What is the purpose of the Technic Axle 6 in set 6847? In set 6847 (Space Dozer, http://www.peeron.com/inv/sets/6847-1) there is a black Technic Axle 6 that has no structural or cosmetic function in the set. I don't have the original instructions or box, so it might have been included to build one of the alternate sets. Or some other reason?
When weight is applied on the structure, the wheels are are creating a twisting motion that will eventually dissasemble the 1x6 beams.



With the extra structural part when a load is applied there is a lot of flexing but the 1x6 beams are staying in place. 



Effectively, the axle is preventing the beams from rotating out of place.