View Full Version : a cooling situation....

07-31-2003, 09:08 PM
Ok, here's the problem: Is it better or more efficient to remove heat, (draw the heat), from an object or to introduce cooling air to the heated object?(to cool it)

The application is a centrifugal clutch on an RC engine, with heat being the main "clutch-killer". I'm in the process of making a new flywheel that enshrouds the the clutch bellhousing; at this point, I can either make it draw heat away from the cluchbell or introduce cooling air to the clutchbell, depending on which way the cooling fns are angled. Which would be the best way? My guess is to draw the heat away, rather than to induce cooling air.

07-31-2003, 10:18 PM
Basically, what you are asking is whether it should suck or blow. I think blowing air on the hot area will work better.

07-31-2003, 10:33 PM
THAT's what I needed to know!Thanks Evan.

Peter S
07-31-2003, 10:58 PM
You might be lucky cooling it, on the other hand, it may just be plain overloaded. Excess heat can be a sign of overloading (ie clutch is undersize), causing it to slip and generate heat.

07-31-2003, 11:22 PM
Where's the "air" comming from? From around a hot engine or fresh air? Car or Plane? It shouldn't matter if you push or pull air. The air displaced has to be replenished with air from somewhere--hopefully, that's cooler air.
In the old kart racing days, we pressed an aluminum fin sleave over our clutches. This pulled air from the outboard side and blew it toward the engine. This helped keep the "bell" of the clutch cooler. It also made it easier to tune the clutch--a more consistant rpm engagement. Copper would be better but that's more weight.

07-31-2003, 11:47 PM
If that is a flywheel, it must be on the engine side of the clutch, and the clutchbell would be on the clutch output shaft. So the two should be on a common axis next to one another, but on separate shafts.

Maybe I don't entirely see the geometry there, but............

Cooling depends on air velocity, if everything else remains the same. Faster flow generally means better cooling if the air goes over what must be cooled. If it goes so fast that it misses some places, slower would be better. Whether the air was pulled or pushed usually is of no matter.

If you pull air out, you have to somehow control where it comes from to ensure that it goes over what you want cooled.

If you blow air you often can better direct it over the right areas.

But, if you have a flywheel "surrounding" the clutch, with the fins outside of the clutch, pretty much no matter how you angle the fins, they will end up slinging the air away. So you will be back to directing the intake side so that the flow is over the clutch.

07-31-2003, 11:48 PM
I think blowing air on it would give a slight advantage.

08-01-2003, 01:05 AM
The idea is to make a heatsink. If you put fins on the "bell" of the clutch, the fins will extract heat from the bell. If the fins are angled, they will move more air across the surface and extract more heat. "Slinging" air off the fins? What up wit dat? How do you think a fan works? Every fan I have blows air from the front, not from the sides!
"Sling" that hot air off using a copper fan as your flywheel. Press a steel ring into it for your clutch contact surface and go racing!

08-01-2003, 02:08 AM
OK... as I understand your setup, you are going to build a cooling fan on the perimeter of your flywheel, outside the clutch engagement area?

In that case, the finned housing idea is best; angle the fan blades to pull air from the coolest intake source. The straightest path for the air to follow will keep its speed up, but it needs to be pulled from as far from the engine as possible.

The main variable you are concerned with is the difference in temp between the air and the clutch. There only difference between suck and blow is which side of the fan you are sitting on.

08-01-2003, 03:08 AM
When you pull air from an area you have little control where it comes from. This is a common issue in cooling a computer. I sell computers that have a fan on the left side of the case. If it exausts air from the case the effect on CPU temp is slight. If blows air into the case it is directed onto the CPU and lowers the temp by 12 degrees. Either way, there must be a way for the cooling air to enter and exit the area to be cooled.

Look at it this way. Use your shop vac in suck mode and put your hand a foot from the nozzle. Feel anything? Now put it in blow mode and try the same. Different? I would say so.

08-01-2003, 03:48 AM
I won't pose an opinion on the subject. ButI will express my objection on the writers posing of the question. If you really want an unbiased answer, you can't say what you expect the answer to be. Good luck, and I know best answers are often the ones that you don't expect.


