Torque Calculations for Gland in Cylinders 1

Can you share a picture? Preferably a sectional view along the longitudinal axis.

Regards,

Cory

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here is what the gland and barrel looks like.

Yes, you can do that. Do you know what force you wish to generate?

Regards,

Cory

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I need to generate a torque that would prevent the gland from seperating.

Right now i'm using the bearing stress on the gland shoulder as the max stress in the joint due to the fact the gland is weaker than the barrel. Then using a safety factor of 2 i calculate the max force and use that force as the max clamp load. i then use the T = KDP to calculate the torque.

but when i use this method i come up with a huge torque that really isn't required.

thats why i want to use the joint seperation method to determine what the MINIMUM torque can be. but even using this method comes up with a number the guys in the shop have never even came close to torquing these glands.

btw: Bore of barrel 3" and gland shoulder OD 3.5", rod dia. 1.5, @ 2750 operating pressure.

Hi gordohongo112

Normally joints are designed so that the male (bolt) fails before the internal thread, the calculations are based on the tensile area of the bolt and the shear stressses and area's of the external and internal threads.
Have look at this site and go to lecture 28.


In addition from your drawing it looks like the thread engages before your 'o'ring seal gets seated in the bore, as
screw threads are not the most reliable device for aligning
components I would ensure that the 'o' ring is seated on the bore before the screw thread starts to engage that way your not trying to screw the 'o' ring into the bore before seating.

regards

desertfox

The low torque (and hence low clamp force) calculation would be based on leak resistance. You would need to determine what force is needed to provide a seal against the fluid. Is your pressure in pounds per square inch? What is the fluid?

Regards,

Cory

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desertfox: thanks for the input and advice. Currently we do pre-assemble the gland with all the seals before we install them into the cylinder.

Right now i am calculating the shear stress on the threads, the tensile stress in the gland due to the clamping force and the bearing stress on the gland shoulder due to the barrel. out of these calculations it seems like the bearing stress is the highest then followed by the shear and tensile. this is why i'm using the bearing stress to determine my clamping force.

CoryPad: thanks for your reply.

the pressure is in 'psi' and the fluid we use is some hydraulic oil, i don't know the type, but can be easily determined.

i did some flaring calculations in the barrel threads, is this what you mean by leak resistance? because if the flaring stress in the threads is enough to deflect the barrel wall then it would tend to leak.

Based on what I see in your image and assuming an internal pressure loading, the torque on the gland is not significant as long as it is snug enough to prevent loosening during service. I do not think that you have to worry about gapping of the gland from the housing as the pressure load will go through the threads into the barrel (unless you are pulling on the head of the gland, then you have to look at gapping with this load).

A bigger concern of mine is the fatigue of the barrel at the start of the threads on the lefthand side of the image. Your image does not show any thread relief detail, and so the SCF at the end of the thread may be very high. Even if you add a thread relief, this area will still cycle with pressure loading. To aliveate this, the gland nut should bottom out on an internal shoulder in the barrel which is left of the thread relief. Then you need to ensure your torque is high enough to prevent gapping and loss of preload.

It is also improtant to check to make sure the gland will not pop-out due to the radial expansion of the barrel from pressure and/or the radial force generated from the thread angle. This done by calculating the radial load, and looking at the overall radial displacement. Once this displacement is calculated, recalculate thread shear based on the new average diameter. Note that the thread shear area has now decreased compared to before.

jetmaker

is the radial stress considered the flaring stress in the threads given by the following equation?

flaring stress = (P/A) + (6M/t^2 * alpha)

Also the piston will be hitting the gland putting some external forces on the threads.

Hi gordohongo12

First get a reasonable value for torque on the gland, forget
the bearing stress for the present and base the torque on the tensile stress and shear stress of the threads as per the site I posted.

What I meant about the seals was that they should be sealing in the cylinder before the screw thread on the gland engages in the cylinder and not just that seals were preassemblied.

regards
desertfox

desertfox: i have calculated both tensile and shear and the stresses are a little higher in the bearing on the gland shoulder, that is why i used that as a base.

the torque values i get when i use the resulting forces from the maximum stress value at the desired saftey factor is still at least 3 time more than what the guys in the shop use for a rule of thumb.

i need to figure out a method or calculation to determine weather an applied load is adaquate enough to consider a safe spec.

the groove that is before the thread is for a o-ring and a backup ring, so this design is already sealing the threads.

gordo,

The equation you sited about the stress, where did you get that from? It looks to be in a correct form, but not sure on the P/A (which area), and the second term I would obtaine from Roark (where M is the radial force acting at the thread mid-length).

jetmaker

jetmaker: i have a photocopied sheet outlining flaring stress calcs. its refrencing NFPA/T3.4.7-1975. i tried searching the standard on the national fluid power website but couldn't find it.

flaring stress = (P/A) + (6M/t^2 * alpha)

A - min. cross sect. area at undercut
P - tensile load on threads
M - Applied moment per unit length of circumfrence
t - min wall thickness at undercut
alpha - equation factor

with this i determine the flaring stress and caluclate the safety factor.

is there a equation for deflection where you determine the actual diameter like you said in your previous comment?

also where could i find a refrence for Radial stress?

Hi gordo,
The minimum axial load you need to generate on your gland during tightening is that given by the fluid pressure multiplied by the cylinder cross-sectional area acting within the cylinder bore. Example - if your bore is 3 inches and your pressure is 2750 lbs/sq in then your axial force is 19429 approx.
If you put this into your earlier equation T = KDP this should give you the minimum torque your gland requires. Obviously if there are any other forces acting on the gland they will also have to be taken into account.

With the axial force I calculated earlier, you should be able to work out the shear stress on the threaded portion and also calculate the tensile stress on the root diameter of the gland.
You can use the formulas in the link I posted earlier to determine whether the internal or external threads are overstressed.
The O-ring acts as a radial seal and therefore sealing is independent of gland tightening torque. If you wish to calculate the expansion of the cylinder bore due to the pressure then I would look in a text book for Lam'es equations which deals with pressure vessels.

The only time that leakage will occur between the gland and the cylinder bore is if the O-ring seal cannot compensate for the expansion of the cylinder bore. If this is already a tried and tested product then you have no need to worry about leakage.

Going back to my earlier comment about engaging the screw thread before the O-ring seal is seated in the bore, the point I am trying to make is, having assembled the O-ring onto the gland and inserting the gland into the cylinder bore, it appears from your diagram that the screw thread will engage before the seal is in the correct position ie, this means that to get the O-ring into the cylinder bore you have to screw the gland into place.

In doing this, this means that you have to screw the O-ring down into the tapered portion of the cylinder bore and finally into the reduced cylinder bore where it starts its job as a seal.
Because threads are not the best thing for aligning two components, it means that you could possibly damage the seal whilst screwing the gland into place. It would be better in my opinion if the O-ring groove was moved axially further away from the screw thread so when you offer the gland into the barrel the O-ring seal has already passed the tapered section and is sat in the final bore before you tighten the gland up.

Regards,
desertfox

desertfox: If i were to use the axial force i get a torque of 947 ft-lbs given the thread is 3.25 nom. which is still too high. (gland material yield is 45 ksi) the guys in the shop are used to numbers around 300-350 ft-lbs without having problems.

still can't think of a method to determine a number in this range.

and i see what your saying about the o-ring and i do agree the best way to seat the seal is your method but most of the time due to tight space restrictions that cannot be accomplished.

Why does the installation torque matter? The seal will do it's job even if the gland is only hand tight. The pressure in the cylinder will not act to gap the joint.

-b

bvanhiel: would the piston hitting against gland cause gapping or loosening in the thread?