What applies spring tension to the camera's apature adjustment ring lo

I had removed the lens mount on my a58 and in doing so the apature adjustment ring fell out. I reinstalled and now I sometimes get an error message on the camera. I noticed when comparing to a Minolta film camera the apature ring is under spring tension which causes it to return to position after rotating. The a58 ring does not do this. Did I lose a part or install it wrong. Thanks

Sailloco@aol.com