Author Topic: Let's talk about crankshafts--machined, built up, etcetera  (Read 3734 times)

Offline Brian Rupnow

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Let's talk about crankshafts--machined, built up, etcetera
« on: October 06, 2017, 10:12:08 PM »
I have tried my hand at just about every kind of crankshaft construction I know of. Many successes, some that went terribly wrong, and some that I "made do". I have found that the absolute best material for a crankshaft machined from solid, is 1144 stress proof steel. Unfortunately, I am only able to buy it as round stock, so there is a lot of "hogging" to end up with a bar of flat material. It is a marvelous material, because as parts of it are machined away, internal stresses don't try and make the rest of it move or twist. I turn the material between centers, using a lathe dog to make sure it doesn't slip when I am machining it. This is an excellent way to make a crankshaft for a single cylinder engine with only one "crank throw" on it. There are numerous "how to" posts on the internet about machining a crankshaft from solid, so I'm not going to try and repeat it all here.
   I find however, that if you try and use this method to make a crankshaft for a double or triple cylinder engine, it is very difficult to keep things all moving "true", and you get into some very scary moments when turning the con rod journals with the tool stuck out a ridiculous amount.
   Many of the crankshafts I have made from "built up" pieces press fitted and Loctited together give satisfactory results, but there are pitfalls in this method as well. I traditionally use plain cold rolled steel for my crankshafts and crankshaft webs, and get good results. However, there is a caveat in doing this, and it has to do with "press fits". I have "on size" reamers, and reamers that are 0.0015" undersize. Cold rolled steel generally comes in .0005" undersize, so you don't really get a 0.0015" interference.--You get a 0.001" interference. I have always thought that .001" interference is not enough, so on many pressed together crankshafts I use "drill rod" because it comes in "on size" and you do get a full 0.0015" interference.
--After building the most recent crankshaft in my "back to steam" thread, I am of the opinion that .0015" interference is dramatic overkill, especially if you are building to a design that has thin web plates (3/16" in my case".) I think that .001" interference would have been sufficient. The mere fact that you are pressing so hard to get pieces together sets up stresses in the main crankshaft that will cause it to move far out of alignment. When the pieces of the main crankshaft are cut out between the web plates, these stresses become less, but they don't completely go away.--I ALWAYS use Loctite when assembling a built up, pressed together crankshaft. The Loctite rep says that even though the majority of the Loctite will be pushed away by the press fit, enough remains at a microscopic level to strengthen the joint by at least 30% over not using Loctite at all.
---The problem I see with the crankshaft I just made, is that the web plates are too thin in relationship to the crankshaft diameter. There simply isn't enough material in a 3/16" plate to make a really sound inflexible joint with the mating crankshaft.
---In my opinion, if I was designing an engine from scratch with a built up crankshaft, I would try and make the thickness of the web plates at least to the same thickness as the crankshaft diameter.
---I "cross-pinned" the joints on my crankshaft with 1/16" diameter steel dowels, but with such a thin web plate I don't know if pinning the joints gives a more secure joint, or makes it weaker because you are removing 1/3 of the cross section of the web plate.
--I am giving some thought to tapered reamers and tapered pins at the joints if I do this again, but I really have very little experience with tapered pins and reamers, especially with the small diameter (5/16" to 3/8" diameter) crankshafts I generally use.
----In my early days of crankshaft building, I silver soldered some built up crankshafts together, but the results were questionable. The joints were extremely strong, but the application of heat tended to pull the crankshafts "out of true" which sort of defeated the purpose.
---That is everything I have to share with you about crankshafts.---What do you guys find works best?---Brian

