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?