How to properly align the rear wheel to the bike (granny bike, single speed) I have an old bike in which the rear axle is like you can see in the pictures below.

It has pedal-brake. I've been struggling with the wheel alignment with respect to the bike. Each time I put it in, I align it as much as I can making sure the chain is straight (the cogs are aligned) and then I press the axle nuts as much as I can trying to keep that alignment. After a bit of cycling the wheel moves a bit in such a way that the front and rear cogs are not properly aligned, making the chain to go off the cog very easily.

What am I doing wrong? How can I solve this?

 

 

EDIT: could this chain tensioners fit my bike?
I suspect you either are missing the lock washer (toothed lock washer, picture follows) on the drive side or the one that is there is worn out. Place one outside of the dropout, between the dropout and the hanger for the rack/stand.

And the promised picture of the washer I meant: