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Join Date: Feb 2019
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Help With Project

02/06/2019 12:11 PM

Hi! This is my first post on here. I need to purchase an air cylinder, most likely hydraulic. The cylinder will have a forming die attached on the end and will be used to crimp two small stainless steel tubes together. The crimp will occur at an offset of .004 inches from each end of the tubes. They are thin tubes, as the ratio of the inner diameter and the thickness are less than 20. How can I calculate the pressure required to execute this task? Is there a specific set of formulas? Any help is greatly appreciated.

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#1

Re: Help with Project

02/06/2019 12:47 PM

Why do you need a pneumatic cylinder for a hydraulic press application?

You need to know what kind of stainless steel it is....in case any heat treatment is needed pre or post...

https://www.azom.com/article.aspx?ArticleID=2874

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#3

Re: Help with Project

02/06/2019 6:50 PM

Hi, its 316L. They will be crimped together and then heated in a furnace.

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#2

Re: Help With Project

02/06/2019 6:14 PM

You said, "Crimp two small stainless steel tubes together."

Do you mean to join two small stainless steel tubes together? As in making the two inseparable, as in welding/brazing/bonding?

Crimping simply means to squeeze. (In my experience)

If you can describe what you wish the end result to be, maybe there is hope.

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#4

Re: Help With Project

02/06/2019 6:53 PM

Yes the end tooling will crimp the two tubes together making them almost inseparable to where at least the two tubes will not be sliding after the crimp. Brazing takes place after and then the furnace. So my thought is that the outer tube is radially deformed from the crimping force, causing a change in initial and final radius.

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#5

Re: Help With Project

02/06/2019 7:42 PM

OK, so you have two tubes of different diameters that you want to crimp or swage together to fix them in place for brazing.

You do not want the inner tube to collapse, I'm guessing?

Again a narrative description would help. What you have (two tubes of differing diameters) that you want to raze together, one inside the other...... and the resulting product will look like what? A tube within a tube.

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#6

Re: Help With Project

02/06/2019 7:55 PM

Yes. Two different tubes, each different lengths and diameters. They are both thin tubes and the diameters are not significantly far apart in measurement at all. I do not want the inner tube to collapse and want them to be fixed in place for brazing. Each end will be crimped. Also, being different lengths, the offset from each end of the outer tube will be about .004 inches. The final product will indeed look like a tube inside of a tube (not exciting at all haha). To keep the tubes from initially moving out of place during crimping (.004 offset from each end) I was thinking of 2 air or hydraulic cylinders, one on each side, to crimp each end at the same time.

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#7

Re: Help With Project

02/06/2019 8:09 PM

You might be able to do this with a setup like a hose ferrule crimper. There are versions that work like Vise-Grips® and others that use a hammer. I don't know if any will fit your tube size.

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#8

Re: Help With Project

02/06/2019 8:26 PM

I agree. This could be a simple hand crimping operation.

Trial and error should produce results in less than 5 tries. Dies could be simple two plates with a hole to crimp the outer tube.

If automating, force is second to proper crimps and simple stops can produce the desired crimp.

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#10

Re: Help With Project

02/06/2019 8:40 PM

I have to at least have a proposal to back the equipment I will purchase which is why I'm hoping on finding a proper formula to calculate the required psi for some sort of cylinder. The company wants to go forward with these and is set on the specific idea for some reason. The pressure will be initiated through a pedal via the operator. I have found several other methods, but or some reason they want the air cylinders or hydraulic cylinders.

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#12

Re: Help With Project

02/06/2019 9:34 PM

Cylinder applied force is not important. Use one that's plenty strong.

A simple rig like SE showed is a start.

Again, do not worry about the force required, it's not important. Just use plenty. It's the shape of the swage/crimp/forming tool that's important. You need a uniform crimp all around, that's all that matters.

It may need to be in three sections, but you can workout the details,

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#9

Re: Help With Project

02/06/2019 8:26 PM

The company is looking to use a simple method that involves the simplicity of stepping on a pedal to initiate the crimp. Im not really sure which formulas I need to find the required pressure needed to crimp the tubes.

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#13

Re: Help With Project

02/06/2019 11:30 PM

It's difficult to know how much force is plenty if you don't know the minimum force required. You need to exceed the elastic limit of the alloy you are using (Young's Modulus), stated in psi, typically, to the yield strength, where plastic deformation occurs. 316SS & 316L vary between 25,000 and 30,000 psi. It sounds like it might be similar to the formula for ultimate shear strength of a round column, but if I understand the crimp concept, then you need to calculate the shear strength of a double wall column that can slip on one half of the compression application.

A good structural engineer will know how to calculate the required strain on the tube, once they have the exact dimensions of the die, as the contact area and shape will determine the applied force, and the amount of metal that has to be moved.

Steel tube is stiff up to a point, but you do have some risk in locating the crimp accurately, and with your 0.004 inch location, the crimp die could end up buckling the center slightly, as the displaced metal has to go somewhere.

Also, you don't mention whether the inside of the tube needs to retain any of the original properties of the original tubes, crimping will add a weak point to both tubes and the assembly which may affect the structural properties of the assembly.

