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Step 10: Joining Parallel Plates: Preloaded Spacers

There's a special category of "using little round things to join parallel surfaces together" which I want to discuss separately, and that's using through-bolts with spacers.

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:
  1. 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).
  2. 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.
Yet another incredibly informative and well written instructable, nice job! Love the FEA’s and especially enjoyed your notes on set screws. My goto sources are always McMaster and ServoCity. Similar to your RoyMech site I’ve used http://www.gizmology.net/ for reference many times.
I knew I was forgetting something! Gizmology has been added to the end - I may sprinkle relevant links in the middle too.
http://web.mit.edu/2.75/fundamentals/FUNdaMENTALS.html is the correct link now, it's a great resourc, especially for offline use. THANK YOU FOR THIS INSTRUCTABLE!
<a href="http://web.mit.edu/2.75/fundamentals/FUNdaMENTALS.html" rel="nofollow">http://web.mit.edu/2.75/fundamentals/FUNdaMENTALS.html</a>&nbsp;whoops.&nbsp;
nice one
Re using one part to template tthe other, &quot;dimpling&quot; -- mention transfer punches here?
Probably worth it. I was definitely in &quot;slummin' high school&quot; mode then, when we didn't have a set of center punches much less transfer punches! I'll look to adding it in.
Wow that was incredibly comprehensive! Are there still robot combat competitions going on?
Hell yeah. Primarily small weight classes and these days grassroots-level and builder run. The big event is RoboGames: http://robogames.net/index.php and Combots: http://combots.net/, and on the east coast, NERC: http://www.nerc.us/ <br> <br>Various other local clubs and organizations exist also. A current listing of events is on buildersdb: http://buildersdb.com/
This might be my new favourite Instructable. Great info!
Thanks for sharing that. <br>One suggestion to add for using set screws in transmitting torque on shafts (only works if the shaft and hub are the same material and ends flush) is to use the set screw as a key - drill and thread the keyway parallel to shaft on the joint between the shaft and hub.
Wow, that is some invaluable mechanical design info. I thoroughly enjoyed the read and I feel like I just took an engineering class, an incredibly fun one. Seriously, amazing detail, thanks a bunch! <br> <br>Simply out of pure curiosity, why weren't taped holes used over T-slots more often? I'm guessing that it didn't fit the 2D fab theme of the class? <br> <br>PS: Working in that shop must have been like a dream come true, I'm olive green with envy :)
Purely as a matter of convenience. The t-nutted holes are not nearly as strong as a properly drilled and tapped hole due to the number of inside square edges. It's a matter of recognizing when the structural loads in the device can be borne by material-on-material interference (the slots and tabs) and then having the fasteners (t-slots) only be there to keep it all together. There are far more instances when drilling and tapping is stronger than using t-nuts.
Excellent! Thank you! <br> <br>Suggestion for an addition: how do you align parallel guide rails on which bushings will slide? Also, how do you keep things sliding freely when temperatures change? <br>Specifically, in my 3D printer project I have an aluminum carriage supported by 4 bronze bushings that slide on guide rails. The print-bed is bolted to the carriage and is heated. As the print bed heats up, it expands, applying force to the bolts that stand it off the carriage, which in turn bend the carriage, which in turn misaligns the bushings.
The design you describe is a classic &quot;overconstrained slide&quot;. It's very sensitive to change in the center distance (gap) between the two rails regardless of what you do to the bushings. <br> <br>Generally you have only 3 bushings - two on one axis to constrain it against planar motion (up/down, left/right) and against tilting/pitching, which are two rotations. And one on the other to make sure it does not pivot on the first axis (rolling). The result is only one motion possible (along the rail). Four bushings adds another constraint which is technically unnecessary, and for it to not impede the motion of the slide, they all have to act perfectly in line and on the same axes. Any misalignment of the rails or of the bushings, then would seize up the slide, as you've noticed. <br> <br>The solution is usually to use 3 bushings - two on one rail, one on the other, and also 'float' the 3rd bushing on a mount which is compliant to misalignment in the center distance. For small applications it's sufficient to just use one of those rubber-mounted self-aligning bushings. <br> <br>In addition, your issue seems to involve flexing of the entire carriage structure which can bind up the two-bushing side too, unless they are also self-aligning. Short of isolating the hot build bed from the carriage, perhaps one or more of the bushings on the two-bushing rail should be also flexible types. It's less rigorous machine design but also a practical solution.
Sup everyone, <br> <br>Feel free to chat amongst thyselves and ask questions. Interesting discussions could very well get folded into the document for everyone to reference.
Well done.
That was great! Now can you come over and help me build my Spencer Aircar?

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