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Torque Calculations for Gland in Cylinders

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.   

RE: Torque Calculations for Gland in Cylinders

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

Regards,

Cory

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RE: Torque Calculations for Gland in Cylinders

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

Regards,

Cory

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RE: Torque Calculations for Gland in Cylinders

(OP)
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.

RE: Torque Calculations for Gland in Cylinders

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.

http://www.utm.edu/departments/engin/lemaster/machine_design.htm

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

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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RE: Torque Calculations for Gland in Cylinders

(OP)
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.

RE: Torque Calculations for Gland in Cylinders

(OP)
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.

RE: Torque Calculations for Gland in Cylinders

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

RE: Torque Calculations for Gland in Cylinders

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

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

 

RE: Torque Calculations for Gland in Cylinders

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

RE: Torque Calculations for Gland in Cylinders

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

RE: Torque Calculations for Gland in Cylinders

(OP)
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.

RE: Torque Calculations for Gland in Cylinders

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

RE: Torque Calculations for Gland in Cylinders

(OP)
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?

RE: Torque Calculations for Gland in Cylinders

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

RE: Torque Calculations for Gland in Cylinders

(OP)
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.

RE: Torque Calculations for Gland in Cylinders

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

RE: Torque Calculations for Gland in Cylinders

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

RE: Torque Calculations for Gland in Cylinders

hi gordohongo112

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

Hi Again

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

(OP)
desertfox: yes thats correct.  i want to apply an adaquate preload to prevent gapping or loosening over time.  the o-ring is lubricated during installation with white grease so it slides in a little smoother.  would you consider the threads to be lubricated since the threads will likely be in contact with the hydraulic fluid?

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

(OP)
ok i will scan some hand calcs for you.

RE: Torque Calculations for Gland in Cylinders

Quote:

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

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

(OP)
bvanhiel: no we haven't had any failures, i just need a method to calculate the torque, validating torques in case something does go wrong.

how would i go about creating a torque spec?

RE: Torque Calculations for Gland in Cylinders

Maybe you are going about this the wrong way.  It seems to me that the consensus is that the gland nut will not gap, but the preload is there more for prevention of loosening during service.  If this is the real reason for the preload, then I would suggest looking at the friction between the gland and the cylinder.

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

As an aside, I agree with bvanhiel that an anti-rotation feature should be incorporated into the design if loosening of the gland is a concern.

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

gordohongo112,

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

Hi gordohongo112

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

(OP)
currently we don't have any locking mechanisms, we have before machined a few nothces on the gland shoulder and just punched part of the barrel into it.

RE: Torque Calculations for Gland in Cylinders

I've got the same thing on the spindle nuts of my car.  I'd just stick with it.

RE: Torque Calculations for Gland in Cylinders

(OP)
would the threads be considered lubricated? there is not coating, but the threads would be in contact with some hydraulic fluid.  

RE: Torque Calculations for Gland in Cylinders

Interesting joint.

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

(OP)
ok thanks for your replies, i am going to use the power screw method since it makes sense.  i'll use it to calculate the point where the joint is self locking then the required load to overcome the friction forces.

RE: Torque Calculations for Gland in Cylinders

(OP)
is there a way to determine the Frictional forces in a thread?

RE: Torque Calculations for Gland in Cylinders

(OP)
is Force of Friction = friction coeff. * (F/cos*alpha)

alpha = thread half angle
F = load
 

RE: Torque Calculations for Gland in Cylinders

hi gordo

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

(OP)
desertfox: i want to determine a way to calculate a Torque that would be sufficient enough to create a frictional force in the threads and the collar to withstand the maximum axial pushing force created from pressure.

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

Provided that the thread form is self-locking, any torque will do, and the forces generated by pressure improve the situation.

RE: Torque Calculations for Gland in Cylinders

hi again

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

The clamping force of the threads is additive to the piston force. These forces and the pressure induced swelling reduce the capacity of the threaded joint. We have gone to acme stub threads for greater durability on this type of cylinder design. Please lubricate the threads. If water can get in the threads will rust and the capacity will be reduced even more and make repair difficult. Locking the head from turning is more important than super tight.

Ed Danzer
www.danzcoinc.com
www.dehyds.com

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