Aluminum Pulley and Steel C3 bearing

[trutech]

Hi all,

I am new here!!!

I really really need help with aluminum 6061-t6 pulley I am working on. A steel bearing will be press fit into it, and supposrted by Internal retaining clips on each side!

I read a little about thermal expansion, difference between a metal and aluminum pulley. Metal has to be .002"(.051mm) and aluminum at least .003"(.076") I use the metric system, I find it much easier!

I am confused as to how small the opening of the aluminum pulley has to be?? I made a few with .0035"(.089mm) It was extremely tight to get the bearing in! More tight than the metal pulley! When it finally went in, the center of the bearing that supposed to turn was really hard to turn, as compared to when it is in the metal pulley, it is easier/smoother!

Any ideas?? What do you guys think??

[RWOLFEJR]

Hi Trutech,
There are several things to consider with what you are doing. Selecting the fit is one aspect but there's more to it.

In your example the tighter fit in the aluminum bore locked the bearing up. The bearing wasn't designed with a little extra running clearance to allow for the fit you gave it. So the first thing I'd look at is do you really need that much interference in the fit. If the temperature change isn't more than a couple hundred degrees and this is a small diameter then just back off on the fit. If you feel you do need that fit then buy a bearing with more internal clearance to deal with the tighter fit.

Also... bearings with rolling elements... balls, rollers, needles... will typically want to roll under load before they will tend to want to slide on the inner or outer races. All bearings have an ideal set of circumstances they are designed to operate in. Selecting the right bearing design and appropriate fit for that bearing is very important to the life of the bearing. Proper installation and lubrication are also critical.

If you want to share more of the specifics of your application, I'm sure we could help get you where you need to be.

Good Luck,
Bob

[PinkertonD]

Usually the bearing manufacturer will provide a size range for the machined hole for the bearing. Given that both parts are lightly pressed together, the amount of expansion difference will be negligable unless the temperature swing is huge or the bearing diameter is huge.

If the bearing outer diameter is less than around 3" and/or the operating temp range is within about 200F you can forget the thermal coefficients.

This is one of those cases where experience overrides technical specs.

[trutech]

Thank you all for the reply!!

Here is more information of what I will be using:

The diameter of the billet pulley is 71mm(2 3/4"), width 19mm(3/4"), it is for a V-belt! I will be using Internal retaining clips on each side, just like the original pulley! The bearing is 35mm in diamter. The bearing I plan on using is RBtech 6202RS/C3. I heard it is a real good quality bearing! {Commercial link removed - please see our linking policy} If you go down to page 6, 6202 row, 15mm Bore, thats what I'll be using!
The engine Temperature gauge, 220 is in the middle, and 260 is the highest it goes. The engine usually reaches just under 220 even on hot days with A/C, so lets just say it is for 260 to be safe!!

With all the information provided, what do you you guys think the bore of the pulley should be?? Also when it will be machined I will have to put in a tolerance of +/- .025mm, since I really need an accurate number for the bore and the bearing press fitting. Example, If I want the bore of the pulley to be 34.925mm, then I would have to make the bore on the CAD 34.900mm with a tolerance of +/-025mm, meaning it will be anywhere from 34.875mm to 34.925mm.

What do you guys recommend I do, with all the info I gave you??

Thank you!!

[RWOLFEJR]

You've selected a supplier, so why not just give them a call...? I never heard of the folks you're excited about (!...?) but the book I like to reference when selecting bearings and fits and applications etc. has all that information at the front of the book!! (Wooo-Hooo!!)

You picked C3 for greater than normal internal clearance so you have a little extra allowance for press fit! (Yippeee!) Not sure why only one seal?...! Guess you're allowing for re-lube!! (Wooo-Hooo!)

Sorry... Whe I read things with a lot of exclamation points I get all worked up too!

Good Luck,
Bob

P.S. (See Topic "Types of forum spammers" in FOO section of this forum.... "Link Request Spammer?")

[PinkertonD]

Well, given those specs, I have to say there is nothing special required there. Unless RPMs are not into 6-figures, pretty much any pulley bearing combo will suffice.

As RW mentions, and I stated previously, check with the bearing manufacturer for fits and tolerances. Tolerances are defined to suit the application and not necessarily set to 3-Metric decimal places just because you can. e.g. 34.875mm = 1.3730314" . In this application, standard machining tolerances from Machinery's Handbook will suffice and if in doubt a little dab of Bearing Mount locking fluid, will take care of sleeping well at nights.

[trutech]

You've selected a supplier, so why not just give them a call...? I never heard of the folks you're excited about (!...?) but the book I like to reference when selecting bearings and fits and applications etc. has all that information at the front of the book!! (Wooo-Hooo!!)

You picked C3 for greater than normal internal clearance so you have a little extra allowance for press fit! (Yippeee!) Not sure why only one seal?...! Guess you're allowing for re-lube!! (Wooo-Hooo!)

Sorry... Whe I read things with a lot of exclamation points I get all worked up too!

Good Luck,
Bob

P.S. (See Topic "Types of forum spammers" in FOO section of this forum.... "Link Request Spammer?")

Is there a bearing you recommend I use? If so, would they have a website with all the information? You mentioned, not sure why only one seal, and re-lube? What you mean by that? Sorry, I am not a bearing expert, I am just good at CNC machining parts and making pulleys in Billet Aluminum.

Thank you!

[RWOLFEJR]

Hi Trutech,
I don't want to pitch for any specific brand of bearing. I'm sure most all of them are fine. Some names have been around a long... long time. Those big name manufacturers all have websites with access to their general catalogs online. Also I see that some manufacturers call "RS" Rubber seals (plural or two) whereas the book I was looking at calls out 2RS for seals both sides and RS means one seal. It's an old book though...?

