How many fasteners?

Over the years I’ve developed a gut feeling about what will likely hold and what may not, but I’m curious what rules of thumb (or even better, math) others use to determine the number of fasteners they need when joining parts. Some things, like compression load of bolts, is relatively simple. Other things, like shear load of bolts, less so. Rivets are also fun. Curious what people do.

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we just add as many fasteners we think we can fit tbh

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This is the exact thing I would like to avoid.

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that’s fair

Honestly the best lesson learned came when my first boss as a college intern sat me down and went “right now we use xx faster for this handle, figure out if it’s right”. My gut said it was fine, the math said it was 3 sizes bigger than it needed to be.

There are some calculators out there like this Fastenal one for understanding loading. https://www.fastenal.com/fast/services-and-solutions/engineering-calculators/load-calculator

I will say that threaded fasteners are nice, but boy do I love some rivets. Steel are completely worth the very small extra weight compared to AL.

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Fasteners are the most prevalent form of barf that robots leave on the field. Keep the ones on your robot fastened. I personally do not think this is an area where more is better.

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Some very VERY basic FEA is always an amazing tool. We did this a lot this year making sure our shoulder bolts could be as small as we wanted for our arm joints.

Rivets weigh nothing (and cost virtually nothing in bulk).

We use ALL the rivets!

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Sure, but there is a difference in time and effort for machining and riveting 8 holes vs 80 holes. I’m trying to figure out what an appropriate factor of safety is with these things so I can provide guidance to my students that doesn’t come down to “use your 10 years of experience to look at it and just know”, which is currently my only method.

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Zip ties, duck tape, and hope for the best. Don’t recommend this.

The proper way is to estimate the maximum forces your parts will experience and provide support for that plus a 50% margin of error to be extra safe but this usually requires mapping out forces which isn’t always worth the effort.

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My first thought was the do the simple math on the maximum load the system would apply to the joint and distribute that over the bolts, except I wasn’t sure how to determine:

  • Appropriate factor of safety (I have heard everything from 1.5x to 3x)
  • What a realistic shock load would be
  • Which “maximum load” I should use (stop to full speed, full forward to full reverse, full speed into a wall)
  • Appropriate shear load for bolted fasteners (I think rivets are pretty straightforward, but bolts depend on which part of the bolt you’re using)
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If it supports a static part usually take the weight of the parts it supports and calculate for gravity as the force (even if that would assume your bot is upside down dangling from a ceiling) and then multiply by 2 usually, for moving parts a bit more math but take the maximum acceleration of the part and its weight to figure out the force it induces and add that to the gravity value, if the part has a mechanical stop for example you need to use the maximum speed and treat it like an acceleration value since top speed into the stop will exert the force of acceleration to zero extremely fast, mechanical stops should also account for the force of impact with a 3x safety usually.

Maximum load should always be top speed to zero on collision where applicable, frames & parts with mechanical stops mostly

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I just make sure I use more rivets then 1501, and figure I should be good

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There is also a time and effort for figuring out what the minimum number needed is. And even then it’s based on a bunch of assumptions.

The CNC doesn’t really care how many holes a part has and the rivet tool takes about 2 seconds per rivet. And rivets look purty:

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Your tools might care though :frowning:

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Build it, then try to break it. If it breaks, address the failure. If all of your rivets sheared then you probably need more rivets. Build/break/repeat.

If you assume that other teams are also doing this, you can guestimate how many fasteners should be required for a certain type of joint by looking at other robots. Collectively, we tend to converge on the right number.

This approach uses no math, but it’s pretty effective. A rule of thumb.

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When it comes to fasteners, I would not say there is a strict “rule of thumb.” Some things I consider when determining how many fasteners are needed for a part include the weight of whatever I’m trying to fasten, the load on whatever I’m fastening and in what direction that load is being applied, the material of whatever I’m fastening, and what type of fastener I’m considering to use. Hope that helps!

(You can also perform FEA if you have a tight weight constraint when designing)

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Or just use like 6x 10-32 bolts, and slap it like a trucker saying ‘that ain’t goin anywhere’

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This sounds like how fields are built in the offseason.

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Build a robot and add fasteners according to gut then strategically remove fasteners until it doesn’t work, then put the 1 fastener back to make the robot work again.

(In reality, we just use bolts for everything, usually like 4 10-32 bolts does the job. We don’t trust ourselves to assemble it correctly the first time so bolts it is.) (a mentor from another team asked when we will start utilizing the rivet technology after doing a check up of our robot before elims) (I swear we will use them next year)