10 Tricks Engineers Need to Know About Fasteners

You’d expect an engineer to know everything about a basic subject like nuts and bolts right? Well fasteners are one of those topics that seem simple at first but turn out to have much more depth than you expected. What follows are 10 tricks on this ‘basic’ subject that might surprise you!

UPDATED MARCH 2017: It has come to my attention that there is testing data available that makes the counter argument to my first point in this article. In the interest of transparency and good scientific discussion, I’ve provided links to all sources mentioned at the end of point #1. When faced with contradictory test results, if we assume the methodology and integrity of all sources are without fault, it is reasonable to conclude that your results may vary. Many factors could significantly affect testing results including variances from manufacturing process, fastener & clamped materials, heat treating, surface finishes, ambient conditions, and bolt diameter.

1. Split washers have been experimentally proven to be ineffective locking devices and can even aid self loosening over time. And yet I see these things in use everywhere, so what gives?

split washers dont work

In theory split washers (aka lock washers or helical spring washers) are supposed to work by squishing flat between the nut and the mounting surface when you tighten them. At this point the sharp edges of the washer are supposed to dig into the nut and mounting surface to prevent counter-clockwise rotation.

In practice a split washer is unable to gain any purchase against hard surfaces and does not actually prevent rotation. The problem is that split washers make for poor springs and bottom out after only a small percentage (on order of 10%) of a bolt’s total clamping load.

The only time a split washer might prove useful would be for fastening onto soft easily deformed surfaces such as wood, where the washers springiness & sharp edges could actually work.

The evidence against split washers started stacking up in the 1960’s when a gentleman named Gerhard Junker published some of his lab experiments. He invented a machine specifically for testing the effect of vibrations on threaded fasteners. The first thing he discovered was that transverse vibration loads generate a much greater loosening effect than do axial vibrations. Good to know.

transverse axial loading

His second discovery was made by plotting the bolt tension vs vibration cycles to create a ‘preload decay chart’. When he compared the preload decay of a bolt & split washer combo to bolt by its lonesome, he found that the split washer caused the connection to loosen sooner, as seen below.*

preload decay chart

Not to worry, there are better locking options available. Chemical lockers like Loctite, deformed thread lock nuts, and Nyloc nuts should be your everyday go-to locking devices. If you have some money to burn then wedge lock (Nord-lock) washers &
Serrated flange nuts are probably the best way to go.

When lives are on the line you may want to employ a ‘positive locking device’ such as a castle nut or a slotted nut. No amount of vibration will break this kind of connection:

castle nutSince this one is sure to stir things up when you mention it the guys at the office, I‘ve provided my sources below.

A) Article 1 n boltscience.com and Article 2 on Boltscience.com and Article 3 on Boltscience.com, all condeming split washers
B) pdf file from hillcountryengineering.com condeming split washers
D) Awesome video showing actual testing and how preload decay charts are generated.
*E) Alternate Testing Video #1 making the counter argument in favor of split washers.
*F) Alternate Testing Video #2  making the counter argument in favor of split washers.

.

#2. Double Nutted joints with jam nuts are affected by order of clamping. While I’m talking about bolt locking techniques, I’ll share another interesting one: For double nut connections involving the use of a jam nut & a standard nut, it REALLY matters what order you install them in.
The jam nut should go on first! Otherwise the effectiveness of the nut pair is greatly reduced. Double nut Source.

 

double_nut_Jam_nut

Before I move on to the next one I need to clarify the difference between static loads and fatigue loads. Static loads do not change over time. If a bolt is rated to yield at 3,000 lbs of tension, any static load less than that will not have a permanent effect.

However, if you were to vary that applied load over time you can fatigue the bolt until it breaks using less than 3,000 lbs! In the same way that a small stream can carve out the Grand Canyon, fatigue loads gradually chip away at the structural integrity of fasteners over time.

static vs fatigue

#3. The relationship between fatigue load and the number of cycles until bolt failure occurs can be predicted using experimentation. It turns out that you can make reasonably accurate predictions of the cycle count at failure by performing as few as three experiments (though I would recommend doing at least 6 to attain some real accuracy). All it takes are a few data points and a regression line to create a high cycle fatigue life curve (aka an S-N curve).

fatigue life

We don’t put the ultimate static loading on the graph, but if we did it would be the highest point on the ‘applied load’ axis and the zero point on the ‘cycles’ axis.

