Ok, I'm not a rocket scientist but this has me stumped. I have a new hot water hose for my washing machine. I replaced the O-ring on the faucet side of the the connection, tightened and went to use. After about 10 minutes of use, the connection becomes loose and starts to leak. I've tightened this to the point of pain in my hand. What could be the cause?
Washing machines vibrate - an inevitable side-effect of an unbalanced drum turning - so anything not very tight will work its way loose over time.
The clamps usually used to compress the seal/o-ring are designed to be tightened using a screwdriver, and you want to get these clamps really tight. The o-ring will deform quite a lot as you compress it, but it is designed to do that.
Also, before you put the hose on, make sure the surfaces which will connect together are clean and dry - this will help reduce slippage.
Before installation, remove the flat black rubber washers from the hose coupling and inspect them. Do not use plastic washers. They don't last long and aren't designed for a washing machine hose. The general rule for connecting a washing machine hose's female swivel nut to the male water valve is "Hand tight and a quarter turn until snug with a pair of pliers or channel locks." Take care not to cross thread the swivel nut on the washing machine's plastic solenoid inlet valves threads.
Jeff, why did you only buy one new hose, when manufacturers warn you to replace what I consider to be substandard washer hoses?