrential Lock for Off-Road Vehicles, Int. J. Vehicle Structures & Systems, 3(4), 241-246. doi:10.4273/ijvss.3.4.05 (cage). This is done because the friction associated to gripping wheel provides the required tractive force [5]. These friction plates are loaded using kind of dished springs that are mounted such a manner that the loading pattern proportionally varies with the torque. For this type of differential, some arrangement for load sensing is needed. Non-slip or limited slip type differential is costly and complex in design. Further, it cannot be used in front wheel drive vehicles. Dog clutch type diff-lock consists of a dog clutch and cover jaw which is fitted with cage. Dog clutch is engaged with cover jaw by mechanical shifting fork. This type of differential design is comparatively simple and less costly than non-slip ones. This design is unable to distribute the torque as per loading like non-slip type differential. Dog clutch type differential is still more familiar in commercial vehicle segment especially in developing countries. In India, army vehicles have few relaxations in performance (e.g. pass by noise) and add on accessories like RUPD, FUPD, SUPD etc. for certification as offroad vehicles. In order to get this certificate, off-road vehicles are required to pass all the mandatory norms of heavy commercial vehicles in line with some specific requirements. Among these, rear axle with diff-lock arrangement is one of the key requirements for certification of military vehicle as an off-road vehicle. In this paper, a typical Banjo type rear axle with dog clutch type differential is used for certification of offroad vehicle on heavy duty chassis dynamometer using coast down techniques. The developed dog clutch type differential is briefly covered in Section 2. Since there are no specific standards available in automotive industry to validate rear axle with diff-lock, the

ACRONYMS AND NOMENCLATURE: DVP AWD CMVR RUPD FUPD SUPD GVW FAW RAW Ft Rf

Design Validation Plan All Wheel Drive Central Motor Vehicles Rules Rear under Protective Device Front under Protective Device Side under Protective Device Gross Vehicle Weight Front Axle Weight Rear Axle Weight Tractive force Tyre rolling friction coefficient

1. Introduction Differential lock (sometimes denoted as “Diff-lock”) is situated inside the differential housing of an axle. In automotive market [1] unlocked and locked type differentials are widely used. An unlocked (or open) differential provides an equal torque to each of the two wheels. However, both the wheels can rotate at different speeds due to unequal requirement of tractive force on each drive wheel. In contrast, a locked differential distributes the torque unequally to both the drive wheels to rotate at the same speed regardless of different traction requirement on each wheel. In this case, both the drive wheels function as a common shaft in an axle due to the engagement of diff-lock. Non-slip or limited slip type and dog clutch type differentials are widely used locked type differential designs [2 – 5]. In first case, a friction factor is intentionally introduced in the differential using clutch friction plates between sun gears and differential cover 241

Biswas et al. 2011. Int. J. Vehicle Structures & Systems, 3(4), 241-246

cylinder prior to put “ON” the diff-lock switch. That time diff-lock becomes engaged. When diff-lock switch is “OFF”, return air will exhaust to the atmosphere via the exhaust port of the solenoid valve.

objectives of this work are to develop a Design Validation Plan (DVP) and preparation of the vehicle for testing on a heavy duty chassis dynamometer.

2. Design & Construction 3. Design Validation Plan (DVP)

In-house Banjo type heavy duty axle (with un-locked differential) was redesigned to introduce a dog clutch type diff-lock. It was needed to redesign the following – a) Axle shaft RH, b) Differential cover (LH & RH) and c) Differential housing cover rear. Further, a dog clutch and a cover jaw are incorporated inside the differential housing. For shifting the dog clutch, a fork on the spline of axle shaft RH and its shift cylinder is used. The shift cylinder is mounted on the differential housing cover rear. Fork and shift cylinder are carry over parts from other diff-lock axles of organization. Fig. 1 shows a cross-section view of the rear axle with dog clutch type diff-lock. Appropriate tolerances, surface treatment and spline/teeth were taken into account during redesign of existing parts and design of new parts. Pneumatic and electric actuations are available for dog clutch type diff-lock operation by means of a shift cylinder. In this work, electric actuation is used for operation of rear axle with diff-lock. This system consists of a solenoid valve with electrical connection, air connection, diff-lock switch, diff-lock indicator and electrical connection with pressure switch at the axle end. The shift cylinder has two ports. First port is working for suction and return of air. Second port is used for fitting a pressure switch. Fig. 2 shows the schematic operation of the proposed rear axle with diff-lock. Difflock indicator works as a safety indicator for vehicle. It glows only when the diff-lock is engaged. The minimum pressure required for operation of the shift cylinder is 6.5 bars. The solenoid valve passes the air to the shift

To prepare a DVP for validation of rear axle with dog clutch type diff-lock, it is important to know the application area and its loading pattern of army vehicles. The considered axle is used in 2-3 variants of army vehicles. The loading requirement is approximately 8200 kg. From the survey, it was clear that there are no over loading case in army application in India. However, we considered a 10% over load on the rear axle. The newly designed rear axle was fitted to an army vehicle having GVW of 12180 kg. Table 1 list the loading pattern for the vehicle durability test. Table 1: Vehicle loading (weights) pattern

