peg in hole - no play fit that locks but is easy to insert and remove 4

[lamby88]

Hi, This is my first post on the forum. Hopefully someone will be able to help me out.

The problem I have is getting a no-play fit between a tube and a hole. The part in question is around 33mm OD (and min 20mm ID), around 800mm long. The requirement is for this part to fit into a hole and lock into place providing virtually no play in all directions and lock in place BUT is required to be easily inserted and removed (with some kind of catch?). With a standard peg in hole fit with plastic parts and moulding tolerances even low CPK values will mean that a worst case fit will translate to a large amount of play at the end of the 800mm tube.

Ideas I have been experimenting with so far are non-locking taper fits (7 degrees or more). This works well however im struggling to work out how to keep the tube held down. The variation in tube/taper size diametrically means that the parts can vary vertically by a few mm and a catch would be required to work over this distance. The taper works well as long as the parts are held tightly face/face against each other. Any lift of the tube will result in the parts breaking away from each other when load is applied.

This seems like quite a universal engineering problem however im still finding it difficult to find an application where it may be used already. Any ideas would be much appreciated.

edit: just to add, the inside of the tube on both male and female parts (min 20mm) has air-flow and needs to be free of any restrictions.

Kind regards

 

[racookpe1978]

Double wedge. The second wedge pulls the first wedge into the hole: The pioneers used it for their early cabin walls and roof beam connections.

Now, this starts the problem of locking the second wedge into position so it doesn't move and allow the first to loosen. use a screw to lock that wedge since the forces are low.

 

[MikeHalloran]

Make the hole square, or square-ish, with walls that are easy to deform, but stiff enough to hold the peg.

Look _very_ closely at how Legos lock together.



Mike Halloran
Pembroke Pines, FL, USA

 

[lamby88]

Thanks for the replies both of you.

The double wedge is something that I have been looking into. The first wedge being the taper fit between the two parts and the second wedge acting against the tubes. Like you say though the difficulty is trying to lock the second wedge in place (as well as ensuring it does not lock under load). The part needs to be quick and easy to remove by a user, and ideally from the other end of the tube. This could mean a push/pull rod or a wire connection down to some kind of mechanism. It does mean the mechanism needs to be catch like in its operation rather than something that needs several turns.

The peg in square hole is an interesting idea. This would indeed ensure no play between the parts. I fear that maybe with the variation in tolerance would still maybe create a part that is difficult to assemble at worst case (ensuring it is still held sufficiently at the loosest fit). The other concern would be that if the parts are flexible enough to deform to each other they will also be flexible enough to allow some movement when a load is applied to the end of the 800mm tube (considering even low forces will become very high over the lever length) I will certainly look into this though.

 

[ajack1]

Could you not have a two blade “propeller” on the end of the tapered tube?

This could be posted through two slots and tighten against two angled face, a quarter turn should then lock the part in place and take up any tolerance issues.

 

[lamby88]

I like that idea also. Im going to have to start experimenting with these when im back at work tomorrow.

One thing that I wanted to try and achieve was a positive lock (ie. a click) to ensure that the user has some feedback to correctly assembled parts. A typical sprung snap catch comes to mind but due to the variation in tolerance a normal single pivot catch would end up having self opening geometry, or a couple of mm of lash back which is not acceptable. I would therefore need something else to stop the catch from opening like a wedge.... which just goes back to the double wedge idea.

all this feedback is much appreciated and all the ideas are helping greatly!

thanks

 

[btrueblood]

How about a compression fitting, eg. Swagelok?

 

[ornerynorsk]

Search "expanding diameter pin". Just a thought.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[lamby88]

Very true ornerynorsk!

All the ideas are defiantly helping.

btrueblood:The end product will be produced in high volumes so using off the shelf systems like that is not really an option due to cost. I was wondering if a tube lock like that could be engineered to allow the tube to be easily inserted and removed and locked and unlocked with some kind of push/pull rod. I think they are fairly simple systems though and cant see how it would work.

 

[gadkinsj]

probably a variation of the double wedge, but run a threaded rod down the center and have the cylinder cut into 3 pieces with opposite angles at the interfaces so that when you tighten the threaded rod down there is compression.

 

[btrueblood]

Well, yes, I was suggesting you copy the design of the tube fittings, not necessarily use one of their products (unless it worked...). Take a look at cheap camera tripods: some of them use a variant of tube compression fittings, done in nylon or similar plastics, and some better ones use a toggle clamp to drive the cone rather than a screw thread.

 

[lamby88]

The camera tripod is a good analogy. Ill take a look into that, I have a few camera tripods laying around!

thanks

 

[ornerynorsk]

I'd agree with BT, a collet type of arrangement might be a very good approach.

It is better to have enough ideas for some of them to be wrong, than to be always right by having no ideas at all.

 

[lamby88]

WIth a collet type mechanism, I assume the taper would still need to be over 14 degrees inclusive in order to make sure it does not lock down against the tube? The part needs to be easily removed with the flick of a switch/pull rod type arrangement

 

[1gibson]

Probably doesn't help here, but my first thought for questions like this is usually to go look up some Ikea assembly instructions. You never know what you will find...