Classic screw and nut(or sleeve?) system 4

[pawanhv]

hi ,
I am a bit confused here and i would like your help here please. I have a classic screw and nut(or sleeve?) system and I have attached a pic of it for your reference. the torque is applied to the screw and causes the translation of the sleeve. I have derived the relation between the torque applied on screw and tensile force generated on the sleeve. Now I want to find the minimum value of tensile force on the sleeve required to overcome the frictional force generated due to the sliding of this sleeve in a cylinder. I just want to know how do I calculate the friction due to sliding of sleeve in the cylinder? Thanks. In the pic the thing in green is supposed to be the sleeve. sorry for the image I dont have a CAD software.

 

[GregLocock]

And who said the art of sketching was dead? (It's not, but it is very very sick)

OK, so the compression in the sleeve will cause it to burst outwards, by poisson's law, and will then be constrained by whatever hole it is in, causing a pressure at the interface, and then the friction between the sleeve and the hole will resist any attempt to move the sleeve axially.

At this point I'm going to tip toe away, it seems to me there is either a plug and chug solution or the answer can be infinitely complex.

If the sleeve is rubber like and the hole is steel and the initial fit is tight, then poisson's ratio is 0.5, mu is 1, and life is relatively straightforward. If the initial fit is loose, the sleeve is steel, and the hole is concrete, then go and test it.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[IRstuff]

Isn't the sliding friction essentially a design parameter? Isn't it generally traded vs. how sloppy the movement is?

TTFN
faq731-376

 

[som1973]

Tangential Force = Co-efficient of Friction for power Screws x Normal force

When the tangential force F overcomes the frictional force between two surfaces then the surfaces begins to slide relative to each other.

The Co-efficient of Friction for power Screws is dependent on material of Screw & Sleeve (Collar)

See below Co-efficient of Friction values for various material combinations

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

 

[pawanhv]

thanks everyone for your reply... effectively, the sliding of sleeve occurs when the tangential force overcomes the frictional force. so imagine if the outer surface of the sleeve has threads too and is screwed inside a cylinder having internal threads. So when torque is applied on the screw causes the sleeve to tmove linearly as well as rotate. I know it gets complicated here , but does anyone know how to model analytically such a system ? any hints please?

 

[GregLocock]

Ah, well at this point I feel like a complete mug for replying to your first completely inaccurate post.

You have still completely failed to identify whether we are talking about soft sleeve/hard socket, or whatever.

Good luck, I'm inclined not to play 20 questions.



Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[Engdoitbetter]

I don't think your system will work.
And here is why: provided that the screw rotates with constant velocity (i.e. equilibrium conditions), the couple you apply to the screw is balanced by a couple applied by the thread to the screw. Due to thread peculiar geometry, an axial force is generated and applied on the screw, which is balanced by an external load (in this case, friction).
Therefore the sleeve undergoes ONLY axial load: if the sleeve is externally threaded, it won't move.

Stefano

 

[Engdoitbetter]

Sorry I have to correct my last post: the sleeve actually undergoes torque and axial load, both.

 

[hydtools]

If there is not enough friction between the sleeve and the bore, the sleeve will spin with the screw and there will be no translation. If there is a little more friction, the sleeve will both rotate and translate. With even more friction, enough to resist all the screw torque, the sleeve will only translate.
You need to know how much normal force there is between the sleeve and bore in order to know how much friction force, normal force * mu, is present.

Ted

 

[rb1957]

i'm confused ...
what sort of fit is there, between the sleeve and the hole ? (sliding ?? clearance ??)

how does twisting the screw translate the sleeve ? is the head of the screw captive (so that twisting the screw thread drives the sleeve) ?

presumably there is positive postioning of the sleeve ? something like a small spring (that's being compressed as the sleeve translates) and probably another one above the sleeve.

 

[1gibson]

Key the sleeve to the cylinder?

 

[pawanhv]

hi rb1975,
thanks for showing interest in the post, basically I started off the post by asking a question with basic configuration where the rotation of screw causes linear motion of the sleeve (this happens by blocking the sleeve to rotate by a flat surface on the outer suface).

In the second configuration I wanted to quicken the mechanism (I wanted the sleeve to move laterally from position A to position B faster, so I wanted to see if by threading on both sides of the sleeve can be better than just on one side), so removed the external flat surface and threaded on the outer surface of sleeve so it rotates as well as translates. I did the initial analytic analysis and it seems that if the thread pitch of sleeve inner surface is greater than the pitch on the outer surface , this can be achieved .

Can there be any other parameter other that pitch and friction coefficient that can effect this? thanks hydtools, engdoitbetter for your reply too

 

[pawanhv]

Can anyone give me the reference on how to calculate these kinds of structures, so I can do a detailed analysis of this?

 

[hydtools]

Is the screw constrained so that it cannot translate? If so, the sleeve will not move if threaded externally as in the second case.
In the first case, increasing the pitch of the thread will increase the translation of the sleeve per screw revolution.

Ted

 

[rb1957]

ok, if the sleeve has a flat (or some sort of anti-rotation device) troquing the screw will translate the sleeve. mind you there's a bunch of mechanical design going on that we're not talking about ... fixing the head of the screw, the fit of the sleeve in the bore, lubrication, ...

draw a free body of the sleeve. the torque applied to the screw will be reacted by a couple onto the sleeve, which will apply a couple to the body around the sleeve. this should also show you the forces translating the sleeve.

to speed up movement of the sleeve, i'd suggest changing the pitch of the thread. this is what links the translation of the sleeve to the applied twist of the screw.

 

[pawanhv]

hi ted, yes the screw cannot translate. can you explain why you think it cannot work ?
if it helps, the sleeve has thread (groove or female) internally as well as externally ..the threads(males) are provided on the bore and screw.

 

[rb1957]

i don't see how adding an external thread on the sleeve (between the sleeve and the body) will help translate the sleeve. if the head of the screw is restrained to the body then twisting the the screw will translate the sleeve. that anti-rotation device is going to be worked pretty hard.

wouldn't it be simpler to thread whatever is restaining the screw to the body, so twisting the screw causes it to translate. then the sleeve is simply fixed to the screw.

 

[pawanhv]

hi rb1957,
yes I agree that it is difficult to translate the sleeve with blocked rotation, thats the reason I taught of the second config. According to the design, the sleeve has to move for fixed number of rotation of screw and disengage the screw and bore(body) (by moving the sleeve). thats the reason I cannot think of fixing the sleeve. Thanks

 

[rb1957]

so it's a one time deal ? once the sleeve disengages the screw it won't re-engage (easily), no?

i think once you draw the free body diagram you'll see some of the difficulties in using a flat. what about using two tabs, slotted into the body ?

 

[rb1957]

what about a square sleeve ?