Stretching a thin rod?

[ssbowtie1]

Hey guys, this is my first post here.

Say you have two plates that are evenly spaced (10inches or so) and you want to stretch a thin rod (.1inch) across the plates. What would be the best way to do this? I tried threading the rods and using nuts on each end to stretch the rod, but the threads stripped out since the rod was so small.

I hope that made sense, thanks!

 

[rb1957]

how much are you trying to stretch the rod ? ie what strain are you imposing ? what stress ? what load ?

can the rod material handle this load ?

could this load been applied to the rod thru a cross-pin, something like a 0.05" dia pin ? if the rod can handle this load, what about the pin ?

if the rod can handle the stretching load, could you manufacure loops on the ends of the rod and pull on these ??

 

[MiketheEngineer]

Could you use some type of jamming ferrule that tightens around the rod as you PULL the plates apart??

 

[racookpe1978]

What were your thread spec's? Fine or coarse?

As above - What rod diameter? How long were the threads? How much expansion did you need?

 

[tr1ntx]

Compression couplings, perhaps. Sounds like you need a miniature version of the method used to post-tension concrete.

There's a "socket" which the end of the rod goes through, then you slide on the split collar. The socket is tapered inside so that as it is pulled, the split collar bites into the rod and is compressed against it.

Or you could try, instead of hex nuts, some sort of long internally threaded gizmo that would give you much more shear surface of the threads. If it's a .1" dia rod, what the heck did you thread it, #2 or metric? Is there a 2.5mm thread?

You can get #2 hex spacers, fully internally threaded, that are 3/4" long (McMaster). That would give you much more shear strength than a hex nut. #3 threads would be better, but that is a "secondary size" and probably nothing is available.

 

[racookpe1978]

To the original poster: You said 0.1 inch diameter rods - but that's NOT a standard thread diamter, does NOT fit any standard nut, any standard diameter rod, any standard bolt hole, any standard die set .... etc.

So, if you used a tap-and-die set for a rod that's undersized from a nbr 10 thread (3/16 nominal or .183) or a nbr 8, nbr 6, nbr 4 etc. , then your "assumed" thread isn't going to hold anything but fingernail polish.

If you are converting to/from a metric rod, then you need to specify a metric dimension and metric threads.

That tiny a dia rod isn't strong enough to stretch much regardless of units, but using the wrong nut/rod thread engagement will kill your attempt.

 

[asimpson]

Sounds similar to way spoked wheels are tensioned so should be possible. I have never seen them wring the threads.

Alternatives: Weld large diameter to rod ends. Bend rod ends around anchor or rod end.

See below for some ideas

http://www.google.ie/images?hl=en&b...isch:1&sa=1&q=collet+shaft&aq=f&aqi=&aql=&oq=

http://www.google.ie/images?hl=en&r...=UTF-8&source=og&sa=N&tab=wi&biw=1660&bih=769

What is the purpose of the exercise? Do you wish to stretch the rod beond its elastic limit?

 

[ssbowtie1]

Wow I didn't expect this kind of feedback so quickly!

To answer a few questions, the rods will be stretched along two plates and the requirement given to me is for it to be "tight". There isn't an exact stain requirement given to me, nor does the person know how tight he even wants it to be until he tests it.

I got two rods, one 3mm rod which will have m3 threads put on, and one 1/8" rod which will have 5-40 threads. I got hex spacers (thanks tr1ntx) which should allow me to stretch the rods tight. From there I will get feedback from the client and readjust the design as needed.

Thanks again everyone for the replies, I will keep you all posted with results.

 

[drawoh]

ssbowtie1,

According to my Machinery's Handbook, a mechanic can break any bolt up to 5/8". You are going to have to work out a strategy for tighening your threads. "Make it tight" will result in a broken rod.

Since your rod is 10" long, you can specify that they turn the nut finger tight, and then specify how far to further rotate the nut. My very quick estimate is that for M3X0.5, you can do 3/4 of a turn of your nut without reaching yield stress. This allows for a very approximate understanding of "finger tight". I am assuming your rod is of soft material, and that your structure is rigid. You should do the calculations yourself.

JHG

 

[dhengr]

Ssbowtie1:
I think you might stand a better chance of pulling those wires 3mm or 1/8" if you made some end buttons, shape per your design and tensioning methods. Maybe just a cylindrical shape which can receive a larger male thread and nut. Pull the button, drop some shims btwn. the button and plate, and release the pulling device. Drill a 3mm+ or 1/8"+ hole through the buttons, and then drill a small countersink on one side of each button. Push the wire through the hole so it projects approx. its dia. on the countersink side and puddle weld the wire to the button. This might give you the potential of developing the full strength of the wire, if you experiment a bit with methods, sizes and weld process.