These three design features provide effective sealing and give a smooth, free-running movement to the valve door, while allowing rapid winding of the handwheel:
1)      The use of rollers, in a cam-following action, to guide and control the movement of the door, as it approaches the stainless steel seat. The cam action clamps the resilient seal tightly against the seat and prevents dragging the seal across the face of the seat.
The reverse of this action, when opening the valve, quickly separates a seal 'stuck' after a lengthy period of disuse, then allows rapid winding.  (A telescopic control column means that the handwheel is non-raising, also promoting rapid use.)
2)  Ball bearing support, both rotational and thrust in two directions, is provided  for the control screw.
3)  The control screw is a large diameter, jam-resisting, stainless steel 'Acme' thread, engaging a large bronze nut.

The exploded view below shows the arrangement of guide rollers and profile tracks in the side frame of a valve.  (The illustration is actually of a Batescrew square profile sewerage valve, using the same principle.)