Barrel Nut Threads Vs. Gland Threads (Cylinder Threads)

The rod guide has to be centred in the cylinder. If you only use a thread then there is NO centring and there is a high risk of wear in the bushing and in the cylinder where the piston can touch the wall. If you need an internal thread because of limits in the external diameter then provide a good centring for the bushing and a ring "nut" for the fixation.

So the advantage is alignment and installation only or is there a strength advantage?

I concluded the installation advantage would be:

1) Using a barrel nut design would be quicker for disassembly and less machining is required for the barrel, since only the barrel OD is threaded. Also, easier to get to the seals since the gland just slides into the barrel and is retained by the nut which makes it more concentric. The disadvantage is (2) component is required (barrel nut and gland)

2) Using the threaded gland design, disassembly might damage the threads on the gland and concentric issues. Also, if adequate clearance is not provided for the barrel internal threads it might be difficult to get the piston seal inside the barrel damaging the seals. The advantage is only (1) part is required (gland).

I would like to know if there are any strength factor involve?

There is a slight advantage in the thread carrying capability. Under load the internal thread will expand the tube thus the thread shear section decreases. It is the opposite if the thread is on the tube out side. From the wall fatigue point of view the two solutions are equivalent. Only one remark the nut in the case of an external thread has to be thick wall in order to accept the load without expansion.

In fact I met rams with thread generated failures in both designs. Your fear about harming seals at assembly is easy to avoid with a correct design allowing the introduction with guide chamfers.

Your diagrams only show the gland. Depend on the cylinder. If it is one that the swab has to be inserted in through the gland end. Then if its threaded internally you take the chance of tearing the packing on the swab. Also if threaded internally or externally you have an issue with centering. The gland is under force. It is hard to control the distribution of the force across the threads. The cut of the threads reduce the thickness of the tube. The angle of the force caused by the threads may cause the tube to fail. That is why most glands are retained with internal snap rings. The external thread is just a retainer for the gland. Would preform the same as a snap ring. Centering not a issue. Though the tube is still threaded externally the force on the gland is straight up. There is not much of an issue with thread stress.

actually there is more than a slight advantage to strength for external threads (barrel nut) because the shear area increases non-linearly with increasing diameter. the problem you my face is making sure that the barrel nut is thick enough to withstand the hoop stress generated but you usually have room to increase the nut's o.d. in such situations, whereas if you are limited to an I.D. thread, you usually don't.

Due to the flank angle (30°) the hoop stress is low even with a not too thick wall but the deformation can be important in comparison to the thread height and thus reduce the loading capacity. This leads to the stiffness as criterion and NOT the hoop stress level. The possible second criterion could be the axial stress in the most loaded thread where due to the notch effect some higher values could be reached.

Last but not least for big threads the pitch is almost constant for large diameter variations so that the shear area grows linear within the diameter range for which the pitch does not change.

If you are interested I can mail you a graph for the shear section for metric threads.

Hello Nick Name, thank you for the replys and I very much interested in the graph for the shear section for metric threads. thank you

send on the private channel your mail and I shall send it

Glad to be of help

You are assuming metric threads, that is not necessarily the case. we could be discussing imperial or even acme threads, the OP has not stated. Not everyone in the world uses metric threads.

And my point was about the increase in circumferential area with increasing diameter leading to much higher axial strength. and the hoop stress was not about the threads but the internal pressure flaring the tube.

1- As far as I know (fortunately I do not know all as some pretend) there is also using Imperial Threads to change the diameter without changing the pitch.

2- About 35 years ago Machine Design (the most serious American magazine) printed a paper about Metrication" and made the prognosis that within 15 to 20 years the USA will use metric threads. It appears that the assumption was wrong!

3- If the thread is on the outside of the cylinder it does not change the axial force which is-again as far as I know- only function of the INTERNAL cylinder diameter. Normally the thread is in a zone where the bushing is already sealed so that the wall is not directly under pressure. The use of an external thread does not change the internal diameter and in your comment you mention the nut wall which has low or no influence on the cylinder wall deformation, any way no good designer will build the cylinder wall stiffness on the nut wall thickness since as everybody knows a thread needs a gap to be assembled (even if small) and even if when tightened 2 flanks are in contact there is a helix gap on the other side of the thread and any radial movement of a thread will be possible till this gap is filled.

4- The differences between metric and non-metric threads are small and a transfer of principles is possible.

5- A question was not ACME the supplier of all devices for the coyote in his fight with the road runner ?

you still aren't getting my point. the axial load is developed by the piston effect from gland. The shear area is the annular distance between the crest and the root diameter multiplied by the pitchline circumference. clearly the larger the thread is, the more area you have, regardless of the pitch. it isn't rocket science.

ACME threads have a pressure angle of 14-1/2 degrees. there are two types, full acme and stub acme, they are almost all imperial. Since they have such a steep pressure angle, they are much stronger than conventional v threads. Since there are no set standards for pitch, it is up to the manufacturer to chose what pitch they want to use, but they tend to be even inch increments (because of availability of tooling). the formulas of course work with metric numbers just as easily, but in industry the "metric" acme threads are all really imperial threads that have been translated to metric to satisfy stupid EU rules.

The US will never be metrified, some industries have and will remain metric, but most will not.

typical dumb ass Texan reply. Just go off an execute some more prisoners.

I am afraid you did not get my point or at least did not want to take a position against or for it.

In your comment you write about "nut wall" and related hoop stress and NOT about cylinder wall thus my explanations related to stiffness more than to stress level.

Rocket science makes also use of bolts and nuts and may be you do not know there were failures due to those "simple" components.

As for your last comment I would like to tell a story about relativity:

It was the time when parts of the world were still "white" and explorers had to risk their lives to discover land and people living there. One of them became friend with one of the tribal chiefs and once asked him why does he have such a peculiar smell. The answer came immediately: do you think that you do not have a peculiar smell for me?

When you write that the EU rules are "stupid" you can expect that some sensitive person (not me since I am the one who thinks that dogs bark and the caravan goes its way) would consider this as an arrogant comment and a proof that you consider "america überalles". You should be aware of the fact that as well some American rules and behaviours can be considered from other as stupid or illogical. I do not consider it as true since every body can have his own opinion and way of thinking. The problem in the globalized economy of today is that although America is still in position #1 the rest of the big markets as China, Europe, India aso go metric or are already metric. So that spare parts not metric will be more expensive and will compromise the competitiveness of products.

It is an error many times made in the West considering that intelligence and creativity are only in the West. According to my humble opinion intelligence is spread in an uniform way over the world and only knowledge is not. So when knowledge will be available all over the West will have troubles to maintain its leading position if not a constant progress is done. there are several examples I am sure you are aware of and it is not the place to mention them.

Being conservative as you claim to want to stay is a future problem the USA will be confronted with.

I really appreciate the comments guys, much appreciated