Lets talk about Crankshafts--Machine, built up, etcetera

[Brian Rupnow]

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

 

[Herbiev]

Some great information there Brian. Thanks for sharing.

 

[arc]

Thanks for sharing Brian,. I'm a beginner at crankshaft building but the last one built I machined the two ends and throws from solid.The crank pin was of a harder grade of steel. Then I broached the throws and milled a keyway in the crank pin.I pressed it together with a .001" fit . It worked out really well.

 

[JCSteam]

I think this post illustrates the H7 fit which im quite happy to learn more about

 

[nautilus29]

I think this post illustrates the H7 fit which im quite happy to learn more about

Are you referring to the metric tolerancing charts? If so H7\h7 is more of a slip fit tolerance as the hole always has a -0 for the low limit\ and the shaft always has a +0 for it's upper limit. A N7\n7 chart is probably closer to the press fit he's talking about.

Disregard this if I'm understanding you wrong lol.

 

[nautilus29]

--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.

Great post Brian I learned alot from it! I'd agree with you that .0015 is overkill for a crank of that size. This is just my experience (and a guess at why, not real engineering data), but is seems like a smaller hole can't expand as much a larger hole. For example I was just pressing together a .189 pin in a .188 hole and it took a significant amount of press to mate the pair. The same .001 press for a 1 inch hole will slide in with much less effort.

I've had issues with smaller bores in the past getting tore (with .0015 to .002) unless a significant lead in was put on the front of the shaft. This would cause the shaft or pin to be out of location since the bore was compromised. I now try for .0005 to .001 press and have had much better results. I haven't made my own crank though, so I'm not sure if .0005 would be sufficient.

 

[Brian Rupnow]

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.

 

[fcheslop]

Hi, for multi cylinder cranks Iv found and its only my personal experience
When machining from the solid stress re leave the blank first .I simply heat it to cherry red and let it cool slowly buried in the fires ash
as each throw is machined I then put a jacking nut and bolt across the webs and hold it in place with a wrap of tape.
For built up cranks I tend to use silver steel and what evers to hand steel wise for the webs.
Ream the the webs and put a couple of shallow grooves on the pins were the webs sit ,
Assemble with a cylinder fit compound I think its loctite 641 as it gives upto 30 mins play time before it grips depending on the ambient temp. then drill and ream for taper pins these are also given a spot of glue
Iv never had any luck hard soldering cranks they just seem to end up black and twisted
Just my two bobs worth take it or leave it
cheers

 

[bouch]

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

Don't know if this "works best", but its arguably the best crankshaft I've ever made...

For a Stuart Turner #1, so a single cylinder engine and a 1/2" shaft with a 1" offset in to the crankpin (2" bore) Each web is also 1/2" square, IIRC.

I bought a "partially machined" casting set and the Stuart cast crank was well beyond the point of salvaging it, there was nothing remotely close to right about it.

I built up a crank out of stock, making everything 1/32" oversize in each dimension. (that is, 17/32" dia shaft, and 9/16" square web) Once all the parts were made, I put it together and added taper pins in each joint to hold it solid. I then disassembled, fluxed everything up, re-assembled (pushing the the taper pins in with a small arbor press!), and silver soldered everything together.

Once it had cooled and all cleaned up, I then treated it as a cast crankshaft, and machined it between centers on the lathe to finish dimension.

WAY MORE work than necessary, but I made one hell of a crankshaft!