Offline ART

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Re: Let's talk about crankshafts--machined, built up, etcetera
« Reply #1 on: October 06, 2017, 11:29:35 PM »
Hello Brian. I have made several built up crankshafts for model hit and miss engines, and all have been successful. This is my process. I use water hardened drill rod for both the main shaft and crankpin as they have true diameters. I make the webs from " scrap bin" mild steel, and fabricate them together as a matched set. The holes are drilled then reamed .oo1 over the diameter of the respective shafts. I drill cross holes through the shaft holes usually about .062 for a .250 or .375 shaft, and I do not drill the holes through the shafts at this time. I use finishing nails which are about .062 diameter, and have used this same process with my model Little York engine which has .187 thick webs. After cleaning all the parts in Acetone to remove any oil I dry assemble the 4 parts on a flat firebrick, and make sure they are in correct position. I place a drop of Stay Clean liquid flux into the 4 cross holes in the webs, and insert 4 small pieces of Stay Brite solder into the holes. I think the Stay Brite melts at about 450 degrees, and I use a propane torch to gently heat the entire assembly until the solder melts then I let it air cool. After it is cooled I thoroughly wash the assembly with water to remove any flux residue then re-drill the cross holes through the shafts and press or hammer in the finishing nail pins until they come out of the other side of the web. I then file the pins smooth with the side of the webs, and then mill out the center section of the main shaft which I left in one piece until this last operation to ensure proper alignment. I have used this same process for a built up crankshaft .625 in diameter with .125 nails as pins and have had no problems. I know there are many ways to get good results but this has worked for me. Earl

Offline Ian S C

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Re: Let's talk about crankshafts--machined, built up, etcetera
« Reply #2 on: October 07, 2017, 11:35:11 AM »
I only have one of my hot air engines with a built up crankshaft, it is my smallest one with a 3/8" bore and 1/2" stroke. As it's a beam GAMMA type motor it only requires one crank on the shaft.  The shaft, and crank pin are 3 mm dia, and the crank webs are 2 mm thick, the Loctite is still hanging on, the shaft is just a firm push fit in drilled holes.  The reason for making it this way was to fit ball races in the big ends. Couldn't find a pic of the crankshaft, but this gives an idea of the lay out of the little motor.
Ian S C

Offline gbritnell

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Re: Let's talk about crankshafts--machined, built up, etcetera
« Reply #3 on: October 07, 2017, 02:40:01 PM »
Crankshafts. This part like a lot of others in the model engine world comes down to desire, necessity and or just plain functionality.

 Let's start with desire. Some builders want to replicate the prototype crankshaft meaning in general that it is made from one piece. In the full sized world this can mean the original was cast, forged or in today's world CNC machined. We as builders don't have the forging capabilities nor would we probably want a cast crank but we can manually machine or CNC machine said part. I personally have used what's known as C.R.S. (1018 steel). Not generally a good choice, machining is tough and warpage is guaranteed. The next step up would be 12L14 steel. This is a leaded free cutting steel that doesn't seem to warp as badly as 1018 but any rolled steel has stressed applied in manufacturing that relieve themselves as material is removed. The last material would be1144 stressproof steel. I was put onto this material many years ago by a good friend and fellow modeler and have never use anything else. It machines very nicely and distortion is minimal. Whatever distortion there is can be cleaned up in the final tool passes. There are some fellows who use 4140 steel. Personally I don't see a need for this high grade tool steel for the usage these model engines see but then it comes down to desire. Desire can also be limited by ability and tooling availability.

 Necessity somewhat ties into desire. Is it necessary to use high grade steels for a crankshaft when in general the engine gets pulled off the shelf and run several time a year or in my case I attend 4 or 5 shows a year and run my engines for a couple of hours intermittently. Is it necessary to build a crank grinding machine when the ends don't justify the means. To respond to someones question about how they made a crank and their response is " it's made from 4140 or 4140 HT. and it was ground to size on a crank grinding machine that I built" generally isn't understood by most of the curious people at a show of exhibition.

 Lastly is functionality. Will the part, as made, hold up to the intended usage. Even in some of the most elaborate models, the Ferrari of Mr. Sceleri (sp) comes to mind, what is the intended function? It's generally to showcase the builders ability and dedication to the hobby and not to propel the vehicle down the road on a daily basis.

 Now let's talk about construction. I started out like many of my fellow builders by fabricating a crankshaft, steel crank webs generally made from CRS and shafts made from drill rod. My first exercises were to press and pin. This was followed by lightly pressing and silver soldering. In most cases these were simple single or at most double throw cranks. The processes worked for these kinds of cranks but any more than 2 cylinders and it became too much of a job trying to keep everything in line and relatively distortion free.  My first one piece multi cylinder crank was made for my 4 cylinder OHV engine. While quite small it presented the same machining exercises that would be required in making a larger or crank with more throws.
 
