Fastener Design by NASA
Screws, bolts, nuts, washers and their diameters, lengths and pitches take considerable effort to design correctly. While a quick build might get away with using whatever you have around the garage, when it counts you need to make sure to get it right.
When it does matter, there is a lot to consider: material selection, coatings, and vibration resistance are just the start. Some things to consider:
- Should the fastener be lubricated, and if so, with what?
- How can I predict, and prevent, corrosion?
- How much clamping force and shear will the fasteners hold?
- How much torque should I use to tighten them?
Choosing a head is one of the easier steps when picking a fastener
If any of this sounds familiar or you have any of these questions, it’s likely it’s been analyzed by the engineers over at NASA. If there’s any place where it matters most to get it right, it’s space. Not only could a sheared bolt be lethal, but there’s no hardware store or provision for a quick replacement if something fails.
Fortunately, they’ve put together a handy resource, the NASA Fastener Design Manual from 1990. This is one of those evergreen documents that keeps giving useful information decades after its publication. It’s a valuable fastener guide, but it is the beginning of a very deep rabbit hole so watch out. We have busted our knuckles, had things break at the wrong time, cussed while removing a nut from a long-corroded bolt (and sometimes just cut it off).
I know most of you don’t want to read the whole NASA guide, so we’ve taken one for the team. To hopefully spare at least one soul from a headache or frustration, here is a summary of tips from the guide and our experience with fasteners.
Know What You’re Looking For
It’s best to know the purpose, dimensions, and pitch for your fastener before you go to the hardware store. Bringing a sample of your nut, bolt, and washer will certainly come in handy. All the screws start to look the same when you’re staring at a wall of 4,000 of them.
NO NAKED THREADS!
Always use some sort of lubricant if you’re going to be taking something apart more than a couple times. My favorite is Never-Seez on machinery that will require future maintenance. It makes it easy to remove the bolts, especially if they are in a corrosive environment. You can pick some up at your local hardware store or on Amazon for pretty cheap. For those that live along either coast, ocean air is surprisingly corrosive. And in the case of stainless steel hardware it has a bad characteristic of galling without lubricant even when the fasteners are new. Check out our blogpost on galling for more details.
Not all Washers are Locking Devices
The average hardware store split-washer, aka a helical spring washer, is not a locking device and the nut can still come unscrewed. This washer acts like a spring until you tighten it down, then it transforms to a normal washer.
Despite their appearance, once tightened these spring washers do nothing to prevent loosening
Watch for Over Hanging Threads
There is etiquette for how many threads should show past the nut: as few as possible. One or two threads for fasteners smaller than ¼ inch diameter is fine. Anything more is “wasteful” and your future self will not like the extra work required to remove the nut once that long bolt has corroded. These long bolts usually stick out too far where they should not belong and the risk of injury increases.
Important Application? Never Re-use Fasteners
If it is a critical application, DO NOT REUSE THE FASTENERS. It’s better to grab an extra pack at your local hardware store, than to have something fail in the future due to old or worn components. Your instruction manual should tell you if this is the case, but when in doubt, throw it out.
Too much Torque on Small Fasteners is a problem
With small fasteners, It is very common mistake to over-torque them and have your screw break off in the hole. The smaller they are the harder they are to remove - be careful tightening them past their purpose.
Rivets are fasteners too.
Our list may not get you to space, but I hope it helped you save some time. The NASA Fastener Guide has some extremely valuable reference material and is some of the more impressive technical art you will see. (that’s right – most of it was done by hand)
You can see and download the full guide on NASA’s Website here:
© Evolvent Design, 2020