Steel Vs Titanium Fasteners - Preload / Clamping Force

[jack20]

Hi,

I am trying to determine the difference between a steel and titanium fastener of the same diameter and length when torque tightening to a standard value.

Obviously the steel fastener is stiffer than the titanium and I believe this means the titanium fastener is likely to have a higher preload and a be under more tension than the steel? I need a quick simple way to demonstrate this can somebody help point me in the right direction?

Thanks in advance

 

[rb1957]

I'd've thought it was an E issue (steel being 2x Ti).

But for the same torque, I'd've expected Ti to be 1/2 the preload of steel.

But I'd expect that Ti would require a higher preload than steel (in order to support the same load).

another day in paradise, or is paradise one day closer ?

 

[SwinnyGG]

If the thread pitch is the same, and you apply the same torque, you get the same preload- but because of the E difference you get that preload with twice the elongation in the titanium bolt.

 

[drawoh]

jack20,

The standard torque equation is T=CDF, where F is the tensile strength, D is the major diameter and C is the friction factor. High strength steels and titaniums have similar strengths, so F is approximately the same. If you lubricate the joints, the frictions probably are similar. Elasticity is not a factor if you tighten using torque.

This is a crude model. We know that screw torque converts very approximately to tensile force. If you tighten by watching the turns of the screw, titanium will be very different than steel.

How accurate are you trying to be? There are FAQs here on bolted joints. You can read these and work them out for steel and titanium.

--
JHG

 

[jack20]

Thank you for your answers, I understand this and it matches my pre load calculations.

I am trying to demonstrate why a steel fastener rotates under load and an identical fastener made from titanium at the same torque doesn't. I would be very grateful to hear your theories.

The friction between threads may be higher for the titanium fastener if it is under more tension than the steel so I assume my issue may not be preload and may be more related to tension?
This means to get the same elongation on a steel fastener I would need to double the torque?

 

[EdStainless]

While the loads are the same for the same torque the elongation of the bolt isn't.
The Ti bolt will stretch twice as much as the steel one.
Perhaps this is causing enough thread interference to prevent loosening.
Another possibility is that the friction on the Ti fastener is higher and helping prevent rotation.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[SwinnyGG]

I assume my issue may not be preload and may be more related to tension

Preload = bolt tension and bolt tension = preload.

Do you know that the loading in both applications is exactly the same? Thread friction will certainly be different between the two materials- so if the torque is being applied dry, it is very likely you aren't actually getting the same preload.

 

[Nescius]

The standard torque equation is T=CDF, where F is the tensile strength, D is the major diameter and C is the friction factor.

I think you meant F = the clamping force.

 

[Tmoose]

" a steel fastener rotates under load "

I'm having a hard time PICTURE-ing what is happening.

Titanium is generically famous for not getting along with other metals in a sliding relationship, even when well lubricated.
In the early days titanium connecting rods had bronze inserts inlayed into the big end faces that just slide against the crank journal cheeks.
Nowadays the rod BE faces are treated with fancy DLC coatings etc.
DITTO Titanium valve stems.

Maybe the Ti bolt and nut are galling in place.

 

[drawoh]

Nescius,

If I am doing the calculations, it is the tensile force that I want. Of course, this also is the clamping force.

--
JHG

 

[Nescius]

Understood, drawoh. When you said "tensile strength", you're not meaning the mechanical property of the bolt material, but the discrete "strength" of the fastener itself.

That'll clarify for future generations of young engineers that read this thread...and try to plug 150ksi into that equation.

 

[EdStainless]

Do the bolts have precision ground length that you are measuring (say by UT) so that you know exactly how much they are being stretched?
How are you measuring the actual clamping force?
Torque alone does not cut it.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[tbuelna]

You might consider other factors like CTE mismatch between the bolt and clamped structure materials that occur with temperature changes during operation.

 

[rb1957]

i was wondering ('cause I can't be bothered to do the calcs) but isn't the general torque/preload relationship true for steel fasteners (which we'd use 99% of the time). I'm thinking ... torque > twist > elongation > strain > stress > preload. With two bolts, same except for material, apply a torque, reasonably same rotation and elongation and strain, but different stress (E) and different preload.

thoughts?

another day in paradise, or is paradise one day closer ?

 

[SwinnyGG]

No.

If you assume rigid bodies and no friction, the preload achieved as a result of applied torque is a function of the thread pitch and diameter of the fastener, and nothing else.

All the material does is change what the fastener's elongation is at that level of preload.

In the real world of course, friction exists and bodies aren't rigid, so it becomes more complicated- but the basic principle is still true.

 

[Nescius]

rb1957,

Think of it this way: threads are nothing but a wedge/ramp that is wrapped around something. The angle of the ramp is the helix angle of the threads, which depends on the pitch and diameter of the screw.

 

[rb1957]

i quite get that, hence torque gives you elongation (assuming friction and same thread form); but going from elongation to load uses E (strain > stress)

another day in paradise, or is paradise one day closer ?

 

[SwinnyGG]

You have the relationship backwards. You can't determine elongation without knowing E. Torque tells you bolt tension, bolt tension tells you elongation.

 

[rb1957]

ok, apply a torque to a bolt.

This twists the nut, which moves axially along the bolt due to the thread, which stretches the bolt.

This stretch strains the bolt and from Hooke's law we know the stress and hence the load induced by the torque.

Therefore the preload in a Ti bolt of the same form as a Steel bolt will be less (about 1/2) ... assuming the same friction, etc.

another day in paradise, or is paradise one day closer ?

 

[CWB1]

Tmoose hit the nail on the head. Titanium fasteners are notorious for galling which is why you are seeing the difference in retention.