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Step 18: Planar Linkages, and When You Can Use Mostly Tightened Nuts

It's time for me to create an exception for one of the big three rules I set out at the beginning. Under certain circumstances, you can use a mostly-tightened nut to make flat linkages.

This is one area which I have not personally built anything, so all the illustrated examples will be from others, but it's a tactic that is commonly attempted enough that I think addressing it is important.

Use prevailing-torque nuts

Also just called "locknuts", these nuts have a bit of deformed thread or a big chunk of nylon plastic in them which tightly grips the thread. These cannot be spun on a screw with your finger - generally, at least a wrench or set of pliers is needed, and a driver bit for the matching screw. The idea is you can tighten the nut to a very well known degree and have them stay there. If the mechanism needs less slop, you can tighten the nut a controlled amount.

Use back-to-back nuts

Called "jam nuts", or literally 2 nuts tightened against each other, this method is less reliable for setting an exact tension but is handy for when you do not have locknuts available. A dab of threadlocking compound before tightening helps give the threads some extra friction.

Use washers with the nuts, preferably plastic ones.

The washer helps to add a small amount of spring compliance to the joint, and also acts as a bearing surface. It helps prevent the material from torquing on the nut or screw directly and causing it to unpredictably tighten or loosen.  Plastic washers give bearing properties as well as more compliance than bronze washers.

Make the mating faces wide

In a typical unsupported planar linkage, the two links are basically braced against eachother. Making them wider not only adds rigidity in flexing, but the wider faces have more leverage against the flexural forces. They are less likely to twist and bind. Lubricating the faces, or using a bearing washer between, also helps.

There's another cheap way of making a planar linkage which involves pop rivets (blind rivets) that is popular with small mechanisms (like on 2.007 robots!):

Paper shimmed rivet linkages

This method uses a pop rivet as the hinging mechanism, since they're easy to install. Insert a small piece of paper or other very thin and sturdy material into the space between the two links, and apply the rivet. Normally the rivet would tightly clamp the parts together, but with the shim in, there is a small gap on the order of thousandths of an inch.

After the rivet is installed, slip the paper shim back out. The artificially installed slop allows the linkage to move freely. This method does not allow for much post-installation slop tuning, but is fine for prototyped mechanisms or things that are only carrying light loads.

The example picture of the 2.007 robots (Images 3 and 4) show locations where this method was used.
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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