Preload - Taper connection with screw compression

[Brew88]

Hi,

After some googling i've been unable to find an answer.

I have a male tapered shaft with female thread in the end, which goes into a body with female taper. The whole thing is then nipped up with a screw from the other side. The screws head is also tapered as is it's mating face. Thats my best effort at explaining you may find the picture easier to understand.

My question is how would I go about calculating preload for a system such as this

 

[MikeHalloran]

It sure looks like you will be expanding the female member, a lot, as the screw is drawn up. That part, you can treat as a press fit. It gets messy where the outside surface of the part is not a right circular cylinder, and as the male part approaches the bottom of the female taper. Whatever you assume about thread friction and friction on the taper will affect the results significantly, as will the actual coatings/ lubricants/ degree of cleanliness used on the actual thread and actual taper.

The female part looks like a u-joint clevis. If you are expecting to transmit much torque through a taper (that probably distorts permanently and plastically on assembly if I am eyeballing it right), things get really interesting.



Mike Halloran
Pembroke Pines, FL, USA

 

[Brew88]

I thought it might be a bit of a minefield, I think i'll ask ansys whilst I dig out some of my old books.

FYI......The taper is included for fine adjustment rather than torque transmission although the connection should be able to take 20Nm without loosening. The joint will be subject to tensile forces of up to 100N and compressive forces of up to 500N. The part is a titanium medical part which will be machined to a high tolerance, and assembly clean and dry.

Thanks for your reply

 

[MikeHalloran]

Doesn't Ti on Ti gall rather badly?



Mike Halloran
Pembroke Pines, FL, USA

 

[CastMetal]

Mike,

Ti on Ti threads are subject to cold welding, but most medical applications have Ti anodized.

 

[rb1957]

isn't the cross screw the primary torque load path ?

when is the cross-screw installed ? on final ass'y ? then you've got only one shot to adjust the length. no?

personally, i don't see what the tapered thread is buying you. plain (unthreaded, untapered) shafts would give you the same adjustment. no?

i'd've thought that preload is generated only after the taper bottoms out, and the two parts are bearing against a flat.

if the cross screw is only to fix the length, why not use a jam nut whihc would allow adjustment after ass'y ?

 

[Brew88]

Mike- Unfortunately our hands are tied since Ti (Grade 5 - Ti6Al4V) is the only material we can use when dealing with nickel allergies, CoCr is the preferred material.

rb1957 - There will be no cross screw as there will be no bone to anchor against, the device will span two joints thus transmitting all the torque and other forces. The idea of the shaft taper is to provide Morse style fixation between the body and shaft, and the taper on the screw head will hopefully prevent the screw backing out. Oh and the image is a little misleading the shaft taper won't bottom out

 

[zekeman]

Unless I am missing something, it seems to me that you can get the preload forces by using a little geometry and a lookup table of interference fits vs force for cylindrical fits which should be valid for very small taper angles.

Since the relative radial motion of the taper is tan@ times the relative axial motion; a dial indicator should do the job.