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Re: [GMCnet] Fan Clutch [message #61445 is a reply to message #61437] Sat, 24 October 2009 11:24 Go to previous messageGo to previous message
Tin Gerbil is currently offline  Tin Gerbil   United States
Messages: 236
Registered: October 2006
Location: Vancouver Island, B.C.
Karma:
Senior Member
Ken;
The rule of thumb we were given when building belt driven equipment was
a one half wrap on a 6" pulley, using a 3/8" belt will transmit a
maximum of 5 hp before it slips excessively. IIRC the reason for
switching to a viscous coupled fan was because under normal driving
conditions a car fan uses up to 17 hp. Yes, I realize these two
statements do not match. As your engineer said you needed two 3/8"
belts, I will assume 10hp. What no one has addressed, is the fact that
the fan and shroud in a GMC are reducing the air flow through the
radiator when the fan clutch is not engaged. Ken has pointed out that
the nature of air is that it will go everywhere but where you need it.
When the automotive industry switched from engine driven fans to
electric, at first they used shrouds. Most vehicles have now done away
with shrouds on electric fans. They did this to improve air flow
through the radiator under normal driving conditions. They cool better
without a shroud. Depending on the design of the radiator, it takes
approximately 5 mph to get air moving through the radiator. We know
this from Honda who have the electric fans blowing forward, out, through
the radiator on all of their large motorcycles. At 5 mph, they shut off
the fan and depend on ram air. If you putt along at 5 mph, as in a
parade, you quickly boil over. The fan never comes on over this speed.

What one of the brilliant mathematicians (who appears to know a lot of
chemistry) in the crowd does not seem to be able to calculate is the
FACT that a Heavy Duty Fan Clutch contains viscous fan clutch fluid of
10,000 CTS. A Light Duty Fan Clutch is using 3,000 CTS fluid. The
heavier fluid drags the fan, around using more horsepower at all times,
not just during maximum couple. This is why I was able to document
better fuel mileage with my diesel trucks when using Light Duty Fan
Clutches. 10% coupling 99% of the time vs 33% coupling 99% of the time.
1 hp 99% of the time vs 3.3 hp 99% of the time. A simple way to
observe this is to paint one fan blade, start a cold engine and let it
idle for one minute. Shut it off and count the number of revolutions
before a light duty clutch stops. Put on a Heavy duty clutch and do the
same test. I really should have taken KenB's advice, as most people
just won't get it.

One other point on fan clutches that so many have experienced failures
with. You can carry a simple locking plate made of 1/8" aluminum. It
has a "U" shaped cutout that slips into the wrench slot on the shaft.
You use two of the fan clutch mounting bolts to lock the shaft to the
clutch. This should have great appeal to those who like to know the fan
is working, as it will now be roaring all the way home and they will be
in true, severe, 100%, heavy duty fan clutch heaven.

Most fan clutches can be repaired by people with very little skill at
minimal cost. If you can get it off the engine, you can repair it. But
then Y'all probably want to switch to a Severe Heavy Duty Clutch anyway.
Send me a PM if you want the rebuild instructions.

Gordon "ad nauseum"



Ken Burton wrote:
>
> Larry wrote on Fri, 23 October 2009 12:36
>> FYI, did a search for 15- 4208 at ACdelco.com and got this:
>>
>> CLUTCH,FAN BLADE
>> Part Number: 15-4947
>> Requested part 15-4208 has been superseded.
>>
>> HUmmm...wonder which one this really is...light, normal or heavy duty?
>
>
>
> I have no knowledge on the 15-4947.
>
> The work I did on these clutches with AC Delco engineering was several years ago and the recommendation at the time was 15-4208. The engineer I worked with had me get the fan diameter and fan pitch for him. Denny Allen had one out so I got the measurements from him. With that data he recommended the 15-4208.
>
> He was able to find the specs on the original GM part number we used and said the specs on the 15-4208 were about the same.
>
> At the time I asked about the 15-4644. He said it was for a minimum 2" larger diameter fan with at least one more inch in the pitch. He also stated that it needed at least two 1/2" wide belts to drive it. When I told him some GMCers were using the 15-4644 with two 7/16" belts, he asked me how their fan belt, alternator, and water pump bearing life was. I told him I did not know.
>
> He said the 15-4208 needs a minimum of two 3/8" belts to drive it. He saw no problem when I told him ours were really two, or three with the optional AC installed, 7/16" belts.
>
> The most recent one I bought was in July and I forwarded that one to Canada so I never even opened the box.
>
> All I can say is someone needs to buy one of the new ones and try it. I also see the new part number is $66.00 where the 15-4208 was $50.00
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Gordon '74 Canyon Lands "Tin Gerbil" Vancouver Island, B.C.

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