# Difference between a spring's k-value and rate constant?

As everyone learns in physics class, Hooke's law states that a spring's force is proportional to the distance it is stretched/compressed. The constant of proportionality is usually referred to as $k$.

I am currently working on a project for which I need a spring and I found online that I should probably use a "K25" spring for my application. I assumed that this K is the same $k$ that is commonly used to represent the spring constant.

However, when I actually started shopping for said spring I have found that it seems to be different. Springs are listed with both a "rate constant" and a K-something value. See this catalog list* from McMaster (the K-value is at the end of the part number, i.e. 9637K25).

The two numbers don't seem to be related. For instance there are two K25 springs, one has a rate constant of 28.53 and the other has a rate constant of 83.

So what's the difference between these two numbers, what do they represent physically, and what is their relation to the spring constant $k$ from Hooke's Law?

*For some reason McMaster's website doesn't allow me to link to a subsection of springs, I am looking in the "Cut-to-Length Compression Springs" category.

• The McMaster rate constant in these tables is: (free length)*(spring constant). See this thread and this thread. – mg4w Oct 9 '16 at 9:06
• do you have some reason to think the number after the k in the part number is anything other than the manufacturers identifier number. I'm inclined to think if the "k value" had a technical meaning it would appear in the spec table, not just in the part number. – agentp Oct 24 '17 at 15:44