Design of an off-highway truck

¶ … sales of off-highway trucks is one of the most profitable and most competitive motor vehicle design segments worldwide. The sector of the vehicle industry that is collectively known as Off-Highway machinery represents a substantial business. (the machine divisions of Caterpillar turn over about $10bn annually. Other big players include John Deere, Komatsu, Volvo and Liebherr.) the companies involved are not in general household names. The machines they manufacture range from the mundane to the spectacular and are all the product of a sophisticated design and development process. The sector is very competitive and manufacturers are constantly working on improvements to machines to achieve greater efficiency and greater productivity. In this paper we present the design of a MAN off-highway truck having the following specification;

The scenario/background information

I am a member of a small team developing a proposal for a new off-highway truck whose primary function is the haulage of timber in remote forests. There is likely to be a military spin-off as a vehicle to transport military hardware in difficult terrain. The military spin-off will use the same basic vehicle structure. I must prepare an outline design of the main vehicle systems including powertrain, suspension and steering that will be used for costing and for discussion with potential customers. The vehicle will have a wheel base of 5.0 m and a track of 2.1m. The unloaded weight of the vehicle is 20t and the load carrying capacity is 20t. When fully loaded the weight distribution is 40% on the front axle and 60% on the rear axle. Given the likely adhesion conditions, the powertrain will drive all axles. The gradeability of a vehicle is expressed as a percentage (%). For example, the figure 25% means that for a horizontal length of I =

100m, a height of h = 25m can be overcome. The steepest grade for the proposed vehicle is 40%. Static friction can fall to 0.5 under the worst conditions the vehicle is likely to encounter.

Minimum turning radius is 6.5m at low speed and 20m at 20 km/h. All turns must be done without any wheel leaving the ground.

Technical approach

As noted earlier, the technical specifications are as follows; the design of new off-highway truck whose primary function is the haulage of timber in remote forests. There is likely to be a military spin-off as a vehicle to transport military hardware in difficult terrain. The military spin-off will use the same basic vehicle structure. The vehicle will have a wheel base of 5.0 m and a track of 2.1m. The unloaded weight of the vehicle is 20t and the load carrying capacity is 20t. When fully loaded the weight distribution is 40% on the front axle and 60% on the rear axle. Given the likely adhesion conditions, the powertrain will drive all axles. The gradeability of a vehicle is expressed as a percentage (%). For example, the figure 25% means that for a horizontal length of I =100m, a height of h = 25m can be overcome. The steepest grade for the proposed vehicle is 40%. Static friction can fall to 0.5 under the worst conditions the vehicle is likely to encounter.

Minimum turning radius is 6.5m at low speed and 20m at 20 km/h. All turns must be done without any wheel leaving the ground.

Survey of competitors

The main competitors are

Mercedes Benz

Marc

DAF

Renault

Navistar

Paccar

Daimler-Freight Liner & Western Star

Scania

Powertrain selection

The selection of the most appropriate powertrain is important in the design of off-highway trucks since it has a direct influence on acceleration and fuel consumption according to Rafael et al. (2009).Fuel economy and acceleration performance must therefore be at the forefront of all vehicle designs. Developing vehicles that achieve both optimum fuel economy and optimal acceleration performance is critical to the very success of any automotive design and development. The unloaded weight of the vehicle is 20t and the load carrying capacity is 20t. When fully loaded the weight distribution is 40% on the front axle and 60% on the rear axle. Given the likely adhesion conditions, the powertrain will drive all axles.

Suspension geometry design and assessment

Steering design

Turning circle

When the vehicle is cornering, each wheel must go through a turning circle. The outer turning circle, is to our main subject of interest. This calculation is never precise because when a vehicle is cornering the perpendiculars via the centres of all wheels never intersect at the curve centre point (Ackermann condition). Additionally, while the vehicle is moving, certain dynamic forces will always arise that will eventually affect the cornering manoeuvre (MAN,2000).

The formula used.

Vehicle Model T31, 19.314 FC

Wheelbase lkt = 5000 mm

Front axle Model V9-82L

Tyres 315/80 R. 22.5

Wheel 22.5 x 9.00

Track width s = 2058 mm

Scrub radius r0 = 58 mm

Inner steer angle ?i = 50.0°

Outer steer angle ?a = 30°30' = 30.5°

1. Distance between steering axes

Calculations 17 lkt

Outer turning circle j js a0 I r0

r0 r0

TDB-172

j = s - 2r0 = 2058-2 58

j = 1942

Therefore

Theoretical value for outer steer angle

3. Steering deviation

4. Turning circle radius

Axle load calculation

Performing an axle load calculation

To optimise the vehicle and achieve the correct superstructure ratings, an axle load calculation is essential. The body can be matched properly to the truck only if the vehicle is weighed before any body building work is carried out. The weights obtained in the weighing process are to be included in the axle load calculation. The following section will explain an axle load calculation. The moment theorem is used to distribute the weight of the equipment to the front and rear axles. All distances are with respect to the theoretical front axle centreline.

Weight is ever used in the sense of weight force (in N) in the following formulae but in the sense

Braking and dynamics control

The braking system will comprise of the following components;