Air con belt keeps coming loose!

[GixerJim]

Hi there,
Any one else suffered from air con belt keeping coming lose after adjustment? The nut on the pulley is tight, the adjusting screw is tight but after going for a drive the pulley works it way down the slot- lossening the belt. Mine has no locknuts holding the adjusting bolt tight in place? Should I have any other further nuts/bolts/clamps?
Any ideas would be apreciated!
Thanks in advance!

[Apache]

Shouldn't need any locknuts. The one on the pully locks the pully in place, the one at the top just tensions.

If it is slackening, maybe try a a nylock (M8?) on the tensioner?