I'd suggest you do a search "bearing nomenclature." Or type in "ABMA" (American Bearing Manufacturers Association)... Or "Standards rolling element bearings." The hits that pop up will have a ton of information on the wonderful world of bearings.

Far as your application goes... You didn't say what sort of fit you're after on the shaft or bearing inner race. You say you need a window of .002-ish or .050mm for machining. You might want to try to tighten that up some... that's a pretty big window for shooting for a fit on a small bearing. If you want a light press on the shaft as well as the bore then your fits become more critical and a window of .002 on both would end up with some failures when the ends of the tolerance windows are hit.

I'm not going to tell you that with a C3 suffix bearing a fit anywhere from size to -.0025 for your bore will be fine... As was said a few times now... Call the bearing house and have them tell you what fit to give it.

Good Luck,
Bob

[trutech]

They are actually 2RS, double seal, just noticed it.

I emailed the company that sells the bearings, they sent me the specs on the bearing. They emailed me a chart with information, most of it is dimensions, weight of the bearing, and some other stuff. Like I mentioned I have no clue about bearings and bearing fit, so if you can help me along. I am making a couple of Billet pulleys, only one of these will require a bearing, the rest are bolt on. I'll post the information that I don't understand.

"Outer Diamter"
35mm +0 -0.009

"Pitch Dia."
25.0

"Radial Internal Clearance" for C3
Min: 11 micrometers Max: 25micrometers

I really don't understand these. Make sense to you guys??

Thank you!

Do you guys understand this?

[RWOLFEJR]

So... You either asked the manufacturer the wrong question, or they misunderstood what you were asking. Myself... I'm more comfortable with picking up the telephone and speaking with a real live person than e-mails. Seems to get to the point much quicker that way. When the person you were talking to was telling you the intimate details of the bearing construction you could have simply interrupted and said no... no excuse me... but I just want to know what size I should bore my hole?

If you require +/- .001" in your boring process... then shoot for a bore of 1.377" (+/- .001) and call it a day. You'll be fine in that window.

[trutech]

I emailed a supplier that sells them and they sent me the chart. I will call the manufacturer directly and see what they say!!

So thermal expansion doesn't affect the size of pulley I am working on?

The thing that threw me off with all this info was when I was googling about this stuff and I came across this website and this thread over here http://www.engineersedge.com/wwwboard/posts/11510.html

On another forum, someone mentioned to me, when making the billet aluminum pulleys, I would have to go -.003". They told me -.002" for metal and -.003" for aluminum.

On the link I posted of the other thread on this website, You mentioned, 2nd to last post, "I'd go .003/.0035 shrink and standard bearing and would feel really good that it won't go anywhere and it'll spin fine." So I made my first batch of pulleys -.0035" I thought thats what I had to do also.

Sorry for all this, just trying to understand everything and get it right.

Thank you again for the help, I really appreciate it!!

[RWOLFEJR]

Hmmm...
So you have some history going on with this. .0005 wasn't enough. You locked up at .0035 interference... Oops... my bad. Next thing I'd try is the C3 bearing.

I didn't do any of the math on the difference in the rates of expansion. So let's look at the expansion thing... Aluminum is .00001244 and carbon steel .00000633 this is a difference of .00000611. So at 250-ish... change of about 180 x .00000611 = .0011-ish per inch or about .0015 difference in growth at 35mm if both are heated evenly. More significant than I would've guessed at such a low temperature.... Going on difference in expansion alone it'd look like you'd want to have a minimum of .0015 interference... More like .002?

So next thing to do is find out how much interference can the C3 bearing tolerate. Some of the interference will move the bearing and some the pulley. I'd place my bets on C3 working well for you at .002 - .0035. But if this were a project of mine here at work I'd call John (my bearing house guy) and ask him what it'll take to be certain.

[PinkertonD]

Given the relatively-slight temperature Delta and component diameters, I cannot see expansion being an issue. This sounds more to me to be an issue of machining accuracy and maybe surface finish of the machined portion.

Working to thousandths of a thousands, I would fire a Machinist/Designer/Engineer taking this long over a simple bearing fit issue.

[jboggs]

Hey guys! As entertaining as all this back and forth is, (and I am grinning big time) I feel compelled to interject, yet one more time, how difficult it is for me to understand why so many people have such an aversion to picking up the phone, and calling a manufacturer, and talking to someone who is PAID to make sure you understand how to use their product correctly. I don't know how many times just in the last few months I have had to suggest that to folks on this forum and others. Its like it never even crosses their mind. Yes, some have said they emailed a company. So what? If that answered their questions they wouldn't be posting them on these forums. Don't get me wrong, these forums are WONDERFUL places for an exchange of information and ideas, but "go to the source"! Nobody was born knowing the ins and outs of internal clearances in bearings. And damned few were ever taught anything about it in school. Unless you've worked with it, you don't know about it. There is nothing wrong with asking the acknowledged experts, and they work for the companies. Most of the good ones will help you with the math and make sure you don't miss some critical issue. Try it; you'll be glad you did. You old farts know I'm not talking to you.

Maybe its because the old well-worn paper catalogs that were worth their weight in gold in engineering information are pretty much a thing of the past. Or maybe because most of the experienced engineering talent has been "downsized" so few good mentors are left. Or maybe because most of the current generation communicates as much by texting as by talking. Who knows?

I'll crawl back in my hole now. Carry on.

Now back to our regularly scheduled program ...