Torque Calculations for Gland in Cylinders
Torque Calculations for Gland in Cylinders
(OP)
i need to calculate the torque required when fastening a gland into a barrel. (Gland is externally threaded and Barrel tube is internally threaded)
Can i treat this as a joint, where i consider the unthreaded portion of the barrel contacting the gland shoulder a washer?
gland material is ductile iron 65-45-12.
barrel material 1026 steel.
thanks in advance.
Can i treat this as a joint, where i consider the unthreaded portion of the barrel contacting the gland shoulder a washer?
gland material is ductile iron 65-45-12.
barrel material 1026 steel.
thanks in advance.





RE: Torque Calculations for Gland in Cylinders
Regards,
Cory
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
RE: Torque Calculations for Gland in Cylinders
RE: Torque Calculations for Gland in Cylinders
Regards,
Cory
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
RE: Torque Calculations for Gland in Cylinders
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.
RE: Torque Calculations for Gland in Cylinders
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.
http
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
RE: Torque Calculations for Gland in Cylinders
Regards,
Cory
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
RE: Torque Calculations for Gland in Cylinders
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.
RE: Torque Calculations for Gland in Cylinders
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.
RE: Torque Calculations for Gland in Cylinders
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
RE: Torque Calculations for Gland in Cylinders
flaring stress = (P/A) + (6M/t^2 * alpha)
RE: Torque Calculations for Gland in Cylinders
RE: Torque Calculations for Gland in Cylinders
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
RE: Torque Calculations for Gland in Cylinders
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.
RE: Torque Calculations for Gland in Cylinders
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
RE: Torque Calculations for Gland in Cylinders
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?
RE: Torque Calculations for Gland in Cylinders
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
RE: Torque Calculations for Gland in Cylinders
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.
RE: Torque Calculations for Gland in Cylinders
-b
RE: Torque Calculations for Gland in Cylinders
RE: Torque Calculations for Gland in Cylinders
Well the torque they are using in the shop may still result in the axial force I quoted, it depends on the K factor, if the k factor was approx 0.06 then the 300lbf-Ft would give you 19427lbf axial load.
Is the thread lubricated prior to assembly or is it done dry?
Also the 'o' ring would generate friction when the unit is pressurised, but would also act against you during tightening.
I assume your concerned that if you don't get the preload on the gland correct then the pressure overtime may loosen the gland.
Why don't you post your calculations and lets have a look at what your doing.
Remember also that because the shop floor have been using 300/350lbf-Ft doesn't always mean its right.
regards
desertfox
RE: Torque Calculations for Gland in Cylinders
yes the piston could loosen the the gland, have you any info
on how hard the piston impacts on the gland?
desertfox
RE: Torque Calculations for Gland in Cylinders
the k factor i'm using is .18 from shigley's for lubricated threads. i guess technically they aren't during instalation. there's no plating on the threads, its machined material against machined material.
RE: Torque Calculations for Gland in Cylinders
RE: Torque Calculations for Gland in Cylinders
It will not cause gapping, as the joint that you would torque against is on the opposite side of the load and threads. It will potentially add vibration which could lead to the gland working itself off. The torque you need to resist this vibration is not (directly) related to the hydraulic pressure or the strength of the materials involved.
Are you solving a real problem (ie failures have been observed)? If so then you may want to put a positive locking feature into the nut (roll pin, wire nut, etc). If not I think you are trying to solve the problem of creating a torque spec the wrong way.
-b
RE: Torque Calculations for Gland in Cylinders
how would i go about creating a torque spec?
RE: Torque Calculations for Gland in Cylinders
Choose a friction force that will overcome the rotational force generated by the piston when it hits the gland nut. I would look at the theory of power screws to obtain this number. Remember that the impact load on the gland nut will be a dynamic one and not static.
jetmaker
RE: Torque Calculations for Gland in Cylinders
RE: Torque Calculations for Gland in Cylinders
RE: Torque Calculations for Gland in Cylinders
If you are already using a fine thread profile, this will help against vibration (or is that a coarse thread... a little help here guys...) and another option is to use a tabbed washer that fits a spline in the gland and engages a feature on the cylinder.
jetmaker
RE: Torque Calculations for Gland in Cylinders
What you are calculating is the maximum torque you can apply without destroying the assembly. This is normally what you do with a fastener, as normally gapping = fatigue failure. Your case is different, as the applied forces are not transmitted through the preloaded area. What you need to calculate is the minimum torque required to keep the gland from backing out with vibration.
I don't think you're going to find a nice neat way of doing it. The best way would to test several assemblies with different preloads and observe the results. That way would also be rather time consuming and (probably) expensive. If you have many units in the field and no failures, then I would just document the current assembly technique.
If there are serious repurcussions of the seal failing then I would create a positive locking feature that would keep the gland from turning once assembled. This might be crush feature like those that retain automotive hub nuts, a flanged washer as jetmaker suggested, a cotter pin, set screw, etc.
-b
RE: Torque Calculations for Gland in Cylinders
Thanks for the calcs I'll have a look over them and post later.
Although I would say if you provide a means of locking the thread of the gland and cylinder you won't need to worry about loosening.
regards
desertfox
RE: Torque Calculations for Gland in Cylinders
RE: Torque Calculations for Gland in Cylinders
RE: Torque Calculations for Gland in Cylinders
RE: Torque Calculations for Gland in Cylinders
However, classical threaded fastener/joint theory does not apply to it.
Here is a little though experiment to explain why:
Instead of having a length of thread, let's simplify the joint to say that there is a single, perfect (no clearance) thread located at the center of the existing real thread.
When the plug is torqued, the portion of the plug between the perfect thread and the "head" is placed into tension.
The portion of the plug between the perfect thread and the pressure inside the cylinder has zero stress.
Apply pressure inside the cylinder.
The portion of the plug between the perfect thread and the "head" has exactly the same tension as before. Pressure in the cylinder has no ability at all to change this tension.
The portion between the cylinder and the perfect thread is placed into compression. Pressure in the cylinder is the only thing that can apply load to this part of the plug.
The force within the plug is not constant.
Classical joint theory relies on the load in the fastener being constant.
RE: Torque Calculations for Gland in Cylinders
RE: Torque Calculations for Gland in Cylinders
RE: Torque Calculations for Gland in Cylinders
alpha = thread half angle
F = load
RE: Torque Calculations for Gland in Cylinders
if you go to the link on my earlier post, go to lecture 27 which deals with power screws
desertfox
RE: Torque Calculations for Gland in Cylinders
if i were to use this maximum pushing force as the load in the raising power screw equation would this be correct?
RE: Torque Calculations for Gland in Cylinders
RE: Torque Calculations for Gland in Cylinders
for a normal vee thread the effective coefficient of friction is given by Ue = U x (sec B)
where B = half the angle of thread
where U = 0.2 which is the average coefficient of friction for metal on metal.
desertfox
RE: Torque Calculations for Gland in Cylinders
Ed Danzer
www.danzcoinc.com
www.dehyds.com