Determine the dynamic load acting on the chassis of multipurpose forest fire fighting vehicle

Multi-purpose forest fire fighting vehicle is produced in Viet Nam include a combination of fire fighting equipment such as high-pressure water pump, create corridor fire insulation cutting machine, vacuum and high wind speed bowling machine, extinguish the fire sand blast apparatus that is mounted active three axles vehicle. The paper presents how to determine the dynamic load acting on the chassis by 3D modeling method. The results of this study are the input parameters for assessing the durability of the chassis of multi-purpose forest fire fighting vehicle.


Introduction
Multi-purpose forest fire fighting vehicle was designed on the basic URAL 4320, three active axle, used mainly in the military to increase mobility on the road surface. Shape of base vehicle Ural 4320 and multifunction forest fire fighting vehicle shown in Figure 1   The main equipment of multi-purpose forest fire fighting vehicle are shown in Figure 2 [3,4]  1) Tree cutting mechanism; 2) Tree cleaver; 3) Body; 4) Water sprayer; 5) Seat; 6) Cabin; 7) Water tank; 8) Back cover; 9) Pipe; 10) Table; 11) Plotting small nozzle; 12) Floorboard; 13) Dig system; 14) Lawn cutting mechanism.

Some initial assumptions
To building modelling, there are some assumptions: • The wheels always contact the road surface; • The modelling includes front axle, middle axle, rear axle are considered absolute solids; • The resistance of tire is linear elastic; • Mass of cab and container are distributed symmetry around the longitudinal and transverse planes; • Speed of vehicle on the road is low, constant; therefore, inertial and air resistance low and considered zero [2]. By applying Newton's second law of motion, we establish the differential oscillation system as follows [2]: 2  3  4  2  1  1  3  1  3  3  4   1  2  3  4  2  1  1  3  1  3  3 u3  3  3  31  32  3  3  2  5  3  3  31  32   2  2  2 u3  3  3  31  32  3  3  3  3  31  32   2  2  2  The values of the geometry and dynamic parameters of multi-purpose forest fire fighting vehicle are given in Table 1 [1,3].

Determination of dynamic load
In this survey, the author uses a simple sinusoidal shape is used because of its simple structure and easy to predict the results [2]. The height of the bump is determined by the formula: With a constant velocity of the car (v) is the height of the bump in time (h) calculated according to the formula: Apply Fz11, Fz12, Fz21, Fz22, Fz31, Fz32 is the dynamic load from the road surface acting alternately of the left front wheel, front right, left middle, right middle, rear left, right rear in verticality. Due to the high level of the forest road, sometimes the height of the road is over 0,35 m, and the movement velocity of the vehicle is about 25 km/h. Therefore, the survey when the wheels pass through the bump has a length of L = 0,5m and the height of the bump (H) varies from 0,1 to 0,4m, the vehicle velocity varies from 5 to 25 km/h, the vehicle is moving straight, not subject to horizontal forces.

Rough evenly two front wheels
Because two front wheels are evenly spaced, the vertical dynamic load acting the left and right wheels must be equal to the front and rear axles (Fz11 = Fz12, Fz31 = Fz32). On the car, the rear suspension design is balanced, to simply calculate whether vertical dynamic load acting on the middle wheel is equal to vertical dynamic load acting on the rear wheel in all moving conditions.

Single bump front wheel
In this case, the front right wheel must pass through the bump, The survey results will determine the dynamic load acting on the chassis through the axle and suspension. Because the two front wheels are not equal, the dynamic load on the left and right wheels is different (Fz11 < Fz12).         181783N (Figure 14). Maximum vertical dynamic load is 6,004 times greater than in the case of static load. The maximum value of vertical force Fz32 = 207840N ( Figure  15), larger than 8,14 in the case of static load.

Conclusion
Multi-purpose forest fire fighting vehicle was designed on the basic URAL 4320, was installed specialized fire figtting to increase weight aa well as change the center of gravity of the vehicle. Because of the factor makes it become unstable while moving, at time reduce details when the vehicle has a large IOP Publishing doi:10.1088/1757-899X/1173/1/012030 9 load, especially the chassis. The result of the study are the basic for stability of chassis to perfect the design of the multi-purpose forest fire fighting vehicle. However, this study only determines the dynamic load when the vehicle through sine-wave form. Therefore, further study is need to determine dynamic loads when vehicles are traveling on forest roads or on roads according to ISO 8608:1995.