Press Tie Rod Nut as Projectile
Press Tie Rod Nut as Projectile
(OP)
A customer of ours asked us to look into something for them. They have a number of large presses. The presses have large bases at either end, tie rods connecting the bases, and a large hydraulic cylinder which creates a rather large separating force.
They infrequently experience failures of the tie rods, where after the failure the tie rod fragment along with the nut shoot away from the base with some decent speed. They welded in restraints meant to stop the fragment/nut, but are now questioning whether these restraints are satisfactory. We had previously offered to help them review the material, threads, and pre-tensioning, but as of right now they're only interested in making sure the guards are OK.
So, my issue is in determining what loading I use to evaluate the guards. The tie rod, when loaded via actuation of the hydraulic cylinder, stores an amount of elastic, or spring, energy. What happens when the break occurs? Does all of that (minus minor losses) become kinetic energy of the fragment/nut? Or is some of that energy shared with the remaining length of tie rod and its nut?
Or rather is the accel of the fragment/nut due to stored elastic energy of the compression of the base and nut?
Is the stored energy of the tie rod equal to the stored energy found in the bases? Looking thru some bolted joint stiffness calcs and finding that energy, I don't believe this to be so, but the more I think about this, the more complex it is seeming to me. And the more complex it seems, the more I think I should just keep it simple and choose a very conservative approach.
I've been looking thru a lot of texts and trying to research this, but just can't find anything that answers my questions, though I have found others who have posted similar questions. I've included a really rough sketch to hopefully convey the basic form of the press.
I appreciate any and all comments, suggestions, etc. that anyone can offer. Thanks.
They infrequently experience failures of the tie rods, where after the failure the tie rod fragment along with the nut shoot away from the base with some decent speed. They welded in restraints meant to stop the fragment/nut, but are now questioning whether these restraints are satisfactory. We had previously offered to help them review the material, threads, and pre-tensioning, but as of right now they're only interested in making sure the guards are OK.
So, my issue is in determining what loading I use to evaluate the guards. The tie rod, when loaded via actuation of the hydraulic cylinder, stores an amount of elastic, or spring, energy. What happens when the break occurs? Does all of that (minus minor losses) become kinetic energy of the fragment/nut? Or is some of that energy shared with the remaining length of tie rod and its nut?
Or rather is the accel of the fragment/nut due to stored elastic energy of the compression of the base and nut?
Is the stored energy of the tie rod equal to the stored energy found in the bases? Looking thru some bolted joint stiffness calcs and finding that energy, I don't believe this to be so, but the more I think about this, the more complex it is seeming to me. And the more complex it seems, the more I think I should just keep it simple and choose a very conservative approach.
I've been looking thru a lot of texts and trying to research this, but just can't find anything that answers my questions, though I have found others who have posted similar questions. I've included a really rough sketch to hopefully convey the basic form of the press.
I appreciate any and all comments, suggestions, etc. that anyone can offer. Thanks.





RE: Press Tie Rod Nut as Projectile
Side note: One of the known hazards of towing/winching is that cables or chains under high tension can be very hazardous when they break. Once upon a time, I was working for a dump truck company. They had a heavy bobtail truck stuck, were trying to get it out with a track loader, and the chain broke at the track loader. The hook from that chain went through the windshield of the truck and broke the rear window.
RE: Press Tie Rod Nut as Projectile
I wouldn't be concerned for exact calcs but I'd heavily overdimension the cage.
Probably not what you wanted to hear but here safety is a concern, a FoS of 10 (or more) would still be acceptable according to me. Probably not worth doing the calcs for.
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RE: Press Tie Rod Nut as Projectile
Have you any pictures of the failed tie rod, nut etc?
Reading your post it suggests the rod is failing and the threaded part of the rod is still engaged inside the nut when it flys across the workshop, if that is the case I would look at possible fatigue in the tie rod assuming the cylinder is loaded cyclingly. If you can get hold of a tie rod look at the fracture face it might show classic marks of fatigue.
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RE: Press Tie Rod Nut as Projectile
Pictures of the failed cages, and better yet, some of the actual ailed cages are required, too, if you'll be evaluating the guards, as directed by your customer.
Does the nut escape thru/around the cage on the first rod breakage, or only after multiple strikes ?
I'm thinking some amount of controlled deformation is almost mandatory if the nut/stub is to be contained. If that deformation is provided by the cage, and the deformation is severe, then the cage only has so many life cycles, which users might not recognize.
Perhaps some replaceable, sacrificial deformable element would make it easier to "reload" the catcher system to full capacity.
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Maybe could even turn into a replacement parts business for someone.
RE: Press Tie Rod Nut as Projectile
je suis charlie
RE: Press Tie Rod Nut as Projectile
The majority of tie bar breakages are due to incorrect/uneven tensioning. In 25 years in this business I have never seen one break, but have heard of instances.
