I draw a distinction between this and just using threaded standoffs because of how the forces interact in the material.
A Little More on Preload
As mentioned previously, preload is the selective application of forces to a structure such that external loads manifested as forces must cancel the preload first, before the structure shifts. A great in-depth preload explanation worth reading through is here, as well as Fundamentalschapter 9, page 16.
The Effects of Preload on Spacers
We aim to exact a slightly different end to using preload on bolted spacers. It's not so much the tensile loading that is beneficial so much as the ability to change the type of loading on the spacer's walls from bending to tension and compression. The total increase in rigidity comes from two main sources:
- The outside of the sleeve is put into compression. A bending load will tend to compress one side more while relieving the other side. If there was no existing compressive stress, then the material will deform more before the same levels of stress occur within it. The stronger the material, the more compressive stress can be added (the stronger the preload). This works until the bending causes the compressed side to rupture (buckle outwards), and the other side to irreversibly stretch (plastic deformation).
- Trivially speaking, the addition of a much more rigid core (assuming the structural material is substantially softer than the bolt material) means the bolt does "feel" some of the fastening force, and contributes to rigidity by virtue of being .... much more rigid. The closer the bolt and material are in elastic modulus, the less this effect comes into play. Typically, high-strength steel bolts are used in aluminum or even plastics, so this method has meaningful contribution.
See image 2 for a finite element simulation of two situations where a Little Round Thing is used to bind two plates of metal together. I've even included and simulated contacts for fake screws! The spacers and walls are defined to be aluminum and the bolts themselves to be high strength steel. An equal 100 lb-force load is exerted on both ends, and the ends are separated by a narrow bridge so they do not "feel" eachother but remain contiguous.
Notice the substantially less deflection on the preloaded side. In this case, roughly 50% less. (Be aware that this is a coincidence - there's no particular arrangement of geometry that guaranteed the 50% - it is not a rule that "preloaded things are twice as stiff as not").
Don't Forget to Tighten Your Bolts!
Image 3 shows what happens when only 5 lb-force of preload is applied to the screw. That's basically hand-tight.
A few other conditions had to be changed as a consequence - I no longer had the joy of modeling the sliding friction between materials on the right side as infinite due to the lack of preload force, for instance. A sliding fit with friction was selected instead for both the bolt head and the shaft of the bolt.
As can be seen, the unthreaded standoff is actually somewhat worse. This can be explained by it being mostly hollow, and therefore deforming more for the same applied force. In real life, the steel bolt would be taking up the majority of the load since it is much more rigid despite being closer to the center (neutral, zero-length-change axis) of the bend.
Long vs. Short: Bending vs. Shear
This method is more useful for longer spans - the effect becomes less pronounced as the standoff length approaches 3 to 5 times its diameter from above, as the loading force embodies itself more as shear than bending. Image 4 is the same structure, with 500 lbforce preload again, but with only 1.5" long spacers. There isn't too much difference in this case.
Why you might not want to use bolted spacers
While they may seem better for many applications, there's some practical downsides to spacers. They require a discrete mating fastener like a nut on the other end. It's always a good idea to add a flat washer to both sides to increase the mating surface and prevent embedding or local plastic deformation. So, in applications where you can't easily reach the other side of the material being fastened (to tighten the nut), or in very soft materials, a bolt-through spacer is not as practical, or perhaps even as strong, as a threaded standoff.
