Author Topic: Built up crankshaft feedback  (Read 2941 times)

Offline petertha

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Built up crankshaft feedback
« on: June 02, 2024, 12:33:04 AM »
Soliciting feedback from you talented experts. I drew this crankshaft up as a possible built-up equivalent version of what was originally part machined from solid bar stock. It's for Jung, horizontally opposed twin, 2x30=60cc total. It just occurred to me what a hefty chunk of material this would be to accommodate the counterweight OD. It measures 48mm dia x 188mm end to end for scale. So a lot of swarf & machining operations & after that, the usual possibility of post-machining distortion even if made from something like Stressproof steel. There are some additional machining subtleties specified which I have omitted for now.

Anyway, I am contemplating making a built up version & here is where I'm at idea wise
- the 3 vertical web elements would start out as basically identical circular discs
- the webs would each have the same center shaft hole and 2 crankpin holes on either side
- visualize that the center shaft would be one single full part bonded to the 3 web elements
- visualize that the crank pins would span 3 elements & be bonded to all 3

Then the tricky bit LOL
- saw cut the segments of crankpins & main shaft elements that don't belong
- with the alignment magically correct & no spring back & assembly sufficiently rigid, drill & insert permanent locking dowel pins or similar to make it mechanically stable for running
- finish turn the sawn ends flush to the web faces leaving the bonded slugs permanently intact in the webs

The purpose of the hole arrangement in the webs is to serve double duty - either hold the crankpin where it belongs or act as an alignment aid to the overall assembly as much as possible. I recognize the shaft / hole fits could be tricky. They cant be anything like press fit or it wont slide through the web elements easily. Too loose & the retainer glue would shear. So I'm thinking 'just enough' say 0.001" oversize hole to provide me some leeway but easily within the limits of what typical Loctite retainer allows. The objective of retainer is just to freeze the alignment position sufficiently to the point mechanically pin the parts & not have anything come loose during this operation.

Now that's the next big question - how to pin it. I watched a video on a Bugatti CS & they had these dowel pins with a flat on one side that wedged against a mating surface near the crankpin. I don't think that would scale easily to my size. I don't ever need this CS to come apart & I don't have the requirement to accommodate a middle bearing (although I've been contemplating that too if it could fit). So could I get away with dowel pins on all the mating surfaces? How should they be positioned for maximum strength?



Offline petertha

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Re: Built up crankshaft feedback
« Reply #1 on: June 02, 2024, 12:39:35 AM »
If I did not explain it quite clearly, hopefully this sketch helps. The crank pin elements would be oversize & slide through all webs & same goes for center crankshaft rod element. Bonded in place, pinned etc.
Then X shows the bits to be removed & finished flush.
« Last Edit: June 02, 2024, 02:25:43 AM by petertha »

Offline crueby

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Re: Built up crankshaft feedback
« Reply #2 on: June 02, 2024, 12:42:02 AM »
The biggest change I would suggest is to the order of your steps. Wait till after everything is pinned (and/or silver soldered, loctited, whatever) to cut away the un-needed sections of the main shaft. That will keep everything aligned during all the other work. The ends of the cut shaft will need some finish milling or filing to get them flush with the inner faces of the webs so they don't catch on the connecting rod end.


I've built both single-piece and built-up crankshafts, and prefer the built-up ones, especially if the crank throw is large. Lots less in the way of tricky setups on the lathe. Personally I prefer taper pins on the connections (with some high strength loctite for extra confidence). The taper pins do require special drills or reamers, so roll pins are another option. Some people like to silver solder the connections, though the heat involved could cause issues on some metals. All a matter of preference. I just build steam engines, so the cranks are taking less force and speed than a gas/diesel engine would impart.

Thats my 2 cents worth!

Just saw your second post (we posted at the same time), and sawing out the un-needed parts last would leave the last little section in place in the center web - not an issue but thought I would mention it. On many engines the two sets of webs have a short length of the main shaft between them, thats what I am used to making.
 :cheers:

Offline petertha

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Re: Built up crankshaft feedback
« Reply #3 on: June 02, 2024, 02:21:37 AM »
Thanks. Just to clarify, the holes with red + serve as temporary, supplemental alignment holes. But also where I would just leave the slug of original shaft material in place with retainer. Its not going to be as strong as virgin stock, but hopefully better than an open hole. The green lines indicate required clearance for connecting rods.

