BSPP STRAIGHT THREAD TORQUE REQUIRED TO COMPRESS RUBBER WASHER SEAL 1

[imagineers]

So basically I am wondering what torque to use on a BSPP fitting similar to this link below. I want to compress the washer inside but my torque value will determine how much the rubber washer compresses. I understand you need durometer value to determine this also, but assuming a 60-70 durometer rubber what would i need to torque a 1" fitting to seal this type of fitting. I cannot find any standard for this anywhere. Thanks

https://storzconnection.com.au/store/Storz-Female-Adapter-38mm-1inch-Aluminium

 

[EdStainless]

The similar ones that I have used were designed so that you torqued against the metal, the design was based on the correct compression.

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P.E. Metallurgy, Plymouth Tube

 

[imagineers]

I am not sure what you mean. So there are also whats called a banjo seal with is a bonded washer used for BSPP fittings which have their own torque value, however this is a rubber flat washer

 

[Compositepro]

What Ed describes is usually done with an o-ring in a groove. You tighten until you get metal to metal contact, and then the groove depth controls the amount of compression of the o-ring.

With a flat gasket, as you have, you only need to compress the gasket about 10%. The torque is not very critical. It will rise quickly as the rubber compresses because of the large contact area. That seal design works fine for garden hoses.

 

[imagineers]

I understand but is there some kind of conversion for torque on a straight thread to compress the seal a certain percentage so you do not overcompress or undercompress.

 

[Tmoose]

Page S5 here might be a starting point -

https://www.parker.com/literature/Tube Fittings Division/Assembly_Installation.pdf

If only to see if the most similar torque destroys the rubber washer.
Discussions about lubricated and un-lubricated torque will be arriving any moment now.

 

[drawoh]

imagineers,

Does the BSPP fitting squwash the O-ring down flat, or is there metal to metal contact? As the designer, I like metal to metal contact because it keeps torques simple.

--
JHG

 

[EdStainless]

For ones without grooves where you are actually crushing the rubber seal we used the amount of rotation.
Hand tighten to initial contact and then add 1/4 turn or whatever is appropriate for you application.

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P.E. Metallurgy, Plymouth Tube

 

[imagineers]

Here is a bit more info. You can see what I am talking aout. Essentially I would be torquing the fitting to compress the washer to seal the outlet of a water pump.

 

[imagineers]

So yes I saw the 150nm force for a Form A sealing washer but I believe thats for a bonded seal washer

 

[SwinnyGG]

Loading for a seal of this type should be based on a compression percentage- this is very easy

-measure your seal thickness
-determine how much compression you need (10% as stated above is a reasonable starting point)
-calculate how much the fitting needs to move axially to compress the seal 10%
-using the thread pitch, calculate how many turns (or, more likely, what fraction of a turn) will result in 10% seal compression

Then instruct your people to install that fitting finger tight, and then add 1/4 turn or whatever you calculated


Also note that the seal design you've shown is likely to result in seal extrusion- you don't want a flat gasket to hang over an edge if you can help it. If this is any type of critical sealing application, and I were the designer, I would look for a fitting that uses an o-ring in a groove in the sealing face of the female fitting.

 

[MintJulep]

The seal design that you show is not consistent with the design intent of BSPP.

 

[SnTMan]

Forget torque it will likely never be reliable, the "additional turn" method per EdStainless and / or jgKRI is your best bet. You might also consider some type of thread locker to retain your seal.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[imagineers]

definitely will use thread sealant. The fact the manufacturer even has this thread on an outlet without the typical design to seal it makes no sense. However I have seen fittings with this exact design for BSP triclamp fittings so it is done just not a standard from what I can find.

 

[racookpe1978]

It appears, despite the excellent recommendations above, that you have fixed in your mind the "classroom theory" that "Rotational Torque is proportional to Linear Movement of a straight threaded item rotating on a straight-threaded fixture."
Thus, you persist in asking "What torque will create enough movement of the straight-threaded fitting on the straight-threaded fixture to force my rubber gasket onto the flat surface?"

Linear movement is NOT proportional to torque in the real world of dirt, mediocre threads, corrosion, and bad mechanics in a hurry assembling threaded items with bad (or missing, or mis-calibrated, or broken, or stupidly-used) torque wrenches and adapters.

As above, (1) determine how much compression distance of the rubber gasket you need, and then determine how much over-compression will cause failure (extrusion or tearing) of the rubber gasket.
(2) Chose a prudent value between the two.
(3) Determine the number of nut-flats that will create that compression distance. NOT THE TORQUE!
(4) Issue assembly and repair instruction plainly:
"Assemble with a new gasket finger tight.
Turn, with a wrench, 2 more flats to compress the gasket."

 

[racookpe1978]

Note that Swaglock specifically recommends/specifies NOT USING thread sealant on their straight-threaded connections!

 

[braddles90]

What's the application here - you mention water pump, is this a hose fitting for an engine or something?

Considering the type of hosetail fitting you're showing here, in real life I would doubt that anyone actually working on this is reaching for a torque wrench or considering the number of turns - eyeballing a quarter turn on that fitting is about the best you would get, certainly not accurate.

What's the pressure/temperature of the working fluid? If this is the outlet the pump manufacturer has selected, maybe its not a critical connection, and as long as you tighten it up and tighten it a bit more if it leaks, that's ok...

 

[3DDave]

One thing that is generally true - rubber is incompressible. Any force required goes to elastic deformation, which is highly constraint dependent. Rubber also has very high thermal expansion coefficient so an amount of deformation that is suitable at one temperature won't work at another.