Forge DV o-ring
07-19-2001, 05:49 PM
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Forge DV o-ring
<center><img src="http://im1.shutterfly.com/procserv?si=00002937301920010720013015568.JPG&ps=0 &rx=640&ry=480"></center><p>I tested the originally supplied Forge DV o-ring material and found it to be an acceptable choice for this application. Previously reported tests of the pistons (Nylatron & aluminum) and housing showed that dimensional changes with temperature were not a problem. The reason my piston (Nylatron) stuck open is still unknown.
Graph details:
The o-ring was tested over a temperature range of -150°C to 180°C (-237°F to 356°F). The specimen was subjected to an excitation force at 5 Hertz. Testing started after the material was held in air at -150°C for 20 minutes. As the material became warmer it also became less stiff (decreasing modulus). This is a natural consequence of decreasing density with increasing temperature. Above -50°C there is a noticeable increase in molecular freedom. Co-operative motion peaks at -25°C. Tan delta, a relative indicator of viscoelasticity, peaks at -16°C. The stiffness of the o-ring is quite stable above -10°C (14°F) with little change from room temperature all the way up to 180°C (356°F). The natural decrease in stiffness with temperature is offset by the greater variation in entropy with strain. You need to remember that the potential energy stored in a strained material (such as a spring or this o-ring) is comprised of two components - enthalpic & entropic. The greater atomic separation caused by the increased temperature reduces enthalpy (as can be seen in steel at room temperature or this material below -50°C) can be offset by the change in entropy during straining of a highly conformable long chain molecule (as seen for this o-ring at temperatures above -10°C).
There is no way that this material selection is responsible for the sticking of my Forge DV. Nor, as previously reported, are the small changes in dimensions of the parts (pistons & housing) with temperature. Investigations in other areas are needed.
Graph details:
The o-ring was tested over a temperature range of -150°C to 180°C (-237°F to 356°F). The specimen was subjected to an excitation force at 5 Hertz. Testing started after the material was held in air at -150°C for 20 minutes. As the material became warmer it also became less stiff (decreasing modulus). This is a natural consequence of decreasing density with increasing temperature. Above -50°C there is a noticeable increase in molecular freedom. Co-operative motion peaks at -25°C. Tan delta, a relative indicator of viscoelasticity, peaks at -16°C. The stiffness of the o-ring is quite stable above -10°C (14°F) with little change from room temperature all the way up to 180°C (356°F). The natural decrease in stiffness with temperature is offset by the greater variation in entropy with strain. You need to remember that the potential energy stored in a strained material (such as a spring or this o-ring) is comprised of two components - enthalpic & entropic. The greater atomic separation caused by the increased temperature reduces enthalpy (as can be seen in steel at room temperature or this material below -50°C) can be offset by the change in entropy during straining of a highly conformable long chain molecule (as seen for this o-ring at temperatures above -10°C).
There is no way that this material selection is responsible for the sticking of my Forge DV. Nor, as previously reported, are the small changes in dimensions of the parts (pistons & housing) with temperature. Investigations in other areas are needed.
Thanks John!
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07-19-2001, 07:49 PM
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The Forge Valve has a.....
bore diameter = 50mm
piston diameter = 48mm
O ring (inside) diameter = 48mm
O ring cross section = 2mm
O ring seat depth = 1mm
These are rounded up sizes but very close as measured with my Mitutoyo digital caliper.
Do the math. and you find that the O ring is supported by a seat depth of only half its cross sectional width. Also you find a total piston to bore clearance of 2mm. This is also the cross sectional width of the O ring.
In my opinion some bind of the O ring to the piston due to piston expansion or lubrication issues could cause the roll-out of the O ring from its seat and sticking of the piston.
piston diameter = 48mm
O ring (inside) diameter = 48mm
O ring cross section = 2mm
O ring seat depth = 1mm
These are rounded up sizes but very close as measured with my Mitutoyo digital caliper.
Do the math. and you find that the O ring is supported by a seat depth of only half its cross sectional width. Also you find a total piston to bore clearance of 2mm. This is also the cross sectional width of the O ring.
In my opinion some bind of the O ring to the piston due to piston expansion or lubrication issues could cause the roll-out of the O ring from its seat and sticking of the piston.