Why is this useful? Now that you know bolts can break from small fatigue loads, imagine trying to build a bridge using riveted or bolted connections. How could you trust that you ever had a big enough fastener? It turns out that fatigue loads below a certain threshold will never cause the fastener to break.

As a very general estimate, a bolt will require an infinite number of cycles to break if the fatigue load is around 30% (+/-15%) of the ultimate static load. You can expect the bolt to break in a few thousand cycles if the fatigue load is about 80% (+/-10%) of the ultimate static load. (Note: the exact percentage can vary dramatically based on material composition and ambient conditions.)

 

#4. (UPDATED) For maximum strength tighten bolts up to the yield point…For maximum durability, don’t! There is a common misunderstanding that a bolt within a securely fastened connection is impervious to outside forces if hey don’t exceed the clamped load of the connection.

That is, the myth says a bolt clamped to 500 lbs won’t experience additional stress unless outside forces applied to the clamp exceed 500 lbs.  This is not so! In fact ANY additional load, no matter how small, will add to the tension in the bolt. But not at a 1:1 rate.
Think of pulling on a fastened connection as if it were 2 stacked springs. Both springs stretch measurably, but the weaker one stretches more. Part of the external load is absorbed by the joint and part by the fastener.

pic 1

 To be crystal clear, as you tighten the nut the bolt will compress the two parts together. The bolt itself has an internal reaction force equal to the amplitude of the compression force, but the bolt itself is in tension.  If you were to graph the tension on the bolt while you tighten the nut, the plot would look like the graph below. To get the greatest clamping force out of the bolt we would to tighten it all the way up to the yield point. Any more force and the bolt will enter the plastic region and permanently deform.

force displacement curve

In practice engineers don’t design that way. Since any additional force will begin to yield the bolt, you want to give yourself some margin for error.  Engineers select a bolt tension that is somewhere between the calculable minimum functional clamping force and the yielding force…. while also accounting for error in the tension measurement method.

(Sources: Shingley’s Standard Handbook  & Article on Fastenal )

#5. It is actually quite difficult to determine the exact load the fastener sees during clamping. We now know how important it is to avoid over tightening a bolt, but how do we know when it is yielding?

   For everyday purposes the clamping force can be approximated by measuring the tightening torque. You can look up the recommended tightening torque for a given fastener size in my bolt sizing calculator or in a table like the one found here.  An alternate method is called the ‘turn of the nut’ wherein you tighten the bolt until it ‘feels snug’ before rotating it another 90 degrees to ensure adequate tightness.

Those methods work OK for most things, but some critical applications require you to be certain of the clamping force (think spacecraft or large weights above your head). The torque method has difficultly accounting for friction and lubrication, but at least the torque is mathematically correlated to the clamping force. On the other hand the turn of the nut method uses rotational displacement to bypass lubrication affects, but it doesn’t even consider forces at all.

There are better options though. Load indicating washers can accurately verify bolting loads by squishing open a paint sack after reaching a specific load. The drawback with these is that they only work once.  http://www.boltscience.com/pages/tighten.htm The other option comes from a company called smart bolts who came out with a fastener featuring a built-in tension indicator. This is by far the most accurate method of measuring bolt clamping load. On the other hand, a single box of these bolts can cost around 10 times as much as a standard fastener!

smart bolts

Neat, I just wish I could afford one.

How different tightening methods compare in terms of accuracy.

Tightening_Method_Accuracy.JPG

#6. If you’ve ever designed a part with a threaded hole, you may have wondered:How many threads do I need to make a strong connection?’ The answer is that it varies, but six at most.

Bolts actually stretch very slightly when force is applied, which causes the loading on each thread to be different. Because of this stretch, when you apply a tensile load on a threaded fastener the first thread at the point of connection sees the highest percentage of the load. The load on each successive thread decreases from there, as seen in the table below.

