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Classic screw and nut(or sleeve?) system
4

Classic screw and nut(or sleeve?) system

Classic screw and nut(or sleeve?) system

(OP)
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.

RE: Classic screw and nut(or sleeve?) system

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: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Classic screw and nut(or sleeve?) system

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

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Classic screw and nut(or sleeve?) system

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/c...

RE: Classic screw and nut(or sleeve?) system

(OP)
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?

RE: Classic screw and nut(or sleeve?) system

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: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

RE: Classic screw and nut(or sleeve?) system

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

RE: Classic screw and nut(or sleeve?) system

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

RE: Classic screw and nut(or sleeve?) system

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

RE: Classic screw and nut(or sleeve?) system

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.

RE: Classic screw and nut(or sleeve?) system

Key the sleeve to the cylinder?

RE: Classic screw and nut(or sleeve?) system

(OP)
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


RE: Classic screw and nut(or sleeve?) system

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

RE: Classic screw and nut(or sleeve?) system

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

RE: Classic screw and nut(or sleeve?) system

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.

RE: Classic screw and nut(or sleeve?) system

(OP)
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.

RE: Classic screw and nut(or sleeve?) system

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.

RE: Classic screw and nut(or sleeve?) system

(OP)
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

RE: Classic screw and nut(or sleeve?) system

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 ?

RE: Classic screw and nut(or sleeve?) system

what about a square sleeve ?

RE: Classic screw and nut(or sleeve?) system

(OP)
yes exactly its a one shot application. Once you are done with it ! its finished .. what you mean to suggest is two projections on the exterior of the sleeve which mates with the two female slots on the body (or even a square sleeve) would be better than the cylindrical sleeve with a anti rotation flat to move it laterally? thanks again rb1957..

RE: Classic screw and nut(or sleeve?) system

either would react torque better than a flat

RE: Classic screw and nut(or sleeve?) system

(OP)
if I think the other way around , like I said you about the second configuration where the ideal case would be the pitch of thread on the ext surface of sleeve to be greater than at the internal surface ,turns out if the pitch is very very high it is infact a straight line . so your suggestion is infact the ideal case for me second configuration. thanks a lot rb1957.

RE: Classic screw and nut(or sleeve?) system

not sure i follow that logic ... if the sleeve is threaded on both faces, i see problems if the threads aren't the same pitch ... one thread will be driving a displacement based on a rotation; if the other thread is a different pitch it'll want to displace more (or less) for the same rotation. maybe the rotation of the outer face isn't exactly the same as the inner face (as the wall of the sleeve strains against these two different displacements). i'd expect the sleeve might displace the average of the inner and outer displacement; whatever, i expect it'd be complaining like a sob !

RE: Classic screw and nut(or sleeve?) system

A sleeve with a threaded ID and OD will not work. The stationary screw will prevent any translation pending due to the OD thread. Likewise, the stationay outer housing will prevent any translation of the sleeve due to the effort of the screw. The sleeve, when threaded on both ID and OD is captured axially between two fixed elements.

Ted

RE: Classic screw and nut(or sleeve?) system

i'll buy that ... 'cause the sleeve isn't turning it can't travel down the external thread.

RE: Classic screw and nut(or sleeve?) system

The proper approach is some sort of twisting constraint, i.e., making the sleeve square, or keyed with a flat or slot, or even just having a single wall constraint, as is done in typical lead screw stages:
http://www.edmundoptics.com/mechanics/positioning-...
http://search.newport.com/?q=*&x2=sku&q2=T...

In the latter cases, the speed of the motion is strictly limited by the screw pitch and turning speed.

p.s. I agree that outside threading will not work; if anything it's more likely to jam in place, since there's insufficient torque on the sleeve to cause it to rotate, since the inner screw drag will cause it to want to rotate in the same direction as the screw, but it will actually need to rotate in the opposite direction

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

RE: Classic screw and nut(or sleeve?) system

(OP)
well all my arguments above about the external sleeve thread was based on the calculations shown in the attached file. I found by kinetic analysis alone that if the pitch in the interior of the sleeve is less than twice the pitch on the outside i could arive at what i am looking for i.e. quick displacement of sleeve .

