Loctite on steel axle in Delrin tube: experiment - chemistry questions

[mmagliaro]

As many of you know, I am building the axles in my 0-6-0 by having drill-rod half-axles in each wheel and pressing them together
into a Delrin tube. 

I have started assembling and quartering them, using Loctite 680 retaining compound.  680 is not a thread locker.  It is a "retaining compound" which is supposed to be particularly for holding smooth shafts inside bearings or tubes - i.e. not threaded nut/bolt applications.

Before I started, I conducted a little test.  I stuffed two pieces of drill rod into an spare axle tube with Loctite.
First, I cleaned everything with acetone.  The next day, it was "pretty hard".  It felt like it would hold in an N Scale engine.
But I was able to bend and wiggle and get the axles to pull out.  I was kind of hoping for more.

Then I tried it again, but this time, I ground a small flat on the tip of each axle.  I inserted the axles.  Then, I drilled a #80 hole
through the Delrin tube down to the flat on the axle, and I poked a short piece of .0125" phosphor bronze wire in there, dipped in
Loctite.

In about 10 minutes, it was like iron.  I would have to wrench it apart and tear the axle tube to pieces with pliers to get that joint
apart.  No joke.

Needless to say, I will do this on my real axles.

But what's going on here?
I am guessing that:

a) Loctite bonds best in the presence of metals, and particularly metals like copper, brass, or plain steel.  It doesn't cure as well
or as fast when touching stainless or other polished steels (which I think would include the drill rod).  So the little bronze wire may have been the catalyst that got it to cure to its hardest.

and

b) This approach got me a cavity full of Loctite, provided by the flat I put on the axle, bonded to the bronze wire.
I no longer had to depend on the somewhat dodgy bond with the Delrin tube.  Essentially, I have a rock-hard slug of Loctite in there, with the bronze wire buried in it, formed hard against the flat on the axle.  I have made a mechanical joint, rather than just depending on the stickiness of an adhesive.

I'd like to know what your thoughts are on this, about how Loctite works, and why it worked so much better this way.


(I am doing this in a jig to keep the wheels plumb and quartered the same.  All I can say is.... man, my quartering better be good, because there will be no getting these things apart now!  )

[Chris333]

Always wondered what made Loctite react. If you leave a dot out in the open it doesn't  harden.

[Cajonpassfan]

Max, you continue to amaze me. I have enough trouble with #80 drills without drilling steel axles sideways
At first I figured you wanted belt and suspenders, but those can be removed. This is more like stapling the pants to your butt; let's hope they're on right
Otto

[PROPULDUDE]

From loctite...
Anaerobic adhesives remain liquid until isolated from oxygen in the presence of metal ions. Though anaerobic applications differ widely, in most cases the adhesive provides high shear strength and temperature resistance. Anaerobic adhesives fixture in minutes at room temperature and fully cure within 24 hours. Full cure of anaerobics may be achieved more rapidly using short-term exposure to heat (e.g. 30 minutes at 120°C). In laymans terms, it cures when not in the presence of oxygen. Learned that in Tech School years ago..

[mmagliaro]

I didn't drill through the hardened drill rod axle.  I only drilled through the plastic tube down to the flat spot on the axle,
and I shoved the bronze wire in so its tip was up against the flat.

The bronze wire had a drop of Loctite on it, so imagine, inside the tube, there's this little air space where the flat of the axle is, and that airspace is full of Loctite with a bronze wire stuck in it.   I *guess* that blob of Loctite hardened up really good because of
the bronze, and since it is against a flat on the axle, and wire is stuck into it from outside the tube, nothing can move (unless I
destroy it).   I *think*... but I don't really know.  That's why I'm asking.

