Build your own crankshaft

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Anne from Little Britan
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Build your own crankshaft

Post by csonics » Wed Nov 18, 2009 4:16 pm

Posted on behalf of stevey_frac:

stevey_frac
Lighting the Boiler


Joined: 28 Jul 2009
Posts: 37

Posted: Wed Jul 29, 2009 10:01 pm Post subject: Build your own crankshaft
Hi Guys,

I've kinda worked out a lot of the trickier details for the engine I want to build, however, I don't know the best way to the crankshaft.

The idea I'm working with at the moment, is to use 1/2" stainless steel round, and use some steel webs, then sink a 1/4" hole, tap it, then put a stainless bolt through the joint, holding the web and the crank together.

I'm worried it would flex, and there would be a lot of wear. Especially if you start loading it up with 300 lbs turning force.

Thoughts?

-Steve
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steamboatjack
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Joined: 01 Oct 2007
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Posted: Thu Jul 30, 2009 2:03 am Post subject:
reproduced below, hopefully is a small item I did for the "funnel" magazine on this subject. Steer clear of stainless steel its not as strong as you may think, use a good H.T. alloy or straight carbon steel.

In the pages of funnel there has been numerous items concerning crankshafts and their construction for steamboat engines and probably more tears shed than in any other part of construction. My personal view, is that the methods discussed below are the most suitable and I would not consider any other method bearing in mind that steamboats often operate in conditions of a very high torque to power ratio what is known as a “torque rich” condition, this is often due to the idea that even small boat engines should run at 200 or 300 revs per minute turning a huge propeller, a view I do not necessarily share.

Solid construction, many crankshafts are machined from solid using stock material, If you have a Dean Smith & Grace or another fine large capacity lathe made in Yorkshire, or the midlands all well and good. Carbon steel should be selected of at least 600 N/mm2 tensile strength. I use EN8 generally for this type of work, its easy to machine and is of sufficient strength providing the dimensions are sufficient and (most important) the change in sections are provided with generous radii. The radius at the crankpin and main journals should be at least 0.125 of the journal diameter and the bearings should have a chamfer sufficient to clear this. Ideally the journals should be finished on a crankshaft-grinding machine, automotive repair engineers can usually do this, the grinding process will accommodate the radii and there is no need to leave the crankpin centre bosses on. Modern crankshaft grinders use a wheel head that reciprocates to suit the eccentricity of the crankpin in question.
Should you be a dab hand at pattern making, then S.G. iron could be used although bear in mind the strength of the material will be less than EN8 and the radius is even more important. Many years ago at marine engineering college I saw a film that showed the construction of a crankshaft for a Doxford oil engine (I don’t use the “D” word as the oil engine was invented by Herbert Ackroyd Stuart an Englishman). Anyway the chaps at Doxford & Sunderland had a large forging which was to be made into two crank throws with a main journal between (part of a crankshaft) this was a U shaped forging and one leg of the U was lowered into a pit and the centre part heated up to red heat, the other leg was then twisted though a large angle (forget what) to form a two throw piece. If this can be done with a forging weighting many tonnes I am sure it could be done with a profile cut slab of EN8 to form a two-throw crankshaft. Obviously stress relieving and a large machining allowance are essential, but it could save on material for anyone wishing to try (I have never done this myself).

Fully built construction. A fully built crankshaft is one where the main journals and the crankpins are shrunk into holes in each web. The main shaft is kept as a single piece until the shrink process is complete and then cut out before final skimming of the web inner faces if required. This type of construction has the drawback that the fillet radii as mentioned above cannot be used. Many large marine steam engines were built by this method using a web profile similar to fig AA. These were often fitted with pins installed axially along the join between the main shaft and the web. The fitting of any kind of pin is NOT recommended. In the case above the pin could in fact compromise the integrity of the interference fit and pins fitted radially are of limited use as torque transmitters, any pin hole produces a stress point, don’t use them. The same could be said of oil holes, these should only be contemplated with solid crankshafts and if used all drilled ends should be carefully radiused. The surface finish of shrink fits should be as good as possible, grinding is best but careful turning and polishing will suffice, the idea that a “rough” surface will grip better is nonsense and purposely roughed surfaces should be avoided.
The correct interference fit is 1/600 of the diameter concerned.
i.e. 1.67 thou per inch of diameter, obviously a tolerance needs to be used but it should be a close to this as possible. The selection of material is very important and 600 N/mm2 tensile strength is again the minimum particularly for the webs. The minimum web thicknesses both axial and radial and the ligament width between pins shown on the drawings must be adhered too to avoid over stressing the webs, which would lead to failure of the shrink fit.
I personally would not use the fully built method unless I was constructing a very large shaft, as I believe the “semi built” method is superior,

Semi-built construction. This is the method used for large marine crankshafts weighing 300 tonne or more and transmitting over 100000 H.P. the main reason for this method is two fold.
A) A large radius can be accommodated on the crankpin/web corner. And B) The vertical distance between the diameter of the crankpin and the main journal can be made small (which is not possible with two adjacent shrink fits) and therefore the journal diameters can be made larger for a given stroke of engine.
Each cylinder unit consists of a pair of crank webs and the crankpin machined from solid, these are then shrunk onto the main shaft as in fully built construction. If the profiles shown in fig BB or CC are used the web units can be machined from round stock with a minimum of waste and machining time. As with solid construction it’s often easiest to chop out the bulk of the crankpin area using a milling machine and dividing head rather than the lathe.

