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lbhsbz
03-11-2011, 04:11 PM
I have an automotive brake application where the caliper is experiencing "cooking" of the dust boots around the pistons due to radient heat from the pads. We've stepped up to silicone boots which last considerably longer, but still get burned up within a few months. These vehicles are abused, no doubt, but thats out of my control. I have to come up with a solution.

I've seen titanium heat shields for race applications that snap into the piston in an effort to reduce the heat transfer to the piston and the fluid..but that gets pricey real quick.

I'm exploring other more economical solutions.

The material will be in contact with the heat source, but not with the thing it's protecting. I was thinking that aluminum might be a good idea here because since it's making contact with the heat source, the reflective qualities of stainless or titanium will not be as important as the heat transfer properties of aluminum.

Thoughts? The shield material will be approximately 1/16 thick, at most.

ckelloug
03-11-2011, 05:19 PM
Polished aluminum is a good heat reflector. If the aluminum is only 1/16 inch thick, there will be relatively little heat conducted into the plate itself. Some basic physics suggests it will be fairly effective as a heat reflector. But the part of the heat that is absorbed rather than reflected will heat the aluminum. Brake dust will tend to blacken the plate, lower reflectivity and increase absorbtion. The only question I have is whether an aluminum plate covered in brake dust will melt with sustained exposure to braking temperatures.

--Cameron

winchman
03-11-2011, 06:39 PM
There's probably enough of the piston sticking out past the boot to install a thin aluminum disc. How about making them out of junked hard drive discs turned to the appropriate diameter. They're usually free, readily available, and have a mirror-like finish.

I have no idea how what they're made of or how they will stand up to high temperatures, but it would be good to see someone use them for something.

lbhsbz
03-11-2011, 06:39 PM
Well, They make aluminum pistons for calipers, and those don't melt....possibly because they're cooled by the fluid on the backside. Hm. What as the heat transfer of aluminum with a higher melting point?

bob_s
03-11-2011, 07:57 PM
Well, They make aluminum pistons for calipers, and those don't melt....possibly because they're cooled by the fluid on the backside. Hm. What as the heat transfer of aluminum with a higher melting point?

Copper has about double the thermal conductivity of aluminum with a melting point of 1064"C (1947 F)

darryl
03-11-2011, 08:43 PM
Hmm. Maybe there's a better pad material that doesn't absorb so much of the braking energy. Ideally, the disc takes the heat and dissipates it- the pad should remain far cooler. Maybe the pads you have on there are designed to take lots of the heat and prematurely cook the braking system- you know, cheap pads but more money spent for more frequent servicing :(

Willy
03-11-2011, 10:05 PM
Have you thought of perhaps running cooling air ducts to the brakes?

Rather than try to get components to survive the heat, get rid of the heat.

Not sure what application you are using the brakes for, but it seems to be what everyone that tortures their brake system relies on.

http://i76.photobucket.com/albums/j31/250willy/e9e6ce66.jpg

http://i76.photobucket.com/albums/j31/250willy/544dae48.jpg

http://i76.photobucket.com/albums/j31/250willy/bd58668a.jpg

Arcane
03-11-2011, 10:10 PM
There's only a few things you can actually do to reduce the heat. First is to maximize the cooling of the rotor. More airflow to it by use of ducting (if possible) and by using a different rotor design that will pump more air through it. You have probably already considered this option. Second is the shielding you are inquiring about. There's a couple of ways to do this one, the easiest is just to fit a thermal barrier between the piston and pad. Another way is by using a phenolic piston. This insulates the dust boots from conducted heat and has the added benefit of insulating the brake fluid from heat also. You can use aluminum or stainless steel for a thermal barrier, both are relatively cheap. Personally I think the stainless steel would be the better choice.

Heat is transferred by conduction, convection and radiation. All three methods are affected by the emissivity of the material and the thermal conductivity (it's reciprocal...thermal resistivity). The oxide layer affects it as well as the surface finish. A change in the operating temperature will effect a change in a materials thermal conductivity as well. It's not a simple thing to scope out!

dian
03-13-2011, 04:40 AM
titanium pistons are used on racing applications. i have not seen phenolic ones. do you have a source for these?

ikdor
03-13-2011, 05:33 AM
I'm a bit puzzled that the dust boots are suffering from the radiant heat while they're touching the scorching hot piston. I would expect the problem to be conduction.
As you're trying to reduce the heat transfer trough this shield I would think that you'd like something with the least heat conduction instead of something like aluminium.

Not sure how much money you want to spend but pistons out of machinable ceramic sound really sweet :D

Igor

vpt
03-13-2011, 10:01 AM
I second the air ducts!


On a side note, my ford excursion has composite pistons in the calipers.