Comparative Strength of Loctite

Being an inquisitive soul, I have been making some experiments with the relative strengths of Loctite.

First to set the scene. Some of you will be familiar with this as being the mandrel gear for a pre-historic Myford S7 with the Mk 1 QCGB,

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The one on the left is the proprietary Myford 30/12T original, the one on the right is my version of the same thing with 17 teeth on the smaller gear. I made a 17T gear on a stub shaft, and soft soldered a 30 T gear onto the stub.

I was curious to know if I could have used Loctite, and how strong is the soft solder anyway.

So here is a dummy gear cluster, with hexagons cut instead of the two gears:

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For the sake of completeness here is a picture of the two component parts:

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There are, as you will have noticed, remnants of Loctite on the interface between the two parts. To scale the area of the interface, that stub is 0.625 OD +/- about 3 tenths, and the bore is 0.375 ins long by 0.625 OD - minus nothing plus maybe 5 tenths. So a "place fit" rather than a push fit. I didn't make any attempt to polish the mating surfaces - the bore is straight off a reamer and the stub is straight off the lathe tool, which is exactly how I made the parts of the original gear cluster.

So I tried assembling the two parts with differing grades of Loctite. Then I broke them apart in torsion holding the large nut (41 mm a/f) in the vice and applying a measured torque to the 19 mm a/f "pinion" with a torque wrench. To be strictly fair the torque is applied only one sided - so it is a cantilever load on the join - but that is how the assembly will be used in real life. I applied successive and increasing torques to the assembly increasing in 5 ft-lbs increments until the joint failed. The two torque wrenches I used are in good condition but not formally calibrated and are the micrometer type.

This is what I measured:

Loctite 638 12 hours cure, failed at 60 ft-lbs, withstood 55 ft lbs.

Loctite 638 repeated, 17 hrs cure, same result

Loctite 243 (Nutlock) 2 hrs cure failed at 15 ft-lbs, withstood 10 ft-lbs

Loctite 542 (Thread Sealant), 3 hrs cure, failed at 30 ft-lbs, withstood 25 ft-lbs.

I then tried soft-soldering the two parts together, using 60/40 resin cored solder with additional active flux, this failed at 110 ft-lbs withstood 105 ft-lbs.

If anyone can suggest other grades of Loctite which might be of interest to get into the magic three figures I'd be interested to try them out.

Rgds Simon

It would interesting to compare with epoxy and superglue.

Of the three 638 is the only one really meant for that type of use so I would have expected that to be stronger than the other two.

Did you use anything particular to clean the mating faces as that would affect strength too.

I am surprised that Loctite 638 didn’t more closely approximate the soft-solder result.

MichaelG.

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P.S. ___ You may find the link in my post of 27/0202017 useful, or at least interesting:

https://www.model-engineer.co.uk/forums/postings.asp?th=125269

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P.P.S. ___ Sorry the ftp doesn’t appear to be working any more 

Edited By Michael Gilligan on 22/06/2020 16:01:39

Crazy idea...I wonder what would happen if you just drilled and tapped an M2 down the interface?

pgk

According to my copy of the datasheet for 638, steel on steel, fully cured:-

Compressive sheer strength 29 N/sq mm, 4200 psi.

Torque to IOS 10964:-

M10 black oxide bolts with mild steel nuts :- breakaway 57 Nm, 42 ft lb ; prevail 22 Nm , 16.25 ft/lb

3/8 x 16 tpi steel bolts & nuts :- breakaway 25 Nm, 18 ft lb ; prevail 9.8 N/m , 7 ft/lb

Quick guide to the definitions here :- **LINK**

Clearly there is a considerable dependance of torque strength on materials being bonded.

The breakway torque is what the Loctite adds to the actual tightening torque of the fastener.

Loctite isn't that strong as an adhesive. As always adhesives get their strength from bond area rather than absolute material properties. The influence of the surface properties of the material(s) being bonded is considerable and hard to control in ordinary workshops.

With solder the absolute strength of the material is greater and its relatively easier to control surface properties. basically clean and use the right flux.

Clive

Simon Williams - Are both parts joined in your test mild or plain carbon steel with low nickel content?

