Interference Fits on Tapered Shafts
Interference Fits on Tapered Shafts
(OP)
Would someone please point me towards some equations for calculating the required interference for transmitting torque through a 8620 case hardened tapered (11.5deg) steel shaft to a 6061-T6 aluminum hub? Can I just use the equations for cylindrical interference fits and pick either the large or small end for the dimensional input values? The shaft has a M6x0.75 external thread and a nut is used to press and hold the hub to the shaft. My end goal is to calculate the required torque on the nut to provide the calculated required interference fit.
Things I am fuzzy on:
How does the case hardening affect how I calculate the strength of the 8620 threaded portion of the shaft? I want to find a max torque value that won't exceed the strength of the threaded portion of the shaft.
Do you use the strength of the core or the case? I have HRC values for the both the core and the case hardened portions of the shaft. Is that enough information to be able to find a rough tensile strength?
Thank you,
Casey
Things I am fuzzy on:
How does the case hardening affect how I calculate the strength of the 8620 threaded portion of the shaft? I want to find a max torque value that won't exceed the strength of the threaded portion of the shaft.
Do you use the strength of the core or the case? I have HRC values for the both the core and the case hardened portions of the shaft. Is that enough information to be able to find a rough tensile strength?
Thank you,
Casey





RE: Interference Fits on Tapered Shafts
Ted
RE: Interference Fits on Tapered Shafts
RE: Interference Fits on Tapered Shafts
I am not the best person to address your questions regarding the determination of the required interference. But once you determine the interference that you require, use pull-up to achieve it, not torque on the retaining nut. And, it if it is possible, I always prefer to use heat rather than brute force to achieve the pull-up.
Johnny Pellin
RE: Interference Fits on Tapered Shafts
11.5° is an awfully steep taper isn’t it? That’s certainly not a ‘self-holding’ taper. The retaining nut will be needed to hold the hub in place, on that steep a taper, once it is where you want it. But, I do agree with JJPellin that that nut is not the way to try to get the press fit you want. Why don’t you ask the people who make that off the shelf 3W engine, what ever that is, how to deal with their equipment. A good sketch with dimensions and some working forces and loads would help you explain your problem, shaft size, hub torque, etc., if you really want some constructive answers. We can’t see what you are imagining from here. You also have to deal with two dissimilar materials in that hub/shaft press fit, and that can be tricky. There have been a couple threads on that issue in the past on this mech. forum. I don’t remember the titles of the threads, but a fellow by the name of Desertfox and I participated in one of them about six-eight months ago. So, do a search and see if you can find it, or look through his or my postings on the mech. forum for that thread.
RE: Interference Fits on Tapered Shafts
Regards,
Cockroach
RE: Interference Fits on Tapered Shafts
11.5 degrees included looks mighty close to the 1;10 used to mount the clutch on many snowmobile cranks. No key, just friction from the interference fit. I don't have any numbers, but I expect the clutch moves way up the crank taper. They are installed with big torque on the center bolt, and removed with BIG torque on the puller bolt. The clutch hub is kind of thin, as I recall, at least on an old Salisbury.
RE: Interference Fits on Tapered Shafts
Regards,
Cockroach
RE: Interference Fits on Tapered Shafts
I would like to point out that I do not subscribe to the believe that case hardening threads is a bad thing. This is contrary to the commentary above. For over twenty years I have been case hardening threads by liquid nitration without any problem whatsoever. I strongly feel that, and have argued against this supposidly high stress concentrations and cracking at the thread arguments. Cracking and propagation of cracks is a Charpy phenonema and typically originate from sites of imperfections such as notches or dislocations within the material. Often this is due to incorrect manufacturing and poor quality thread cuts. Material selection and strength via notch toughness mitigate these effects.
I say mitigate because every thread is prone to failure, not necessarily by the root cracking phenonema. Line pipe threads made from stainless steel comes to mind. These are typically poor candidates for transverse loading and are prone to metal pickup off the flanks of the threads where the faces are high load bearing surfaces.
But to allude to case hardening as a poor practice as a post machining process, cart blanche, is simply nieve and a wrong statement. On the contrary, I fully recommend hardening of the threads by liquid nitration in order to reduce galling and oxidation in the joint as a result of galvanic corrosion as a secular phenonema.
