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Mike Halliwell does a good job in uncovering some of the structural 
aspects of DIN and Yoke regulator connections. While shear stress and 
axial stress are present in yoke connections, I suspect the culprit for 
yokes and DIN connections to be stress primarily caused by bending 
(stress is defined as the applied force divided by the area over which 
it is applied). Unfortuantely in a linear structural system all three 
stress sources add to create a combined stress state. The axial force 
on a typical yoke with a 3000 psi fill would be about 1154 lbs. A brass 
yoke with two "arms" with dimensions of .25" x .6" x 1.25" would have 
and axially applied stress of about 3078 lbs/sq.inch and would stretch 
about .00025" due to the axial force caused by the air pressure acting 
on the O-ring seal face. The 3078 psi stress is well below the failure 
strength for the brass material. The ideally largest gap through which 
an O-ring, with a typical hardness in SCUBA applications, will not 
extrude with a 1500psi pressure load is about .003". The axial 
stretching due to the pressure accounts for about 10% of the maximum 
ideal gap. The reality is the gap can be somewhat wider. In static 
conditions I have used .003" gaps in excess of 10,000psi with no 
problems. So axial loading alone will not cause extrusion in a properly 
connected yoke connection. Now if the regulator body somehow is in 
direct shear, the recess that the regulator seal face fits in will tend 
to limit the lateral motion and thus the shearing in the yoke arms. 
Still this add somewhat to the combined total stress.

On the other hand, if you bang the regulator body you tend to arc, or 
bend, the yoke arms. You create a bending force which is a function of  
mass of the equipment and yourself, the velocity at which you are 
traveling, and the deceleration time to stop. The deceration time can 
almost be instantaneous and the mass can be large if your harness is 
tight. The applied bending force tends to rock the seal face on the 
regulator away from the O-ring, but it does not do it evenly. It 
creates a wider gap at some segment of the O-ring. The pressure force 
which tends to seal the O-ring in the gap will now cause the O-ring to 
extrude through this widened gap segment. The yoke arms may or may not 
be permanently deformed. If not the gap may open during the impact and 
the O-ring extrude; the gap would attempt to close and would cut the 
O-ring. If the yoke is permanently deformed the O-ring would extrude 
and the gap would be uneven around the seal. I have yet to see a yoke 
break off from an impact (this doesn't mean it can't happen), but I 
have seen bent yokes and extruded O-rings. As Mike said there is a lot 
of material in the yoke arms; but there is also a long distance over 
which the bending occurs (1.25"on one of my yokes). This tends to 
distribute the stress such that the yoke would generally not break but 
may be deformed on large impacts.

The DIN connection suffers the same applied forces. Only in the case of 
a DIN connection the bending force (more correctly the bending moment) 
acts on a very small cross-section (small 2nd moment of inertia for 
those who care) of metal at the joint of the hollow tube at the body of 
the regulator. Since the connection into the tank valve is very rigid 
compared to a yoke, the energy of the applied force is concentrated at 
the joint on the regulator. On regulators with small tube diameters 
such as the Poseidon this may cause breakage (I've seen two broken 
Poseidon DIN connections as a result of falling tanks). DIN connections 
with a large cross-section of metal at the connection (greater 2nd 
moment) will fair better. This is probably why the US Divers DIN 
connection mentioned in a previous post faired better than the 
Poseidons?

I prefer the DIN connection mainly due to the captivated O-Ring and the 
compact valve-regulator arrangment; however I used yokes for quite some 
time on a set of 72s with no failures. I believe an extruded O-ring in 
a yoke connection is more likely to happen than breaking off a DIN 
connection in the water. It appears most of the DIN failures are due to 
falling tanks => so don't drop the tanks.  And there is no substitute 
for good technique!

Thanks,
Doug Chapman

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