Lower Collar Type ( Data Sheet DS 820 )

This nut has a longer piston which is externally threaded and fitted with a load retaining locking collar. A gap is created between the body and the locking collar when the nut is pressurised. The gap is a combination of the compression of the bolted joint and gasket, if fitted, plus the elongation of the bolt.

In use the nut is pressurised until the hydraulic jack develops more than the desired pre-load. The locking collar is tightened. The pressure is released and the pre load is transferred onto the locking collar threads where settling of the threads causes some of the pre load to be lost. This is more critical in short bolt applications where the bolt elongation may be small. The pre-load loss on transfer to the collar becomes less significant on longer grip length bolts.