Usage of Hexagonal columns instead of Circular???

[Tsiolkovsky]

Theory tells us that under a compressive load, a circular, hollow, cross section is the most efficient in carrying the load. This is because a circular cross section does not have in it a "least moment of inertia". i.e. there is no "weak point" to flex about like a flat ruler for example.
Can we then agree that a hexagonal column is less efficient because there are six sides among which, there are least moment of intertias. so if the member had to buckle, it would do so about any of those six sides. Whereby a circular profile could buckle among any infinite combinations.

However, I noticed on "face drill" machines used on mines that a long steel piece connects the percussion hydraulic oscillator to the drill bit. This long steel piece is a hexagonal profile, not circular.

Now i'm thinking why so? and I immediately think that it definitely has something to do with the fact that it is a dynamic loading scenario not a static scenario. Furthermore, the frequencies of these percussion bits is so high (no idea maybe >60hz?)it may interfere with the natural frequency of the long steel piece connecting the drill bit.

So maybe a polygon shape somehow distributes the buckling one direction then along the other side and other side and so on. whereas a circular one might buckle once and keep buckling along that same spot and fatigue will set in much faster because the same side is flexed upon.

I actually dont even want an answer, hehe, a clue or hint or a "youre thinking in the right direction" will do me just fine. I want to see if I can figure it out.

 

[Tsiolkovsky]

Shouldn't of used the term "buckling", just replace that with the word "flexing".

 

[MintJulep]

Cheap and "good enough" trumps efficient.

A hex can be cheaply and easily fabricated from flat plate.

 

[Tsiolkovsky]

But remember; these shafts break often and are under tremendous stress (I was told). I think the fatigue gets to it quickly, so, efficiency becomes extra important. Could be wrong though.

 

[ornerynorsk]

A hex makes a convenient wrench flat, it provides a means of driving rotationally without relying on shear pins, and I believe the load is more torsional than compressive in mining down-hole tools, although that depends on the guy running the rig.

 

[IFRs]

Hex may be stronger if the pipe diameter is equal to the distance across the flats because the bends extend further out.

 

[hydtools]

Tsiolkovsky, how do you mean most efficient?
Like IFRs suggests, compare hex with round having distance across hex flats equal round diameter.
Hex shape has an inherant drive shape. Drill steel rods are upset by forging to create a drive shank and collar. For these directly impacted drill steels the hex shank fits the hollow hex drive of the drill motor. Failure usually occurs under the collar at the section area change.
Impact rates are in the range of 600 to 1500 impacts per minute, depending on the size of the drill.

To use a round shape would require creating a transition piece to create a feature to transmit rotational drive. Drills which use striking bars can use round drill steel. One end of the striking bar engages the rotational drive of the drill motor. The other end is threaded and attached to the round drill steel may means of a coupling.

Ted