Pull force on two cylinders 1

[kcjBoiler]

Hi,

It's been a while since I've posted. I have a question and I haven't flexed my physics brain in a while. Hopefully someone can answer me. I have a question about pull force. I'm sticking a plastic cylinder insider another plastic cylinder so that the two cylinders stay together. I'm trying to achieve a specific pull force specification, meaning I want them to stick together, but they can pull part at "X" lbs of pull force. My current cylinder design is at half of the pull force needed. My question is, is there a way to calculate how much further I need to push the inside cylinder into the outside cylinder? Is it twice as much? 1/4 as much? I'm sure there is an easy equation for it, I'm just not 100% sure what that is. It's the distance I'm after.

 

[TugboatEng]

It sounds like you are describing an interference fit.

https://www.tribology-abc.com/calculators/e3_8.htm

 

[hydtools]

You would need to tncrease the interference to increase the friction force.

Ted

 

[kcjBoiler]

Thank you, so in other words, I'd need to have a larger diameter of the inner cylinder in order to increase the force, not necessarily fitting it further inside the outer cylinder.

 

[LittleInch]

A drawing or sketch always helps, but I think in theory if you want double the force then insert the smaller cylinder twice as far. Because as you push the inner cylinder inside you are increasing the surface area without affecting the interference pressure between the two. If the surface area remained the same, e.g. pushing a cylinder of length smaller than the amount inserted, then the force would or should remain the same as you insert it further. The key is increasing the surface area as the inner cylinder is inserted. I think.

The issue will be whether by doing so this impacts or affects the frictional force between the two surfaces?

Or increase the diameter of the inner cylinder in order to increase the force between the two surfaces and hence the friction per unit length of insertion.

Like we've said before though, friction is an unreliable friend. Never there when you need it and turns up more often when you don't...

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[rb1957]

friction an unreliable friend ? friction is the worst enemy ... always around when you don't want it, effed off when you do ... like management ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[mfgenggear]

kcjboiler

please make a drawing of what exactly you are trying to achieve.
a partial press fit (interference fit) depending what press it is will distort the inner cylinder if only partially pressed, unless
the bore is machined after press. what is the current press how was it assembled. shrink fit, or brute press. what type of axtial forces and how much.and what would be the
press required to maintain a safety factor

 

[dvd]

Plastic can have a large coefficient of thermal expansion. If there is any thermal gradient, there is going to be a possibility for a significant swing in the separation forces. Additionally, the out-of-roundness and tolerance discrepancies of these cylinders will create a situation which is different than your calculated values.

 

[MintJulep]

Plastic tends to relax over time. So whatever force you achieve "today" will almost certainly be less "tomorrow".

 

[Stick.]

Some plastics (nylon being a main culprit I think) will also change dimensions depending on humidity/moisture level. Couple that with the other issues already brought up and I'd be seriously considering a different method for controlling your pull out force unless you're ok with the resulting range of pullout forces.

 

[HTURKAK]

Today I read the tread 3 times and still could not get the full picture ..

My questions, are the inner cylinder solid and outer cylinder is a pipe ?

Two options , pulling interior cylinder against exterior tube or pushing interior cylinder against pulling exterior tube...

These are totally different story..

A sketch or dwg will help to get better responds ..









Tim was so learned that he could name a
horse in nine languages: so ignorant that he bought a cow to ride on.
(BENJAMIN FRANKLIN )

 

[IRstuff]

How often do they need to separate? This does not sound like something that can be repeatably performed, since there will changes to the surfaces even from the first extraction.

Have you considered a detente approach?

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

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[kcjBoiler]

Hello Everyone,

Thank you for the responses thus far! This has been very helpful even though the whole picture wasn't presented and I could definitely been more clear in what I'm trying to do.

Currently I'm inserting a plastic machined part which has some structure to in internally but for practical purposes is a tube (Let's call that Part A) into another plastic tube (Part B) such that Parts A and B are able to hold together. Both the OD of Part A and the ID of Part B are smooth. I'm Inserting A into B about 10mm and getting a certain pull force measure in lbs. I can machine Part A to be a little longer, and was wondering how much longer I would have to make it in order to double the pull force. I wasn't sure if I made it 5mm longer it would get me 50% more or 200% more. I was hoping for an easy calculator to give me an approximation so I wouldn't have to waste time machining the part through numerous DOE iterations. Maybe I'd get lucky and only have to make the part 2 or 3mm longer since I'm sure this curve isn't linear. At some point it wouldn't be practical for me to make Part A longer, I would just redesign A or B with some type of mechanical interference to increase the pull strength. Something like ribs for example.

I can work on a drawing to help, but was really just looking for a more broad answer than getting into specifics. I really appreciate the answers thus far, they are really helpful and already helping me come up with other ideas.

 

[mfgenggear]

Kcjboiler
Now from lack of information will give lack of better answer.
Make a couple of test parts like 3 assemblies. Drill a hole at the end apply an exact know weight and see how much weight lbs will take to pull it a apart. I think you will be surprised.

 

[IRstuff]

I'm Inserting A into B about 10mm and getting a certain pull force measure in lbs. I can machine Part A to be a little longer, and was wondering how much longer I would have to make it in order to double the pull force. I wasn't sure if I made it 5mm longer it would get me 50% more or 200% more.

Seems to me, the longer you machine A, the skinnier it gets and the easier it is to insert/extract it from B, which is the outer piece.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

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[LittleInch]

I think your "curve" is linear or close to it.

What would make it non linear?

10mm though is very low and seems difficult to get repeatability.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[enginesrus]

Maybe say what the reason is, and what it is used for. Then you will get some new out of the box idea's of how to accomplish the task your trying to achieve.
The question about pull force, with all the mentioned variables, I'd say start experimenting.

 

[chicopee]

A machine design text book or a mechanical engineering handbook will provide the equations of force extract or insert parts subject to interference fits.

 

[Windback]

Usually I use "press fit" equation from "Advanced Strength of Materials" textbooks and that equation yields the interface pressure. The interface pressure is one of the big contributors. If possible I prefer to use very similar materials with similar strength to avoid the complexity.

For the pull force required to disassemble the cylinders, you need to consider the friction coeff. at the interface too.

First I would calculate the fit pressure at the interface. From pressure i would obtain the theoretical radial resultant force at the mating area. Then pull force should be equal to the resultant force times static friction coeff. -here we assumed the parts are perfectly fitted, which is not that possible in reality so, you might need to test and validate it in case further sensitivity need.-

Lastly, not sure what's your plan but If you are planning to assemble and disassemble it so frequently, the pulling and forcing the mating parts may not be the best design option, these assembly procedures usually deform parts and damage the fit over time.