Hope this helps you out somewhat.
Regards,
Cockroach
RE: Interference Fits on Tapered Shafts
The issue I am looking into now is periodic thread breakage when the hub is installed. The customer currently has us torque the hub nut to 200in-lb with a small application of Loctite 271. I believe I found the threads dhengr was referring to, but I have not had a chance to review them thoroughly. I also need to do some "light" reading in the papers Cockroach provided.
RE: Interference Fits on Tapered Shafts
http://artec-machine.com/_images/documents/hydraul...
but it assumes material for shaft and hub are the same.
Why are you using aluminium for the hub?
RE: Interference Fits on Tapered Shafts
RE: Interference Fits on Tapered Shafts
RE: Interference Fits on Tapered Shafts
I would use the core strength in the calculations, not the case strength. The core supports the case, if the core gives way the will fail.
Ted
RE: Interference Fits on Tapered Shafts
I think I can shed some light on your thread failure, firstly using a very approximate formula for axial load derived from torque applied to a screw:-
F = T/0.2*d where T= torque
d= bolt/screw dia
F= Axial load
0.2= friction factor
convert the torque quoted in your earlier post to Nmm and this gives 200lbin equal to 22652Nmm.
F = 22652/0.2*6 = 18876.8N
Now if you take the minor diameter of an M6 screw (fine) its 5.08mm
Next divide the tensile area into F above
stress = f/A = 18876.8/(3.142*5.08^2/4)= 931Mpa
The yield for the material in the hardened condition is about 690Mpa according to the site:-http://www.tatasteelnz.com/downloads/CaseHard_AISI8620.pdf
for upto 25mm section.
All that said my initial thoughts are your tightening the shaft thread to tight, however the threads don't fail every time, so there as to be reasons for that,I would put these down to an inaccurate friction coefficient in my formula above because it will change depending on thread geometry and surface finish of threads and mating parts and the yield stress quoted for the material is usually a minimum or guidance only.
The above fomula for axial load related to torque also shows how inaccurate using a torque setting is, for example just change the friction coefficient to 0.1 and for the same torque setting you can get double the axial load.
desertfox
RE: Interference Fits on Tapered Shafts
My calculation shows the area under the nut to be 58.32mm^2 and dividing that into the axial force in my earlier post gives a stress of 323.6Mpa compared with a yield stress of 276Mpa for 6061-T6 material, if you use a plain washer between the hub and nut then the stress in the hub doesn't exceed 216Mpa based on the same axial load.
It is also prudent to note that the tolerence on preloading the shaft screw by torque wrench is about +/- 25% so even with a plain washer between hub and nut its possible to get to 270Mpa of stress on the hub face just on this tolerence alone.
Can you tell us what the interference on the taper is?
Also can you tell us more about the screw failure, is the screw breaking off as I suspect or the threads shearing?
desertfox
RE: Interference Fits on Tapered Shafts
Attached is a picture of one of the failures. I don't have enough experience to be able to tell what type it is.
The nut that is used is a custom nut made from 4140. It has an enlarged bearing surface with an area of 88.89mm^2.
Something that I need to look into is the nut running out of threads and shearing off during tightening. After taking a second look at the break, it appears that may have happened. In that case, the 5 or 6 failures we have had may be attributed to dimensional inconsistencies in the hub that allows it to run out of threads. It really is a poorly dimensioned drawing and doesn't control the critical features very well. What I don't get is that the production guys just grabbed another core and the same hub that the threads failed with the first time worked with the new core. I haven't been able to find the other cores that have broken to make a comparison.
I don't know what the actual interference is with the current tightening procedure. I may try and measure before and after torquing dimensions of the hub relative to the end of the crank shaft on our CMM.
RE: Interference Fits on Tapered Shafts
RE: Interference Fits on Tapered Shafts
I am on site so my getting on eng-tips is limited but weekend I can get a better signal.
The point of my post is to indicate to you that at the torque setting you're using can over stress
the male thread tensile area, the reason there not all failing is because the yield of the material used in the calculations is a minimum and in practice it may be higher, also the friction factor may vary as told in my post above.
The failure to me seems to be very brittle and I can see indications of multiple cracks around the screw thread tensile area.
Did this failure occur on tightening the nut at the assembly stage?