In 1993 Volvo launched their new FH series of heavy trucks. Developed to meet market demand for lower operating costs and higher payloads, while maintaining Volvo's image for safety and environmental concern, the truck cabs make extensive use of high strength steel to reduce weight. Weight reduction of the order of 30% has been achieved over the previous generation of truck cabs.
Traditional spot welding techniques gave problems when joining the high strength steel material, in that the fatigue life of the spot welded joint did not increase with an increase in yield strength of the steel. This resulted in premature failure of the firewall to engine cover joint during fatigue testing.
The immediate solution was to spot weld reinforcement plates in the affected area, but this resulted in added weight and cost. Following extensive testing, however, the problematic spot welded joints were replaced by self-piercing riveting. The reinforcement plates were removed and a truck cab with a riveted firewall to engine cover joint was fatigue tested to simulate twice the expected life of the truck without failure.
There are 42 rivets per cab, and production was 150 cabs per day.
Although the static strength of the riveted joint was lower than the equivalent spot weld, and the tooling investment costs for the riveting equipment were $43 000 higher than for spot welding, the advantages and savings to Volvo of adopting self-piercing riveting technology, as listed below, far outweighed the initial increase in equipment cost.
- Removal of the reinforcing plates.
- Improved fatigue life of the cab structure.
- Improved working environment, without sparks or fumes.
- Similar automation techniques to spot welding.
- Savings of $244 000 per year in production costs.
Following the success of this project, Volvo are considering extending use of self-piercing riveting to other applications in their range of heavy trucks.
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