Can exessive grease influence the torque for straigh thread connection 2

[johnchrc]

If too much grease is used on threaded connection (Stub Acme/O-Ring Connection), can this cause problems with connection?
Recently, I made-up a 6" SA Thread connection that had two O-Ring grooves in box. Seal land was located on nose of pin thread. Total connection length was quite long (~ 5-1/2 inches). Thread made up by hand initially but torque to rotate quickly increased (before o-ring seated) and eventually required ~ 6,000 ft-lbf torque to shoulder (way too high). No galling. Movement was smooth, just tight. Used high temp seal grease for threads and seals that have been using for years. In addition, I made up three more identical connections but applied much less grease, just enough to apply very light layer to threads and seal surface. Connections were much easier to make-up.
I have been warned in the past not to use too much grease and I try to comply and have never had problems (at least in recent memory). I could see how grease could build up and get trapped between threads as connection is made up. Also, pressure could increase on seals causing additional seal friction resisting rotation.
Is too much grease a valid concern?
What problems will result?
How do you know how much grease is adequate?

Thanks for your help.

 

[desertfox]

Hi johnchrc

Yes generally if a lubricant comes into contact with threads it reduces the friction factor, what this means is that the amount of torque required to produce a given pre-load is reduced.
So if the torque figure given for the joint being assembled is intended to be without lubricant and lubricant is used, then using the intial torque will more than likely over stress the threaded connection.
Have a look at this site:-

http://www.markserv.com.sg/torqueinfo3.htm#Thread

coefficient

desertfox

 

[unclesyd]

You might have taken up the slack provided in the threads because of the long connection, I agree with the above post in that the will be lower to achieve th same seating stress, but it shouldn't affect the torque required to turn the assembly.

 

[johnchrc]

Thanks or the feedback...

The connection is meant to have lubricant to prevent galling and reduce make-up torque. This is a straight thread and there is no requirement to torque connection beyond several hundred ft-lbf because there is not concern about the connection backing off.

The lubricant/grease is to allow the o-rings/back-ups to slip over seal surface without damage. The seals are static.

I just don't understand why the torque went up well beyond what it should have just to shoulder the connection. I can only assume excessive grease created the problem.

I'm making up 7" OD Pipe with couplings and adapters as required for the test apparatus. Without o-ring the thread should screw on by hand eve without lubricant.

 

[desertfox]

Hi john

Can you post a pic or sketch, we can see better then what your connection looks like.

desertfox

 

[MikeHalloran]

It sure sounds like you're reducing the volume of a small cavity, e.g. between the seal bore and the threads, and the usual exit path for air, along the threads, is sealed by grease when you use too much. You may even be attempting to compress a small volume of grease; it doesn't happen.



Mike Halloran
Pembroke Pines, FL, USA

 

[johnchrc]

attached is simple sketch. Let me know if need more info.

 

[johnchrc]

Agree with you Mike. It must e acting like an NPT or tapered connection and the grease is acting like a seal material. I would like to know more about this and how to manage application of grease on threads, seal lands, seal bores and slick joints.

Thanks for the help. Hopefully somebody will have a written xplanation or guide for this. Maybe a hole drilled in through thd OD into box will allow pressure to bleed.

Falling asleeppppppppppppppppppppppppppp. Later...

 

[hydtools]

You are trapping excess grease between the thread and seal. The grease cannot exit the cavity because of the seal on one side and the closing thread clearance on the other side of the cavity.

Ted

 

[redpicker]

Yep,it's called hydraulic lock. By slowing how fast you make it up, it can help, but the easiest solution is to use less lubricant. If there are o-rings in the back of the box, you could be damaging those seals.

rp

 

[Tmoose]

Hi Johnchrc -

I guess there is a "compression space" at the end of the female thread, but stub acme threads have real clearance between major and minor diameters, and back lash as well, so grease and pressure would seep out over time. Grease applied to the male part would tend to be wiped off when the parts were assembled.

Unless the actual joint had threaded together easily without grease, I think it is most likely some feature of the thread machining was wrong. Examples might be female thread was not machined "deep" enough, or the male thread was not long enough, so the thread runout section on one component was engaging. Or the male thread started too close to the end of the part, so it was advancing into the female thread runout.

