<VV> Fan belt tension (Clarification)
Mark Durham
62vair at gmail.com
Fri Apr 13 09:29:48 EDT 2012
So Doc, first thanks for the exacting engineering briefing. Can we then
summarize by saying the engine will grow more than the belt and the
belt will tighten up some as the engine reaches operating temperature,
not so much from the belt getting hot, but from the engine's expansion?
Mark Durham
Sent from my Windows Phone
From: RoboMan91324 at aol.com
Sent: 4/12/2012 18:02
To: virtualvairs at corvair.org; cmckinley313 at verizon.net
Subject: <VV> Fan belt tension (Clarification)
Chuck and All,
I would like to clarify some misstatements made on this subject.
It has been stated by another that fan belts thermally expand at a rate of
about 10 times that of steel, aluminum, etc. This is completely wrong for
several reasons as follow.
1. Even if the belts were made completely of polymers, the polymer would
expand at 3.3 times that of aluminum. (0.000041 vs. 0.0000123 Inches per
Inch per degree F rise). This is not close to the 10x claimed. Of course,
far worse, it was claimed that the belts (not polymer alone) had a 10x
expansion rate which leads to the following.
2. Our fan belts are not merely made of polymer. They are composites
made of polymers combined with fibers that are intended to keep the belts from
stretching under either tension or thermal expansion. The relative rates
of thermal expansion are 0.000017 for a fiber reinforced polymer vs the
previously stated .0000123 of aluminum in inches per Inch per degree F rise in
temperature. This calculates to approximately 38% more expansion for the
belt as compared to aluminum for the same temperature rise. 38% is a far
far cry from the 1000% claimed. Even this 38% differential is moot due to
the following.
3. The crankcase of our engines reach a significantly high temperature
when driven. Anyone who has accidentally touched a hot engine can testify to
this. Of course, being air cooled, our Corvair engines get hotter than
water pumpers. While the engine is running, the belts we use have a very
benign temperature rise because the only way they can absorb heat is from very
short duration contact with the crankshaft pulley. The flexing and
friction also causes temperature rise. Counteracting this heat gain,
the belt is
flapping through fairly cool air in the chamber above the engine. As you
know, this air is constantly pulled into the upper chamber by the engine's
fan and has no time to heat up before being drawn into the engine by the
fan. It is very close to the ambient temperature you and I enjoy outside the
car. In addition, a moving section of the belt is being significantly
cooled due to the air stream which is pulled into the fan at the mouth of the
"Turkey Roaster." This is what is called forced convection cooling and it
is significant. If the fan belt were to reach actual engine temperature on
a regular basis, yes, it would stretch but we would be replacing them
every few hundred miles or sooner. So, while the belt has an approximate 38%
greater thermal coefficient of expansion compared to the aluminum crankcase,
the belt's temperature will only rise a few degrees while the engine can
rise hundreds of degrees. Remember, the coefficient of thermal expansions
is defined with "per degree rise in temperature." In the actual operating
environment, the engine expands much more than the fan belt in our Corvairs.
4. As stated above, polymers and rubber expand at 0.000041 Inches per
Inch per degree F rise. The fact that the included fibers prevents the belt
from growing significantly along its length does not negate the laws of
Physics. The polymer must expand. If it cannot grow in length due to the
fibers, the expansion must relieve itself somehow. It does this by "plumping
up." In effect, the belt grows both in depth and width when it heats up.
The net result is that the belt will get tighter on the pulleys despite its
greater coefficient of expansion. You might call this a "reverse" thermal
coefficient and it is likely that Engineers take this into account when
designing belts far application in hot environments. Of course, this growth
in cross section will be minimal because, as stated above, the temperature
rise of the belt will be minimal.
For all the above reasons, I think we can accept my contention that the
Corvair engine grows more than the belt in real world use.
Next, the belts on water pumpers stay on and last longer because they are
in-line unlike the odd configuration of our Corvairs. They do not twist as
much as 90 degrees before rushing on to the next pulley as they do on our
cars. This is one reason they can be adjusted tighter. This was hinted at
by another.
Next, on water pumper fans, the clutch decouples the greatest rotating
inertia in the cooling system; that being the fan blade assembly. To verify
this, go out to any water pumper with a fan clutch and turn the fan with
your finger. It turns extremely easily. Because of this, the effect it has
on a fan belt when changing engine speeds is nearly non existent.
Lastly, on water pumpers, if there are multiple driven components like
A/C, power steering, etc. they usually have multiple belts so that each belt
is not subjected to too much slippage when RPMs change quickly. The
rotating inertia is spread between belts during acceleration and
deceleration. Of
course, this isn't the only reason to use multiple belts. Serpentine
belts usually work alone but they have the advantage of "dynamic"
tightening to
allow slippage as necessary without excessive wear. Again, these are
in-line (on a two dimensional plane) so there is little danger of popping a
belt off. Our Corvairs' belts drive only the fan and the alt/gen. If there
is an A/C compressor, it has its own belt and it is in-line.
