alternator/power steering help please

[jono172sport]

hi everyone
got a 2001 172 sport. when i turn on full lock while stood still the steering makes a horrible screeching noise like the alternator belt is slipping. ive adjusted the belt so its tight but it still screeches, on further inspection i noticed while turning the wheel one of the pulleys the belt turns stops and struggles to turn under load and the screeching is caused by the belt slipping on it anybody know what this could be? is it somthing to do with the power steering pump?

any help will be awesome
thanks
jono

 

[jono172sport]

also forgot to mention if the car is rolling or if you rev it slightly it only screeches half as much and the pully manages to turn

jono