On the pics below, the black collar on the tie bar end has been marked by the manufacturer to get the same tension as designed and as built. (The tie bar has to be heated to enable getting the correct pre-tension on re-assembly)
Note the pics have rotated 90 clockwise somehow - after spending half hour on Photoshop getting them correct orientation!
Tie bar clamp in situ:
Tie bar with clamp off and extracted:
Tie bar clamp and M20 bolts:
Safety restraint around clamp cylinder studs:
www.tynevalleyplastics.co.uk
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RE: Press Tie Rod Nut as Projectile
Regarding why the tie rods themselves fail, I'm fairly certain it's fatigue since I'm sure they don't properly pre-tension the tie rods. They say they get 10 years out of them, so they're not very interested in doing the work to extend the life.
I don't have any info on how their existing guards may have failed...I was told that they had a failure at another facility. I've requested pics of this, or some description of the failure, but have not received anything as of yet. I'm gonna make a point that I can't continue on without that information.
I would expect that after a strike the guards may need to be replaced...like a bicycle helmet, once it's employed to do it's job, it gets thrown away.
Yeah, this is the part of this that has me confused...I'm trying to figure out the dynamics of how much energy goes into propelling the fragment/nut versus the share of it that would go with the rest of the tie rod, as it has a huge impact on this.
Thanks again, I'm always impressed by the helpfulness of this community.
RE: Press Tie Rod Nut as Projectile
The force exerted by the nut is resisted by the base which must then either compress or move / bend, ever so much. given that the movement is likely to be low, but the bending moment very high, this is what gives the nut the force / velocity, IMO.
The energy is in the base, not the rod, but is equal to the energy in the rod (force x distance).
Think of it this way, if the base on the LHS was effectively solid / very big/stiff, but the base on the Right much more flexible, as you tighten the nut the base on the RHS moves, storing energy caused by force x movement. As the nut breaks, the base returns to its lowest form of energy, imparting some/most of said energy to the nut.
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RE: Press Tie Rod Nut as Projectile
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Press Tie Rod Nut as Projectile
What I run into with the energy balance is that essentially the tie rod is a (relatively) soft spring in tension, and the bases are very stiff springs in compression. I have not quantified it, but the stiffness of the bases has to be a decent multiple of the stiffness of the tie rod (analogous to a typical bolted joint where the clamped materials have a much higher stiffness than the bolt).
Spring energy then being E = (1/2)*stiffness*(delta^2), I don't get the two to equal each other. The attached pic is a really simplified example of this. Perhaps the conservative approach is to use half the stored tie rod energy.
RE: Press Tie Rod Nut as Projectile
je suis charlie
RE: Press Tie Rod Nut as Projectile
https://encrypted-tbn0.gstatic.com/images?q=tbn:AN...
RE: Press Tie Rod Nut as Projectile
RE: Press Tie Rod Nut as Projectile
RE: Press Tie Rod Nut as Projectile
Do you have a photo?
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Press Tie Rod Nut as Projectile
It is a decent amount of energy though, about as much as the muzzle energy of the largest shoulder-fired rifles. It would also be like hitting a 350 pound animal with your car, going 22.5 mph. That would certainly do significant damage...and animals are much softer and less dense than press parts. Of course, such comparisons are fun, but not necessarily directly applicable.
I'd guess that the energy is manageable without a highly specialized solution. In other words, not like the examples of big tires or flywheels exploding. For testing, you can achieve 33 fps by dropping the "projectile" from a forklift or something.
RE: Press Tie Rod Nut as Projectile
The square tubes are the guards they currently have.
RE: Press Tie Rod Nut as Projectile
My concern over that as shown is whether the nut could squirm around it or over it if the bar isn't dead centre, plus the fracture might make it fly off slightly off centre.
Two of those slightly apart would work, but for energy / force you need to use the stored energy / force in the bar I think which just instantly transfers to the nut.
Remember - More details = better answers
Also: If you get a response it's polite to respond to it.
RE: Press Tie Rod Nut as Projectile
I believe the wire rope element might absorb the blow softly if it were sized right. It probably wouldn't be too expensive, either. Most any shop could make it, except for the swaging operation.
Caveat: It's a completely untested idea that I pulled out of the air...on a Friday morning...while eating a sandwich.
Edited to add: It might be less of a hassle to use a bundle of smaller wire ropes, like 1" diameter, independently swaged. Weld the bundle of swaged fittings (not the rope itself) together. In the case of the giant wire rope, swaging might not be needed. I'd imagine that the wire rope could simply be inserted into the tubing a generous amount and held in by some less sophisticated method...like through bolts, or welding (away from where the tubing would flex).
RE: Press Tie Rod Nut as Projectile
RE: Press Tie Rod Nut as Projectile
RE: Press Tie Rod Nut as Projectile