Spring sag

[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.