Press fitted axis 2

[marcuscouceiro]

Dear All,

Here at company we have to install an axle onto two (3) bearings. After using Lame´s equation I was able to find the pressure generated due to the interference between the outer diameter of the axle(wich is solid) and the inner diameter of the bearings.

My question is regarding the necessary force to remove the axle from the bearings. Since I have the pressure at the surface of the axle I could use the F=uN formula to find the necessary force to remove (or install) the axle.

Based on your experience which value of ´u´ should I use? So far I have found values from 0.74 to 0.8 but since we have both surfaces really polished I was wondering if I could use another value.

If you know another value that could be used could you also inform where I could find it?

Thank you once again for your valuable help.

Marcus Montenegro

 

[CoryPad]

Likely you should not use 0.74 to 0.8 - those are values shown in handbooks for surfaces in a vacuum. If you have a more traditional environment, 0.2 [±] 0.1 should cover almost any variations. If you need a more precise value, testing is the way to determine it.

 

[marcuscouceiro]

Hi CoryPad,

Thank you so much for your quick answer. Can we consider our depot facilities a ´traditional environment´? It´s a large area where we do maintenance in trains.

Is this value (0.2 +/- 0.1) based on your experience or based on a book? I will probably use your values but just as a clarification.

Thank you once again,

Marcus

 

[kingnero]

.2 to .3 are common values for clean metal-metal contact, I don't know however how surface grinded or polished areas compare to standard... there will be *some* less friction due to lower Ra values...

 

[desertfox]

Hi marcuscouceiro

Have a look at this site it gives lots of friction data:-

http://www.roymech.co.uk/Useful_Tables/Tribology/co_of_frict.htm

you need to scroll down a bit to see the tables.

desertfox

 

[Tmoose]

I'd have a BIG powerful press available, with stout pulling fixtures. Depending on assembly conditions there may be a certain amount of galling or pick-up when pressing thing apart.

Preserving the axle may make cutting the old bearings off a safer, more efficient choice

 

[dhengr]

Marcus:
Are you pressing bearings or wheels on a railcar truck axle? Bearings and axles have a very tight toleranced dia. difference to assure a fairly light press fit. And, the bearing is pressed to a toleranced location on the end of the axle. Wheels and axles have a dia. tolerance difference too, and the wheel is expected to be within a length tolerance on the axle also. But, in pressing wheels you must match the dia. differences to assure that you get a specific range of pressing forces, in the process, to assure an adequately tight press fit to keep the two together. The railroad industry sets these standards, here in the U.S. that’s the A.A.R. You always want more press power than you might need, with very fine distance (length) control, and except to learn to match the dia. differences to achieve the end result you want, a friction coef. calc. is not too significant. Shop experience, methods, and machining quality soon sets your standards, rather than these types of calcs. Talk to your bearing, axle and wheel suppliers they will give you much better guidance than most of us here can. My specs. on these matters are not up to date and they may not comply with your railroads standards, either. Most railroads here have a special shop that does nothing but this work, it is not done in a depot or light repair facility.

 

[marcuscouceiro]

Dear Gentlemen,

Thank you all for sharing your experiences. Here are my comments:

Mr. CoriPad and KingNero,

I will use 0.25 in my calculations instead of 0.8. This will give me a lower axial stress and it´s within your experience.

Mr. desertfox,

I had seen those tables before but they gave the 0.8 value for the coefficient of friction, which is very high and not applicable according to Mr. CoriPad and KingNero. Nevertheless, thank you for your help.

Mr. Tmoose,

This procedure is done during the assembly of a gear box and therefore I can not cut the bearings. Sorry if I was not clear enough.

Mr. Dhengr,

As mentioned above, this is done during the assembly of the high speed pinion of the gearbox. Since the documentation that we have is 35 years old we do not have a lot of information regarding procedures. What I can tell you is that the end side of the pinion fits inside two bearings and the bearings are already installed on their housing. According to the manual(as old as hell)the drawings we somehow pull the pinion through the bore of the bearings and this is done with a screw. So, we apply torque on the screw and it brings the pinion up and installs it onto de bore. Sorry for such a long text. I do not know ifI made myself clear so feel free to ask if you have further doubts.
As soon as I finish I will post the calculations for the tool I am trying to build.
It would be nice if you all could giveyou opinion and share you experience on that.

