Anti-seize compound for 1/4-28 threads

[Andrew Seltzman]

When dismantling the cube on my fusor yesterday, I had one of my 1/4-28 bolts (McMaster 92196A325) seize up when it was being removed and cracked off a thread or two from the bolt into the threads on the cube. These cracked threads to the bolt occurred at the tip of the bolt and the very top threads on the cube. The bolt was under very little force, being used to dog down a viton o-ring

The cracked off threads were easily removed, however it left the top 3 threads on one of the cube holes slightly deformed. This will be an easy repair using a 1/4-28 tap on the threads, however it could have been much worse if it seized up deaper in.

I have noticed that on the mcmaster bolts, the threads are rougher compared to the threads on silver plated 12 point bolts designed for vacuum use. Back at georgia tech, we used an anti seize compound on out vacuum system bolts to prevent them from binding up during bake outs, which worked very well, however the coumpound gets everywhere and is hard to clean up.

I would like to use the silver plated bolts, however they only come in a 12 point variety, not in socket head cap screw variety, which is easier to work with using Allen keys in tight spaces. I know the threading is a better quality(smoother), but how well does the silver plating work as an anti-sieze?

Since I will be using some sort of thread lubricant on the cube, what would you recommend? I may just use the nickle anti-seize from mcmaster, however I would like to use something more vacuum compatible and easier to clean. Would boron nitride powder or graphite work well as an anti-seize?

Andrew Seltzman
www.rtftechnologies.org

[Wilfried Heil]

We're using silver plated bolts throughout. I've never seen one seize yet and they can be removed and reassembled at least a dozen times before the silver plating starts to wear off. No additional anti-seize lubricant is needed here.

You can get silver plated hex head bolts too, e.g. at MDC Vacuum. Make sure that only the bolts are plated, not also the nuts, otherwise you will solder them together.

[John Futter]

apiezon L with silver powder added is what we use

[Andrew Seltzman]

Is that L grease with silver added?
Can you make your own anti-seize like this?

How well would silver plated bolts with boron nitride powder lubricant work?

Andrew Seltzman

[derekm]

Graphite/MoS2 based compounds are sold as high temperature anti seizes look for Dow corning MOLYKOTE® D-321R SPRAY. This has service temperatures +425C to -180C. priced around $30 a can - I 've still go some 321R from before the 1980's!
Even used on the Space Shuttle.
http://ntrs.nasa.gov/archive/nasa/casi. ... 004345.pdf
the FAA insists its use
http://www.thefederalregister.com/d.p/2 ... 8-02-13185...

I use it cos my father found out about it and reckoned it was the business for lubricating moving parts on high temperature industrial ovens n pollution control equipment...

[John Futter]

Yes you can make it like this 2 of grease to one of silver powder by volume.
its expensive way of doing it, but a 4.5 inch CF cube isn't cheap either.

Our biggest problem is the five sixteenth UNF bolts and 8mm bolts are very similar and each go into the other by a couple of turns =disaster if a spanner is bought near.

in the pics on the other post you will see all CF's labelled with thread info.

PS I would suppose that copper powder would also work with the grease although I've never tried it.

[Andrew Seltzman]

I actually may have discovered the cause of the problem, while repairing the 2 or 3 deformed threads on my cube(the repair went beautifully, completely fixed), I noticed that our standard 1/4-28 H3 taps would not free fit in the tapped holes in the cube. After a little research on taps I discovered that the holes in the cube were probably taped with 1/4"-28 H1 taps. The difference is that the H3 tap generated a 0.0015" oversize thread, while the H1 tap generates a 0.0005" oversize thread, eg, the H1 thread is a much closer fit.

Most standard threads are tapped with an H3 tap, so the rougher mcmaster bolts and nuts go together without binding. When putting a rough bolt in an H1 clearance hole, it has a much greater chance of binding due to the narrower clearances. The smooth threaded silver plated 12 bolts designed for vacuum fittings take this into account. The smooth thread and the silver plating for anti-galling prevent the bolt from friction welding into the tapped hole.