 I guess at this point it goes to desire. My desire was to make a one piece crank so I planned a course of action, made the fixtures, bought some metal and jumped in. Were there setbacks, of course! The worst I can think of was I had completed about half of the machining on a 4 cylinder crank and was turning the throws. I set up for the next throw and thought I had my tool in the right place, but of course I didn't, and when I started the lathe it went 'kachunk' into one of the webs and naturally bent the whole thing like a pretzel.
 
 I enjoy this hobby and the challenges that it presents, to a limit. After making at least 8 multi-cylinder crankshafts I have the process down pretty well and would never try to fabricate one. When Steve Huck was building his first Demon V-8 engine he experimented with segmented cranks and had issues with machining and assembly. That's not to say that it won't work but it became more of  task when there are easier ways.

 Ultimately it's whatever each builder wants to accomplish. If you're happy with your work and techniques then by all means keep at it but it is rewarding to think out-of-the-box and try a different approach.

gbritnell
'

Talent unshared is talent wasted.

Offline Doc

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Re: Let's talk about crankshafts--machined, built up, etcetera
« Reply #4 on: October 07, 2017, 02:49:53 PM »
Very nicely put and I totally agree!

Offline Brian Rupnow

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Re: Let's talk about crankshafts--machined, built up, etcetera
« Reply #5 on: October 07, 2017, 04:49:09 PM »
Something very important that I forgot to mention in my first post---I never drill and ream the crankshaft webs as separate pieces. I stack all of them together and either use a machinists clamp or sometimes even a bit of weld to hold them together as if they were one single entity. That guarantees that you are not building in a bias for misalignment of the crankshaft. Also on the sections of crankshaft or conrod journal that assembles with the web plates, I make them about 1/2" longer than they will end up at and using 280 grit sanding strips I put a slight and gradual taper in that 1/2" that is going to be trimmed later. This ensures that the shaft doesn't start in crooked when pressing into the web plates. The extra length is trimmed on the bandsaw later. Since I use 638 Loctite on all of my pressed together crankshaft joints, I don't recommend using an abrasive wheel in an air tool for this trimming, because the heat it generates can weaken the Loctited joints nearest to where you are cutting

Offline Jasonb

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Re: Let's talk about crankshafts--machined, built up, etcetera
« Reply #6 on: October 07, 2017, 05:11:14 PM »
I've done several from castings which are quite easy to do and would not be put off another engine if it had a cast crank, usually they are from SG iron which is more like steel to machine.

On all but the smallest built up cranks I tend to go for silver soldered construction which seems to work OK for me and I don't get the distortion Brian seems to maybe the propane heats the whole job rather than local heat from O-A, never had to resort to straightening one of my cranks with a hammer.

Again unless it is a very small crank I don't use a reamer, I much prefer to bore to size mostly on the mill using a boring head. Unless the stock is bang on size a -0.001 reamer is not much use for a press fit, need more clearance on soldered joints anyway. Never stack the two webs together, always machine separately.

I've never pressed a crankshaft together.

As for machining from solid I usually try to do that from cold rolled flat bar which does not seem to move much, if I have to do it from bright flat or round I will rough it out and then leave it to settle, don't seem to get problems this way.

Online Alyn Foundry

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Re: Let's talk about crankshafts--machined, built up, etcetera
« Reply #7 on: October 07, 2017, 06:10:20 PM »
I've made a few !!  :)

Mostly composite, Ground stock Silver Steel with mild Steel webs and Silver soldered.

I, like Brian, machine the webs in pairs using a dab of Loctite 638 to hold them together. Like Jason I then use a really flat firebrick to lay the assembly on for soldering. My flux of choice is Tenacity number 5 and to ensure ease I cut pieces of Silver solder and place at each joint. If you heat the assembly from the outsides in I found little to zero twist once the shaft has cooled.

