Threaded Piston and Nut Joint
Threaded Piston and Nut Joint
(OP)
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?
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?





RE: Threaded Piston and Nut Joint
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
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RE: Threaded Piston and Nut Joint
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?
RE: Threaded Piston and Nut Joint
Superbolt,Inc. Multi-Jackbolt Nut and BOlt style Tensioners
www.superbolt.com
StressCut
Ilya B.
RE: Threaded Piston and Nut Joint
I would be less worried about gapping and more worried about the stress range in the threads and fatigue.
RE: Threaded Piston and Nut Joint
RE: Threaded Piston and Nut Joint
RE: Threaded Piston and Nut Joint
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 lbf-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 lbf-ft to generate 80,000 lbf.
This should be straightforward.
Regards,
Cory
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RE: Threaded Piston and Nut Joint
http://www.superbolt.com/applications.html
We use these in the same type application as you state.
RE: Threaded Piston and Nut Joint
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.
RE: Threaded Piston and Nut Joint
RE: Threaded Piston and Nut Joint
I don't think your calculations are correct: 900 lbf-ft of torque on a 1-3/8 in diameter thread doesn't generate > 79,000 lbf.
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
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RE: Threaded Piston and Nut Joint
ok, thanks for the link i'll look into it.
RE: Threaded Piston and Nut Joint
Ed Danzer
www.danzcoinc.com
www.dehyds.com
RE: Threaded Piston and Nut Joint
RE: Threaded Piston and Nut Joint
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
www.danzcoinc.com
www.dehyds.com
RE: Threaded Piston and Nut Joint
do you use the threaded piston and nut method as well? is there a way you calculate the required torques?
RE: Threaded Piston and Nut Joint
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
RE: Threaded Piston and Nut Joint
Ed Danzer
www.danzcoinc.com
www.dehyds.com