RE: Classic screw and nut(or sleeve?) system

(OP)
I heard this concept has been used before and I became interested in that . But I did not find where it is used.  

RE: Classic screw and nut(or sleeve?) system

I don't understand how could the sleeve move: for a given rotation, in a general case, i.e. if the thread pitches are different, the axial movement would be different for the inner thread and the outer thread, and this would be against the basics of the translation movement.
Second, If the sleeve does rotate due to the torque applied on the screw, there will be no relative motion (or a slower one) between the screw and the sleeve, so a slower (or no) axial movement.

RE: Classic screw and nut(or sleeve?) system

external tread won't work (in conjunction with an internal thread). the sleeve is not rotating, the screw is being torqued, the sleeve is prevented from rotating, so how will it translate along the external thread ? at the internal thread the screw is rotating, and the sleeve is fixed; what is allowing the body to rotate with respect to the sleeve (which it is fixed to) ??

the pitch of the thread controls the displacement due to applied torque, coaser thread, more displacement per rotation.

you could thread the sleeve/body interface (what we've called an external thread) and it'll work If the screw is fixed to the sleeve, so that twisting the screw rotates the sleeve.

clear as mud ?

RE: Classic screw and nut(or sleeve?) system

The internal and external threads so constrain the sleeve such that the velocity of the sleeve relative to the body cannot be anything but equal to the velocity of the sleeve relative to the screw. You have not accounted for that constraint. Displacement and velocity of the screw relative to the body are 0 by the screw to body constraint.

Build a physical model and let us know how it performs.

Ted

RE: Classic screw and nut(or sleeve?) system

"....well all my arguments above about the external sleeve thread was based on the calculations shown in the attached file. I found by kinetic analysis alone that if the pitch in the interior of the sleeve is less than twice the pitch on the outside i could arive at what i am looking for i.e. quick displacement of sleeve .

http://files.engineering.com/getfile.aspx?folder=9..."

Where did you get this "file" ?

I wasted 20 minutes reading this nonsense.

I can't believe that any credible kinematic analysis would allow the frame to rotate and in the end he/she gets an "amplification" directly dependent on the ratio of the screw rotation divided by the "fictitious" frame rotation. Fatally flawed and wrong conclusions.

Perhaps a PHD candidate??

Same guy probably can prove 0=1.

Bottom line
OP-- don't believe everything that's written especially when it is not published.

RE: Classic screw and nut(or sleeve?) system

Still trying to make sense out of the paper, I believe the author was using the frame as a reference to capture the screw and complicated a simple problem, so I thought doing it as follows:

Let the screw rotation be N1 and the sleeve rotation N2
The pitch of the internal and external threads are P1 and P2.

I get the translation to be
P1(N1-N2) which must be equal to -P2N2, the translation along the external thread.
Then you can write

P1(N1-N2)=-P2N2

In order to amplify the motion, N2 must be negative. For example, if the amplification= 2,
(N1-N2) = 2N1 , or
N2=-N1
and
2N1P1=P2N1
To satisfy this, the external pitch is
P2=2P1
If the amplification is 3,
then N2=-2N1 and
3P1N1=2P2N1
P2=1.5P1
Now, the trick is to get a small muscular little man to rotate the sleeve accordingly.
That is the problem with all this. I just doesn't work. Moreover, what has the external thread done for the system? Nothing.
Once the little guy has done the spinning, the amplification is accomplished.

RE: Classic screw and nut(or sleeve?) system

The whole notion of "torque" on the sleeve is essentially bogus. The only torque comes from the frictional drag from the lead screw turning. That's not enough to make the sleeve turn against the outer threads. If one wants to change the linear speed of the sleeve, the two possible approaches are faster rotation of the lead screw, or a larger pitch on the lead screw.

TTFN
FAQ731-376: Eng-Tips.com Forum Policies

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