Oh those CERTIFIED plane engines !!!!!!!!!!!!!!!!!

wrote:

...

Hollow shafts are stiffer than solid shafts. The stresses are
all concentrated in the outside wall, with no central material to act
as a fulcrum to stretch the outside on bends. Try bending a piece of
5/8" bar and one of 5/8" tube sometime (same material, of course) and
see the difference. The bar will bend, but the tube will resist bending
until it suddenly kinks. Cranks need to be stiff, especially where they
are loaded with gyroscopic forces, and need to be light, so they're
mostly hollow.


More importantly a hollow shaft that is the same weight as a solid
one will have a larger diameter which gives you much better
stiffness, especially in torsion, for the same weight.

--

FF

wrote:
wrote:
...

Hollow shafts are stiffer than solid shafts. The stresses are
all concentrated in the outside wall, with no central material to act
as a fulcrum to stretch the outside on bends. Try bending a piece of
5/8" bar and one of 5/8" tube sometime (same material, of course) and
see the difference. The bar will bend, but the tube will resist bending
until it suddenly kinks. Cranks need to be stiff, especially where they
are loaded with gyroscopic forces, and need to be light, so they're
mostly hollow.


More importantly a hollow shaft that is the same weight as a solid
one will have a larger diameter which gives you much better
stiffness, especially in torsion, for the same weight.

Yes. The outer diameter of the hollow tube will be slightly larger than
the solid rod. On the drawing board, if the outer diameter is strictly
limited (by crank journal diameter), or slightly limited (if the block
allows slightly larger journals), then changing the crankshaft from
solid for hollow this may or may not be an issue. (Pretty wordy!)

In reality, swapping out a solid crankshaft for a hollow one of equal or
at least sufficient strength and stiffness is not always simple,
prudent, or even possible. (Pretty wordy again!)

Here is a good reference for anyone (regardless of their level of
engineering knowledge):

http://www.engineersedge.com/calcula...re_case_12.htm

You can play with the numbers a bit. Moment of inertia is directly
related to stiffness (in torsion) while (cross section) area is directly
related to weight. Section modulus is directly related to bending strength.


PS- Fred, you (and others in this thread) obviously have a good handle
on this stuff. My post is mostly directed to the audience. Disclaimer,
I'm 99% sure I got the working definitions right. Corrections are
welcome