2 graphite rings (top and bottom anti-extrusion)
DIAGRAM OF PRESSURE DISTRIBUTION FOR THE VARIOUS RINGS COMPRISING THE STEM PACKING, DEPENDING ON THE GLAND THRUST.
It can be seen that sealing pressure on the stem decreases because the thrust of the gland is gradually dampened by the gaskets rubbing against the walls (chamber and stem).
It is obvious that the operating condition of the last ring is about one third less than that of the first ring.
Disadvantages of the conventional standard set
The graphite rings required for ANTI-EXTRUSION considerably increase friction applied on the stem and walls of the camera, subtracting 50% of the TORQUE.
As it is SOFT, the TOP RING does NOT allow sufficient transmission of force to the next ring in the set when loaded by pressure on the gland.
The GRAPHITE RINGS have very little or no sealing properties.
If you take the LOWER GRAPHITE RING into consideration, we can see that, due to difficulty in torque transmission, the ring itself is only slightly loaded and compact and thus highly HYGROSCOPIC.
This can be a drawback in that the end GRAPHITE RING can retain various liquids, such as the water used for the seal test, and this stagnation and consequent leaks can be the cause of known ELECTROLYTIC CORROSION.
