The Swiss Federal Railways (SBB) was faced with challenges to stabilise its system of bolts that were causing high maintenance and downtime costs.
Originally hexagonal bolts had been used, type DIN 931, 10.9, M12x140. Due to the load that was applied transverse to the bolt axis the bolts became instable after a short time in use. They were applied in the wheel drive system. Even though, there was no security risk at any time, SBB was highly interested in finding an efficient solution.
After analysing the reasons for the problem, the engineering teams of SBB and Bossard AG worked in a very close co-operation to develop a stable bolted connection that also increased functional safety.
Various solutions were originally considered - for instance using dowel pins to improve the force transmission. But since the spare parts need to be interchangeable, this turned out to be not the optimal approach. Finally, the solution was the use of a bolt with waisted shank.
This solution has a lot of advantages. There is a low level of loss in tightening force when setting up the connection. That leads to considerable security against it coming unscrewed by itself. The low level of sensitivity to bending reduces the additional forces that occur during thermic strain.
Then, its ability to change shape plastically ensures a high degree of safety when tightening up over and beyond the yield point. Optimised mating surfaces mean that the torsional or transverse stresses in the waisted shank can be eliminated. And the durability is substantially greater than for conventional standard bolts, even under a vibrating load.
"Since we use bolts with waisted shank, we increased the availability of the locomotives," says Mr Hans-Peter Luethi, Strategic Purchasing at SBB. "At the same time, we could significantly reduce maintenance and repair costs."
Furthermore, the bolts with waisted shank prevent displacement against each other. A larger diameter in the segment under load leads to increased shearing resistance. The elastic thin shaft guarantees high safety when tightening beyond the yield point. Moreover there is no loading on curves in the area of the thread and head.
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Bossard AG is based in Zug Switzerland. www.bossard.com