Spring sag 2

[Frankiee63]

Will a steel spring weaken if constant load is applied to it even if the load is within its operating limits? Example: a torque wrench left at 100 ft-lbs for months? Thanks in advance

 

[BEMPE16524]

yes, we call it creep. you can read it here: "Mechanical Engineering Design" by J.E. Shigley and R.G. Budynas, 9th and 10th editions, this topic is typically covered in the sections on material properties under high stress and temperature over time, such as in Chapter 8 or Chapter 10, which deal with fatigue, stress concentration, and thermal effects on materials. These sections provide insights into the fundamentals of creep behavior, calculations for long-term stress effects, and practical considerations for engineering design.

 

[3DDave]

The spring-loaded torque wrench was tested on the Torque Test Channel:

This was not an exhaustive test. I didn't notice a follow-up to see just how far-reaching the effect was.

 

[JStephen]

"yes... under high stress and temperature over time"

This is something I'm rather curious about. As in the case questioned, there's no high temperature at all.
Long ago, I recall hearing that truck leaf springs should not be left under load overnight, so one guy insisted on jacking his dump truck off the springs when it had to be left loaded overnight lest it damage his springs.
On the other hand, I would think high-strength carbon steel at room temperature would be just about immune from this effect. In the case of the truck springs, presumably load bouncing over a pothole is 2 or 3 times the static load, so they're not loaded to the maximum tension normally.
And presumably this would be an issue with structural bolts which are typically loaded to high strength levels, as well as rivets.

 

[desertfox]

Over time the modulus of elasticity of a spring will degrade due to environmental,mechanical, temperature and alternating loads, have a look at this article

Do springs get softer over time? Fact or Fiction? - Technische veren specialist | Tevema.com

Explore "Do springs get softer over time?" as we demystify this common query. Uncover the science behind the durability of springs.

www.tevema.com

 

[EdStainless]

There will be some stress relaxation over time.
At normal temperature and load it will take decades for this to be noticable.

 

[Frankiee63]

The spring-loaded torque wrench was tested on the Torque Test Channel:

This was not an exhaustive test. I didn't notice a follow-up to see just how far-reaching the effect was.

Wow that was unexpected. 20 %??

 

[dgeesaman]

Wow that was unexpected. 20 %??

I just received a used torque wrench from eBay and the (moron) seller had it left at max torque adjustment. And if it was his personal wrench (it had his name written on the case in marker) it likely was left there for a very long time until he was motivated to list and sell it. Tomorrow I'll take it into work and test it on our torque calibration block and report back how much spring sag was there. Mine is a better quality wrench (SK Tools) so it's possible it's higher quality spring steel.

That said, compression set is not loss of elastic modulus. I do not expect a change in modulus or spring rate at a given length.**

** What could happen, is if the spring has enough compression set, that the linearity will change and the dial adjuster of the wrench will only be correct around one specific torque value. I hope that doesn't happen.

Another place where spring compression set is well-established is spring piston airguns. The advice is you can cock the gun and walk around with it all day but at the end of the day you should dump a shot in the dirt if necessary and release the spring force. Higher quality springs definitely hold up better against compression set but they don't solve the issue.

The OP used the word 'weaken' and taking that alone verbatim leads us to a different property, which is allowable stress. The thread title and reference to torque wrenches implies we aren't talking about that. So I won't.

 

[LittleInch]

Quite a lot for me is not knowing what the operating limit actually is and what factor of safety or margin was applied by the vendor.

In theory, if the spring was compressed, as I think this one is, without exceeding even the start of yield at low temperatures and has been relatively static, then it shouldn't creep or fatigue or lose "strength".

I suspect the dump truck was simply overloaded, or had been hence why the myth of jacking up the truck came from.

Only car I ever had with leaf springs one did get a little bent, but that was due to one hell of crash load on it.

 

[GregLocock]

Leaf springs and coil springs don't sag, beyond a preliminary shift as the abutments (generic) dig in, if they are properly designed and not massively overloaded. Sometimes somebody has a fit of conscience and designs a coil spring so it is coil bound before it yields. In cars that is more by luck than judgement, I'd call it a nice to do in some contexts. It has a big downside, you can't scrag the spring to get the check height right and improve the fatigue life. I doubt a cheap torque wrench has good quality steel in the spring, and the design of other parts in the wrench may allow things to move more than they should.

 

[Doug Hunter]

Steel springs will experience "load loss" (less force at the same compression) due to a number of factors. The peak stress in the wire relative to the elastic limit is important. The temperature is also important. However, cycling a spring at high temperature when achieving peak stresses near the elastic limit is when you really accelerate the load loss. I agree with EdStainless about the static loading conditions. Dynamically, you can lose 10% in 100 hours at high temperatures and near-limit stresses. Stainless steels support less stress but are more resistant to temperature dynamically when compared to "spring steels" such as Chromium-Silicon steel.

 

[dgeesaman]

Leaf springs and coil springs don't sag, beyond a preliminary shift as the abutments (generic) dig in, if they are properly designed and not massively overloaded. Sometimes somebody has a fit of conscience and designs a coil spring so it is coil bound before it yields. In cars that is more by luck than judgement, I'd call it a nice to do in some contexts. It has a big downside, you can't scrag the spring to get the check height right and improve the fatigue life. I doubt a cheap torque wrench has good quality steel in the spring, and the design of other parts in the wrench may allow things to move more than they should.

My used torque wrench turned out to have lost significant linearity so I couldn't get it calibrated beyond any single torque value. And that's an SK Tools clicker style wrench with a $330 retail price. Maybe the wrench needs you to explain to it how that's not possible - I'm open to anything here.

 

[EdStainless]

Most of the time when you see changes in springs it isn't because changes in the material properties but rather in the residual stresses.
I have seen high precision applications where springs were formed, stress relieved, load cycled a few thousand times, and then stress relieved again.
Part of the changes are actually dimensional.

 

[GregLocock]

There's more to the mechanism of a torque wrench than a spring. You'd have to check for wear or compliance in all the pivots and the ratchet and so on.

 

[TugboatEng]

dgeesaman, it's quite possible the lubricants in the wrench mechanism have degraded with time.

 

[3DDave]

Didn't see sag, but did see stress relief from mechanical operation of disk springs. We used them to set a pre-load and, after a short time, the pre-load was lost. Ended up having to take the springs and push them nearly flat a half dozen times before use until they would have a repeatable force-holding capability.

If it was mainly lubricant, then the problem would show up all the time. Most use a rocking-lever design that doesn't depend on a sliding or other motion that lubrication would have much effect. It is also more typical for forces to go up with lubricant failure rather than down.