Torque Calculations for Gland in Cylinders 1

[gordohongo112]

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.

 

[desertfox]

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

 

[desertfox]

Hi Again

yes the piston could loosen the the gland, have you any info
on how hard the piston impacts on the gland?

desertfox

 

[gordohongo112]

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.

 

[gordohongo112]

ok i will scan some hand calcs for you.

 

[bvanhiel]

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

 

[gordohongo112]

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?

 

[jetmaker]

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

 

[gordohongo112]

here are what my spread sheets look like

 

[gordohongo112]

Sheet 2

 

[jetmaker]

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

 

[bvanhiel]

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

 

[desertfox]

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

 

[gordohongo112]

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.

 

[bvanhiel]

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

 

[gordohongo112]

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

 

[MintJulep]

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.

 

[gordohongo112]

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.

 

[gordohongo112]

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

 

[gordohongo112]

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

alpha = thread half angle
F = load

 

[desertfox]

hi gordo

if you go to the link on my earlier post, go to lecture 27 which deals with power screws

desertfox