Loading on rear axle FAW (kg) RAW (kg) GVW (kg) Normal conditions 4060 8120 12180 With 10% over load 4060 8940 13000 Selection of total run time, cooling interval and preparation of test setup on chassis dynamometer to actual operating conditions are the main areas in the formulation of the proposed DVP. In-house standard [6] for testing of unlocked differential is used for preparation of the DVP. With the introduction of a dog clutch type diff-lock on the existing un-locked differential, it is necessary to check the performance and durability of the redesigned differential assembly. In general, the life of any army vehicle is considered as 100000 km or 5 years. Accordingly, the diff-lock for any axle is designed to meet this target life.

Fig. 1: Cross sectional view (top) of differential housing

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Fig. 2: Schematic of operation of the proposed rear axle with dog clutch type diff-lock

Since the diff-lock is used only in off-road conditions, its use in total life of the vehicle is lesser than that of the un-locked differential. However, it was decided to conduct the durability test of diff-lock for 10 hours (minimum required for testing of unlocked differential). The durability test was carried out to test the failure of diff-lock or other components of the differential assembly. In order to accomplish this, it was decided to conduct the durability test with exact loading pattern of vehicle for the first 10 hours and thereafter and until a failure, a 10% over load factor is used. The tractive force is defined as the torque required to move the vehicle against the friction between the tyre and road surface. The function of diff-lock is to ensure that an equal rotational speed of LH & RH wheels is achieved by the required distribution of torque on each wheel irrespective of different tractive force requirements by locking both the shafts of drive wheels to act as a common shaft for the axle. The tractive force can be calculated in two ways namely from engine side

[11] and from tyre side [12]. In this work, tyre side calculation is adopted for the coast down test method using chassis dynamometer [7 - 10]. The tractive force Ft(N) is calculated using: Ft  9.81R f RAW / 2 (1) Where Rf is tyre rolling friction coefficient = 0.6. One of the key aspects of DVP is to select the run time and cooling interval for the durability test. Based on the study of lubricants/oil temperature limit of various aggregates (engine, AGB, axles etc) we have selected a run time of 30 minutes and a cooling interval of 30 minutes. There was a flexibility to increase the cooling interval up to 15 minutes as per the demand of off-road conditions. An increase in the temperature of lubricants and oil at various interfaces is quite possible for the maximum torque condition of vehicle during coast down. Hence, the temperatures of lubricants/oil at various aggregates were monitored during test.

Table 2: DVP for rear axle validation with diff-lock

S.No

2 3 4

Test name System level – Shifter mechanism test Vehicle level test Vehicle level shock test Abuse test(at end)

5

Durability test

1

Test specification Remarks Vehicle level shifting checking for 100 times (Engagement and disengagement) All these testing 1. Sand terrain; 2. Mud terrain; 3. Torture terrain done by ODT Forward and reverse at GVW Running change (Engagement and disengagement) Chassis dynamometer simulation condition 1. 4x2 mode 2. Vehicle with actual load condition – 10 hrs 3. Vehicle with 10% over load condition till failure 4. No tractive load in rear axle right wheel This testing is done 5. Full traction on rear axle left wheel in chassis 6. Running vehicle for max. torque condition in first gear till failure dynamometer 7. Coefficient of rolling friction = 0.6 8. Run time 30 minutes and cooling interval 30 minutes (changeable if situation demands) 9. Temperature monitoring of lubricants/oils

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Along with the tractive force value, the following are input to the developed DVP:  Engine data sheet;  Cooling package details including max. coolant temperature limit;  Gear box - Oil capacity and max. temperature limit;  AGB (or Transfer case) - Oil capacity and max. temperature limit;  Front Axle - Oil capacity & max. temperature limit;  Rear Axle - Oil capacity & max. temperature limit;  Tyre - temperature limit ;  Weight details of vehicle;  Differential housing details with lock.

4. Test using chassis dynamometer Chassis dynamometer measures the power delivered to the surface of its roller by the drive wheel of a vehicle. It consists of two rollers which are separately driven by AC motors and two brake units to control the rotation of the dynamometer rollers. The details of test setup are shown in Figs. 3 and 4. This setup simulates various road conditions in terms of speed, torque and loading pattern allowing the vehicle to be operated under the same circumstances as it would expect on the test track or highway by feeding the required rolling and drag resistance to the dynamometer rollers. The most significant part of this durability test is to correlate the test conditions with actual situation.

