Actually, yes. Eventually most any polymer will degrade from something.
Every one has an Achilles heel. Most automotive motor mounts are made from
natural rubber. The reason really is its glass transtiton temperature. It
is around -60°C and that makes the mounts very effective in isolating
vibration even at -40 degrees. The engineers are just very careful with the
temps the mounts see. Some do have a mount installed in hot regions and
there many engineers will use HNBR. Its extremely expensive and not likely
to be available in commonly obtained hardware. If you can get it, EPDM may
not be a bad choice. Its much more heat stable, has slightly better low
temperature performance but it has no oil resistance. If you anticipate
leaks of oil on it, it would be a bad choice.
If its possible to determine how hot the mount is getting, that's a good
start for determining how long it will last. Neoprene's continuous use
temperature is about 80-85°C [176 F to 185 F] meaning it can be expected to
perform acceptably for about 2000 hours at that temperature. Hotter temps
mean shorter life. A good rule of thumb is if the Neoprene part will last
2000 hrs at 80°C, then expect 1000 hours at 90°C, 500 hours at 100°C [212 F]
and so forth. Keep in mind this is just a way to estimate useful life. You
will reach a temperature where it will have zero hours of life and that
point may not be readily apparent.
Just inspect often until you have confidence it looks OK. Be looking
for cracks, crazing or chunks falling out (the usual rubber turning to goo
things). Small radial cracks would be the way I would expect Neoprene to
begin to fail.
Charlie Smith
KIS Cruiser 4021
Charlie,
I'm a bit slow responding - couldn't find my list of materials.
I'm looking at my Aussie PSRU damper requirements and couldn't help notice
someone actually knew a lot about these compounds.
Charlie you mention HNBR= High temp Nitrile temp range ;
HNBR -35* to+150*C
EPDM -50*to+130*C ( no oil resistance)
So what can you tell me about;
VIT=Viton
VIT -20* to +200*C
SIL= Silicone
SIL -60 to +200*C
TFV= Teflon Virgin White
TFV -200* to+260*C
TMB= TeflonFilled Grey
TMB-200* to +260* C
I have a contact who can turn these up for me in any shape I need, but I was
wondering if there were any hidden secrets which would limit performance
like the EPDM ( no oil resistance).
George (down under)
Silicone is good to 260C or 500 F
Teflon is good to 450 C or 750 F
Teflon has no bounce.
Paul Lamar
Working with rubber is great preparation for a second career as a diplomat. If you want something extra from your rubber compound, be prepared to trade an existing property to get it.
HNBR is an excellent overall choice. It has excellent strength, compression set, fuel and oil resistance as well as heat resistance and a good balance of dynamic properties. On the down side, it is exceptionally expensive (the company I work for is one of two manufacturers of HNBR in the world). If the heat your part will see does not exceed 150°C continuously, it is a great choice. Some of the Big Three in Detroit use this for high temp motor mounts.
EPDM is not quite as strong but with the proper cure system, has just as good high temps as HNBR. Down side, no oil or fuel resistance. It will swell to double and triple its original size upon extended contact. Losses in other physical properties are commenserate with the swell. It is relatively inexpensive.
Viton is the DuPont trade name for FKM (FPM for those of you in Europe). Tremendous chemical and heat resistance, so-so physicals and decent compression set. Poor dynamic properties due mostly to lousy cold temperature performance. Given it costs traditionally twice what HNBR costs, its not a good choice for engine mounts. This is what you make fuel injector o-rings from.
Silicone's claim to fame is heat resistance. It can stand 250°C for extended periods of time. Lousy strength, no oil resistance however. Again, not a choice for motor mounts.
As Paul notes, Teflon has very poor dynamic properties. This is the central reason for having rubber in a mount. It also flows under heat and pressure (plastic flow). Not at all what you want in a motor mount. Great heat resistance and chemical resistance though.
Chloroprene (Neoprene) is not a bad choice. It just concerns me that that lower mount will be located very close to exhaust components in some installations. Just know the environment you have it in. If the area is hot, be aware of this and check the mount frequently for signs of heat damage (cracks and surface crazing).
If it were my plane and I could get any elastomer in a mount, I'd use HNBR as its probably the most bullet-proof of the selections. But I can't get that polymer selection in a mount so I'll end up with chloroprene as most everybody else will. Just check it often until you gain confidence in how well it is performing.
Charlie Smith
KIS Cruiser 4021
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