# Calculating Max Pull for M30x2mm Threads on Chrome Rod

• Jimbo86
In summary, the best way to calculate max pull / tensile loading in Newtons that the thread can be subjected to before snapping (not stripping threads, but actually snapping) is to use the three different limits, three different calculations, and those calculations require more information.

#### Jimbo86

Hi,

I'm machining threads on to the ends of chrome rod for use in hydraulic rams.
I have a ram which will see a maximum retracting load of 10 ton (98.1kN). For this I'm considering a thread of M30x2mm pitch. The chrome rod we use has a tensile strength of 550 n/mm2 with a yielding of 420 n/mm2.

What is the best way to calculate max pull / tensile loading in Newtons that the thread can be subjected to before snapping (not stripping threads, but actually snapping)?

Thanks

There is a load at which the rod will break the first time it is loaded.
There is a somewhat lower load at which the rod will yield (stretch) slightly the first time, then break after being loaded several more times.
There is a lower load at which the rod will not yield, but will break suddenly after being loaded many times.

Three different limits, three different calculations, and those calculations require more information. You might consider looking at some factory rams, and copying their designs. Look carefully at the threads on factory rams because, for example, rolled threads are stronger than cut threads.

berkeman
Jimbo86 said:
I'm machining threads on to the ends of chrome rod for use in hydraulic rams.
I have a ram which will see a maximum retracting load of 10 ton (98.1kN).
Is there a reason you have chosen chrome as the material for this application? I would think there would be better material choices for such a heavy load...?

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Jimbo86 said:
I'm machining threads on to the ends of chrome rod for use in hydraulic rams.
Is this the chrome plated piston rod or the four surrounding tie bolts that hold end plates onto a cylinder tube. If the piston rod is induction hardened, it will be hard to cut without first reducing the diameter by stripping the external layer with a ceramic cutter. I would expect a 30 mm piston rod to have a thread with a pitch of less than 2mm. At peak loads the pitch will change along the thread so a lock nut will be needed to prevent creep.

External cylinder tie rod sets are sometimes chrome plated for use in food preparation applications or clean areas. They may be coarser pitch but will always need some attention to thread locking at both ends.

Please confirm if you are talking of induction hardened, chrome plated piston rods, or of tie rod sets.

berkeman
Piston rods have a greater axial compressive force during extension than they have tensile force during retraction. That is because the sectional area of the rod makes a difference to the area exposed to hydraulic pressure.

It is normal to step reduce the rod diameter at the piston end before cutting the thread to attach it to the piston. The step rests hard against the piston and carries the higher load during extend. Thick rods are used for column stability on long push strokes, but thick rods reduce the retraction piston area and so reduce retraction force. That pushes you to use larger diameter cylinders.

It is important to profile the inside corner of the step in the rod to avoid stress concentration at that point. Also keep the cut thread back from that step. Using a fine thread significantly reduces the amount of rod section lost to thread-depth within the piston. The thread can be made longer to compensate. A lock nut, often with a pin, is fitted after the rod is screwed tight into the piston.

It is not unusual to find the situation where vibration or impact during the retract stroke strips the piston and lock nut from the rod thread. The cylinder then extends OK but cannot retract, and there are bits of metal in the spool valves on their way to the tank.

Because the step in the rod seats hard against the piston there is little flexing of the reduced rod diameter inside the piston where it is pretensioned during assembly.

How are you going to couple the rod end to the load? That will limit the tensile force possible in the rod. Maybe remove the chrome and use external collets to lock onto the rod. Then you would have to do the same inside the piston. The rod end attachment need only be as good as the piston end attachment.

In a ‘good’ design should not the weight of the extended structure help in the retraction process thereby somewhat medicating the difference in the pressure area available between extending and retracting of the cylinder? Also, it seems the rule of thumb should be: The retraction load allowed/required should always be less than the allowed extending load.

AZFIREBALL said:
In a ‘good’ design should not the weight of the extended structure help in the retraction process thereby somewhat medicating the difference in the pressure area available between extending and retracting of the cylinder?
Define 'good', mechanical or economic? If it can be engineered to advantage in time, within the economic budget available then OK, do it. Hydraulics are pragmatic.
Sometimes velocity is more important than force. A differential connection can be used to apply the same pressure to both sides of the piston at the same time, so as to get the maximum velocity, or balanced velocities in a constant flow fluid system.
'Medicating' was not the right word, it is a malapropism, but I can't immediately recall the appropriate term. Any ideas anyone. (Assuage, Mediate, Mitigate, Mollify, Moderate, Propitiate, Palliate, Compensate)?

AZFIREBALL said:
The retraction load allowed/required should always be less than the allowed extending load.
That assumes there is space available 'below' the load for a hydraulic jack with a thick rod. The reliability of simplicity often outweighs the use of more complex mechanisms such as a 'hydraulic jigger' with a very thick rod.
https://en.wikipedia.org/wiki/Hydraulic_jigger
https://en.wikipedia.org/wiki/Marine_riser_tensioner

Long thin rods fail by buckling when in compression. There is no requirement that pressure be used to extend the actuator if the mass of the load and attachment is sufficient to keep the rod in tension. When a cylinder is used only in tension, then the large area side of the piston may be used only as a fluid storage tank.

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I meant to say 'mitigating' not 'medicating'. Sorry.

Baluncore