Dropping diameter of crankshaft for the Webster engine.

[deere_x475guy]

I want to drop the dia. of the crankshaft from 5/16ths to ¼ on the Webster engine I am building so that I can use the bearings I ordered from Boca Bearings. They didn’t have anything with a 5/16ths ID in the 99cent list. What do you guys think about me doing this? Will this significantly effect the strength of the shaft? I do want to try to run a small generator with this and plan on adding larger fins for cooling.

 

[wareagle]

A couple of thoughts here, and if I am wrong, the others will surely correct me.

First, if the shaft is turned down at the bearings, this might create a weak spot for a failure to start its campaign.

Second, if you turn the entire shaft down, then the distance between the bearing mounts will affect the amount of stress the shaft can take. It won't have the mass to support the load without deflection, at least from my take.

My thought is to turn the crank down at the point the bearings carry the shaft leaving a slight radius to avoid a stress riser. Mount the bearing as close to the radius as possible. The extra meat on the shaft will help keep it rigid under load, but remember that the weak point will be where the bearings are. In my opinion, you should be okay as this isn't a high horsepower high torque application.

I have to admit, I bit on the $.99 bearings myself, and have quite a few coming.

 

[Bogstandard]

Hi Bob,
Having given the plans a good dose of looking at I can see no problems with going down to a 1/4" shaft.
Wareagles concern over the drop down is a very valid point, and a radius should always try to be incorporated into a turned down stressed part. It is because of this, if you look carefully at a ball race, it should have like a small countersink on the lead in to the hole, this is to allow for the small radius.
But in this case, I would use 1/4" drill rod instead of cold rolled steel, this will help compensate stress wise for the reduced diameter, plus you will most probably get a better fit to your bearing bores.
What you need to do is print out another set of plans and go over them with a microscope and any reference to the original shaft size should be altered to the new shaft size, and only use that set of plans, otherwise you will make a part only to find you have used the wrong drawing set and the bits are rather loose.
I do this type of thing on all the engines from plans that I build, and usually the designer has over engineered the engine, so this type of modification can usually be incorporated without any problems.

BTW, you can get cheaper bearings on ebay, but they come from the far east and you don't know the quality, plus you have to have the qty and size that they say. Boca seem to have got their act together and give you only what you want, and they cost only pennies more.

I hope this helps

John

 

[deere_x475guy]

Guy's thanks for the quick responses. What I am actually doing is using 1/4" drill rod and not turning down the 5/16ths. Sorry I should have been clearer on that.

Bogs I printed out a new set of plans last night and started dropping the dia on all the references I could find.

 

[deere_x475guy]

I am also cutting my own gears and need to change the location of the flywheel height. I need to buy another cutter to make a 13 to run with a 26 I already made for practice a year ago.) Big difference in pitch diameters between a 20p that I already own and the 32p that are called for in the plans.

 

[steamer]

droping the diameter from 5/16" to 1/4" will decrease the bending stiffness by 40%

It goes as the 4 power of the diameter. That is actually quite a bit

Dave