I did think about silver soldering & maybe leaving all the alignment stock in place would be enough to mitigate heat distortion. But in my mind I was hoping to depend solely on mechanical dowelling for mechanical strength even if the retainer started to fade in the future.

So should a tapered pin go right through the web & crank pin? Would I size it to be slightly longer & then finished flush on both sides for maximum engagement strength? Aside from 'keeping things put' I'm having difficulty visualizing how to best orientate pins from a stress standpoint. Just visually eyeballing some of these 3D stress/strain visualizations, seems like the middle web segment is the rubber duck. Maybe because the bearings are closely positioned on either side of the fore-aft webs providing support & the middle accordion is seeing the con rod force. So maybe a big diameter alignment hole in the middle of center web is not a good idea. The crankshaft shaft on the front & rear webs would see mainly shear force which I suppose means bigger pin section area is better? Or maybe intercepting the crankpin through its tangent OD is better than cross drilling through the middle?

On that note, this is a cool Bugatti crankshaft video showing partial disassembly & some of the other attributes that made this engine cool. Some of this subject matter was discussed on some other forum builds, but they are even more challenging with middle bearings.

<a href="https://www.youtube.com/watch?v=RpohTIT7KSA" target="_blank">http://www.youtube.com/watch?v=RpohTIT7KSA</a>

Offline petertha

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Re: Built up crankshaft feedback
« Reply #4 on: June 02, 2024, 02:29:19 AM »
And then there's the sound. OMG. I'll just drop this here LOL

<a href="https://www.youtube.com/watch?v=s0JlQeCeExs" target="_blank">http://www.youtube.com/watch?v=s0JlQeCeExs</a>

Offline crueby

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Re: Built up crankshaft feedback
« Reply #5 on: June 02, 2024, 02:31:32 AM »
Many ways to do it, here is a post where I show the most recent one I did, the pins go through the web completely so they can be trimmed off flush either side, no worries about trapped air making them push out. For taper pins, through and through is the easiest. I put on a drop of red locktite before assembling the webs to the shafts, wait for that to cure, then drill for the pins and tap them in place, finally trim them off. Then go back and trim out the un-needed sections of the main shaft - it keeps everything aligned perfectly till then.


https://www.modelenginemaker.com/index.php/topic,11727.msg278423.html#msg278423

As I said, this is just one of many ways to assemble things, I've seen a variety of other ways on other peoples threads.

And that Type 35 - what fun that must be!

Offline Jasonb

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Re: Built up crankshaft feedback
« Reply #6 on: June 02, 2024, 07:08:21 AM »
I don't think I would want to build that size IC engine with Loctite. I would silver solder and then provided you are confident of penitration the pins are not needed but could be added if you want belt and braces. If you are worried about distortion build it up oversize and then finish turn the crank once all the spare bits of rod have been cut away.

I do build cranks up with loctite but on smaller engines where a bit of kickback won't be an issue. So seldom used on IC engines

If building that one up I would most likely have shorter pins that only span from side to middle web. And would solder the shaft into the middle web so the web ends up solid and stronger not with a hole as per your drawing.

As it is a flat crank you could do it from flat bar and add separate weights which is quite common. If you can't get the flat bar mill some round stuff down to flat and you can also mill out 90% of the waste


Offline petertha

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Re: Built up crankshaft feedback
« Reply #7 on: June 02, 2024, 09:34:49 AM »
Thanks Jason. Just to clarify, my intention for Loctite retainer was only to hold the aligned assembly securely enough so that once cured, it would stay together intact for the pinning (drilling & insertion) operation. The plan was that the pins (of some sort & TBD orientation) would be the basis of mechanical retention. At least that was my hope. Silver soldering would be strong but I just can't see how that localized heat from one joint to the next will not result in some distortion, but that is coming from the perspective of a torch Neanderthal. But, I'm willing to sacrifice some material in the name of R&D, the dream engines only get more complex from here haha.

This is why I'm keeping a sharp eye on that Ohrndorf-v12 build. Its a completely bolt together CS but using alternating segments of press in crank pins & bolt clamped webs. No glue or braze. But there are more intermediary bearings & I think that must be a big contributor to success.

Online Roger B

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Re: Built up crankshaft feedback
« Reply #8 on: June 02, 2024, 10:10:14 AM »
I would tend to make a one piece crankshaft from flat bar and then bolt on the balance weights.

If you decide to use Loctite the joint needs to be carefully designed to ensure the optimum thickness of adhesive. Too little or too much reduces the strength.

