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?
It's structural, but not really mandatory:
The full (4 pages) instruction can be found by googling or e.g. there: http://lego.brickinstructions.com/lego_instructions/set/6847/Space_Dozer
Well, it's not really of structural importance, as the 4 x 6 plate in step 2 holds them together. I had the scanned instructions already, but was wondering if on the box, any of the alternative builds sometimes pictured used it.– RevenDec 29, 2013 at 20:12
Well, it will, to some degree, keep the 1x6 technic beams from coming off the plate above when too much pressure is applied from above. Without that axle, I see the 1x6's coming off all too easily.– gevDec 29, 2013 at 23:15
if they'd put a plate under those beams to keep them in place the vehicle would have no ground clearance (or possibly negative clearance). You could always build it and find out...– MóżDec 31, 2013 at 22:49
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.
2Yay for Science! Nothing like the experimental approach. :)– RevenJan 2, 2014 at 7:42