I work part time for a local bike rental. I'm the sole mechanic for about 250 bikes. Primarily, we rent out the beach cruisers. My number one time consumers are loose and broken chains. What I have noticed is that whenever I tighten a chain, or even replace one, It never has a uniform tension. If I crank it around a half turn, the chain is tight. Another half turn, and the chain seems loose again. Thinking a stretched chain, I'd replace it, yet the problem still persists. I've even gone as far as rebuilding the rear and crank hubs, replacing bearing sets and races. The cups are a little gunked up sometimes, but seem to be in good enough condition after a good cleaning. They're just not as easy for me to replace out, so I have to leave them in. Any ideas?