How much balancing will you need? An opposed twin is in reasonable primary balance although there is a rocking couple due to the offset axis of the two cylinders.
Best regards

Roger

Offline Jasonb

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Re: Built up crankshaft feedback
« Reply #9 on: June 02, 2024, 10:13:01 AM »
You should not really get localised heating as the whole thing wants getting upto heat which tends to happen with a propane torch with say a 28mm burner, unlike an oxy torch which is a lot smaller flame.

<a href="https://www.youtube.com/watch?v=QwlmaAIzUMU" target="_blank">http://www.youtube.com/watch?v=QwlmaAIzUMU</a>

Offline Alyn Foundry

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Re: Built up crankshaft feedback
« Reply #10 on: June 02, 2024, 11:09:56 AM »
Way back in the 1980’s I tried out Loctite 638 high strength retainer, it having been highly recommended by my engineering mentor. Almost a king’s ransom was paid for a bottle! Two crankshafts were prepared, too the letter and fitted to two of my R.L.E. engines. Both failed within a couple of hours running. I was furious, I contacted Loctite on the telephone and they very casually told me that their product was unsuitable for shock loading…. On another occasion a friend tried it out with the addition of taper pins, typically failing during a weekend demonstration.

Since then we have favoured a composite construction with Silver solder only. In our opinion pinning only serves to weaken the integrity of the components.

The only important point in making a Silver soldered crankshaft is to get the whole job warmed up, fully expanded before going in with the solder. This greatly reduces the risk of warping as the metal is allowed to cool very slowly.

Photos are of the latest crankshaft for my Brayton Ready-Motor @ 3/4” diameter.

 :cheers:  Graham.

Offline petertha

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Re: Built up crankshaft feedback
« Reply #11 on: June 02, 2024, 04:53:18 PM »
Hmmm... maybe I should just face my silver solder demons with some test mockups. I did some low brow silver solder testing on mild steel when I was contemplating mating 2 gears together for my radial. I was amazed at how the silver solder penetrated & wicked the ~ 0.001" annular gap. But I also spent a lot of time trying to remove the scale & then dressing down the solder fillet. Maybe that could have been controlled better, part of my inexperience. For this particular CS, the rod widths are quite close dimensionally to the webs elements on either side so I think the fillets would have to be dealt with. That was the part of the appeal (in my mind) of a 'cold' assembly using something like ground rod for the crank pins.

Graham, on your CS you show the exact idea I was proposing where the center shaft goes straight through the web throw elements to assist with alignment & then gets cut away post silver soldering. Did you detect shaft runout once the segment was removed? If so, how much over what kind of length? Do you do anything to assist with crankpin centering like pip marks, or am I overthinking? (see crude sketch)

Offline petertha

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Re: Built up crankshaft feedback
« Reply #12 on: June 03, 2024, 04:27:04 AM »
This is the Ohrndorf 4-cyl boxer built up CS. Similar principle to what he did on V12

Offline Jasonb

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Re: Built up crankshaft feedback
« Reply #13 on: June 03, 2024, 06:50:22 AM »
having the ctr shaft run right through is a very common way of doing built up crankshafts, if you look through other builds you are likely to see them being done that way.

The black scale and the flux can be removed by an acid pickle which will get you back to bare metal.

Offline Alyn Foundry

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Re: Built up crankshaft feedback
« Reply #14 on: June 03, 2024, 10:59:01 AM »
Good morning petertha.

We always start with good quality metal, either precision ground mild Steel or Silver Steel. The main shaft is centre spotted both ends and we now do the keyways too. If you do get a little warping you can easily re true the shaft between centres on the lathe afterwards.

There a a few methods to reduce the risk of solder going where it shouldn’t. Correction fluid is one and the old faithful Plumbers Black is better.

The flux used is very critical to a good job. Not so much on smaller diameters but as the bulk increases you need a flux that will stay active for much longer as the heat increases. Tenacity Number 5 is my weapon of choice. Specifically designed for use with ferrous materials and really long lasting during the soldering process.

As Jason has pointed out, after the crankshaft has been left to cool naturally a bath of White spirit Vinegar is used to gently remove the flux residue ( totally immersed ) and then the metal polished with fine Emery paper/cloth.

This method of crankshaft production is ideal for our models as they are not particularly highly stressed. However I would recommend making from the solid if you are intending to drive them hard.


 :cheers:  Graham.
« Last Edit: June 03, 2024, 11:06:20 AM by Alyn Foundry »

 

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