You said "no galling". When the torque was too high Did you disassemble the joint to view the thread condition? Was a trial assembly done without o-rings or grease? That would have revealed if there was some basic mechanical interference problem with the threads.

Dan T

 

[Ron]

Agree with MikeH....hydraulic pressure issue.

 

[johnchrc]

It appears most folks think grease is creating a partial hydraulic lock. It seems the grease is acting like a cheap sealant in the leak path between the male and female thread. If grease is the issue than at least I can be less sloppy with application.

Reply to questions:
1) I have made connection up without seals by hand.
2) No galling
3) No method to check threads so had to use mating parts. Did't check over wire.
4) Threads are 2G so either lots or little clearance between box and pin
5) The Thread clearance should not be closing... it will remian constant... correct?

Thanks for replies. I will be rebuilding the fixture with these parts several times. I'll update with results.

 

[hydtools]

The thread flanks will close as the male thread is fed into the female thread. The tip-to-root clearance will remain at whatever the tolerances created.

Ted

 

[johnchrc]

hydtools,

Well said. Thanks for clearly stating my thought from (5).

With regards to applying grease to a connection:
I used to place ~ 2" dia. glob of grease at spot on OD @ beginning of pin thread and let grease spread onto thread contact surfaces as pin was rotated into box. Now I try and apply light coat over entire thread. Don't know why I changed. As for o-ring surfaces I always applied a light coat over entire lands.

Later...
CJ

 

[unclesyd]

As you mention galling what are the materials?

Unless less this is a make and break joint I would try to use a dry film lubricant. Static O-rings normally need a very light lubricant for makeup.

Have you pinned down the cause by making up the joint without grease and/or O-rings and what was the torque in this test?

 

[johnchrc]

Unclesyd,

No galling... low alloy steel 36 RC typically phosphated. If using CRA, then I would coat with dry-film lubricant at very least, copper coat, xylan, etc…and peen threads. Lubricant would not be as big a concern. Also, all joints were leveled prior to make-up to help with starting. Supports were use to prevent bending from side loading while making up. These were ~ 7" OD SA thrds.

These were quick turnaround parts and no coating other than spray on moly was used. Therefore grease was applied more liberally than usual. I have concluded the amount of grease and thread length must be the problem. I made up several other identical connections when assembling this fixture but applied the minimum amount (grease) to create a very thin film on the surfaces. I made these up by hand until o-rings seated then finished job with a 36” pipe wrench.

Trial Make-Up
Yes, I made up connection with/without o-rings and grease. Dry it makes up with ~ 300-400 Ft-lbf to bump shoulder. For the problem connection to require ~ 9,000 ft-lbf is, in my experience, unheard of. Now if the grease was partially trapped and could not escape the leak path between thread clearances, I could see how pressure from grease could be a problem. Just pressure acting on o-rings could drive the seal friction through the roof. It seems that a small relief holes in box up 1” or 2” from o-ring grooves could allow excess grease to weep out and may be beneficial in these long threads. I think the grease acted like a sealant, like API pipe dope, to block seal path between male and female threads. Still, if I can prevent this by minimizing or using a different lubricant, that is route to take. I can't think of the grease type but it is cream colored, and used for high temp downhole seals. It's not really anti-galling thread grease as far as I know. We have used it for years without issues I'm aware of.

In the end I guess I'm to fault for using to much grease. However, this is a first

Thoughts?

 

[hydtools]

Do you need the o-ring seal? Perhaps use the existing clearance and over-grease to seal.

Ted

 

[gadkinsj]

I'm going to weigh in and say this boils down to bad threads no matter how you look at it. If its hydro-locking, then there isn't clearance everywhere there should be in the threads. Or there is just some bad threads somewhere causing the resistance.

Can you chase the holes and file all the threads?

 

[MikeHalloran]

The viscosity of grease is such that it will flow only _very_ slowly through the thread tip and root clearance spaces. A radial weep hole or less grease should solve the problem.



Mike Halloran
Pembroke Pines, FL, USA