Has anyone made a laminated composite "Live hinge"?


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cookeaa
cookeaa
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I'm considering a project making a lightweight single wheeled cycle touring trailer, these typically need a horizontal hinge point to accommodate cornering, most simply use a fairly standard bearing/bushing and shaft arrangement of some sort. 

But it occurred to me I could design some sort of live hinge insert using a laminate of some sort, the advantages being it would save weight and possibly be simpler/easier to manufacture, and if it was done as an insert rather than as part of the trailer's structure it would allow me to replace a worn or broken hinge element more easily... 

I know some RC aircraft and cars use laminated composite live hinges and I've definantely seen MTBs that use them as part of their suspension systems, so its not a completely novel concept... 

My real question is what sort of reinforcement, and resins should I use? 
It would need to allow movement in only one plane of maybe 90 - 120 degrees (45-60* either side), and it would be under pretty much constant tensile loads from towing so I guess most/all of the reinforcement would need to be aligned to that primary load path and tolerate lots of lateral flex. 

And just how flexible is a "flexible" epoxy? or should I be looking at other resins? 
FLD
FLD
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Maybe I've missed something but what would be wrong with a small section of rope?  Its flexible, rotatable, cheap, replacable etc. 
cookeaa
cookeaa
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FLD (23/07/2014)
Maybe I've missed something but what would be wrong with a small section of rope?  Its flexible, rotatable, cheap, replacable etc. 


like I said I want to make a hinge connection a single axis pivot point not a substitute for a universal or ball joint... The trailer will have a single wheel, tow it using a bit of old rope and it will spend most of its time being dragged along on its side.... 

I already have a 2 wheeled trailer which uses a spring in a similar manner to what you suggest, it simply won't work in this instance... 

I had envisaged a 100mm long or so laminate with a "Deflection area" maybe 15-20mm wide mounted vertically between the trailer and the towing arm... 

This illustration for a rubber live hinge shows the principle perfectly:

http://www.sdplastics.com/ff2.gif


and I could yet look at something off the shelf like this, but I liked the idea of designing my own solution... 
FLD
FLD
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Ah, makes sense now.  I had missed something!  Personally I would use a machined pin hinge bonded into a carbon bracket but if you want to go full composite you could make carbon blocks that have a piece of thick mylar bolted between them.  I've used mylar hinges in my plane before and they are OK.  Just make sure you pin through it.  I wouldn't recomend the flexible resin for this, I'm not sure it has the flex you need.
Dravis
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The EC flexible Epoxy and Kevlar fabric makes excellent "hinges", just ensure that there is enough length in the flexible part to have enough flex, without  overstressing the "laminate"

I have used it with twaron/CF weave to make hinges for a large transport case ... the long hinge have now lasted for more than a year without showing any cracking or getting "softer"

In my construction it is not really stressed when the case is closed, it was made at a quarter circle on a "mandrel" made from a stainelss steel plate, bent with a 10 mm radius.  The "hinge" is actually made with both normal EL2 resin in the "securing parts" along the length, and flexible epoxy in the radiused part.. At the boundary they just mix and bond together.-- I let the flexible Epoxy cure to gel state,  then added the normal EL2 and  and cured it in an envelope vacuum bag.
It is bonded to the case itself with structural epoxy from EC 

You do get some "distortion" in the weave when the case is fully opened (maximum radius/(almost straight) for the hinge) but it does not seem to affect the strength of the hinge.

The Twaron /CF combined fabric is probably not as good a choice as a pure kevlar fabric, since the CF component may eventually fray or break, but it looks cool on the case, since the entire surface of the case is Twaron/Cf BigGrin

I will try to see if I can post a few pics of it later..

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Fasta
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I have made lots of thin Kevlar/epoxy hinges for a small wing and element but this hinge only moves through about 45 degrees max.

You need to match up an appropriate laminate thickness and size of bridging  hinge to suit your load and use etc.

For my purpose they are just one layer of 170g Kevlar 45 deg and epoxy resin and it bridges a gap of 3mm.




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