Pipe Tape vs Pipe Dope in High Pressure Hydraulics

[s219]

Agree that tape shouldn't be used on hydraulics, but I'd do it in a pinch if I had no paste, being very careful the way I wrapped the threads. I only use paste on hydraulics, but I was surprised to see the loader on my tractor (installed by dealer) had tape on some of the fittings.

As far as the leak, if it keeps happening then I'd suspect the threads (male or female or both) are buggered up. I've even seen poor quality fittings (no doubt Chinese) leak immediately when brand new, and further inspection showed damage to the threads or something out of round.

Theoretically, the tape and paste is only there to lubricate the threads and help you snug them down properly. You should not be relying on the tape or paste to actually make the connection leak proof. It really can't (think about the pressure).

 

[Tractor Seabee]

In my pipefitter days we had a project where after a large hydraulic systems was operating for a few days everything stopped working. They owner had paid off the original contractor due to shoddy work and his crew finished it. My company was called in to find and fix the problems. Some piping was 2" and down to 1/4" tubing. Most valves were pilot operated and had small orifices. We removed a couple non-working valves and found the pilot valves plugged up with Teflon tape residue. Needless to say we had to dismantle and clean the whole system. Luckily there were no long lengths of piping. The suction filter for the pump kept it out of the pump luckily. The repairs cost twice as much as the original system did. Big lesson for our company to never use Teflon tape again.

Years later when I was designing and specifying system I prohibited the use of tape and required Teflon bearing paste dope applied one thread from the end.

Ron

 

[woodlandfarms]

So my machine came with Teflon from the factory. The leaking is on pipe fittings. I am going to tear down the leaking areas and go with the dope. Just need to find stuff that isn't oil adverse.

 

[mcfarmall]

No tape on hydraulic systems. If you wrap it closer than 2 threads or so from the end it can spiral off and contaminate orifices, valves, etc.. Use Loctite PST pipe dope. Also tape and pipe dope DOES seal the threads by "plugging up" the clearance between the root of the female thread and the crest of the male thread. This clearance is inherent in ALL threaded joints, bolts and straight threaded fittings. Straight thread fittings typically rely on a sealing washer made of a softer metal e.g.copper or an o-ring such as in a standard SAE o-ring boss fitting.

There is also a myth that says you don't need any pipe tape or dope on brass fittings...total B.S. The same issues hold true about threads regardless of the material of construction.

 

[Gary Fowler]

No tape on hydraulic systems. If you wrap it closer than 2 threads or so from the end it can spiral off and contaminate orifices, valves, etc.. Use Loctite PST pipe dope. Also tape and pipe dope DOES seal the threads by "plugging up" the clearance between the root of the female thread and the crest of the male thread. This clearance is inherent in ALL threaded joints, bolts and straight threaded fittings. Straight thread fittings typically rely on a sealing washer made of a softer metal e.g.copper or an o-ring such as in a standard SAE o-ring boss fitting.

There is also a myth that says you don't need any pipe tape or dope on brass fittings...total B.S. The same issues hold true about threads regardless of the material of construction.

Having worked industrial construction for 45 years, I have to somewhat disagree with this. A properly made thread does NOT depend on the dope to seal. The thread dope (regardless if liquid or tape) is made to act as a lubricant only so the threads can be tightened properly without damage to the threads from metal to metal galling. It may help a bit on damaged threads so they don't gall up but that is not the primary purpose of the thread dope and any threading procedure will confirm this(google it).
Also while working in the QA/QC role for 25 years, I have seen more threads leaking from OVER tightening than under tightening. Overtightening strips the threads just like on a bolt and nut and will invariably leak under pressure.

I have even seen pipe fitters put Teflon tape on the brass threads of oxy-acetylene gauges when they have nothing to do with sealing the pressure, it is the ball seat from gauge to the tank that seals. The threads just apply the pressure to the joint. Sometimes the gauge ball joint gets dinged up and can be lightly sanded to remove the ding to make it seal again but taping the threads wont work.

 

[Gary Fowler]

One other thing about Teflon tape, when properly applied, i.e. skip the first thread then wrap in the direction of the threads applying not more than two layers, the tape wont contaminate anything on first application. It is when the threads are disengaged that the small Teflon particles may get into the system unless careful cleaning is done both on male and female threads. SINCE this is rarely done in practice, many if not most engineering and maintenance practices forbid use of Teflon on systems that may have fine orifices like on pilot operated control valves.

I use it (tape) all the time around the house, but I take the time to clean everything when disassembling and properly apply it. I also have the Teflon paste but it is very messy and once in contact with hands or clothing, it is pretty much permanent.

 

[the old grind]

What Gary said in post #15. :thumbsup:

(btw, Permatex #2 non-hardening.)

IME 1/2" NPT fittings, if Parker need a 2' Johnson bar to tighten leak-free, if TSC (China) need a 4' bar and some will weep anyway. :mur:

 

[EddieWalker]

What do all own that you use anything on your hydraulic fittings?

I have four tractors, plus a few other things with close to a hundred hoses combined on all of them, and every single one of them DO NOT use anything other then an o ring at the end to seal them. A lot of them don't even have that.

Are you talking about tractors? or something else? That might be why I'm so confused.

 

[mcfarmall]

Reference Post #15. Regardless of our combined years of experience in industrial construction, maintenance, engineering, and machining, one fact holds true. In any threaded joint there is clearance. This is due to the necessity of manufacturing tolerances in everything that makes up our world. Granted, I have seen some crappy pipe threads, particularly male ones, that are the result of chipped pipe dies or lack of cutting oil and have resulted in chunks of missing threads.

In the Machinery's Handbook there is a great treasure trove of information regarding threads, clearances, tolerances, etc.. There is a leak path that is inherent in any threaded joint due to manufacturing tolerances. This leak path must be blocked by one of many methods, tapers (as in 45 degree and 37 degree flare fittings), O-rings, flat seal rings, soft metal washers, Pipe sealant or tape, and the list goes on.

If you assemble a high quality Parker SAE o-ring port fitting into a Parker manifold that is machined to the SAE o-ring port standards and leave off the o-ring, it will surely leak. Not due necessarily to "bad threads" but due in fact to the spiral leak path at the junction of the male and female threads with their necessary clearance for machining and assembly. This spiral leak path can be found where there is clearance between the crest of the male thread and the root of the female thread. If this clearance did not exist the threaded joint would be size for size and these things would be true: It would be very difficult to assemble. It would be very expensive to machine. It would not likely be readily interchangeable with other fittings of like nominal size.

Pipe threads, due to their lack of other leak stopping methods (taper, flare, ball, o-ring, flat seal, etc.), must rely on another means to seal the spiral leak path created during the machining of the components. Use whatever you want to seal up that leak path but I wouldn't use Teflon tape on any pipe-threaded hydraulic system components. Use something that can disperse into the system fluid without harming, blocking or otherwise impairing the operation thereof any components.

Let's get back to the really important topic of "Hydrostatic vs. gear transmissions."

 

[Xfaxman]

What do all own that you use anything on your hydraulic fittings?

I have four tractors, plus a few other things with close to a hundred hoses combined on all of them, and every single one of them DO NOT use anything other then an o ring at the end to seal them. A lot of them don't even have that.

Are you talking about tractors? or something else? That might be why I'm so confused.

Most tractors use hydraulic cylinders with National Pipe Thread (NPT) fittings, that require a sealant on the threads.