Why is ductile iron not used for brake drum usually? 2

[Henryzhang]

Ductile iron has a very good comprehensive performance,why is it not used for brake drums(trucks or trailers)? Is it because of its weak heat conductivity and vibration damping capacity? Can I try have a try? Thanks!

 

[swall]

First, you have to consider why gray iron is typically used for brake drums--the flake graphite. This gives you a good friction surface that is resistant to heat checking. Flake graphite irons also dampen noise better than graphite nodule irons,as you have guessed. So, a drum made from ductile iron would be more prone to heat check cracking over time and not dampening vibration as well.

 

[Henryzhang]

Mr.Swall,Thanks a lot

But I heard that the normal failure of brake drums for truck or trailer are heat fatigue just as you mentioned 'HEAT CHECK' above.I know ductile iron has a very good heat fatigue resistant property.Can you give me some data for detail help including friction surface also? If I have a try to make ductile brake drums,what will happen? Will it be a low braking force due to low friction coefficient and low heat fatigue limit than gray iron?
where can I get these basic data? Thanks!

Best regards

 

[rmw]

Brake drums are also heat sinks as they serve as friction devices. The whole reason of using ductle iron parts on truck/trailers is to reduce the weight of the vehicle by using less mass of a lighter material of the same strength.

Therefore, irrespective of any heat checking/friction issues, the reduced mass of ductle brake drums would defeat the purpose of the brake drums. Note that while disc brake rotors are not as massive as brake drums, they are also specifically designed to disapate the heat, not 'store' it until it can be disapated slowly once the brakes are de-activated.

rmw

 

[swall]

Henryzhang--For starters, take a look at SAE J431, the spec for gray iron castings. There is a section titled "Brake Drums and Clutch Plates for Special Service". This will give you an over view of the current brake drum cast iron grades. Yes, the best grades of cast iron drums eventually fail due to heat checking--you can only delay the inevitable, not prevent it.For an upgrade, I would look at compacted graphite (CG)cast iron, also know as vermicular iron. It has mechanical properties somewhere between gray iron and ductile iron. The graphite morphology would be better than the spheroidal form found in ductile, insofar as providing a good friction surface. Dampening would not be as good as gray iron, however. Don't know of a current spec for CG iron.

 

[NickE]

Doesnt flake type graphite also reduce distortion caused by uneven heating?