Why springs on hub bolts?

[Cord]

Ok, I got my new (to me) hubs (THANKS MARK!) and I'm missing a spring and bolt to snug the hub to the axle. Before I ordered them, I was toying with the idea of just omitting hte spring. Usually I just run the bolts in until they bottom out on the axle. With the bolt bottomed out, there is no need for the spring. So why does Yanmar use them?

 

[alvanko]

My guess would be it acts as a lock washer , constantly applying presure to the bolt. This is only a guess. Can't use a lock washer .