Fig. 3: Overall layout of durability test using chassis dynamometer

Fig. 4: Inside layout of chassis dynamometer

During the test, the left wheel of vehicle is considered being stuck in mud and the right wheel is free to move on air without touching the road surface. At this condition, the tractive force requirement is very high on the left wheel compared to the right wheel. This

condition was simulated by lifting the right wheel as shown in Fig. 5. The diff-lock was engaged. A fan in front of differential and another fan for left tyres were used for cooling purpose. 244


diff-lock met the durability requirement of rear axle for 10 hrs with a tractive force of 23900 N at 2200 rpm on left rear wheels. After 10 hrs, the tractive force with 10% over load in rear axle is applied to predict the optimum life of the diff-lock. Some sound and jerk came from differential and the rotation of left wheels stopped after 4 hrs with a tractive force value of 26310 N at 2100 rpm. Only right wheel was rotating at that time. Immediately the test was stopped. The differential was opened for failure analysis and we observed that all the tooth of the diff-lock were sheared as shown in Fig. 8. Table 3: Temperature output from test for different tractive force

Temperature (oC)\ Normal loading With 10% over Tractive force (N) (23900 N) load (26310 N) Diff-lock oil 100 – 188 96 – 119 AGB oil 98 – 113 109 – 113 Engine oil 103 – 105 103 – 105 Coolant water 89 – 90 89 - 92 Recommended limit 120 120

Fig. 5: Test setup for rear axle with diff-lock validation

Same test conditions were simulated on the chassis dynamometer for highway or test track parameters. There is a vehicle cooling blower whose speed follows the vehicle speed. The rolling and drag resistance were fed to the dynamometer roller to represent the actual road conditions. The rolling and drag coefficient of the vehicle were determined using coast down test as per IS 14785:2000. The design tractive force was given to the dynamometer controller in the form of rolling coefficient. This means that the brake unit will apply the same amount of opposite tractive force in the rollers. This way, the vehicle drives the rollers against the tractive force and wind drag coefficient.

5. Results and Discussion Durability test was carried out on the newly designed rear axle with a dog clutch type diff-lock using a heavy duty chassis dynamometer. Fig. 6 shows the arrangement of diff-lock assembly before the test. The vehicle was driven in first gear in intervals of 30 minutes run time and 30 minutes of cool down time. Differential oil, AGB (or Transfer case) oil, and engine oil and radiator inlet water temperatures were monitored during the test. The temperature measurements are shown in Table 3. It was found that these temperatures were below the max. temperature limit of 120 oC.

Fig. 7: Tractive force vs. Duration of test

Fig. 8: Differential assembly after failure of lock

These results are the positive indication for success of this new methodology for certification of army vehicles’ rear axle with diff-lock for off-road conditions using heavy duty chassis dynamometer. Fig. 6: Differential assembly before test (without Fork)

6. Conclusions

Tractive force vs. Duration of test is shown in Fig. 7. From this graph, it is clear that the tractive force values are constant up to the target test duration for normal loading pattern and there after till the failure for 10% over load case. The newly designed dog clutch type

The newly designed dog clutch type diff-lock for Banjo type rear axle met the durability target for off-road conditions. The proposed DVP and experimental setup using heavy duty chassis dynamometer demonstrated the uniqueness for off-road vehicle certification. As the 245

Biswas et al. 2011. Int. J. Vehicle Structures & Systems, 3(4), 241-246 [3] S.E. Chocholek. 1988. The development of a differential for the improvement of traction control, Proc. IMechE, C368/88, 75-82. [4] H. Ina, H. Izumi, T. Itoh, T. Yamada, and F. Matsuura. 1994. Development of a new limited slip differential, JSAE Review, 15(3), 209-214. http://dx.doi.org/10.1016/ 0389-4304(94)90036-1

testing was continued till the failure of any components of diff-lock assembly and the vehicle was at maximum torque condition, it is possible to find out the expected life of the diff-lock assembly using coast down testing. The proposed testing method using chassis dynamometer indirectly helped to check the power train and the drive train for durability and compatibility up to some extent.

[5] J. Markdahl. 2010. Traction Control for Articulated OﬀRoad Vehicles, Master’s Thesis, Royal Institute of Technology, Sweden. [6] TS: 7582. 2007. Testing of Differential Assembly, 1-6. [7] IS: 14785. 2000. Automotive Vehicles: Determination of Road Load Constants by Coast Down Test Method. [8] SAE J2177. 1992. Chassis Dynamometer Test procedure: Heavy Duty Road Vehicles. [9] SAE J2264. 1995. Chassis Dynamometer Simulation of Road Load Using Coast Down Techniques. [10] SAE J1263. 2009. Road Load Measurement and Dynamometer Simulation Using Coast Down Techniques. [11] TEL 037, Tata Cummins Engine data sheet. [12] Calculations, 2000, 1-29, MAN Nutzfahrzeuge Aktiengesellschaft, Munich, Germany.

Acknowledgements The helpful discussion and support of Mr. Thomas George (ERCJ-Head) and Mr. Subir dey Sarkar (ERCJTesting) of Tata Motors Limited are gratefully acknowledged. Authors would like to thank Tata Motors Limited to give an opportunity to publish this work in this Journal. REFERENCES: [1] R. Gunn. 2004. Trucks & Off-road Vehicles, Motorbooks International, USA. [2] M. Klomp. 2005. Passenger Car All Wheel Drive System Analysis, Master’s Thesis, University of Trollhatten/Uddevalla, Sweden.

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