It’s all in the numbers (but can we believe them?)

Dr. Robert M. Gresham, Contributing Editor | TLT Lubrication Fundamentals July 2014

When it comes to meeting product specifications, a little bit of variation can lead to a lot of problems with your customers.

 

KEY CONCEPTS
• Sometimes a customer sets a specification or requirement for something beyond our or maybe even the world’s capability.
• In the specification, not only should you agree on what is to be made but also on the means of measuring to ensure it has been made properly.
• ASTM shepherds the development of test procedures and equipment for measuring something, verifies the precision and accuracy of the test and publishes a standard to codify the test method.

IN MANUFACTURING, THE FIRST GOAL is to make a given product or part as consistently as possible—that is, always the same. The second goal is to make what we wanted, a product that meets our specification or requirement. Clearly, the two concepts go hand-in-hand—did we make what we wanted and is it the same every time?

Finding the answer to those questions is easier said than done.

Let’s look more closely. First, did we make what we wanted?

Well, sometimes a customer sets a specification or requirement for something beyond our or maybe even the world’s capability. I’ve had customers tell me that they wanted the specification that way because they wanted me to do the best I could. Well, that may sound good, but in the business world it’s not so good.

If you agree to a specification beyond your capabilities, you end up not meeting the requirements of the sales contract—and you might not get paid. Agreeing to an unrealistic specification just to get a sale might help a salesman make his numbers, but it won’t endear him to the company’s manufacturing department, accounting department or, if it results in a lawsuit, to the CEO.

The second issue is a little more complex. How do you know you are making the product the same way every time and to the specification? This gets us into the issue of measurement. In the specification (and/or sales contract), not only should you agree on what is to be made but also on the means of measuring to ensure it has been made properly. When you make the measurement, will you get the right number?

This is where organizations like the American Society for Testing and Materials (www.astm.org) come in. Working with a cross-section of industry, ASTM develops standards for materials and test methods. Regarding test methods, ASTM shepherds the development of test procedures and equipment for measuring something, verifies the precision and accuracy of the test and publishes a standard to codify the test method.



Accuracy addresses the question, “Did you get the correct measurement value?” Bias is a systematic error that contributes to the difference between the mean of a large number of test results and an “accepted reference value.” The accepted reference value is considered the true value and, therefore, bias has to do with the degree to which that true value is not obtained.

Through calibration, bias generally can be reduced or eliminated. Precision has to do with the variability of the test method itself. Does the method provide the same number every time or does it vary and, if so, by how much?

In ASTM parlance, precision is stated in its published standards as the method’s repeatability and the reproducibility. Repeatability addresses the difference between two test results performed by the same operator in the same laboratory with the same apparatus on the same test material under constant operating conditions. Reproducibility addresses the difference between two single and independent results obtained by different operators working in different laboratories on identical test material in the normal and correct operation of the test method.

These are important considerations when a specification is written or a sales contract is agreed upon. Here’s an example of why.

I once had a customer who wanted the consistency of the grease we sold to him to be very consistent (pun intended). Grease consistency is a somewhat ethereal word to describe the apparent thickness, soupiness or whatever of the grease. ASTM D-217 describes the test method of measuring greases and provides a way of categorizing this property (see Figure 1). I suppose a very inconsistent grease is like a brick and a very consistent grease is like a thick fluid, since the numbers go up, although that seems counter intuitive. But I digress.


Figure 1.

Anyway, people refer to grease consistency by its NLGI ranking. As I recall, the customer specified the grease at 305 +/- 5. The product wasn’t much, as we merely mixed an inert solid lubricant into a base grease that we purchased according to his specification. The base grease was a NLGI No. 1, as noted in Figure 1. As you can see, a No. 1 grease can vary from 310-340, but what about a 305? After we mixed the additive, the consistency decreased a little (got thicker), and was on the low end of a No. 1.

However, to address the issue of the consistency of our grease, one should look to ASTM method D-217 concerning the variability of the test method itself (see Figure 2).


Figure 2. Repeatability and Reproducibility

It states that the method’s worked penetration variability for more than one lab and operator is 20 units. Thus, the customer and I could vary by 40 units (+/- 20) and statistically have the same result. As you can guess, with a specification of +/- 5, we could test the same batch and sample of the grease and at least 75 percent of the results would be out of specification. Thus, he could reject a lot, send it back and we could relabel it, send it back and 25 percent of the time it would meet the specification. Hmm.

Clearly, in the design and specification of various systems, consideration of our ability to measure and verify the components that make up the system needs to be given priority at the very beginning. Both customer and vendor QC departments and/or outside test labs need to be able to generate the same numbers on the same samples. Salesmen should not accept orders for the impossible, just to make their numbers.

After all, it’s all in the numbers.

Bob Gresham is STLE’s director of professional development. You can reach him at rgresham@stle.org.