Additional threads beyond the sixth will not further distribute the load and will not make the connection any stronger. 

6 threads at most

So will a bolt break before the nut strips? Yes! Nuts typically have no less than three internal threads, but nut thickness standards have been selected on the basis that the bolt will always sustain tensile fracture before the nut will strip.

#7. Have you ever seen a fastener labeled with a 2A or 3B rating and wondered what that meant?That number-letter combo is used to indicate the thread class of the fastener. Thread classes include 1-4 (loose to tight), A (external), and B (internal). These ratings are clearance fits which indicates the level of interference during assembly.

  • Class 1 is a good choice when quick assembly and disassembly is a priority.
  • Class 2 is the most common thread class because it offers a good balance between price and quality.
  • Class 3 is best used in applications requiring close tolerances and a strong connection.
  • Class 4 is precision tight, typically used for lead screws and such.

#8. All fasteners are available with either coarse or fine threads and each option has its own distinct advantages.
Finely threaded bolts have slightly larger cross-sectional areas than coarse bolts of the same diameter, so if you are limited on the bolt size due to dimensional constraints, choose a fine thread for greater strength. Fine threads are also a better choice when threading a thin walled member. When you don’t have much depth to work with, you want to utilize their greater number of threads per inch. Fine threads also permit greater adjustment accuracy by requiring more rotations to move linearly.

On the other hand, coarsely threaded bolts are less likely to be cross threaded during assembly. They also allow for quicker assembly and disassembly, so choose these when you will be reassembling a part often. If the threads will be exposed to harsh conditions or chemicals, a coarsely threaded fastener should be considered for its thicker plating/coating. Coarse ly threaded fasteners are much more commonly available in the united states.

#9. Would you expect a bolt to be stronger or weaker at very high temperatures? How about at cryogenic temperatures?

Most people answer ‘weaker’ to both question, but being weaker at both temperatures doesn’t even make sense when you think about it. Why would steel be strongest at whatever typical room temperature happens to be?  It’s not.

As a rule, metals are strong & brittle at low temperatures and soft & ductile at high temperatures, within their solid phase temperature range. Room temperature is just another non-extreme point on the curve.

test temperature chart

#10. You can make bolted connections more resistant to shear loads by using clever design instead of larger bolts. For maximum strength, try to use the correct thread length for the connection. In the image below you can see two connections which are identical except that the one on the right has a properly sized thread length. It exposes the bolt shank (rather than the threads) to the applied load at the connection seam.

All else remaining the same, the connection on the right will be stronger because the shank has a larger cross-sectional area and no stress concentrations.

pic 2

Another clever trick is to design connections so that the applied load will be on multiple sections of the bolt, as opposed to just one section. In the images below there are two connections. The one on the right is twice as strong as the one on the left because it would have to shear the bolt off in two places to become free. Also, the single shear configuration can also lead to bending loads on the fastener and loosening of the connection (see #1).

pic 3

#11. Have you ever cursed the day you were born because you just stripped out a Phillips head screw? While it is nice that Phillips screw drivers don’t slip off screws like flat heads do, it’s a real pain when the head can no longer be rotated because the screw head has melted into a hollow cone.

Stripped_Phillips

As frustrating as that is, it turns out that Phillips head screws are designed to strip out via the tapered point and rounded edges. The technical term is called a cam-out and every time it happens the relative surface motion wears out your screw.  Alternate screw heads like torx and pozidriv are specifically designed not to cam-out.

..

If fascinating facts about fasteners are your thing then you should check out  Carroll Smith’s “Nuts, Bolts, Fasteners, and Plumbing Handbook” aka ‘Screw to Win’. Smith’s Engineer To Win is another good one. Actually every book by Carroll Smith is pure gold.

carrol smith book

You might also enjoy reading about a custom MS Excel based EngineerDog Bolt Sizing Calculator here.