The design of the crimp is also critical, assuming the center of the tubes are unsupported, as it seems it could be easy to crush the tube in the process of making it small enough to restrict movement.

You might get better results, repeatable, dimensionally stable, if you belled one end of the small pipe, and fitted it to the larger pipe, if the crimping die design proves difficult.

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#11

Re: Help With Project

02/06/2019 8:50 PM

I think I would split the end of one tube and make an expansion dimple, male in one and female in the other, so that when pushed together the internal male end will be able to contract because of the split and pop into place...

...sort of like this but much smaller ridge....

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#14

Re: Help With Project

02/06/2019 11:47 PM

You could use a tubing expander and just use a pressure fit....we use these on condenser tubes...

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#22

Re: Help With Project

02/08/2019 10:05 PM

They’re used to expand tubes on the tube sheet of a heat exchange or fired boiler.

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#15

Re: Help With Project

02/07/2019 4:59 AM

OK, so I'm imagining the smaller tube being completely inside the larger tube as two concentric items. Originally I thought you meant the crimp was like a step down in diameter and you were crimping where the diameter change would be.

If that's the case, then using a central mandrel could support your internal shape while the external tube is deformed/crimped onto the inner one. An inner mandrel means that your external force needs to exceed the necessary deformation force by an adequate amount to allow for process variability. Hydraulics will give better process control in this circumstance as pneumatics will have equalisation pressure delays and when someone later wants to speed up the process, they will fail to recognise this time dependant portion of the process sequence.

If you are later brazing these together, then the critical thing is to have enough clearance for the filler material to wet into the gap at the temperature you are using. Remember also that brazing both ends will create a sealed space.

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#16

Re: Help With Project

02/07/2019 5:33 AM
1. As you will be brazing the assembly, all you need is a method of temporarily fixing the tubes into their correct relative positions. The inner tube needs to slide over a round anvil to prevent it collapsing under the swageing force. The leading edge of the anvil should be tapered to ease the task of the operator sliding it into position. By machining the back of the anvil with a flat face containing with a .004" concentric groove the offset between inner and outer tubes can be fixed. On foot pedal initiation of the swageing cycle a second anvil can advance into the open end which will push and lock the inner and outer tubes to their correct position. Place a pressure switch into the circuit of the locking cylinder that will trigger the swage operation as soon as locking pressure is achieved. Instead of a full swage which is intended to create a liquid/gas tight joint just indent the outer tube with three dimples (at 120º spacing around the circumference). This requires much less force (you should be able to use pneumatic rather than hydraulic equipment (much cheaper)) and leaves a concentric annulus between the two tubes that will produce a consistent brazed joint. Getting three swageing cylinders at each end to operate simultaneously is not practical so use a single cylinder driving an iris mechanism at each end that will apply force to each dimple point with a high degree of consistency. Once the full swageing force is reach use a second pressure switch to retract both the swageing cylinder and the locking cylinder. Two way cylinders and valves operate faster than spring return cylinders and valves and will speed up the cycle time. You can employ smaller cylinders if you use the full bore to advance and apply pressure. By using pressure switches as suggested the whole control mechanism can be pneumatic with no need for electrics. While swageing is under way for the current cycle the operator can slide the two tubes together for the next cycle. You should target between 20 and 30 units per minute and size your air supply for 40 units per minute (machine operators, especially women on bonus, never cease to amaze me with the speeds they can attain.) A light curtain in front of the machine to safeguard the operator will give a faster cycle time than any other type of machine guarding.
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#17

Re: Help With Project

02/07/2019 5:57 AM

Use a swage tool to expand the outer, or inner, tube to a diameter that will provide a good friction fit when the inner tube is pushed into the outer tube.

It is sufficient for brazing or welding.

This method is used in HVAC all the time to join tubes without using a coupling.

If not meeting your needs,use the same terms and do a further search on Google.

It's there somewhere.I don't think you should expect others to do all of your work for you.

OOPS! Sorry SE,you mentioned the exact thing.

Disregard.

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#18

Re: Help With Project

02/07/2019 11:22 AM

You can do this with one cylinder, two bell cranks and some fixtures.

Sorry about orientation. From above you've got the cylinder, bell cranks and forming die. A guide pin to hold the tubes vertical and step then for the offset. This will protect the inner tube from collapsing.

I'm late for a meeting.

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#19

Re: Help With Project

02/07/2019 2:57 PM

So, the holding fixture/pin is vertical, centered between the die. Two steps, one for small tube and for larger tub to index properly.

Place tube on holding pin, press foot pedal, crimp, release foot pedal, cylinder retracts automatically.

If hydraulic, a simple valve/gauge will allow you to dial in the correct crimping force.

The size of the cylinder is not important, since you can control the force with the valve.

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#20

Re: Help With Project

02/08/2019 3:02 PM

The force should not be critical, as long as it is adaquet. To keep from damaging the tube, the die is what is critical. Once the die is completely closed, more pressure does not do anything. - JHF

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#21

Re: Help With Project

02/08/2019 10:03 PM

As others noted,... check out SwageLok,... if I recall, they have tools for doing crimping

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#23

Re: Help With Project

02/12/2019 9:22 AM

If it's a project, Lyn is the one who is your best bet on this--sarcasm

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