Something is really wrong here. The tension assembly of any machine should be securely attached, regardless of how loose or tight your tension is.

I would recommend joining the Yahoo Vintage Singers Group. Besides being a very knowlegable and friendly bunch, there is document in their Files section on Class 15 tension assemblies. GardenWeb has issues with spam emanating from Yahoo domains, so I can't post the link here.

it's most likely because the tension assembly itself is not on correctly. there is a little 'pin' that goes on under the dial- and if it's not on correctly or in backwards- it doesnt work to stop the dial from coming off