Build up crank shaft?????

[burnit0017]

Hi, I am trying to fabricate a crank shaft from individual parts. I am using set screws to hold all the parts together. It is very rigid but there is a lot of run out; wobble; at each end. Are there any posts showing the correct fabrication method or does any one have any suggestions how I can get better results???   

[philjoe5]

You didn't mention if this was for an IC engine or a steam engine.  I'll assume IC, then I'd use robust pins.

It looks like you have 3 centers of rotation.  I'd take a billet of steel long enough to make 4 webs + waste.  Drill the three holes of appropriate diameter while holding the work in a 4 jaw with the proper offsets.  Part off, or slice off the 4 webs with a saw.  Face the webs to spec thickness.  Place three rods through the 3 holes of each of the webs and place the webs at their respective locations with spacers.  Put a drop of loctite on each web to hold the whole structure together.  Now pin the webs to the appropriate rods.  Then use a hacksaw to cut off anything that doesn't belong.

I've used this method for a crankshaft with a single throw and have gotten 0.009" TIR on the end of a 6" crankshaft which has worked out OK for me.  BTW how much runout are you getting?

Cheers,
Phil

[burnit0017]

Hi, I am trying to fabricate a OPOC steam engine to power a permanent magnet alternator for small scale battery charging. The run out of the crank shaft when rotated between two bearing pillow blocks is about .027".  It looks like the problem is with my drill press table. When I press on the table with my hand it will deflect as much as  0.05". So the holes that I am drilling are not perpendicular to the face of the disk.

Today I will drill some new holes using the drill on my mill, hoping for better results. thanks for the info.

 

[gbritnell]

A multi piece crankshaft requires very accurate machining of each piece because of the stack-up of dimensions at final assembly. If your shaft to bore tolerance is more than .0005 and then you press a set screw against it it's going to cock. Now do that 3, 4 or 6 times more and the total runout could be quite substantial.
As Phil mentioned there are better ways to do it.
gbritnell

[Ian S C]

Here's the diagram I have on the Stirling engine forum, for those who have little in the way of equipment to make anything(there seems to be quite a few of them).  I'v not tried on a multiple crank engine, but I'm sure it could be done.  I prefer to hack my cranks out of a solid lump of steel, but my smallest motor (3/8" bore x 1/2" stroke) was built this way, holes drilled with a hand drill for 3 mm shaft, and ball races for the big ends.   Ian S C

[burnit0017]

Hi, thanks for the info. I rotated the disks 90 degrees and drilled new holes. Big improvement using the drill on my mill. The next problem I need to solve is hole placement. I am finding it very easy to drill a hole in the wrong location. Is there a method I can use so all the holes are in the same location on all 4 disks????  I do not have a DRO on the mill.

[philjoe5]

I would drill all 4 holes in a single piece of stock, then either saw or part off the disks.  Face them so they're true.

Cheers,
Phil

[peatoluser]

Drill & ream one hole in each of the 4 blanks.
 Mark out a piece of flat bar with both hole centres.
drill and ream one hole in this
fit a plug of silver steel (drill rod) in this hole
line up the other hole centre, and clamp bar to table
then fit each blank over the dowel, clamping this down as well ,and starting with a centre drill or spotting drill, drill and ream the other hole
you may have to slightly reduce the dia. of the dowel with emery cloth to help ease the blanks on & off it
Although I prefer to use a similar method on the lathe (no table deflection that way)

yours

peter

[burnit0017]

Hi, thanks everyone for the great suggestions. I increased the throws to 5/8 and turned down the end to 3/8. This created a nice shoulder for the disks to sit on. I also fabricated a simple clamp to aid in assembly. Then for the ends of the crank shaft I used some 3/4 round stock than mounted the crank shaft on the lathe between centers and used a dog to turn down the ends to 5/8. This reduced the run out to about 0.002. Good enough for my project. Thanks again.

[burnit0017]

Hi, I changed the fabrication method a small amount. I made 5/8 spacers and use 3/8 rod for the center. I made a smaller shaft and the run out is still about 0.002. When I turn down the ends from 3/4 to 5/8, I am having difficulty. Is there a method to prevent from making them to small???