The circular upper end of the upper bellows has an internal felt ring that must be FORCED over the formed retaining lip on the lower end of the condenser unit (or the similarly shaped diffusion unit). This momentarily compresses the ring so that the upper end of the bellows can be pushed upward over the retaining lip.
Once this is done the felt ring expands to make a moveable light seal with the outside diameter or the condenser/diffuser housing.
When the lamp house & condenser (or diffuser) unit is raised as needed, the lip pulls the upper end of the bellows upward with it causing the folds to unfurl. The size difference between the smaller diameter hole in the felt ring and the larger diameter retaining lip on the tube of the condenser or diffuser unit keeps the upper end of the bellows above the lip.
As the lamp house is lowered the bellows folds close. If the lamp house needs to be lowered even further, the unit retracts downward into the upper end of the bellows to accommodate.
On page 4 of this PDF you can see the diffusion unit (same size and shape as the condenser unit) identified as item #2. The formed annular ridge on the outside of the cylinder is the retaining lip for the upper bellows.
If the upper end of the bellows doesn’t stay above the retaining lip in normal operation, then the felt ring might have worn out or lost is resiliency and need replacing. Beseler might have the ring available. Otherwise a used upper bellows unit can be bought on eBay—usually with the upper negative stage plate glued to it.
Last edited by Ian C; 11-13-2013 at 08:53 AM. Click to view previous post history.