08-01-2003, 08:47 AM
Increase the size of clutch.

Add cooling fins.

Add a ducted fan cooling system to draw outside air past the cooling fins and exhaust it out the back. Expect a large loss in horspower from the use of a fan on the motor (no free lunch).

Only the first two suggestions offer any reasonable success. The third may be viable if you already have excessive power. Try them in order given.

08-01-2003, 11:59 AM
I'd like to increase the the size of the clutch, but, there's no room for expansion in that area, so the next best thing is cooling. From the general consensus, introduced cooling air is preferred.

A ducted unit is out of the question, if for nothing else, but, for size.

CCWKen, my fans in the house also blow air from the front, but, there are fans that blow outward, such as heater/blower fans found in automobiles. Now, I don't really know all that much about HVAC, but, I'm pretty sure that the shape & placement of the fan blades dictate airflow and direction. Where the "cooling" air is coming from is from the front of the engine, ambient air, but, even air around the engine is cooler than the clutch!

As for the clutch being overloaded...it is! The clutch is designed for a 21 size engine, the engine I'm using is a 70, which comes with a 21 size clutch. There are materials available for use as clutch material, but,finding the right one is like looking for a needle in a haystack, especially, if ya don't know what's out there. Materials such as DuPont Vespel works extremely well from a heat standpoint, but, doesn't hook up well. I do have a sample of material that has been used in clutches for high-powered RC engines from Hibbing Friction. Right now, I'm using a small piece of turned down Fortal for the clutch material and appears to be working out, tho, cooling it would add some longevity to it.

CompositeEngr, that's the way I'm going to go, a flywheel with a finned "housing" around the clutch bell. We'll see how it works out.

08-02-2003, 07:14 PM
Lowering your gear ratio will also help tremendously. Good luck.

08-03-2003, 02:56 PM
Apparently, the general consensus is to provide cooling, rather than to draw the heat. I would have figured REMOVE THE HEAT. That being said, there are now 2 other quandries:

1. The number of cooling fins and angle

2. Should the fins be "open" at the end or "closed"? The reason I ask this is this engine spins at 18k RPM, and if I decide to make these for a 21 size engine, they spin upwards of 40k RPM. At these speeds, I do have concerns of the fins "flying off" and possibly injuring someone, due to the "open" style. Any thoughts on this?

Here's a pix of the prototype with "open" end and "closed" end. Because my math was off on the placement of the fins, I'm using this one as a guinea pig.


08-03-2003, 07:52 PM
If you want to cool, you have to remove heat. You don't APPLY cold, you EXCHANGE heat!
Your "fins" won't work well in the current configuration. By all means, provide a captive ring for the fins.

08-03-2003, 11:19 PM
I think I would allow a bit more room between the "blower fin" ID and the clutch bell. The cooling air passes thru that space, you want to leave room.

I would also extend the blower shell to the front of the clutchbell.

My choice would be to then make the front half or even 2/3 of the blower solid, to force airflow all the way back to the rear. That way it will go over the full clutchbell OD area.

Front portion can be thinnish, as it needs only to not fly apart.

Then put a number of straight slots around at the rear to let air out. If you can go larger diameter at the rear, do so, it will make the slots longer.

The longer the bore of the slot the better it will induce airflow. Too short a slot and it won't "sling" air as well, you just make an "air stirrer".

No need to tilt the slots, it won't help much if any.

Your goal is to suck lots of air past the bell so that it takes the maximum heat away.

The more airflow, the less the air temp rises as it passes, and the better the cooling. Heat flows from hot to cold things. The cooler the air (in this case), the faster the heat flows into it.

If you chamfer or round the edges of the slots on the inside, it will help airflow.

If you could put a stationary housing around the clutchbell and up the front of the blower, you could just make a largish diameter blower with short stub fins and nice long radial slots between them. Like a jet engine centrifugal compressor.

That would likely work best of all.

[This message has been edited by Oso (edited 08-03-2003).]

[This message has been edited by Oso (edited 08-04-2003).]