At 01:02 PM 6/29/02 -0400, Gordie Bird wrote:
>Are fans balanced?
It is my experience with MGA fans, both real world and professional
(mechanical engineer), that the original fans are inherently well balanced
when they leave (left) the factory. The fan hub is stamped on precision
machined production dies and is quite precise and symetrical from one blade
mount to the next. The blades are stamped on precision machined production
dies and are virtually and functionally identical and of equal weight. The
fan is assembled with rivets that are interference fit in the holes when
completed, so there is no discernable misalignment from one blade to the
next. You would need a setup jig and precision instruments to measure the
variation in blade length. New replacement parts may have a different story.
The logical conclusion is that if your fan has a variation of 1/8" in the
length of the blades it is either bad manufaacture or it has been somehow
deformed (bent in various ways) since leaving the factory. This will
likely cause it to be out of balance. Exactly how much out of balance is
not possible to say without dynamic measurement. However, positioning one
blade 1/8" farther from the center of rotation that other blades is likely
to cause a vibration (maybe even a noticeable vibration), and over time
this vibration can be detrimental to the water pump shaft bearings and to
the fan itself. The process of bending and/or straightening the blade
after initial manufacture can result in microscopic stress cracks that can
over time propogate in to larger cracks and eventual breakage of the
fan. Vibration will enhance and accellerate this process.
When the fan breaks one blade will separate from the assembly and fly off
on a tangent, which could be in any direction. Because of the dynamics of
centrifugal force and gravity the most likely direction is between the 10
and 11 o'clock position when viewed from the front. The force of this
action depends on the speed of rotation at the time of separation, usually
in excess of 3000 rpm or at long term constant cruising speed, but in any
case it is a lot of force and energy. This may physically take out the
upper radiator hose connection in the radiator top tank (MGA and early
MGB), and/or the blade may penetrate the bonnet. Or if you're more lucky
it might bounce off of the inner fender or some other part of the chassis
in the immediate vacinity and come to rest on the front suspension cross
member or fall on the ground. Resulting damage could be anything between
very little or very significant, up to and possibly including personal
injury, and the risk of being on the bad end of the scale is
significant. Laws of probability indicate that this will most likely
happen while the car is in motion with the bonnet closed, so the likelihood
of direct personal injury is fairly small. But the likelihood of
significant physical damage in addition to destruction of the fan is
moderately high.
Bottom line is that deformed (or badly fabricated) fan blades are virtually
impossible to straighten (or grind) properly back to original
configuration, and doing so would induce a much higher probability of
breakage in the future. When you have a deformed metal radiator fan you
should without question replace it immediatly and do not attempt to repair it.
>I have a new fan where three of the blades are 1/8" longer than the other 3.
New? As a matter of public safety I would like to ask where this part was
obtained?
>Those three longer blades then hit the shroud.
>Can I grind down the long blades?
If this is indeed a new and unused part I would return it to the supplier
and demand a replacement or a refund. A riveted metal fan assembly with
that much misalignment or inconsistency from blade to blade is indicative
of either damage in handling or poor manufacture, neither of which should
be acceptable. It may or may not be out of balance as received, but if you
attempt to grind back some of the blade tips it will surely be out of
balance when you finish (and I suppose it would void any new part
warrantee). Bringing it back into balance would require some form of
dynamic balance measurement (bubble balancing at the very minimum), and the
removal of more material from the heavier parts of the assembly. This is
not a home garage project, and I know of no professional entity that would
assume the liability to do this for you.
I personally would rather use a 40 year old fan that is straight than a new
fan that is not straight. If new straight fans are not available, then we
may have a procurement problem, but this is not cause to install known or
suspected hazzardous parts on your car. If it was only a museum piece,
maybe. If it is going to be run and driven, that would be a definite
not. Send it back.
$.02,
Barney Gaylord
1958 MGA with an attitude (and one broken fan in 44 years and 225,000 miles)
http://www.ntsource.com/~barneymg
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