[narrowminded]

I suspect you've got it. Plus, even if it did fail due to some errant load, at some point I think I'd rather that joint fail than a load that could break the wheels you made.  You could always go back at the joint if you had to but the wheels?! Ouch!   If you read up on Loctite's site they describe this adequately for the use without necessarily all of the gory detail.  Bronze and brass are highly "reactive" so cure rather quickly.  The insertion of that pin through the delrin is an arguably clever idea as it affords a small mechanical connection to the poorest functioning part of the joint, the plastic, while including the reactive metal to aid in the cure of the material that is mechanically linking all of the parts.  Might be a little overkill but being relatively easy to do it sure can't hurt and that's always good.  I like it.

[mmagliaro]

That's kind of how I felt.  After a wheel set is glued and quarted, it is easy to drill two little holes into the Delrin and insert pins and then snip them off flush.

I made the flats on the shafts in line with the crankholes on the wheels, so I could tell where the flats were after I pushed them into
the axle tube (good thinking, eh?)
And it doesn't have to be that precise.  The flats are, of course, much larger than a little #80 hole, so I just have to be close and I am
getting the pin down in there against the flat.  The strength on my test piece was astounding.  Just about 1/16" of inch of shaft is actually pressed into these tubes, and man oh man, the joint makes it like they are one piece.

[Chris333]

Max. I'm sure you have it covered, but i recently bought a Starrett pin vise, the smallest of the set. I have no idea how I lived this long without one!

[peteski]

Max. I'm sure you have it covered, but i recently bought a Starrett pin vise, the smallest of the set. I have no idea how I lived this long without one!

Most of the time I use the Tungsten-Carbide drill bits with 1/8" shanks and I don't even need the pin vise. I simply use its 1/8" shank as a handle. That give me very precise control of the drill bit.

[mmagliaro]

Funny you mention that, Chris.  About a month ago, I bought the larger Starrett pin vise (the 166C) because the pin vise
I had for my larger bits broke.

You ain't kidding.  There are everybody else's pin vises (also known as "crap") and then there is the Starrett.
The 166A (the small-bit one that can go down to #80 and beyond) is definitely on my list as the next tool I will buy.

Surprisingly, they aren't too awfully expensive.  Yeah, they are $25, not 6 or 10 bucks like the cheap hobby pin vises,
but wow... just .... wow.

In any event, drilling a #80 hole in the side of a Delrin is not hard to do.  Sharp bits help.   

[daniel_leavitt2000]

I wonder how it would react if the half shafts were splined.

[Ed Kapuscinski]

My thoughts... I love any thread that Max starts that has "chemistry questions" in the title.

[rschaffter]

If a piece of bronze or brass rod works, you might try electroplating a bit of copper onto the steel axle...

[mmagliaro]

I'm sure that putting a spline or knurl on the steel axles would help grip a lot, and in fact, would probably get rid of the need
for Loctite in the first place.  However, I don't know about you guys, but I doubt I could put a spline or knurl onto a shaft and get it
to go into a hollow tube without being off-center.   I figured out how to make a jig so I could accurately center-drill a Delrin tube - that was
enough trouble.  I don't think I could get a spline to go in on-center.
(aside: drilling Delrin, or any plastic for that matter, is tricky, because it flexes under the slightest drilling load and it's hard
to truly get a bit to bore right down the middle... so I bored a hole in a heavy piece of brass, into which I can tightly put a piece of
Delrin rod.  That keeps the walls of the rod from trying to spring outward while the bit goes down the middle.)

rschaffter    Copper plate the steel rod.  What a great idea!   And I just happen to have some copper plating solution here.

[peteski]

Copper plate the steel rod.  What a great idea!   And I just happen to have some copper plating solution here.

I think your joint with a flattened axle and a small brass  rod was so strong due to the mechanical connection of the hardened adhesive and the brass rod in a hole in the axle tube.  Delrin is too slippery for any adhesive to strongly adhere to it.  But I'm very interested in how strong will the glue joint be when using copper plated axle.

As far as linear knurling goes, it does make for a joint which resists twisting, but in my mind I think that the scoring it produces inside the Delrin tube will weaken the tuibe. Those scores might be starting points for stress cracks later on.