Why do Parts Break?

[Kiwi_Roy]

A couple of guys talking about parts breaking
https://www.youtube.com/watch?v=QiIT6G3GU4o

I don't know that they have a solution

[Galactica]

Hey.  I’m just over a week since my surgery.  All went well and I’m recovering.  But I’m hoping I’ll need some help.

I’m hoping to look at a CB750 to get. I won’t physically be able to do much for a few months at least.  If I manage to get the bike, I’ll need help to pick it up and maybe some place to stash it until I’m able work on it.  I’m hoping to see it soon.  I just hope I’m not too late.

If anyone might be able to help, please let me know.  I’ll keep you posted on whether I even get the chance to see it.

[hardrockminer]

There is a lot of good info there but I disagree with his opinion on roller bearing crankshafts.  The Z1 crank is built on roller bearings and is pretty much bullet proof. 

Generally there is a factor of safety built into the design of any part.  (One of my old mechanical engineering profs called it a factor of ignorance.). You design a part to withstand a multiple of the expected stress.  This is because fabrication processes aren't perfect.  They can leave voids or inclusions that lessen the theoretical stress limit of the material.  When the multiple gets too low the part will fail sooner than a similar one with a higher multiple.  If it's a wear part it will have wear specs.

My Dad was a WW2 aircraft engine mechanic for most of his service.  Their training initially required them to use a file to bring parallel surfaces on a block of steel to within one thou of an inch over 4 inches of length.  He said later he found out how important this was when he began working on spitfire engines where the head was mated to the barrel without a gasket.  Imagine how little wear would cause the engine to be taken out of surface.

[rz5mark]

A buddy tells a similar "file" story during his machinist apprenticeship.

[Galactica]

Hey.  I’m just over a week since my surgery.  All went well and I’m recovering.  But I’m hoping I’ll need some help.

I’m hoping to look at a CB750 to get. I won’t physically be able to do much for a few months at least.  If I manage to get the bike, I’ll need help to pick it up and maybe some place to stash it until I’m able work on it.  I’m hoping to see it soon.  I just hope I’m not too late.

If anyone might be able to help, please let me know.  I’ll keep you posted on whether I even get the chance to see it.

Oops, wrong thread.

[Wheels]

Ross.  I will call you later today and see if we can help

[Galactica]

Ross.  I will call you later today and see if we can help

Thanks John.  I might not even get the chance to see it.  I dithered for a few hours.  They have someone going to see it this morning.  I’m second in line.