Abstract
A number of demands are made on modern commercial vehicle transmissions. Low fuel consumption, small size and light weight have to be considered more and more in the development phase, along with a defined fatigue life and the cheapest possible manufacturing costs.
The most important characteristics of a transmission are fatigue life and reliability. If it is over-rated it will be too heavy, too large and too expensive. If it is under-rated the reclamation costs will be high and the reputation endangered.
Because fatigue life is influenced by so many aspects, it is impossible to carry out tests under all the peripheral conditions one can think of. The method critical component spectrum described here uses all the fatigue life factors and all the conditions of use to calculate the component spectrum which leads to the greatest damage and the fatigue life which results from this. Based on this information a specific constructive adjustment and a component test can be carried out. This means that these demands can already be taken into account during the development stage of a transmission. This method can be used when developing new products as well as when adjusting transmissions to meet changed demands.
The following steps are necessary to obtain the critical component spectrum:
- as many actual routes as possible are registered. This is done as exactly as possible, for example by measuring topography, speed cycles etc while the vehicle is being driven.
- for these various routes a target fatigue life for the transmission is stated (distance and length of time)
- under the measured conditions, a simulation is carried out to calculate the distance of the route / length of time until failure occurs. This is done for each route and for all transmission components of interest. The relationship between the required route (length of time) and the route (length of time until failure occurs) is described as the damage and this information forms the basis of the conclusion reached.
Using the data obtained from all the simulated route and vehicle combinations, it is possible to establish the damage to all significant components. From this you can see what the worst loading conditions are for each component (critical component spectrum).
If simulations are permanently carried out during the development stages it enables you to locate the critical transmission components. It is also possible to assess any changes in the way the bus is used or alterations to the vehicle itself for example an increased torque and consider the effect of this on the fatigue life.
In conjunction with laboratory tests and damage analysis, it is possible to build up a basis of knowledge which can be very helpful in forecasting the reliability and fatigue life.