On a final note, VV has many contributors who are real authorities on
technical topics as well as historical information as regards our Corvairs and
autos in general. Many are very knowledgeable in off-topic subjects as
well. I think we are all thankful for their contributions. I know I am.
Unfortunately, there are a few individuals who seem to feel that stating
invalid information in an authoritative manner makes them appear to be true
authorities on a given topic. This is a sad reality in personal, business and
political arenas as well as here on VV. Of course, they are entitled to
their opinions as are we all. However, it is to our benefit to recognize
these individuals as being less than the experts they try to portray
themselves as. Once identified, we can take their statements with a
grain of salt.
It would be unfortunate if someone were to take misinformation as fact,
apply it to something important and live to regret it.
Not to sound arrogant or self absorbed but you might say that once you
have read this clarification, "all will become clear."
I would be happy to forward a link or links to sites which show the
numbers I have claimed above. Just ask.
Doc
1960 Corvette, 1961 Rampside, 1962 Rampside, 1964 Spyder coupe, 1965
Greenbrier, 1966 Canadian Corsa turbo coupe, 1967 Nova SS, 1968 Camaro ragtop
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In a message dated 4/11/2012 4:09:22 P.M. Pacific Daylight Time,
virtualvairs-request at corvair.org writes:
Message: 8
Date: Wed, 11 Apr 2012 17:33:23 -0400 (EDT)
From: RoboMan91324 at aol.com
Subject: <VV> Fan belt tension
To: virtualvairs at corvair.org, cmckinley313 at verizon.net
Message-ID: <7f08.772da2f5.3cb752a3 at aol.com>
Content-Type: text/plain; charset="US-ASCII"
Hi Chuck,
I don't recall seeing anyone suggest a "very tight" tension on a Corvair
fan belt but I must admit I skip some of the postings. Always tighten
the
belt so you are just able to turn the alternator or generator fan with
one
or two fingers. You said, "just turn the fan" and I assume you do not
mean
the engine's cooling fan. The engine fan has a much larger diameter and
is
much easier to turn even when the belt is too tight.
If you over tighten the belt, you can shorten the life of the fan bearing
(as you said) as well as the alt/gen bearing, fan belt, etc. However,
there are other reasons to avoid over tightening the belt as follows....
1. The engine's cooling fan is a fairly large rotating mass. This means
that it takes energy to spin it up to speed and once it is at speed, it
is
storing that energy. In effect, it is a flywheel. When you accelerate
in
first gear, you bring the fan up to speed. When you shift to second
gear,
the engine slows quickly (sometimes very quickly) and the fan tries to do
the same but can only slow to a limited degree. Because the fan wants to
keep spinning while the damper/crankshaft pulley has already slowed,
something must slip, break or pop off. Slipping is preferable to the
other two and
this is how the system was designed to operate. It is during the
upshifts
and to a certain degree, the downshifts that your belt will be more
likely
to pop off if it is too tight. This is especially the case if you are an
aggressive driver. I believe there is somewhat less risk of this being a
problem if you have an automatic trannie.
2. Pretty much everything expands as it gets hotter. However, aluminum
expands more per degree of temperature rise than many substances
including
steel. As you know, the Corvair crankcase and other components are
aluminum. If your belt is a little tight when it is cold, it will become
too tight
once the engine gets up to operating temperature when the engine expands
and your belt could become far too tight if the engine overheats for any
reason. You can see this if you check the tightness of the belt when
cold and
again immediately after you have taken a drive. (Don't burn your
fingers.
The alt/gen pulley can also get quite hot.)
As a cautionary note, if you get general service for your Corvair at your
local mechanic, check your belt tightness when you get it back. Some
mechanics are conscientious and might check and tighten your belt to what
they
think it should be. Along this line of reasoning, you should also verify
your tire pressure. As you may know, our cars need higher pressure in
the
rear compared to the front which is the opposite of most other cars. The
mechanic may think he is being thorough raising the pressure in the front
but
is actually creating a dangerous situation in his or her ignorance. By
all
means tell him but don't assume that just telling the mechanic not to
play
with the belt tightness or tire pressure is sufficient. The guy who
"writes you up" may not be the guy who works on your car or FC. I have
also
heard horror stories where the garage "helper" tops off the Corvair's
"radiator" with water. You do not want to drive the car with a crankcase
full of
oil/water mixture.
Lastly, always carry a spare belt because you will be going nowhere fast
if
you lose your belt. It is cheap insurance. The belt is often torn up or
permanently bent up when it pops off at speed. You should also carry the
proper tools to replace the belt but at least generic tools can be
borrowed
in a pinch. The proper size belt may not be available where you break
down
on a holiday weekend up in the mountains or in the desert ........
Murphy's Law.
Doc
1960 Corvette, 1961 Rampside, 1962 Rampside, 1964 Spyder coupe, 1965
Greenbrier, 1966 Canadian Corsa turbo coupe, 1967 Nova SS, 1968 Camaro
ragtop
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