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Steel Vs Titanium Fasteners - Preload / Clamping Force

Steel Vs Titanium Fasteners - Preload / Clamping Force

(OP)
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

RE: Steel Vs Titanium Fasteners - Preload / Clamping Force

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 ?

RE: Steel Vs Titanium Fasteners - Preload / Clamping Force

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.

RE: Steel Vs Titanium Fasteners - Preload / Clamping Force

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

RE: Steel Vs Titanium Fasteners - Preload / Clamping Force

(OP)
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?

RE: Steel Vs Titanium Fasteners - Preload / Clamping Force

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

RE: Steel Vs Titanium Fasteners - Preload / Clamping Force

Quote (Jack 20)

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.

RE: Steel Vs Titanium Fasteners - Preload / Clamping Force

Quote (drawoh)

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.

RE: Steel Vs Titanium Fasteners - Preload / Clamping Force

" 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.


RE: Steel Vs Titanium Fasteners - Preload / Clamping Force

Nescius,

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

--
JHG

RE: Steel Vs Titanium Fasteners - Preload / Clamping Force

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. bigsmile

RE: Steel Vs Titanium Fasteners - Preload / Clamping Force

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

RE: Steel Vs Titanium Fasteners - Preload / Clamping Force

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

RE: Steel Vs Titanium Fasteners - Preload / Clamping Force

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 ?

RE: Steel Vs Titanium Fasteners - Preload / Clamping Force

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.

RE: Steel Vs Titanium Fasteners - Preload / Clamping Force

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.

RE: Steel Vs Titanium Fasteners - Preload / Clamping Force

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 ?

RE: Steel Vs Titanium Fasteners - Preload / Clamping Force

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

RE: Steel Vs Titanium Fasteners - Preload / Clamping Force

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 ?

RE: Steel Vs Titanium Fasteners - Preload / Clamping Force

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

RE: Steel Vs Titanium Fasteners - Preload / Clamping Force

Your logic assumes that X amount of torque induces Y amount of elongation, no matter what the bolt material is. This is not the case.

A bolt is a spring, plain and simple. Hooke's law does apply. But the energy stored by the elongation of the two different bolts cannot occur at the same elongation unless the two springs have the same rate, which they do not.

Go back to the spring analogy- we know that if we consider the titanium and steel bolts as springs, that these two springs have different rates- the rate of the titanium 'spring' is about half the rate of the steel 'spring'.

If I hang these two springs from a hook, and hand a 5lb (or 500 or 5,000 or 5,000,000lb) load from the end of each of them, they will stretch different amounts; the titanium spring will stretch about twice as much. The stored energy is the same in both springs (which is the applied torque), and the force applied to the beam (which is the bolt preload) they hang from is the same- the only difference is elongation, because of the differing spring rates.

RE: Steel Vs Titanium Fasteners - Preload / Clamping Force

ok ... I was thinking more in terms of angle of twist (and sloppily using "torque" to describe it).

if you apply an angle of twist to a nut, you'll get the same extension, hence 1/2 the load. But (I guess) you only applied 1/2 the torque to get the same angle of twist (if 1/2 strain energy out, then 1/2 energy in).

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

RE: Steel Vs Titanium Fasteners - Preload / Clamping Force

Ahh.

Yes- if you're using the same angle of twist for the two bolts, you absolutely will get 1/2 the load from the titanium bolt, by imparting roughly 1/2 the strain energy.

The rest of the analogy trickles down. Might want to be careful about using those terms interchangeably in the future..

RE: Steel Vs Titanium Fasteners - Preload / Clamping Force

thx, but I've already tripped on that sword ...

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

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