Choices of Threads 2

[brents]

I have been reading a lot of questions and answers on this BBS and I am way out of my league here.

So I'll apologize now for being lower on the totem pole. Here is my question:

I'm looking at designing a Hydraulic cylinder. I am confused about thread strength..The unit will be a 10,000 psi system with about 20 Tons maximum force. The end of the cylinder will have an end cap screwed into it. The end cap threads will be 2 5/8"-18 with 1" engagement..Is this strong enough to resist the threads from shearing, or should I go more Threads per inch, or even a Buttress thread..

I am trying to avoid using tie-rod tubes to keep it all in place..I know the pressure inside of the cylinder won't shear the threads, but I am wondering if when the piston comes in contact with the end cap at the end of the stroke, will it shear the threads and knock the end cap out??

I hope I simplified my question.

thanks,
Brent

 

[arto]

Look in Appendix B to ASME B1.1 or Machinery's Handbook for calc. methods

 

[Barry1961]

If you are concerned with the kenetic energy of the load breaking the end cap you will need to know the load, velocity and slow down distance. I assume there is going to be some type of cushioning if there is a large moving mass.

Barry1961

 

[brents]

Kinetic energy would not be an issue in this case, the piston would only move about maybe 4 or 5 inches per second, but I could use a rubber bumper if needed..

This problem has always been my weak point when it comes to designing anything..I want to point out that I am not a design engineer, but I do like to design.

I am thinking that using buttress thread form would be total overkill..I don't know..

thanks,
Brent

 

[BillPSU]

brents
Having a end cap screw internally into a barrel presents two situations, Barrel expansion and concentricity issues. When the end cap of the cylinder sees pressure is tries to push out the end of the cylinder. The standard 60 degree vee threads causes the threads to actually expand the barrel of the cylinder. The expansion can be enough to to expel the endcap. This barrel expansion can be fixed by changing to buttress threads, externally threaded barrel with a retainer ring, strengthing the cylinder with a reinforcing ring on the outside.
Concentricity problem occur when you thread the ID of the barrel locating on the OD of the tubing. Even though DOM tubing is the most accurate tubing made there is variations in wall thickness. The typical process to fix this problem is to machine a steady rest diameter on the OD of the barrel locating on the ID of the barrel. The barrel is then threaded locating on the ID at the chuck and the steady rest diameter. Design solutions include 2- piece design of the end cap with a floating end cap and a retention ring.
I prefer a 2-piece end cap design with an externally threaded barrel with a floating end cap and a retention ring. The threads can be repaired. The threads don't cut the seals while the cylinder is being assembled. Concentricity is not an issue. Barrel expansion actually locks the retention ring is place. Detractions are larger OD may not be acceptable in application and slightly higher cost.

Bill

 

[brents]

Bill,

Thanks for your input..My cylinder will be dialed in on the bore so concentricity isn't a problem..The internal thread bore is larger than the cylinder bore so ripping the seals on the piston during assembly is not a problem..

I am still confused here..I have made a cylinder just like what I am trying to do now, except that I used 4 tie tubes around the cylinder..it works great..But now, in this situation, I can't use tie tubes so I was trying to figure out if 60 degree vee threads would be sufficient or if I need to go a buttress thread for added pullout strength..

I don't know any equations to figure this out..

thanks,
brent

 

[BillPSU]

The calculations would have many assumptions. Make one and try it is the solution I would propose. Unless this device would cause major injury or death if it failed.

Bill

 

[Cockroach]

I have recently completed a design on a 25 ton press, similar to the one you are after.

The end caps were screwed onto the hydraulic cylinder, similar to other designs I have completed in the past. Use stub acme threading rather than vee threads or buttress; the cylinder ID required to generate 25 kips does not lend itself well to vee thread, a sizing issue verses profile performance, and the buttress is an inclined thread so you need worry about flush butting at the mating surface between the end cap(s) and cylinder.