Did you degrease thoroughly with effective degreaser (ie immerse/rinse in acetone, laquer thinner, alcohol, several times) before Loctite application?

Did you use Loctite primer?

What was the gap between parts prior to applying Loctite?

Was the application and cure time at 20 deg C or above ambient temp?

All of the above can affect joint strength for Loctite.

I'd say 55 foot pounds torque is still a pretty respectable result! Good to see some real life testing going on. Well done!

When we did some repairs to the gears and bushes inside the Smart & Brown model A apron, there was a problem with the gear and shaft which engaged with the rack. The gear was quite good, but the integral shaft was badly worn adjacent to the gear. There was no way to reduce the whole shaft and building up the worn part would have been expensive. I set the shaft in the lathe and bored the gear end to just below the worn diameter, and parted it off. Then I made a new shaft to fit the hollow gear on. I made the mating parts with 0.001" clearance and not too smooth a finish and used Loctite 638 on them. About a week after re assembling the apron, the joint moved. It was the only thing that could fail in the whole gear train, so the apron had to come to bits again. It had been very difficult to get apart the first time, so I had drilled and tapped a couple of holes for jacking screws,before re assembling, which saved a lot of cursing. Mike set the shaft vertically in the rotary table on the mill and drilled three equally spaced holes axially in the joint and Loctited some needle rollers in, it has been ok since.

Loctite have a very informative website which is well worth looking at.

Edited By old mart on 22/06/2020 16:58:07

Wow, thank you all for your interest and helpful comments.

In no particular order -

Surfaces cleaned immediately prior to Loctite application with acetone on kitchen roll. Actually not impressed with the oil removal results of this - applying Baker's fluid to the join showed beading on the surfaces, leading me to believe that the join needs something a lot more aggressive to be effective at de-oiling the surfaces. But then Loctite is supposed to be tolerant to a bit of good honest workshop muck, so part of my experiment was to see if this was a limiting factor.

Working gap (as near as I can measure it - 1 or 2 thou, probably nearer 1. Certainly there is a positive gap as the two parts slide together without any interference.

No Loctite primer or activator.

Ambient temperature about 16 - 18 C over this last weekend

I too expected 638 to be nearer the strength of soldering - this was a bit of a surprise, though 50 ft lbs /70 Nm is a pretty respectable result and ample for the purpose.

Obviously pinning the two parts axially would effectively lock the two together for all time.

Materials - the 41 mm a/f hex "nut" is EN32B, the smaller pinion and stub are probably EN1A, sold as "GCQMS". From the way it machines there ain't no lead in it.

I like the suggestion of trying the same with superglue and epoxy - will investigate and report back.

Many thanks

Simon

Simon,

Look out for a P.M.

MichaelG.

Good Evening all, just a minor update to complete my experiments with superglue and epoxy.

Step 1 - make another test piece, as the original has now been soldered together, so to make the next experiment as near the same conditions as possible, not to have the surfaces locally contaminated by lead solder, I made another pair of test parts from the same material. As near as I can judge it the sizes and clearances at the interface between the two parts were the same as the original.

Step 2 - degrease with acetone, then glue together with run of the mill B&Q own brand (Diall) superglue. Leave to cure for 24 hours, ambient temp' about 20 C. Withstood 25 ft-lbs, failed at 30 ft lbs

Step 3 - clean carefully and glue with Araldite - the slow setting high strength version. Leave to set for 24 hours, ambient temp about 25 - 30 C (heatwave!) Apply torque as before, joint withstood 75 ft-lbs, failed at 80 ft-lbs.

Step 4 - remove remnant epoxy, degrease and glue with freshly bought Loctite 401. I chose not to use activator, as the instruction sheet for 401 indicates that activator is only necessary on difficult materials and may be counter-productive on "normal" materials such as steel. Instructions also say that full strength is reached in 72 hours, so I left the test piece to cure for 5 days. Ambient temp' was 16 - 20 C. Joint withstood 65 ft-lbs, failed at 70 ft-lbs.

So - soft solder wins way out in front, Loctite 638 about half the strength, but superglue carefully applied and left to cure for plenty of time pips it. My worry here is knowing what result I have achieved - have I got a 25 ft-lb superglue or a 65 ft-lb joint.

Thanks as always

Simon