cluch disk hardness checking what scale

What is the right scale, KG load ,and penetrator type .500 ball ???

 

Rockwell "S" scale is the industry norm. 100 kg load, .500" ball.

 

Normally, the "S" scale with that big of ball is for thin soft metals like bearings and such, and harden steels use the Rockwell "C" scale, most importantly, check the hardness tester against a calibrated master.

 

Mike, not doubting you or Justin, but the disks must be awfully soft to be tested on that scale. Curious, what should the readings be on a disk that is considered acceptable?

I'm a pg guy so I'll admit that I don't know diddly-squat about clutch disks, but I am knowledgeable in the heat-treatment process and have done my fair share of hardness testing from rubber to case-hardened metals, written quite a few hardness testing procedures and have never heard of using the "S" scale for something like clutch disks.

 

All of the disc manufacturers use the RS scale because the material is fairly soft in comparison to what you would normally be testing on the RC scale, like heat treated steels. If it wasn't, you may as well be using mild steel as friction material. It needs to be softer than your heat shields and floaters so it can wear and dissipate heat to maintain proper coefficient of friction and torque capacity. The RS scale is used for the friction material only though, not the disc carrier or the hub.

Every clutch disc ever made has variances in hardness between the "pads" of friction material. I couldn't tell you why, that is beyond my scope, but never the less, that's the way it is. When you get a clutch disc, you can actually see how it was sampled for the Rockwell by looking at the BB sized indentations on the friction material. We usually do three pads on each side, sometimes all six. You take your readings from each measurement and average them together to get the disc's Rockwell number. We generally see variances from pad to pad around 2-3, but can be as high as 6 or 7 with certain formulas.

 

We at Rear Gears use your Ace clutch discs and are very happy! What I am interested in learning is what hardness do most of the alcohol funny cars use and what differences are there between Ace and Boninfante as far as hardness? Thanks
Danny

 

Hey Danny,

Most run anywhere from 58-67 depending on the personal preference of the owner/crew chief. The difference in discs tends to come in the formulation and conditions during sintering rather than the hardness or manufacturer. This is why you can order the same exact thing once a year from the same place, three years in a row and notice some differences in the way the clutch discs are running. A batch of powdered metal mix can only make so many discs and you can only cook so many discs at one time. The biggest trick is consistency. Just like with race cars, you can do everything identically every time and you'll see some major variations in the outcome unless you know how to adjust for changing conditions.