Threaded Piston and Nut Joint

[gordohongo112]

I'm working on figuring out a proper methond to calculate the torque for a threaded piston with a nut. this application has a high operating pressure.

I calculated the amount of torque that would be required for the nut without a threaded piston and the amount required for a joint that won't seperate was too high. it would cause the threaded rod to fail. this is why the piston is threaded as well to create an additional torque to reduce the amount of torque required on the nut.

my question is how would i calculate the proper torque?

 

[CoryPad]

Are you using the nut as a jam nut to prevent loosening?

If the threaded rod cannot generate enough pretension to avoid separation, why do you think threading the piston helps? The pressure on the piston still creates rod tension whether the piston is threaded or not, and the rod still reacts the entirety of the applied force (pressure times area). You should not thread the piston, rather, you should have a clearance hole and use the nut to develop pretension on the rod. Thus, you have a long grip length and a joint that will resist fatigue and loosening better.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[gordohongo112]

the nut is a stover nut.

here are the specs:
Rod material yield = 100ksi
lubricated (k = .18)
operating pressure = 4500 psi
thread on rod = 1 3/8-12UN
Extend Force Created = 79,742 lbs
Retract force Created = 65,605 lbs

Torque required for the joint not to seperate is about 900 ft-lbs. this amount of torque would yield a clamping force of around 43,636 lbs, and with the external force applied would cause the rod to see 66,674 lbs. which is too high, that is why i would like to thread the piston and torque it on to reduce the amount of torque required in the nut.

if i torqued the piston on at 450 ft-lbs (assuming the unthreaded portion is a washer) this would create a clamp force in the piston and reduce the required torque on the nut by half.

then i was thinking torquing the nut on at 450 ft-lbs and compressing the piston, in increasing the clamping force in the piston. also by doing this it would reduce the tensile stress in the rod.

does this seem like the right method?

 

[StressCut]

For heavy loaded connection of piston to road there is a good solution what proposed by
Superbolt,Inc. Multi-Jackbolt Nut and BOlt style Tensioners

http://www.superbolt.com

StressCut
Ilya B.

 

[MintJulep]

Threading the piston will focus ALL of the load onto the first threat, and guarantee that the rod will fail in fatigue there.

I would be less worried about gapping and more worried about the stress range in the threads and fatigue.

 

[gordohongo112]

i am taking into account the shear stress in the threads and tesnile strain in the rod. we have had this joint come loose in the past and we have upped the torque.

 

[gordohongo112]

StressCut: thats a nice concept i could use in my newer designs but i'm just trying to calculate the right torque for this current design.

 

[CoryPad]

gordohongo,

Your assumptions are completely wrong. You do not increase the piston rod capacity by tightening the piston and nut as different parts.

I don't know what equations you are using. But I will share with you a standard analysis for this type of joint. There is an exact example of this type of joint calculation in VDI 2230 Systematic calculation of high-duty bolted joints, available from Beuth:

http://www.beuth.de

It is standard to use a clearance hole on the piston - you need a stiff joint and an elastic bolt to produce high fatigue resistance.

You need to generate a clamping force that is greater than the applied force so that you have low alternating stress in the bolted joint (which gives high fatigue life) and so that you have high loosening resistance.

If you targeted a clamping force of 80,000 lb_f-ft, you are generating a stress in the piston rod of ~ 60,000 psi. This is approximately 60 % of yield, which is low by fastener standards.

Using the very basic torque equation T = k D F, you need to apply ~ 1375 lb_f-ft to generate 80,000 lb_f.

This should be straightforward.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[unclesyd]

When you work the tension in the shaft required to hold your piston you could give Superbolt a call and they can provide a tensioner that will eliminate any torquing problems. You can also give them the physical parameters of your shaft material and what percentage of the yield you want to use and they can go from there.

http://www.superbolt.com/applications.html

We use these in the same type application as you state.

 

[gordohongo112]

CoryPad: My assumption was to torque the piston and nut seperatly to reduce the amount of stress in the threaded rod. Reducing both in tensional stress and shear stress in the threads, while increasing the clamping load in the piston.

i am using shigleys bolted joints equations.

in order for a joint sepearation factor > 1 i require 900 ft-lbs of torque. this amount of torque does put ~60 ksi of tensile stress in the rod. this yields a safety factor under 2. i have also considered fatigue in the fluctuation in loads from the extend and retract and have come up with a 2.29 factor of safety.

 

[gordohongo112]

corypad: the website

http://www.beuth.de

is in german and requires me to purchase to view?!

 

[CoryPad]

Your assumption of reducing stress is wrong.

I don't think your calculations are correct: 900 lb_f-ft of torque on a 1-3/8 in diameter thread doesn't generate > 79,000 lb_f.

Beuth is a German publisher, but their site can be viewed in English by clicking the "English version" button in the upper right corner.

The guideline VDI 2230 is not a free document. It is the most comprehensive bolted joint calculation system (171 pages of information), and it is worth the nominal fee required for purchase.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[gordohongo112]

corypad: the >79,000lbs is the extend force created from the cylinder. 4.75" Barrel Bore and 2.00" Dia. Rod.

ok, thanks for the link i'll look into it.

 

[EdDanzer]

Hydraulic cylinder piston to rod connection can be very difficult to design. The shoulder between the piston and the rod is a place of common failure. The threads provides clamping force, the pressure induced force (79,742 lbs) is additive to the thread clamping force (43,636 lbs). These forces (123,378 lbs) must not exceed the yield strength of the area of contact for the piston or rod. If these combined forces are greater than the yield strength there will be cold forming and the piston will come loose causing cylinder failure. If you use 100,000+ yield strength material for the piston you will need 1.23+ sq. in. of contact area between the piston and rod to be safe.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[gordohongo112]

EdDanzer: I have taken into account for the bearing stresses between the rod and piston. the contacting area keeping in mind the chamfer on the rod and piston is 2.480 in^2.

 

[EdDanzer]

Gordohongo,

If the rod is 2” and the threads 1 3/8” the picture shows an area larger than 1 3/8” diameter step, a radius, and a chamfer. With a rule scaling the picture the area where the piston and rod contact the largest diameter of the rod is smaller than 2” diameter and over 1 ½” diameter so you will have less than 1.2 sq in. contact area where the piston and rod interface.

This is an area of failure on many of the hydraulic cylinders we repair.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com

 

[gordohongo112]

EdDanzer: Yes i forgot about the shoulder in the rod and didn't take into account for the reduced area on the chamfer on the piston.

do you use the threaded piston and nut method as well? is there a way you calculate the required torques?

 

[desertfox]

Hi gordohongo112

I agree with Corypad you will not reduce stress or achieve what you wish to do by tightening piston and nut on the rod seperately, draw some free-body diagrams of your joint and you will see that your assumptions are incorrect.
Another idea might be (unless the piston needs to come off the rod for maintenance)to use a interference fit for the rod and piston and you can still use the nut for belt and braces.

regards

desetfox

 

[EdDanzer]

We just use a stover nut to hold the piston on. There was some experimentation and warranty costs spent determining the best torque for the nut. Unless a cylinder has port reliefs and is not used for stops using a threaded on piston is a poor design choice. There are no cost savings either. The Danzco Inc. 6K attachment cylinders see 15,000 to 20,000 psi pressure spikes in some applications and will exceed 2000 hours life if the barrel does not fail.

Ed Danzer

http://www.danzcoinc.com

http://www.dehyds.com