Aligning a threaded shaft connection
Aligning a threaded shaft connection
(OP)
Hello,
I want to design a shaft connection using a threaded bore and threaded shaft end to transmit torque. I want to have a straight parallel interferrence fit (on the shafts) above and below the thread to accuractly align the shafts. Does any one have experience with this kind of connection or know of any availible standards.
Thanks,
Sean
I want to design a shaft connection using a threaded bore and threaded shaft end to transmit torque. I want to have a straight parallel interferrence fit (on the shafts) above and below the thread to accuractly align the shafts. Does any one have experience with this kind of connection or know of any availible standards.
Thanks,
Sean





RE: Aligning a threaded shaft connection
Why not just use the interference fit into a standard coupling to transmit the torque?
I say this because depending how you apply the torque relative to the threaded shaft joint it might have a tendency to undo and hence the reason I assume you want an interference fit.
regards
desertfox
RE: Aligning a threaded shaft connection
RE: Aligning a threaded shaft connection
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Aligning a threaded shaft connection
The coupling I mean is one you buy off the shelf.
http://www.lenze.co.uk/luk7.htm
Like the above link.
Can you give more info on your application?
regards desertfox
RE: Aligning a threaded shaft connection
Mike Halloran
Pembroke Pines, FL, USA
RE: Aligning a threaded shaft connection
http
ht
http://www.generalpolygon.com/index.htm
http://www.stoffelpolygon.com/
RE: Aligning a threaded shaft connection
Mike: I have components of a smaller diameter than the ends of the shaft, hence a need for a 2-piece design.
Unclesyd: Thanks, for the info. I don't think a polygon drive is the best fit for me as my torque is very low but I did find the articles interesting
RE: Aligning a threaded shaft connection
Only way I can see for you to do this interference fit and thread is:-
1/ Ensure that the mating shaft ends engage with
each other before the threads engage
2/ This would mean selecting tolerences for the
threads to be as loose as possible to allow
for misalignment whilst still being able to
engage them.
I see problems with this however firstly for engagement of
the threads you after overcome the interference fit you select between the shafts secondly you can't see the threads
your trying to connect.
If your lucky enough to engage the threads, you now have to tighten them by rotating one of the shafts against the interference fit till its fully home and as you don't say what size shafts or interference you intend to use I cannot get a feel for the torque required to make the assembly.
regards desertfox
for there misalignment
RE: Aligning a threaded shaft connection
At the moment I have designed the connection so there are about 5 threads engaged on a 1-7/16 UNEF profile, BEFORE the interferences (top and bottom) engage. The bottom interference engages slightly before the top. This way you can thread the shafts together and then use the threads to pull the shafts fully home. I am concerned a bit with screwing an interference together as I have never seen this done before.
In your post you say the interference must engage first, this is not the case unless I am missing something. It is true you cannot see the threads but I don't see this as a problem. I was thinking of a 0.5-1.5 thou interference on the diameter but haven't done the calcs. yet to see the stresses and torques required.
Sean
RE: Aligning a threaded shaft connection
Provided the clearence in your threaded connection is greater than your interference fit your right in that you
can engage the threads first,the clearence in the threads must also allow for any eccentricity of thread centres during manufacture. I was thinking that the thread
engagement might control alignment of the two shafts.
I have never seen two shafts with an interference fit screwed together and depending on the fit etc you will have to make provision somewhere on the shafts for clamping and rotating respective shafts.
Over what length of engagement will the interference fit be
and what are the respective shaft sizes?
regards
desertfox
RE: Aligning a threaded shaft connection
I just want to make sure we are both talking about apples here so I have drawn a little sketch (is there a better way to post sketches?)
_______________________
|________ |______
^^^^^^^^^^------ ^^^^^^^^^^^------
|
|
______| ______
--------^^^^^^^^^^ -------^^^^^^^^^^^
| |_______________________
With this design 5 threads (^) are engaged before the interferences (_ and -'s) comes into contact. The length of the interferences are about 0.21 inches on 1.455" and 1.390" diameters.
RE: Aligning a threaded shaft connection
To include an image in your post:
(img http://www.mysite.com/images/happy.gif )
but replace the parentheses with square brackets
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Aligning a threaded shaft connection
A locking taper, which is screwed together, has a very high change of picking up and galling when you undo it. (I had a bad experience with this before)
I am also looking at using a locking taper and separate bolt for drawing the shafts together. The idea with the parallel bore and integral thread was suggested to me. I did not have any experience with this type of connection so I posted my question on this board.
RE: Aligning a threaded shaft connection
_______________________
|___ |______
|_____^^^^^^^^^^___--- ^^^^^^^^^^^------
|
Or some variation on that with a similar series of steps on the female part would be easier to assemble. How important is dissassembly?
