|Established in 1952
Tuesday, June 18, 2013
Some of the ball screw manufacturers are very concerned that bearing balls of normal hardness may crush up in service. They typically specify that the hardness be 59 HRC maximum.
One of the tricks in building “O” Ring molds is to polish the toroidal surfaces by rolling a ball of the correct diameter around the groove under high pressure. If one of these balls should crush it would cause an expensive disaster. For this reason the mold makers also specify 59 HRC maximum ball hardness.
I might comment that achieving the 59 HRC is not a cut and dry process. We don’t want to adversely effect the quality of the balls and we don’t want to go much below 59 HRC. We go back and re-temper the balls at 325° F, if this doesn’t quite do it, we repeat the process.
We had a customer who was a large manufacturer of ball screws, who complained to us that some 1/8 inch (0.125”) [3.175mm] diameter chrome steel balls were much too hard. We checked the hardness of some sample from the same lot of balls that were still in our inventory and found them to be just fine. We measured the ball hardness on a Leitz-Miniload-microhardness machine, with a 500 gram load using a Knoop indentor. Not wanting to be blind-sided by the customer we also measured them with the same 500 gram load on the Vickers scale. These measurements were converted to their HRC equivalents.
When this same customer complained of a second set of balls being too hard, our concerns escalated. We franticly explored every possible error source that we could imagine. We have four of these Leitz Miniload, Micro Hardness testers throughout the organization and we used each and every one of them to evaluate these balls, using various loads of 200-300 and 500 grams on both Vickers and Knoop scales.
This being a substantial customer, with a visit to their facility being long overdue, I packed up our best “Scale Micrometer”, some micro hardness test coupons and our test sample of these balls and made the journey to their facility.
This being a large corporation with an excellent metallurgy capability, I felt somewhat intimidated, but I couldn’t imagine how we could be wrong.
After the normal introductory meeting in the mahogany conference room they brought out their stream roller presentation. It was truly overwhelming, with several PhD’s in metallurgy, presenting their case.
After lunch we retired to their very impressive metallurgy lab with its SEMs and X Ray spectrographs. I sheepishly brought out our scale micrometer and took over their Leitz Microhardness test machine, with which I was well versed. The measuring micrometer was dead nuts as every Leitz I have ever checked was.
I pulled out my stop watch, which made a few eyes pop. I checked the drop rate of the diamond indentor and made a minor adjustment to bring it to specification.
I whipped out the first triangle shaped, factory certified, hardness test coupon. It had a 55 HRC equivalency so it was very close to the hardness we wanted to check. Eyes widened when this specimen checked 59 HRC equivalent. Immediately the defensive curtain went up. How did we know this coupon was accurate? Then the biggest gun in the arsenal came out. They had calculated the area of the indention mathematically, which in their minds made their measurements directly traceable to the gravitational constant, so it couldn’t be wrong.
With an overwhelming feeling of confidence I took the hardness specimen to the Rockwell test machine sitting on the same table next to the Leitz Microhardness tester. I gave the machine an eyeball to see how it was set up and without even asking permission I checked our hardness test coupon. It measured 55 HRC, which was right on the money.
In a moment or two it dawned on them, one at the time, that either the Leitz was off or their Rockwell machine was off.
Hardness testing is not an absolute science, there is no direct traceability to any natural standard or mathematical formula.
If you don’t actually check the performance of the test machine in “real time”, that is just before or just after use or even better just before and just after use, you have no idea whether or not it is functioning properly.
The question still left, was why, was the Leitz Miniload Microhardness tester, which is a world class machine, so grossly malfunctioning?
We took a good look at the diamond indentor under a stereo Microscope, and it looked perfect. The scale micrometer was good, the diamond indentor was good and the drop rate was smooth and timed perfectly.
Right then the absolute unimaginable happened. The only variable left seems to be the force applied to the diamond indentor. I am personally very familiar with the Leitz system so when I looked at the 500 gram weight, that they were using, something didn’t look quite right. It was brand new looking, satin chrome plated, with 500 P engraved on the top, but unlike any Leitz weight that I had ever seen it had a rather large diameter hole drilled deep in the very center of the back surface. I asked if they had an accurate scale available. When we weighed the weight, all hell broke loose. It weighed only 438.6 grams. Because the Miniload pan and diamond weighed 15 grams the 500 gram weight should have weighed 485 grams exactly. The 46.4 grams difference is almost 10% of the 485 grams that the weight should have been, so the mystery of the too hard balls was resolved, but how the hole got in the weight will always remain a mystery.
The point that I really want to come back to is that you must check, check, and then check again. Could there have originally been a lead plug in that hole in the bottom of that weight? Had it fallen out. Maybe the results of the hardness test didn’t match the mathematical equations that someone used to calculate the theoretical perfect value and they adjusted the weight with a drill press?
If balls measure too hard, how did they get that way?
Mixed materials can be the cause of balls that are both too hard or too soft.
What may seem like a crude test to sophisticated metallurgists, is to spark test each bar of material with a high speed grinder. In all of our years in the ball business, we have had only one time when this test failed to identify mixed material. In this case, we were shipped some high speed steel in place of some 440C which spark test very similar to the 440C material, that we should have received. When heat treated, this material didn’t harden or more specifically it only hardened to 45 HRC. So outside of our financial loss, this wasn’t a problem.
Two common alloys used to make balls are 52100 chrome alloy steel and type 440C stainless steel. Type 440C when properly heat-treated will be 58-60 HRC while chrome steel will be 60-65 HRC.
Two quick checks: if the material is chrome alloy steel (C/S) it will etch dark gray to black with Nitol, (a mixture of 5% Nitric Acid and Alcohol,) while 440C won’t etch. If these two materials are ground, the spark generated will be entirely different. The 52100-C/S will be a yellow spark with lots of side sparks while type 440C is very red tracers with almost no side sparks.
In the lab, eddie current will identify mixed material.
Another potential problem is that the material has been hardened, but the subsequent tempering did not get done. This will lead to balls that are far too hard.
Over heating could cause this problem, but with all of the
modern control equipment with policemen (a second redundant
system) there isn’t any much likelihood of this happening
on American made products, outside of human errors. Who knows
what is going on with balls produced overseas? I know of at least
one case in which a conveyor system in a giant automobile plant
was disabled by fractured balls that were far too hard and
therefore very brittle. These balls were purchased from an