So from now on, I will use silver plated 12 point bolts with molykote spray (from mcmaster) on the cube to prevent binding.

Andrew Seltzman
www.rtftechnologies.org

[DaveC]

Andrew -

I think you're correct about the differences in thread quality between "garden variety" SS bolts and the Vacuum Grade. Also, the bolt heads on some from MDC or others seem a bit thicker, as are the nuts and washers. I've used both types, but unless you're stuck for time, or cash, I'd recommend the Vacuum grade..

We've used both silver plated (they really ARE worth the extra bucks, I'd say). and the Nickel - based Nuclear grade anti-seize. The anti seize is great stuff, but a big contamination concern. Since we are usually assembling in rubber gloves, it's less of a hand goo-ing issue, but you need to be extra careful that it stays on threads only.

But, once you've had to deal with a couple of galled or seized bolts, especially on a large piece of gear, that you can't take back into the shop for a drillout and retap on a machine.... there is no issue about a few extra cents or a little extra care to prevent a re-occurrence.

Dave Cooper

[Quantum]

A couple of tips:

NEVER screw stainless into stainless of the same grade without anti-sieze. (ceramic grease is best, I believe.)

If you can't use anti-sieze,316 bolts into 304 threads are less likely to gall

Bronze bolts into stainless threads won't gall. (Aluminium bronze is reccomended)

Always run a tap or die down the thread first, using a cutting compound such as Trefolex.

[Doug Coulter]

Andrew,

If you look carefully, there's a difference on McMaster, and we notice it here too. There are both cut thread bolts, and rolled thread. The rolled are one heck of a lot smoother, and this takes care of a lot.
For whatever reason, they also cost less. To make the cut thread bolts work well here, we have to kind of shine them up some and get the rough edges off -- debur them, if you will.

I use a little synthetic grease with MoS2 in it that I got for my gunsmithing, it works great, and an ounce is a lifetime supply.
But if you're tightening flange to flange hard, any bolts will only last a few times -- you're stretching them a little and they get brittle from the work hardening.

[Quantum]

We use Permalon M anti-sieze inside chambers at work.

Once dry (five to ten minutes), it only consists of carbon, and is good for 2000 degrees F when no oxygen is present.

It is definitely suitable for flange bolts, and seems ideal for stainless-stainless threads.

http://www.russprodco.com/PermalonM.html

[Andrew Seltzman]

I actually switched to nylon 6-6 bolts, They are surprisingly strong, and 3 of them per flange is enough to compress the o-ring. In addition they are non-conductive, so if you don't tighten the flange down all the way, the viton o-ring and the bolts forms an insulating break betewwn flanges, assisting in the effort to avoid ground loops. Since the switch, I have had no problems with the system.

Andrew Seltzman
www.rtftechnologies.org

[Chris Seyfert]

A less messy alternative to anti-seize on every bolt, and more affordable than silver plated bolts, is high quality grade 8 yellow-zinc-plated bolts. At my work place we use them extensively on vacuum systems, and have yet to have a problem with seizing, although we have few threaded conflats (mostly through hole flanges).

When we do have to use anti-seize, regular Bostik NEVER-SEEZ works like a charm. Not for in-vacuum parts, of course.

[Quantum]

Using different materials is a good way to avoid seizing (galling), but permalon isn't messy, you apply it with a paintbrush, and allow it to dry before assembly.

It is one of only two lubricants we use inside vacuum chambers. The other is Everlube, which we use for bearings, and which requires baking before use.

[Chris Bradley]

I can't say that I see an occasion when you *need* to design a bolt in a vacuum system where you do up bolts so tight that they might sieze. Don't forget to leave vacuum applied *before loosening* (not removing) all the bolts, then remove the loosened bolts once the vacuum has been vented. I presume I don't need to say that, but, hey, we all forget.. (or didn't know it to forget it!).

Some vacuum flange designs originate from high pressure steam designs, and retain vestigially large and numerous bolt holes to cope with positive pressure. Doesn't mean you need to use them all, nor tighten to the usual spec for whatever large bolt that fits. Copper gaskets will need a compression torque, but once the system is under vacuum, retorque them to a modest setting, *before* they sieze!