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Aligning a threaded shaft connection
Be aware that assembly reduces the volume of two closed cavities, one around the threads and one at the bottom of the female part's bore. You might consider venting those cavities.
The joint is multiply constrained. The threads by themselves would tend to align the parts as the shoulder and female shaft end are drawn together. Both straight bores also tend to align the parts. If all the parts are perfect, it could work. If any of the salient features are imperfectly located, then assembly will induce stresses within, as the various locating features fight each other. The female part in particular seems a manufacturing nightmare, given the tolerances so far implied. You might allleviate some of the assembly stresses with well radiused necks at either end of the male threads; then you have to balance lateral flexibility of the necked areas against allowable eccentricity tolerances.
Absent a complete picture of all your actual constraints, I'd be inclined to go with a locking taper and a long slender drawbolt down the center.
Mike Halloran
Pembroke Pines, FL, USA
RE: Aligning a threaded shaft connection
The internal threads will be a problem unless you have enough quantity to warrant a tap with a long pilot, though you could hold pretty close in a lathe.
I would gun drill the small diameter, then the major diameter for the threads, the bore the larger diameter. This is just one scenario so a talk with a gun drilling shop would be of great benefit.
Another possibility is bore the small dia. then ream; the use a step drill with a non drill pilot or counterbore with pilot to make the thread major dia. then bore the large diameter.
Special Tools
http://www.national-tool.com/index.html
Diamond Hones
http://www.accucutdiamond.com/tooling.htm
Gun drills
http://www.drillmasterseldorado.com/index.htm
Gun drills
http://www.tbt-usa.com/index.htm
RE: Aligning a threaded shaft connection
Yes I envisaged what you have drawn.
I don't understand your diameters of 1.455" and 1.390" unless you mean there the outer diameters of the respective
shafts.
I meant what were the diameters of the two parts that will form the interference.
regards
desertfox
RE: Aligning a threaded shaft connection
If you are custom machining the shafts why not make one shaft with the ends milled to fit your bearings and motor?
Tofflemire
RE: Aligning a threaded shaft connection
To get the sketch to come through right, try using the monospaced text formatting {tt}monospaced text{/tt} but with square brackets. My actual part does have reliefs as you have shown to make assembly easier, I just did not show them in my sketch. (If others want to see Greg's sketch copy and paste it into the reply window.) Disassembly is not the most important feature but it will be taken apart a few times in its lifetime.
MikeHalloran:
One of the shafts is hollow, however you are correct there are two cavities at the start and end of the thread. I will vent these, thanks for the tip. The original design I looked at (and am still considering) has a hollow shaft that is closed at one end, with a short drawn down bolt. Building in flexibility is an interesting idea.
Desertfox:
These are the interference diameters; 1.390" is the standard tap drill size for a 1-7/16 UNEF thread. Although it may change slightly depending on the desired thread tolerance and clearance between the first interference face on the male threaded shaft and female thread. 1.455" is a slightly larger diameter than the major diameter of the thread (1.4375").
Tofflemire:
I have components which mount in the middle of the shaft which are have a smaller bore than either end, hence the need to split the shaft into 2 pieces.
Thanks to everyone for all the help, it is greatly appreciated!!
RE: Aligning a threaded shaft connection
I have missed something here I thought the interference fit would be formed by the larger shoulder diameter behind the male thread and the larger bore in front of the female thread.
Earlier you stated the interference would be between 0.5 and 1.5 thou, theres a lot more than a thou between 1.390 and 1.4375.
Could you please explain as I am now confused.
Regards
desertfox
RE: Aligning a threaded shaft connection
RE: Aligning a threaded shaft connection
I reccomend you only have one interference fit on each shaft. If you try to have two one before and after each thread you will have alignment problems on assembly due to the fact that the interference fits will fight against each other unless they are perfectly in line and concentric
which will be very difficult in practice, this doesn't include alignment of the threads either.
You say your torque requirement is low I cannot see the need for two interference diameters.
regards
desertfox
RE: Aligning a threaded shaft connection
1)You are "pressing" the two shafts by rotating the threaded parts, so the axial force is delivered by the screw threads,along the helical parts which will in general put an eccentric axial force on the two shafts, making it virtually impossible to cause the press fit without significant deformation.
2)In effect you are trying to cause press fit by rotating the two mating interference parts, another imposssibility
3) And if he manages to do it, he is virtually guaranteed two nonaligned shafts that are cold welded , not what he was looking for
4)And how does he dissasemble it? Another impossibility
In summary, think he should be strongly discouraged from this misadventure and get him to consider standard practice (which some of you are) and not continue to entertain new methods (fraught with difficulty) for an old problem.
RE: Aligning a threaded shaft connection