Fully built using vacuum brazing. I am building two engines currently which came with partly finished crankshafts, this has apparently been vacuum brazed, I am not familiar with this construction method but believe it is a “high tech” process used a lot in industry. The integrity of the brazed joints is probably good but the design problems of the lack of radii are still a concern especially as this is a three-throw crank running in six main bearings, the cranks are radially pinned but this is probably for assembly reasons. The photo shows the second crankshaft as received from the brazing process, I intend to reduce the size of the hideous balance weights and fillet out the right angle internal corner on the webs, a very poor idea both mechanically and aesthetically, had I built these myself I would not have fitted balance weights at all on a three throw crankshaft.
Should you consider the use of balance weights I would suggest the web construction as in fig DD this is a fairly traditional design for launch engines, additional weights can be attached on the insides provided the tapped holes are well away from the shrink fit or they can be part of the web if fully built, either way its obvious that the weights should clear the connecting rod. Personally I believe that weights even on a two crank ninety-degree shaft are rather a waste of effort as you can never fully balance such an engine without recourse too much complication, but that’s another subject.
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stevey_frac
Lighting the Boiler


Joined: 28 Jul 2009
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Posted: Thu Jul 30, 2009 8:18 am Post subject:
I don't have the equipment to do any of these methods. I don't own a lathe, or a mill. Pretty much i have a drill press, a metal cutoff saw, and various hand tools.

I wanted to stay with stainless for the corrosion resistence, but perhaps i can use a carbon steel and just keep it oiled all the time. I think for a single cylinder with a 1" bore, the stainless will be alright. It still has about 70% of the strength of a carbon steel i think.
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stevey_frac
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Joined: 28 Jul 2009
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Posted: Thu Jul 30, 2009 12:23 pm Post subject:
I bought the steel just now. I stayed clear of the stainless, and got cold-rolled steel, which i think is carbon steel. We'll find out anyways.

Because it was so much cheaper, i was able to do get larger pieces, for less money, including a nice thick plate to mount it all on. The final version will have to be mounted upright, but this little test engine will probably only produce a horse or two, so i'm not worried about it


Why i think i'd have a hard time making the shrink joints. is how do i make a hole slightly smaller then my shaft? I don't think i can buy a drill bit marked as "slightly less then 3/4".

As of right now, my plan is to weld them, and hope it dosen't warp to badly.
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Maltelec
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Joined: 23 Sep 2007
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Location: Cumbria, UK
Posted: Thu Jul 30, 2009 3:55 pm Post subject:
Don't discount Loctite. The high-strength stuff is seriously strong as long as the metal is cleaned with acetone or electronic cleaning solution. It wouldn't be my choice but then I have the machines (and I hope the skill) to make a shrink fit.

Saying that I have just loctited a crank for a 3/4" bore and stroke model engine. For a model which is never going to do any real work I see little point in going to all the trouble of making a super strong crank shaft.

Quite different for a real working engine.
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stevey_frac
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Posted: Thu Jul 30, 2009 7:12 pm Post subject:
What's the exact kind of loctite your talking about??

There's a whole bunch out there.
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steamboatjack
Urchin


Joined: 01 Oct 2007
Posts: 39

Posted: Fri Jul 31, 2009 1:55 am Post subject:
below is the drawing I forgot to post!
steve, without a lathe your choices of producing a working crank are very limited, I would find a friend who can do it for you, if however you cannot I could only suggest the following:- buy some "silversteel" don't know what its called in the US but its precision ground round stock, and use a reamer on the holes making sure you ream the webs in pairs. assemble with loctite "high strength retainer" in UK its 638 or 601. You could also cross pin using a spring pin of say 3/32 (for a 3/4 dia crank). keep the main shaft in one piece only cutting out the throws on completion.
regards jack
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mcandrew1894
Full Ahead


Joined: 11 Oct 2007
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Posted: Fri Jul 31, 2009 3:14 am Post subject:
A good substitute for "silver steel" here in the US is 0-1 tool steel.

This comes in turned ground and polished (TGP) and is to size and round.

It's a good start for a crank

Dave
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stevey_frac
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Joined: 28 Jul 2009
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Posted: Fri Jul 31, 2009 10:13 pm Post subject:
I haven't gotten to the crank yet. I'm having trouble boring out the cylinder.

I put in a 3/8" pilot hole, and i'm try to get my 1" bit through it, but its skipping really badly, and throwing the piece around, and i just can't hold it by hand. I need some sort of funky vice or something I guess? Not sure what i'm gonna do with it yet.

Steve
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Maltelec
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Joined: 23 Sep 2007
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Location: Cumbria, UK
Posted: Sat Aug 01, 2009 3:00 am Post subject:
A large vice or screw it to a large bit of wood is a good work around.

Its cast iron I assume? If your drill is a Morse Taper with a flat bit at the end it may also have a hole in the end, this allows you to hold the drill with a point in the drill chuck and then use a spanner to turn the drill by hand. A slow but steady method of making a hole.
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stevey_frac
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Joined: 28 Jul 2009
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Posted: Sat Aug 01, 2009 8:53 am Post subject:
Actually, it's a solid steel block, and i'm boring it out with a multi-purpose drill bit. The steel is very hard!. I think I may try claming some wood around it, then clamping the wood to the table.

The purpose of using the steel was to ensure that the block won't be a weak point up to any pressure I could reasonably generate.

it's a 1" bore in the middle of a 2.5" square block, so there's 3/4" of steel between any potential steam and the outside world. That should hold over 1000 pounds without difficulty, not that I'd ever run it that high.
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