What is a Pack Nut?

[Troutsqueezer]

I see this pack nut gets used on a number of engines, particularly Elmer's, and generally on the steam chest. What does it do? From appearances, it looks like I could get by without it.

I ask because it calls for a 1/4-40 thread which isn't in my current tooling inventory and I'm too lazy right now (and cheap) to order a tap and die of that size. :noidea:

 

[Jasonb]

It compresses the "packing" gland around a rod such as a valve or piston rod. You will loose air/steam without one.

You put a few winds of packing material like graphite yarn or PTFE around the rod and the nut compresses it at the bottom of the threaded hole.

On larger models thay are usually retained by studs and adjusted with nuts & locknuts, like the ones on this cylinder

Jason

 

[kd7fhg]

Hello!
A pack nut or packing nut holds the stem of a valve in place, it also acts as a retainer for the stem or shaft packing allowing it to be adjusted (or compressed) around the stem or shaft preventing leaking.

Rex

 

[Metal Butcher]

Hi Trout. On a small engine like the one your building a small loss of air due to the elimination of a 'pack nut' is insignificant.
The stuffing box spigot should be drilled or reamed for a 'close running fit' around the 'valve rod' to keep the leakage to a minimum.
I have been eliminating the pack nuts on my recent builds. I feel they are pointless on engines that run on a pound or two of air. They can add a good amount of unwanted friction on a very small engine.

On page #3, pictures #55 and # 56 you can see how I eliminated the pack nut. If there is leakage, it can't be seen or heard while the engine is running.

http://www.homemodelenginemachinist.com/index.php?topic=6552.30

"To each his own."

-MB

 

[Troutsqueezer]

Thanks guys. I didn't think about pressure escaping from that opening. That makes perfect sense. I'm glad I asked.

MB, I noticed you didn't use one and that made me think maybe it was there more for authenticity than anything else. Without the packing material itself showing in the plans any place, I couldn't see any usefulness.

I'll make the clearance as close as possible without impeding the motion of the rod then.

-Trout

 

[Metal Butcher]

Trout, If you make the spigot the same size as the plan, you could always add the 'pack nut' later when you get a set of 40 TPI taps and dies. You'll probably need them anyway, sooner or later.

-MB

 

[mklotz]

40 tpi is a standard micrometer thread - 0.025" advance per revolution. Having the threading tackle for this thread is handy if you ever intend to make anything that incorporates a screw used to make a measurement. Your mind will already be programmed to read it. Infinitely more useful than 1/2-13 in such an application.

 

[tel]

Agreed Marv, a very handy series to have on hand.

As far as 'pack nut' goes, the term baffled me for a while as well, substantially because it is not a nut. Then the penny dropped that it was the 'GLAND' - the bit that screws into the STUFFING BOX. Having sorted that out personally I would not omit it, you can do without just ticking over on air, but if you ever come to use steam you will need it there.

 

[Troutsqueezer]

I may well run this thing on steam down the road. I think I will do as MB suggests and leave the spigot sized for the pack nut just in case.

From the sound of it, I would do well to go ahead and order up some taps and die in this size range. I found that Enco sells the individual pieces, around $13 each for the die and tap. If I were to buy the "series" or set, where would be a good place to look? I tried LMS and Grizzly. Enco sells them individually but that doesn't seem cost effective.

Oh, what is "PTFE"? Any chance of having some of that in the kitchen drawer?

-Trout

 

[tel]

Oh, what is "PTFE"? Any chance of having some of that in the kitchen drawer? Smiley

Every chance - ordinary plumber's teflon tap - twist a bit of that into a 'string' and use it as the packing - works like a charm.

 

[putputman]

Trout, I see no reason why a 1/4-20 or 1/4-28 wouldn't do the same job. You probably have taps & dies for them. It is the packing that is doing the job. The nut is only holding it in place.

 

[Metal Butcher]

Trout, I see no reason why a 1/4-20 or 1/4-28 wouldn't do the same job. You probably have taps & dies for them. It is the packing that is doing the job. The nut is only holding it in place.

Hi. Your suggestion may be a good one but I respectfully disagree.

A few years back I tried the very same idea. It didn't work very well. It seems that you need the fine adjustment that a 40TPI offers to make accurate compression of the packing material. With the coarse thread the packing was either too tight or too loose. I found the adjustments to be some what frustrating. Actually, very frustrating. This must be the reason that the fine threads are specified in most of the plans by Elmer, and probably other Model Engineers too.

Of course this is just my opinion. Feel free to correct me if I'm wrong.

 

[Troutsqueezer]

It occurred to me to use a 1/4-20 but I figured the number of threads in that short barrel/spigot would be too few with the coarser thread and that was the reason for the finer thread. Not knowing what the pack nut was for, it never occurred to me that it could also be an adjustment thing.

Thanks guys.

-T

 

[compressor man]

Oh, what is "PTFE"? Any chance of having some of that in the kitchen drawer?

-Trout

PTFE is what teflon really is. Teflon is merely a trade name

Chris

 

[Deanofid]

You should do fine with 1/4-28, Trout. I have at least one engine that uses similar sized glands with that thread, and it runs fine. I use Teflon tape for packing.

If you tighten either the steam chest or cylinder glands too tight on small engines, it can slow them down, or stop them. The thing is, it's not supposed to be tight. Just snug enough so it will upset the packing a little to form a seal.
If the engine stops when you tighten the gland, it may not be from the packing being too tight at all, but from not having a concentric bore in the gland. Then, when the gland is loose, it will move a bit as the shaft goes in and out, but when it is tightened, even thought he packing is not tight, the off center bore will rub on the shaft and stop the engine.

And, the gland/pack nut looks nice, too.

Dean