An overnight " pickle " in Vinegar removes every last vestige of flux residue and the crankshaft is finished, bar keyways. BTW. I always centre spot the ends before soldering just in case of a twist.  ;)

When 638 first came out in the late 80's I spent the vast sum of 18.00 on a bottle and built a couple of cranks for some R.L.E. commissions, they both failed within hours. I made a very irate telephone call to Loctite's technical department only to be told that the so called 3.5 Ton shear strength on an 1 1/2" diameter shaft was torsional only, shock loads were to be avoided. Luckily the adhesive found many other uses and my heavy investment wasn't wasted. 

My two youngest sons have been " rooting " through my workshop.... They have found a couple of rejected sets of R.L.E. castings.... They're underway.... I'm trying to talk them into making a pair of pseudo forged cranks just like the agricultural engines were. Well they're trained blacksmiths for goodness sake. I hope there'll be more to report on this thread in a couple of weeks time. 

Cheers Graham.

Offline Jasonb

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Re: Let's talk about crankshafts--machined, built up, etcetera
« Reply #8 on: October 07, 2017, 06:21:06 PM »
This is about the nearest to a "forged" looking crank that I have done for the Ruston Hornsby, rest of the bar it came from in the background



Though this cast one for the Fowler has  asimilar look to it for less effort.



J


Online Alyn Foundry

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Re: Let's talk about crankshafts--machined, built up, etcetera
« Reply #9 on: October 07, 2017, 06:31:28 PM »
Nice.

The R.L.E. uses a 7/16" diameter crank. We're hoping to bend some 12mm " Black Iron " bar just like....


Cheers Graham.

Offline Brian Rupnow

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Re: Let's talk about crankshafts--machined, built up, etcetera
« Reply #10 on: October 07, 2017, 08:01:15 PM »
Graham--I am most impressed by that video. Thank you for sharing that.---Brian

Offline steam guy willy

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Re: Let's talk about crankshafts--machined, built up, etcetera
« Reply #11 on: October 07, 2017, 08:16:01 PM »
crankshafts ,Ive made a lot.
but failures were
too few to mention,
some were built up from stock,
and some, i think, were quick rejections

The learning curve,
was pretty steep,
but then, allow me to say,
it was always a means to an end,
and of course, i did it my way, !!!     

Frankly, it always turned out right in the end !!!
« Last Edit: October 07, 2017, 08:19:36 PM by steam guy willy »

Online Alyn Foundry

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Re: Let's talk about crankshafts--machined, built up, etcetera
« Reply #12 on: October 07, 2017, 08:17:46 PM »
Graham--I am most impressed by that video. Thank you for sharing that.---Brian

You're welcome.

Sadly Rob the blacksmith passed away last year, thankfully his skills have been preserved for posterity.

Cheers Graham.

Offline Jasonb

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Re: Let's talk about crankshafts--machined, built up, etcetera
« Reply #13 on: October 07, 2017, 08:26:57 PM »
I think getting the bends tight enough so you can still get the crank between the nearing blocks will be the hardest thing, not so bad on that engine or an old portable where they are far apart but not so easy on the RLE.

I think Rich (Firebird) had a go at one in his portable thread but that had PB images so can't look back.

Offline petertha

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Re: Let's talk about crankshafts--machined, built up, etcetera
« Reply #14 on: January 14, 2018, 07:46:35 PM »
Perhaps an example of a successful 'component assembly' crankshaft is the Ohrndorf V12, maybe one of the longest aspect ratios out there. And the engine obviously runs & runs well for typical model use. I have the plans & can attest to various throws & journal bits & pieces, although the (German) instructions & (lack of) assembly jigs are a little on the thin side. I suspect like most things there is some technique involved in the glued & pinned type buildups.
https://www.engineman.de/en/products-page/plaene/plan-14-zylinder-sternmotor-duplicate/#Extracts_from_the_plans

The Jung V6 & V8 are similar partial / lateral split crank assemblies, albeit a more funky tooth like union.
http://www.cad-jung-shop.de/epages/62479729.sf/de_DE/?ObjectPath=/Shops/62479729/Products/00-Z0008-0

I've seen a 1/4 scale opposed 4 aircraft type engine with 2 crank sections joined as a lap within a bearing.

These projects are above my current pay grade, but I find it interesting the amount of ingenuity model engineers have applied to this issue, taming distortion & facing some formidable machining issues.