J B Weld

[Roger B]

I was planning to make the crankcase of my Junkers opposed piston engine from welded steel plate. Having thought some more there will be problems with distortion and in reaching some of the runs.

As an alternative I am looking at using 5 or 6mm aluminium plate held together with JB Weld (I will use some dowel pins and M3 bolts to keep it together for assembly).

As it is an opposed piston engine most of the compression and firing forces will be bending moments on the crankshaft rather than loads on the cylinder block and crankcase. The crankcase is essentially a box so it should be fairly rigid.

Good idea, bad idea??? 

[crueby]

JB Weld claims a lot, but I would never use it to hold metal parts under those kinds of forces, unless the bolts were taking all the forces and the epoxy was just for fairing in curves inside the corners, that sort of thing. Maybe its just that I don't mix it exactly in proportion, but its adhesion on metal is not as structural as it would be on something porous like wood. At least do some test pieces on scrap!
Chris

[Jasonb]

I'm sure Ramon would say go for it, look back at his wide a wake thread to see the tests he did on it. With some M3 bolts I'd be happy to put it under load.

If you can't reach some parts to weld would silver solder flow into the joints? I sometimes use a mixture of welding and then soldering as there is no risk of the initial welded parts moving at soldering temps. It would also be possible to fabricate the crankcase by welding etc and leave machining allowances so final things like bearing locations and cylinder mounting surfaces can be machined after any possible distortion.

[petertha]

Just a general armchair engineering comment here, but when you look at many successful  multi-cylinder model engine crankcases, most all are made from aluminum. Well, how about the ones replicating or intending to fly where weight is a factor. They end up looking like a block of swiss cheese by the time the front, rear, cylinder holes & crankshaft recesses are hogged out. Meanwhile the crankshaft is much more meaty & attention paid to the reciprocating parts. What is more difficult is brazing or otherwise bonding other parts to it.

Are you talking about JB bonding halves to make a permanent shell, or is this more for attaching bosses or features that would have shown up as part of a casting? Can you show a specific picture as example? Another consideration is there is such a thing as minimum thickness for adhesives. JB is very viscous so with enough clamping/retention pressure,I think you could inadvertently starve the joint without some keying or fillet features.

This topic is interesting to me too. I have made some spare time test coupons of pieces of aluminum with well fitting interfaces, attached them with good quality Loctite retainer. They survived machining & 'carving' across both surfaces. Depending on the finishing, finding the joint with naked eye is hard unless you know where to look. And it certainly would withstand any kind of temperature a crankcase would ever see. So with some careful planning it might be a good construction mode for more complex shapes to assemble components into what is a final or intermediary shape for further machining.

[Vixen]

Hello Roger,

JB Weld, despite it's name, is only a filled epoxy adhesive: a very strong and hard setting adhesive, I must add.

I would not recommend relying on JB Weld alone for any high load, high stress assemblies. For those applications, the use of aluminium alloys precludes welding, silver solder and brazing. For high load, high stress, aluminium alloy assemblies, I would mechanically screw or bolt the components together (more bolts the better) and use the JB Weld only to grout and seal the joint gap.  Abrading the mating surfaces, before applying the JB, will help key the epoxy to the metal.

Cheers   

Mike

[ozzie46]

Worked absolutely  fine on my Mastiff opposed 4 cyl. crankcase. Been ok for alt least 10 years.

Ron

 

[jcge]

In general terms, epoxies can work well particularly when loaded in shear, less so in peel, but I'd steer clear of direct tension.
John

[Chipswitheverything]

At my place of work we used to use Redux epoxy of various kinds, specified for jointing of structural aluminium assemblies for aircraft componentry, made then by Ciba, now I gather made by Hexel, USA I think.   It seemed to have an etching ability on the alloy, and be reinforced with micro fibres.  Though we were not using it for aircraft work, filleted joints made by it seemed to be very strong .  Might be worth checking the product out for suitability....  Dave

[Roger B]

Thank you all for your thoughts 

The crankcase will be around 90mm square and 150mm long with an offset central divider. Normally the crankcase is transmitting the tension force between the cylinder head and the main bearings however in this case the main force will be from the scavenge pump mounted on the top piston.
I think that rigidity will be the key factor in the design of the crankcase rather than strength directly, hence the box design.
I have attached a picture of the prototype and the current stage of the design.

[GWRdriver]

For decades (it seems like) old Coots over here have gone on about the miraculous cure-all qualities of JBWeld.  "Got a hole the size of a Shilling in your cylinder wall?  No problem - just daub some JBWeld in there and hit it with a file."  Early in my model engineering life I fell victim to an epoxy cure for an all-brass fabricated assembly.  Sure, that oughta' work, I was told, but it was disastrous, so since then I've been skeptical of miracle epoxy products.

Recently I fabricated a flywheel (5"Ø) of mild steel for a model engine and decided to use JBWeld as the adhesive, primarily because it would never be exposed to oil or heat at beyond room temperature, nor to mechanical forces beyond its own weight.  So far so good.  It's a very tenacious adhesive and before long I'll have another application for it . . . fairing out weld fillets on a fabricated steel smoke box saddle for a 7+"Ga loco.  That WILL see heat, and possibly oil, and we'll eventually see how that works out.

[petertha]

I assume you are talking about fabricating (with JB assistance) the lower case indicated in red & the upper cylinder stuff is attached?

I know nothing about this engine so pleading complete ignorance. Maybe this is a distant cousin, but it almost looks like the lower section encompasses some mid section through to where the exhaust manifold exits, because its all a uniform material color & I don't see any hold down fasters. What a fascinating project, I'm going to have to pay attention to this.

https://www.canalworld.net/forums/index.php?/topic/118548-junkers-clm-twin-cylinder-diesel-engine/

[cwelkie]

I wish you every success on your project Roger - however you decide to proceed.

For a little more support in the "go for it camp" I've attached a couple photos of a Fuller & Johnson Farm Engine I've done.  The crankcase is all aluminium assembled from quite a few separate pieces using JB Weld.  The pump too was assembled with JB Weld.  I've experienced no problems with any of the joints.  Granted this is a rather "low load" situation but it did save taking out a mortgage to buy the brass or other solderable materials.

FWIW,
Charlie