Heat Shrink Tolerancing

[Phamjt]

I'm currently working as a mechanical engineering intern and I was assigned a research project involving heat shrinking two metal components as a possible alternative to welding.

The main resource I am using is the machinery handbook (26ed) which does give me formula's for calculating allowance and temperature. Additionally I am using tables found in the following resource (also shown in machinery handbook)

http://docplayer.net/41324995-Shigl...sign-tutorial-3-16-press-and-shrink-fits.html

Talking to my supervisor he suggested calculating the tolerance using that table for I believe the maximum interference. He also mentioned netting the tolerances of both heat shrink (FN5) and Sliding/Running Fir (FN2). Unfortunately I haven't had much experience involving tolerancing yet in school so I was wondering if this was possible and how I would go about solving it? Currently I am using the example as shown in the link as it closely resembles what I wish to accomplish in my project.

Thank you for your time and I look forward to any replies that may help me better understand the thought process I should have when researching this project.

 

[drawoh]

Phamjt,

Go to the ANSI fit tables. Locate the FN fit table, and the column for FN5. What you should see are the recommended tolerances for achieving an FN5[ ]fit, for a range of sizes. Everything is in inches, with the tolerances shown in thousanths of an inch. If you are working in metric, you will have to do conversions. Note how the ANSI fits use ISO tolerance classes.

Now, you have an arithmetic problem, which you ought to be able to solve fairly easily.

--
JHG

 

[JNieman]

The tables are commonly referenced in many, many texts as they are fairly agreed-upon 'rules of thumb' for common materials. I would be wary of using them universally or without knowing within reasonable doubt that the amount of interference is acceptable for your material properties.

The text I use for referencing those fit tables is the "Engineers Black Book" (

https://www.engineersblackbook.com/)

with a tab stuck to that page (among others) because it's just so very quick to pull out. If you don't have one - it might come in handy.

Machinery's Handbook is much more informative and thorough; an indispensable text. The black book is just a faster reference book for a handful of things.

 

[Phamjt]

Phamjt,

Go to the ANSI fit tables. Locate the FN fit table, and the column for FN5. What you should see are the recommended tolerances for achieving an FN5 fit, for a range of sizes. Everything is in inches, with the tolerances shown in thousanths of an inch. If you are working in metric, you will have to do conversions. Note how the ANSI fits use ISO tolerance classes.

Now, you have an arithmetic problem, which you ought to be able to solve fairly easily.

I have been able to calculate those values from the table however my supervisor wants me to look into multiple factors. His thought process was it would be a FN5 fit but when you slide the hub onto the rod there would need to be an additional sliding fit tolerance if that makes sense. For now I am looking into calculating this value then I will be able to talk to the materials engineer to determine the maximum temperature I can apply to the hub before the material properties change.

Is it possible to incorporate two tolerance types (Sliding and heat shrink) or is it as simple as just looking at FN5 and getting the values?

 

[JNieman]

If the part would slide onto the part before you heated it up, it wouldn't be a shrink fit.

The whole point is that the hub /won't/ fit onto the shaft /until/ it is heated up. Then when it cools to normal operating (or room) temperature, it binds tightly on the shaft with a very large force required to overcome that friction.

It is not possible to incorporate two tolerance types so opposed to each other, as no one diameter size will suit both tolerance zones simultaneously. You also cannot control a diameter at one temperature while controlling the diameter at another. You choose the material with the thermal properties you desire, control the diameter at one temperature, and hope the result at your secondary temperature is as expected.

And unless you plan on machining the bore of the hub at the elevated temperature, you shouldn't bother putting that 'hot' diameter on any drawings unless it's purely for reference.

Long story short- figure out how much interference you want in the operating-temperature-state. Machine it to the tolerances stated by that fit class. Then you figure out how much heat increase is required to expand the hub to the appropriate size that it fits on the part. The "appropriate size" is not a tight-tolerance diameter. It is "bigger than the shaft" and "not hot enough to compromise the properties of the hub or shaft" basically.

 

[Phamjt]

Thank you JNieman, I may have interpreted what my supervisor said wrong as I was under the impression he wanted me to somehow use a sliding fit tolerance for the problem while also considering heat shrink tolerances. There is a few additional information I need to look at but for now I can calculate the tolerance of the hub using the tables. The material is Ansi 1045 so it should be reasonable to use the tables on that type of steel. On top of this I can use a few equations that apply the material properties to determine how much we can expand the part and at what temperature we should use.

Is there any other aspects I should be looking into, or is this problem a lot more simpler then expected?

 

[KENAT]

Well I'm glad you sorted that out, I couldn't' work out what was meant by combining 2 different fits which is why I didn't' respond.

In terms of things to consider, if even when heated it is still an interference fit you may want to look into the insertion forces required.

I believe there is a FAQ on this in one of the other forums, maybe

http://www.eng-tips.com/faq.cfm?pid=404

of course the friction of a heated part may be different etc. but at least worth thinking about.

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