[Doug Coulter]

Chris,
modern CF flanges have no basis in the old ASA/ISO steam stuff other than that both tend to be round. What you say is true indeed for things with O rings (we only finger tighten ours at STP, and don't touch them after that, the vacuum forces hold things together fine with the bolts now loose), but....

CF's with the copper gasket are designed to be scientist/grad student proof -- both of which classes have kind of a reputation as not being too safe/skilled (true or not) with hand tools. Believe it or not, you need all that thickness and all those bolts to eliminate chances of warping under the forces involved. You can get by with less if you are careful and don't mind some flexing in vacuum vs atmosphere, and I often do. But I've also had stuff ruined by warping, it's educational. You don't forget after paying for a new one and welding it back on the system.

In fact, I often get to reuse the Cu gaskets because I didn't tighten them up to spec, just to the point it stopped leaking. I get perhaps 3 uses, less if there is temperature cycling that will sometimes cause a leak due to the very different thermal tempcos of Cu vs SS. Doing it "by the book" eliminates that one by pushing the copper near, but not past the elastic limit so it can spring back a little when it shrinks more than the SS during cool down. Tightening it to spec gives it more room to spring back into.
Not usually a problem here as I only bake to about 100c or so. But the hot end of my fusor tank regularly exceeds that, and those bolts are tight indeed.

Metals DO cold-weld, no matter the tension or other conditions (quicker when warm and especially when very clean, the effect is used in industry well below the melting points), and if those bolts are in the vacuum (Which isn't what Andrew was talking about here I think) the effect is pretty serious -- ask the guys who had to break things off the Hubble recently, or the guys who thought their solar arrays and antennas in spacecraft would continue to be steerable in that nice clean enviornment. Or anyone who mixes motion and vacuum. Only a few things work in that case when the surfaces are "vacuum cleaned". And most of those have troubles of their own, like teflon cold flowing, MoS2 getting into bad places and decomposing, and so on. Even brass on iron can't be counted on to not seize in vacuum conditions as many have found out, including me.

If you don't think metals cold weld even when pretty loose, try removing some plug in printed circuit boards from something many years old. As a used to be service tech, I can say, yes, they do weld similar metals or dissimilar. Forces can get to be destructive just to unplug a board after it's been in there long enough, even if the contracts are gold on gold (solder or tin are notoriously bad this way).

By my actual measurements, so do bullets and brass (I am a hobby bench-rest shooter and make my own ammo for that; after awhile, it gets less accurate due to this effect, which I have accurately measured -- the force to pull a bullet from brass can double in a couple months, and get crazy in a couple years to the point of maybe even not being safe to shoot any more). And that's somewhat different metals involved, so the lattices don't just line up, but it happens anyway. In fact, this is an issue our military did a lot of research into as it was causing troubles for them, and they wound up settling for lots less accuracy in favor of more safety for old ammo -- they put anti seize compound in there, which also serves as waterproofing, another requirement for long storage in condensing atmospheres, not to mention what happens in theatre.

Andrew broke off some threads. These were cut-thread bolts, which have sharp corners, and therefore what a mech engineer would call stress risers that weaken them a goodly amount.
I got some of those bum bolts too, which is why I recommended what worked for me -- get rid of the sharp edges and stress risers, then use some lube. What kind doesn't seem to matter as much as what will stay in there and not evaporate or decompose, it all seems to work if those are not a problem.

[Quantum]

Chris, the reason they sell silver plated bolts is to prevent seizing.

Stainless-stainless is notorious for galling, even at moderate torques. stainless-aluminium can also seize.

It's always advisable to use anti-seize of some sort where possible.

[derekm]

Whats so different between a conflat and a car head gasket?
how many here follow a tightening pattern and use a torque wrench?
The last head gasket I changed was copper (an abestos/substitute)

[Chris Bradley]

Direction of the pressure.

But I will stand down my comments, as duly corrected by those more experienced with copper gaskets (and big wrenches!).

[Quantum]

The knife edges, mostly, I think, Derek. Apart from that, not a lot.