Nylon/Steel Composite Roller - Thermal Question

BACKGROUND:

I was asked to mfg some small press rollers approx 40mm OD x 40mm lg. For trial purposes I made six each of these things with a steel tube core, straight knurled the OD end to end which increased the OD by 0.020" - 0.022". Then turned and bored a nylon sleeve with a 1.000" ID and pressed it over the core. I didn't hear anything of them for a few months then I get an RFQ for 50 of the same with a request to sharpen my pencil. The money we're talking about here is absolute chicken feed to begin with, so I made a change to the design which I thought to be minor yet allowed these things to be churned out on my CNC lathe. All ops except the knurl which was done manually. The change was instead of knurling the entire length of the core, stop 6mm short of the ends so as not to disturb the close tolerance bearing bore. I thought this to be a major requirement since one of the customer's complaints was a gritty bearing feel from a competing vendor. (FYI - the other vendor(s) are peddling rollers of differing materials.) Well it seems this change has bitten me in the ass. It's now over a year later and they are asking for the original version as the nylon sleeve won't stay on the core in the batch of 50 as did the first batch of 6. BTW - I offered to build with a retaining ring at each end and the offer was rejected... go figure.

QUESTION:

I've yet to see this process and getting information from the engineers is akin to eating tomato soup with a knitting needle. The ONLY bit of information I have been able to ascertain is the product temp this roller is in contact with (almost 100% duty) will see a max of 125°C ( 257°F). Is anyone here with thermal experience that can/will check my numbers and let me know if I'm figuring the expansion correctly?

I have gotten this data from my vendor re: Nylon 6/6 coefficient of thermal expansion to be 0.000055 in/in/°F. I'm SWAGing a delta T of 187°F in which case the nylon would grow ~0.01028". I am basing the expansion of the steel core on a rate of 0.0000075 in/in/°F. With the same delta T the core would grow ~0.00140".

Therefore, I should still have approx. 0,010" of interference fit.

You might be scoring/broaching the nylon as it presses over the knurl, reducing the ultimate interference and retention. If you can fit it into the budget, set up an oven to pre-heat the nylon before forcing it over the steel, reducing the chance of cutting away bore material during the fit.

Or, use a high-temp adhesive on the knurl OD prior to pressing. It will lubricate the press making it go easier, and give more retaining capability if compounded for plastics.

Milland said:

You might be scoring/broaching the nylon as it presses over the knurl, reducing the ultimate interference and retention. If you can fit it into the budget, set up an oven to pre-heat the nylon before forcing it over the steel, reducing the chance of cutting away bore material during the fit.

Or, use a high-temp adhesive on the knurl OD prior to pressing. It will lubricate the press making it go easier, and give more retaining capability if compounded for plastics.

Click to expand...

Yeah, broaching is definitely taking place... I see that now after disassembling one. I did, however, chamfer both ends of the core in an effort to lessen this on the trial rollers. What's puzzling is they claim these rollers held up approx four times as long as the last batch of 50. Of course, there are no hard numbers to back this up which is frustrating...

I do like the idea of preheating the sleeves.

As an aside, I made up one of these rollers and put it into the furnace at 257° F for an hour. I have no way of precisely measuring force on any of my small presses, therefore, I can't quantify the amount of force required to press these elements together at room temp, or the force required to separate them at the max operating temp, but the nylon wasn't even soft to the hand at this temp and certainly wasn't easy to separate from the core at this temp. I think I will go over and throw a sleeve (only) in the oven and see what happens. Thanks for the input!