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Heat shrink fitting and crankshaft balancing.

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picclock:
Hi
I intend to make a crankshaft by using heat to shrink fit the parts together. Does anyone know of a resource or set of rules whereby I can calculate the correct sizes and temperatures to shrink fit the parts?

To explain more clearly, the main shaft of the crankshaft assembly will be 10mm silver steel. So to heatshrink the crankweb, 8mm thick steel, to the main shaft I will need to make a hole that is smaller than 10mm, then heat the crankweb until the metal expands enough to force the 10mm steel into the hole. So I need to know how much smaller than the 10mm to make the crankweb hole and what temperatures I will need to heat the webs to.

This seems to me to be the best way of making a truly rigid and accurate  assembly. I can leave the  main shaft as 1 long piece  and shrink on the crankwebs and big ends. Then I can cut out the parts of the main shaft that are no longer needed. Because its all assembled with the shaft in one piece the alignment should be spot on.

The other thing I need to know is about how to balance the crankshaft for minimal vibration. The amount of metal left on the crankwebs as a counterbalance should be defined by a formula or somthing similar. If anyone can help with any info on this I could possibly stretch to a virtual beer.  :beer:

Many thanks

picclock

John Rudd:
I would have thought the best way to make a crankshaft would be out of one piece turned up in the lathe...less effort than trying to piece together a load of hot bits and then beating them into submission to get the bits to line up...

Mebbe someone else can chime in with a better way... :dremel:

picclock:
Hi John. You are correct but :

1. I want to fit needle roller bearings to the assembly (mains and big ends), which makes it tricky to get them on :-).
2. I reckon I would need a 40mm piece of steel to turn down which is a HUGE (for me) amount of work.  :bugeye:  Also I would find it hard to get the required level of finish for said roller bearings.

I could perhaps turn down sections but I think the heat shrink way should give good results with less effort. Also I can harden the silver steel mainshaft and big end pieces before assembly. Cutting a hardened shaft is not difficult with an inox grinding wheel cutter. I considered 'Bogs' puddleduck engine method but concluded it would be too floppy. I am hoping to get some serious power out of this thing so it needs a good stiff crank. I have some heavy duty arc welding kit if it comes to it, but the shrink idea would probably be stronger and need less cleanup. 

Using the 'shrink' method I can turn down the shaft where it needs to be removed  (by 2mm or so) and supply a lead in for the cankweb to start on. Once it's done removing the unwanted bits is easy. I'm only making a two cylinder engine so it should be fairly easy to do (famous last words).

I also considered tapering the shafts and holes, applying a lot of pressure, then drilling a hole and using a steel pin to lock it, or threading the parts with a small shoulder and locking them with a through pin.
 
Many thanks

picclock

rleete:
Make lands on the shaft, and press fit the parts on.  .001-.002 interference and red locktite will hold up to an amazing amount of force.

picclock:
Hi rleete

We may have a communication problem. When you say lands do you mean flats like if you looked at the shaft end on, it's shaped like a D ?.  Not a problem to do but how do you cut a hole thats D shaped ?

Let me know if I got it wrong.

Many thanks

picclock

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