In my case, the cylinder ID was 3.375 with 4 1/8 - 8 STUB ACME-G-RH threads. You will find 1 1/2 inches of full engagement is sufficient for thread strength. The piston itself is two piece, I had the piston stem screw into the bottom of the piston in order to save machining of a 1 1/2 diameter shaft off a 3.745 OD piston. A small flange positively seated the shaft thus maintaining parallel requirement between shaft and piston. Also, I used a 2-340 o-ring with two back-up rings, the cylinder was bi-directional, 55223 lbf forward force and 46,388 lbf return force over 12.0 inches of stroke. The cylinder is rated for 5.0 ksi maximum operating pressure.

I used AISI 4140/4130 HRc 28-32 for the cylinder and end caps, then liquid nitrated the components to remove free iron off the surface to surpress rusting. The stem was made of chromium steel, then flashed with one-half mil of electroless nickel baked on to HRc 55-60. This would tend to minimize the probability of material pickup and transfer or gauling.

You may want to go with 3/8 NPT port inlets, this would give good fluid transfer between the sump and cylinder housing relative to an off-the-shelf pump. Depending on your design input, you can specify the pump from various manufacturers who can suggest an economical model. I went with Enerflex, they had some nice equipment.

Good luck, hope this helps you out.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[brents]

Thanks for all the input,

My cylinder is 3.00" OD x 2.500" ID DOM tubing..One end is a welded end cap, so Maybe I'll look into machining an acme thread.

How course of an acme thread should I go If I maintain a major diameter of 2.625"?

COCKROACH--Why did you go with a "stub" acme? Is it because of lack of wall thickness? I would like to go a regular acme, thinking it would have more resistance to pullout. I would have about 3/16" wall thickness from the OD to the major diameter of thread if I went a regular acme thread..

thanks,
brent

 

[Cockroach]

I haven't looked at the numbers yet, but 3.0 DOM tubing sounds a little small for 20 ton force output. Watch the wall thickness relative to delivery force required by your design.

Anyway, in a regular ACME profile, 6 TPI is a very common pitch for 2 5/8 diameter. The only differences between ACME and STUB ACME threading is that the latter is considerably smaller in tooth profile compared to the latter, something like 57% or so. There are some machining advantages, but this is apart from design considerations. As you correctly stated, used primarily for thin wall applications where a strong thread is required. A limitation is with the undercut needed as a thread relief. I like to go 1/32 inch off diameter above the maximum allowable major diameter. This would make your wall rather thin at the thread relief, 2 places typical on your cylinder. I would use the minimum wall for your fracture mechanics computation since this represents worst case.

You should use tri-axial stress computations for your cylinder, this will be far superior than other fracture mechanics models. Von Mises-Hencky lends itself well to hydraulic pressure cylinders. In essence, I have used a 1.25 factor of safety at maximum test pressure, 50% above maximum operating pressure. This will remove much of the design uncertainties common to the practice.


Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[ccw]

Hi Brents,
Good advice from everyone. The cylinders we made, used the floating rod end and the screw on cap (two piece rod end construction), with a welded closed end. I agree with the one commenter that it is better to locate on the TG&P inside diameter than having threads on the ID.

Finally, be sure and check the Euler buckling margins for the rod on these high force, high pressure, designs. A buckled rod can be quite dramatic.

CCW

 

[tbuelna]

brents,

A 40,000 lbf end load through a perfectly machined 2.625-18 UN x 1.0L thread would produce a root shear stress of about 5 ksi through the threaded joint. Not too bad, assuming all the threads share the load equally.

However, to get a 1.00 inch long, 18 pitch thread to engage all 18 threads equally is essentially impossible. So, a threaded joint like this must be machined with clearance. And since there is clearance, the threads do not share the load equally, and thus factor of safety must be applied to the analysis. What that factor of safety should be is up to you.

Of all the thread forms, I would recommend a UN"J" thread form. They have a controlled root fillet diameter and will produce a thread with maximum fatigue life. Also, roll formed threads (if possible) will always give better fatigue properties than cut threads.

Regards,
Terry