Properties of Rubber

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I have to make a part that clamps into the inside of the end of a scaffolding tube. I’m thinking of two disk of metal with a disk of rubber sandwiched between and when inside the tube you tighten the bolts pulling the two disks together and squeezing/expanding the rubber so it wedges it’s self in place.

My question was what should be between them. Polyurethane sprung to mind as it’s so durable but it may be difficult to squeeze, It’s two M6 bolts to tighten and the inside of the tube is 34.8mm.

Any advice on types of rubber would be very welcome.
 

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  • #3
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Thanks for the help Scott. It’s so that different feet can be screwed into the end of the tube. It’s very important that the feet don't fall out when it’s being thrown about, in and out of vans ect, but that’s it, it doesn't actually take any weight pulling on it.

Here’s a little picture of what I was thinking of. It dose need to be very durable though, so that it can be removed and put into other bits of tube over and over again to attach feet to the bottom of them.

Foot_socket_small.jpg

Blue is aluminium, Purple is the two M6 bolts, Orange is the rubber/polyurethane and green is obliviously the tube.
 

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  • #4
.Scott
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It’s so that different feet can be screwed into the end of the tube. It’s very important that the feet don't fall out when it’s being thrown about, in and out of vans etc, but that’s it, it doesn't actually take any weight pulling on it.
Super ball material is pretty tough - and is renounced for its elasticity. If there is one that fits the inside diameter of the tube, I would probably go with it.
Also for this application, I would go with only a single bolt. There are two advantages: 1) Only one hole will be needed through the rubber. 2) When squeezing the rubber, an off-center force would cause that inside aluminum piece to jam against the sides of the tube.
But with only one bolt, you would need to keep that inside piece from turning while the screw is tightened. So the aluminum faces that touch the rubber should not be smooth or completely flat. And if a rubber ball was used, the portions facing the aluminum would have to be sliced back to form flat surfaces (as you show in the diagram).
 
  • #5
jrmichler
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An excellent source of rubber, rubber expansion plugs, and ideas is McMaster-Carr: www.mcmaster.com.
 
  • #6
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That would be easier but the fitting needs to have a 3/8ths threaded hole for the attachments and it needs to be central. I can make the second part that the bolts screw into with a smaller diameter so it dose not wedge in place but the user will have to alternate when tightening the bolts. This is the best Idea I’ve got without it becoming considerably more complication to make.
Here’s a picture of what I’m talking about.
foot_fitting.jpg
 

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  • #7
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Expansion plug seems like the perfect one but is it pretty durable the stuff their made of? Super balls are also a good one as they come in all sizes but again do they damage easily? And a brief look at the shock absorbers also came up with some very promising ones that could be cut to size.

My worry with all of these is that I just don’t know what their made of really and how resilient to abrasion damage they would be. Ultimately I can buy a few samples and test them my self I guess but I’m curious if any particular types of rubber stand out as bing tougher than others.

I suppose by asking it to flex like that I’m also asking for a softer material?
 
  • #8
.Scott
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Super balls are also a good one as they come in all sizes but again do they damage easily? And a brief look at the shock absorbers also came up with some very promising ones that could be cut to size.

My worry with all of these is that I just don’t know what their made of really and how resilient to abrasion damage they would be. Ultimately I can buy a few samples and test them my self I guess but I’m curious if any particular types of rubber stand out as bing tougher than others.

I suppose by asking it to flex like that I’m also asking for a softer material?
If your key factor is abrasion resistance, then use the same rubber used in tires.
These are usually styrene-butadiene (SBR).

Here's a link:
https://www.amazon.com/dp/B078T2W9PS/?tag=pfamazon01-20

Or here:
https://www.zoro.com/zoro-select-ru...MIn--G-7372QIVSRtpCh3DDwi_EAkYBSABEgLcLvD_BwE


My experience with the Hasbro Super Balls is that they are pretty tough.
They were designed for High School kids to play with - and one of the favorite methods of play was to slam them down into the pavement so that they could bounce over a house. Although, it was possible to eventually break them that way.
 
  • #9
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Thanks for the help guys. I'm going to buy some samples of the items metioned and do some tests.
 
  • #10
CWatters
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Devices like this are sometimes used to fix toilet seats to the pan. There are several potential problems I have come across that might also apply to your application...

Over time the rubber can become permanently expanded making it hard to remove or insert even when the bolt is undone.

The rubber expand inwards as well as outwards gripping the bolt making it slightly harder to turn. The main problem is not undoing it, rather you loose the "feeling" that you get when a bolt is loose. It can be hard to know if you have undone the bolt enough. Yet undo it too much and the "nut" part comes off and disappears.
I
Sometimes the nut part starts rotating with the bolt so it can't be undone any further.

Sometimes the rubber sticks a bit in the tube. Pulling on the foot to remove it pulls on the bolt compressing the rubber making it stick in the tube even more.
 
  • #11
CWatters
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I would reconsider your approach and look at using something that fits around the outside of the scaffolding tube and clamps on using a quick release over centre strap or similar. Scaffolding poles end up covered in mud, and mortar. Much easier to clean the outside of the tube than the inside.
 
  • #12
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Needs to be on the inside unfortunately. Weight is an issue and exterior clamps are heavier and more bulky. There will be mud and branches and rock and all sorts and they collect and catch on things, especially if there are cams or clasps involved. There is no ingress from the other end of the tube so an interior fitting gives a clean profile which will not carry stuff. An exterior clamp also can not pass through other clamps and fittings so creates installation issues.

If I cant make a rubber or similar material work then I will have to look at a metal expansion plug but this is getting more complicated to machine.

I can make the hole bigger for “gripping the bolt” but also I was thinking of putting a tube in there to prevent over enthusiastic tightening and damage to the expander.

I can put a circlip or bolt or something on the end to prevent “loosing the nut”.

“Rotating plate”. If it’s rotating there’s no friction and no compression of the rubber so it should slide out unless it’s..

“permanently expanded” which given there is a little clearance between the expander and the inside of the tube includes “rubber sticks”.

If the rubber or material permanently expands so it cant be pulled out that’s an issue I cant get round and I would have to move on. This is down to the properties of the material again and I’m unsure here of whether there is something out there that would do the job or not.

Polyurethane came to mind and I wasn't sure why but now you bring this up there are polyurethane products that require memory, even polyurethane springs. I’m not sure how much force they would need to expand though. If I had to guess I’d say super ball rubber, it’s called Zectron (cool name), also has a pretty good memory, though I don’t really know how that would play out over extended periods of compression.
 

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