NASA Fastener Design Manual: Behind the Most Common Fasteners
NASA Fastener Design Manual: Behind the Most Common Fasteners
Andrew Prestridge | September 13, 2020
Fastener Design: the NASA Fastener Guide
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. Here is a starter list:
- Should the fastener be lubricated, and if so, with what?
- When building with high quality wood products, how can I use threaded fasteners?
- 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?
- Does it need a washer?
- What is the purpose of extra threads past the nut, and how many?
- What is the best way to drive the fastener - Hex, Allen, Torx...
Choosing a screw head is one of the steps
Behind the Most Common Fasteners
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). It is a surprisingly useful document and short enough to review at least once for your earth-bound and mars-bound projects.
For those serious designers, makers and builders, the NASA Fastener Design Manual will give you a very complete perspective on all that should be considered for fastener design. A very useful check on your design chops is to review your old projects and test them-honestly. Ours showed bolts that were too large for the load, had expensive threads, would have been better with washers and worse.
For those of us with more of an artistic flare, it also ends up being useful. You may take a while to open all the boxes in a hardware store to see what is available, but the guide can give you a detailed overview for each. Is it complete? No. Fasteners are like software, they are getting improved all the time. But the most common are explained in detail.
I know most of you don’t want to read the whole fastener design manual, 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: different designs, different uses.
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.
The design manual was originally published in 1990 and the list of thread lubricants proudly shows the brand name Never-Seez. It beats all other options with a useful design temperature limit of 2,200 °F.
Fasteners are for Metal, Wood, Plastics and more
Machinists and metal workers will love the manual, but the principles work for all materials. Give it a close look! Before threads many larger metal structures were held together with rivets installed by blacksmiths. Inserts are a great way to hide the nut. They are an additional piece of technology to design for, but they can yield great artistic results for your project while still giving the joint proper clamping force.
Threads, History and how to make them...
The history of threads and pitches is pretty interesting. The ancient and current processes for making threads is pretty interesting too. If you want to know more, check out this article on a gentleman by the name of Whitworth who was instrumental in the earliest "standardization" of thread pitch. The guide is no help in these areas.
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 these washers are tightened they 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. When you look at very well designed machines, the fasteners are a perfect fit. Even though your project or prototype may be done in a rush to capture your new idea, properly designed fasteners can be a source of pride that may actually speed you forward. Spend most of your time on the idea, plan a little time for fastener considerations and it will make your life easier.
Stripped Threads and Heli-coils
It happens. You over-tighten a hardened bolt in a softer aluminum and the threads strip. Or a bolt snaps off down inside a hole, followed by the machinist tricks to drill out the bolt. Again, this can leave you with a larger than stock hole. The application may still limit you to the same size fastener and the Heli-coil or Threaded Insert comes to the rescue.
Installing a Heli-coil
Lockwiring for Critical Applications
Lockwiring for fasteners occurs in critical applications, most commonly in the aerospace industry where a failure is just not an option. The process is manual, tedious, but very effective. A wire is used to make sure the fastener heads cannot rotate once installed. The process can be done on both fastener heads and nuts.
Lockwires holding screws and nuts in place
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. These situations are usually easy to spot and usually the Original Equipment Manufacturer (OEM) will label them clearly on drawings and sometimes even near the fastener. A few machines come to mind: cranes, gondolas, elevators, airplanes and space ships. Basically when a human life is at stake, it is a good idea to make certain of your fasteners.
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. Those tiny screws in eyeglasses, or holding together your cell phone come to mind.
Rivets are fasteners too.
A whole range of rivets, waiting to be remembered as a quick choice when picking a fastener, sometimes it just takes a little getting started.
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)
One of the Most Valuable - and free - Books in Your Technical Library
NASA Fastener Summary
Know what you need
The more you know about your application the better you can specify your hardware
Looks can be deceiving
Split washers look like lock nuts, but don't help once tightened
Don't skimp on the small stuff
When your application is critical, don't try to save money reusing fasteners; it's not worth the risk!