Titanium (Ti6Al4V) Spur Gear

[alpedro]

Hello,
does anyone know the allowable contact stress fot the titanium ti6al4v? i need it for the pitting resistence of the gear. On standard i have only found data or formulas for steel and some other materials (bronze, aluminum), but nothing about titanium.
At the moment i'm considering the formula for steel taked from agma standard (Sac = 26000 + 327*HB [psi])
Do you think i could use the bearing allowable stress?

Thanks a lot if anyone answer.

alpedro

 

[CoryPad]

I don't know of published gear data for titanium alloys. I think you could use the bearing stress for a few cycles, but titanium galls readily, so likely that value will need to be knocked down a bit to account for this.

 

[alpedro]

First of all, thank you!
The bearing yeld strength is 1480MPa, the value i calculated with the above formula is 932MPa.

 

[tbuelna]

alpedro,

Gear pitting resistance is a measure of resistance to surface compressive fatigue loads under hydrodynamic oil film contact conditions. Pitting resistance is only valid if your gear tooth contact is operating with a full hydrodynamic oil film support. If your gear mesh has mixed or boundary contact, your gears will fail (rather quickly) from scoring, not pitting.

There's no theoretical reason why gears can't be made from titanium, as long as you design for hydrodynamic contact. However, titanium has poor thermal conductivity, is expensive, and cannot be properly case hardened. So a titanium gear would have less capability than a less expensive (and smaller) case hardened alloy steel gear. I would imagine that's the reason it is not used for gears: there's no benefit.

Titanium, like most metals, has a surface compressive stress limit that varies with the number of load cycles. The failure mode for surface (Hertzian)contact is usually a surface spall (ie. pitting) due to a sub-surface initiated shear failure. A contact analysis will give you the depth and magnitude of the max sub-surface shear stress for two elastic bodies in contact. The shear stress should be within the allowables published for the alloy and heat treat of your gear material, with regards to fatigue cycles.

If you design your titanium gear correctly, with adequate margins for contact, scoring and tooth bending, there's no reason it shouldn't work. But as I noted, a case hardened alloy steel gear would be smaller, lighter, more durable and less expensive. So what's the point of using titanium?

Good luck.
Terry

 

[alpedro]

thank you very much for the answer. I had already decided to use steel instead titanium, since these gears have to be used for a space application and, even if the steel gears will be heavier than the titanium ones, the former are more reliable than the latter.

thank you again

Goodbye,

alpedro