3/4"-14 NPT threads to hydraulic cylinder 1

[Ingenuity]

I have a 5,000 lb.f capacity pull/tension hydraulic single-acting (spring return) cylinder that I need to connect to a structural steel fixture for a small scissor lift (for a 400 lb machine).

The piston rod end has a male-end with a 3/4"-14 NPT thread and the cylinder base/body has a female-end with also a 3/4"-14 NPT thread.

To connect to my steel scissor lift I am going to get some round stock and die cut 3/4"-14 NPT threads and connect to the female cylinder base, and similarly tap 3/4"-14 threads to a hole of the steel work to connect the piston rod.

3/4"-14 NPT threads have only a small engagement length - less than 1/2" - so my question is, are 3/4"-14 NPT threads structurally capable of developing my hydraulic ram tensile capacity of 5,000 lb.f?

Would have been a lot easier if the cylinder had UNC threads where I could simply use a bolt or threaded rod to make the connections.

 

[Compositepro]

Those are standard fittings for hydraulic ram accessories and adapters/connectors are readily available. Do you really think the manufacturer would use inadequate parts on their product? The rod end fitting is already an adapter. So you are going to build an adapter for an adapter?

 

[Ingenuity]

I am building a single adapter to connect my cylinder to my steelwork - no doubling-up of adapters.

3/4"-14 thread is a taper pipe thread, and not especially 'standard fittings' for structural connection, in my experience.

The manufacturer has some adapters/connectors i.e clevice, bases, etc, that will not work in my configuration.

 

[BrianE22]

I'd say it would easily handle 5000 Lbf. Usually you would have spherical bearings on each end of a cylinder. Are you comfortable with rigid connections?

 

[Ingenuity]

Thanks BrianE22. I want rigid axial connections - there is axis-rotation about an axle that enables the scissor action.

I think I will cut threads to both ends of a 12" length of round stock and test to failure in a 30 ton hollow ram that I have. The consequences of the scissor lift failing would not be desirable.

 

[hydtools]

Look at pressure service for 3/4" pipe and calculate tensile load on the threaded joint.

ID area x pressure = force

Ted

 

[MintJulep]

I can imagine if your application is pushing or pulling a pipe during installation that there might be some sense to using NTP threads on a hydraulic cylinder.

But there is an entire universe of hydraulic cylinders that use UNC, UNF or metric straight threads.

Wouldn't it be better to use a cylinder with a structural thread for a structural application?

Regardless, the manufacturer of the cylinder that you picured supplies thread adaptors, along with the note that "Factory accessories are do not de-rate tonnage."

 

[Compositepro]

Tapered threads are structurally stronger than straight threads.

 

[Ingenuity]

Thank you for all the input.

I already have the cylinder, so do not wish to purchase another for this application, with UNC or other structural threads.

With only 0.339" of thread engagement for hand-tight, that is less than 5 threads.

If I use hydtools idea of 3/4" ID x assume 15,000 psi pressure (say XXS pipe) that is 6,626 lb.f, which is more than 5,000 lb.f cylinder capacity.

I cut some threads onto both ends of some round solid stock this afternoon. Round stock yield capacity is more than 28 kips. I am going to tensile test the sample tomorrow with a 20 ton center-hole ram and pump. I shall post my results ASAP.

Thanks for your assistance.

 

[MintJulep]

@Compositepro

Please take a look at thread725-472409

 

[hydtools]

Ingenuity, tap the thread deeper if you want more thread engagement.

Ted

 

[Tmoose]

Are you sure the cylinder is not threaded 3/4" NPS ?

https://www.spxflow.com/assets/original/PT Catalog 2018_Cylinders_US.pdf#page=10

Despite what the catalog says on some of the cylinder specs pages, a fair number of the adapters on pages 15 an 16 are NPStraight. I would want any of my adapters to tighten face-to-face, not have tapered threads being subjected to ANY potential bending, or tension fed directly into the threads. Especially when adapters potentially could be made from Ch*n*se pipe fittings.

 

[Ingenuity]

Ingenuity, tap the thread deeper if you want more thread engagement.

I have tapped one end of my test piece with 50% longer threads BUT the length of engagement appears near identical to the other end with less threading - I suspect this is due to the taper design where it bottoms-out (?). Less than 5 threads appear to be engaged.

Are you sure the cylinder is not threaded 3/4" NPS ?

Pretty sure it is NPT - 99% confident. My adapters are not going to be made of 'Chinesium' fittings - I am making my own with traceable materials.

This cylinder is a pull/tension single-acting cylinder (not push), so there is direct axial tension on the taper threads. I have options to mount the cylinder body via a collar of 1-¹/₂"-16 threads but still have the piston rod end with 3/4"-14 NPT to deal with.


Off to my 'lab' to break some things!

 

[mfgenggear]

OK lets us know it works

 

[Ingenuity]

Simple tensile test setup.

20 ton center-hole cylinder, manual single-stage pump. Two gauges: 0-2,000 psi and 0-10,000 psi. I abandoned the low-pressure gauge at the first stage of loading at about 1,500 psi.

I did not use my load cell, so failure load is approximate. I need a follower-needle gauge to record max, but not critical for this test.

My 3/4"-14 tap arrives tomorrow so I improvised and used 3/4"-14 galv pipe cap as a pseudo-nut to both ends.

1,000 psi gauge pressure = 3920 lb.f

Failure load at approx 5,000 psi = 19,600 lb.f. Failure at the end with less thread engagement - I counted only 4 threads engaged at the failure surface. Upon failure I re-died the failed bar-end threads and re-tested. Failed at approx 15,000 lb.f.

Failure load greater than the cylinder capacity (5,000 lb.f) I propose to use (by close to a factor of 4) so I am good to go.

Thanks for your help everyone.

 

[hydtools]

Nominal engaged thread shear area is about 0.93 in^2. If material yield is about 35,000psi and shear stress about .5 x yield then calculated capacity would be about 16,000 lbf. That is comparable to your results.

Ted

 

[Ingenuity]

Thanks, Ted.

 

[Ingenuity]

Nominal engaged thread shear area is about 0.93 in^2.

Ted, may I ask how you get an engaged thread shear area of 0.93 in²?

I calc 0.85 in². First time I have done this calc so prone to error.

 

[hydtools]

You are correct. I used .589 for wrenched thread engagement instead of .5457.
I used 1.00 as a straight thread pitch diameter. I did not detail the taper variation.

Ted

 

[Ingenuity]

Many thanks, Ted.