Kind regards

 

[desertfox]

Hi marcuscouceiro

For press fits it gives these values:-


Various values of relevant coefficients of friction are provided below;

•Steel Hub , Steel Shaft unlubricated - force fit ...C. of Friction = 0,07 to 0,16
•Steel Hub , Steel Shaft greased - force fit ...C. of Friction = 0,05 to 0,12
•Steel Hub , Steel Shaft unlubricated - Shrink fit ...C. of Friction = 0,15 to 0,25
•Steel Hub , Steel Shaft greased - Shrink fit ...C. of Friction = 0,08 to 0,16


none of these say 0.8



desertfox

 

[hydtools]

If the bearings are installed in the housing with a press fit, then the shaft to bearing should be a slip to light press. More than a light press may compromise the bearing internal clearances.

Ted

 

[marcuscouceiro]

Hi desertfox,

You are correct. I did not notice the information that you posted before. The table further below gives a value of 0.8 (Mild) Steel (Mild) 0,74 0,57 but these are related to flat surfaces.

Thank you once again for your patience and sorry for my mistake.

Hey hydtools,

Thanks for the info.

Regards to All,

Marcus

 

[marcuscouceiro]

Dear Gentlemen,

I was finally able to finish the project of the power screw and nut for the assembly of the shaft.

Could you have a look and give me you comments?

This is my first design here at the company and I do not want to machine the tool before someone with more experience tells me that it´s safe. Unfortunatelly, no one here will have enough experience to do such analysis.

Thank you for your help,

Marcus

 

[MikeHalloran]

Are you intending for this screw and nut to be driven by a gearmotor? If so, perhaps custom work really is required.

However, for manually powered assembly machinery, and for machines that are used infrequently, it makes more sense to use commercial screws or threaded bar and commercial nuts wherever possible, limiting the customization to a few adapters of relatively simple geometry.



Mike Halloran
Pembroke Pines, FL, USA

 

[marcuscouceiro]

Hi Mike,

Although it will be used frequently it will be men powered. The main problem is the material that we have to use. I am not sure if a screw made of ANSI4140 is easy to find (I am located in Brazil).

Besides that, due to the arrangement, one end of the power screw has to change to 3/4 UNC screw in order to fit the hole already present on the axis and bring it up.

The dimensions of the thread (pitch, do, dc) are standard values (according to the design book that I have used).

What do you think of the calculations?

Marcus

 

[desertfox]

Hi marcuscouceiro

I am a little confused why don't you just have one screw at 3/4 unc ?
How are you driving this screw?, which I assume is going to create the interference assembly.
Your calculations look ok to me but make sure you can generate more torque that what you have calculated, to allow for any inefficiency in the system.

desertfox

 

[MintJulep]

Start by going here and looking at the bearing installation brochures.

http://www.timken.com/en-us/solutions/rail/Pages/default.aspx

Then, go get yourself of copy of the AAR Wheel and Axle Shop Manual.

http://www.aarpublications.com/Publ... Recommended Practices/Section G Part II.aspx

 

[marcuscouceiro]

Hi desertfox,

The tool will be man powered. Now that you mentioned, it seems that there is no reason to use one section with ACME threads and another one with UNC thread since the worst case scenario is based on the UNC thread. Is that what you mentioned? I have posted a drawing of the tool as I have seen before your suggestion.

Mintjulep,

Thank you for the links.

Regards,

 

[desertfox]

Hi marcuscouceiro

Yes I meant if you were making a new threaded rod why not just use one thread and save some machining. However if the threaded rod already exists it doesn't matter, the only other ting I would say looking at your picture, is that if you use the smaller thread for the whole rod there might be a buckling mode to consider in the design.

desertfox

 

[marcuscouceiro]

Dear Gentlemen,

I would like to thank you all for the valuable information provided during this Thread.

I think that right now we have information enough to finish the tool and we will move on that direction.

I hope I can count on you next time.

Regards,

marcuscouceiro

 

[desertfox]

your welcome

desertfox