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43 comments

  1. I don’t think that is the case. I think the spring washer actually (at least in current usage) is to increase preload with a controlled deformation. When you have say a safety item with 10.9 grade steel to the preload it will start to deform the steel of say more than 980 MPA steel. This causes a preload that puts the energy back into the tightening torque and not the strain energy of the steel. Basically it controls your torque limit so you can keep it fastened (without torquing the nut) but still maintaining the preload to prevent loosening from NVH or light brushing torques. Basically, lower loosening and install torques but still same preloads and tolerancing.

    Like

    • That’s the theory, but it just doesn’t actually work out like that.
      I know, it bends my mind too – years of theory being wrong, “such an obviously correct explanation” being wrong, the “one way design” not actually having any effect – but the results back it up.

      Liked by 1 person

    • I always tell everybody it won’t come loose because it’s John tight. i almost always tighten everything by feel and people complain they can’t get it loose because John tightened it. i tell them i tighten till i feel the bolt starting to stretch and then back off. what are those nuts called with a indentation on one of the six faces. i always called them lawnmower nuts because that’s usually where i encounter them.

      Liked by 1 person

      • Im gonna steal that if you dont mind, ‘Make it Michael-tight.’ 🙂 Its been a while since Ive worked on a lawnmower but I cant think of which kind of nut you’re referring to.

        Like

  2. Hello,

    First, thank you for the article.
    Second, I found on youtube this video also from boltscience. From what I see the test shows now that helical spring washers do help. The amplitude in change but still.
    Can you please check out the video, what do you make of this?

    Like

  3. What about the general use of flat washers? My general practice is to always have a flat washer under the bolt head and under the nut, but I’ve ran into conflict over the years where some engineers dont feel it’s required under the bolt head in some instances.

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    • In theory I can’t see why plain flat washers would hurt as long as they are harder than the bolt AND their ID is no larger than the joint through hole ID.
      Washers are used to provide a bearing surface for a nut or screw head, cover large clearance holes, and distribute fastener loads over a large area, particularly on soft materials. They are useful hardware for many reasons but are not intended as a reliable source for vibration protection.
      Also, I wouldn’t use a washer in addition to a serrated flange nut because it would negate the effect of the serrations.

      Like

      • Flat washers allow for a greater clamping force for the same torque on a nut & bolt, as far as I know. The fastener is easier to tighten due to the two bareing surfaces and, obviously, is therefore easier to loosen off too.
        Obviously there are other advantages as well, such as reduced material deformation and increased stiffness and sliding friction around the hole without increasing the fastener size.

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    • Hi Jeff,
      I’d think some of the engineers are concerned about the additional interface if you use a second washer. Some amount of setting occurs at each interface on and also after tightening. One of the advantages of using a washer under the tightened component (either nut or bolt head) is probably better control of under head friction, if a good quality washer is used.

      Like

  4. I stumbled across Bolt Science several months ago. Glad to see that others have too.

    Have you found any information about using conical or spherical seated bolts/screws/nuts to prevent loosening? They’re used all the time on wheels (http://cdn1.bigcommerce.com/server5200/7e8d0/product_images/uploaded_images/fast-wheel-accessories-lug-nut-styles-and-seat-types.jpg). In that case, I think the taper is primarily for centering, but it also serves to prevent transverse motion.

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    • The conical section doesn’t make the fastener more resistant to vibrational loosening by anything more than the fact the cone has a greater frictional area.
      Plus, you do your wheel nuts up really tight! This is best as the correct torque slightly stretches the stud, and tallies with the “washers don’t help” finding.

      Like

    • Preventing the transverse motion is probably the answer. Note, the Juncker test actually tests the joint reliability against loosening due to transverse vibrations.

      Like

  5. Glad to see a format like this. Electrical connections should use flat washers I believe but I have no test data to support that, only experiences. With flat washer and split ring approach, higher torque can be applied and a gas tight seal is made such that oxygen can not start corrosion. With serrated nuts or star washers, lower torque leads to more open circuits and some sort of liquid tape is needed to seal the moisture out which assembly and service people hate. Ring terminals have been in use for 100+ years and SAE J1908 Electrical Grounding Practices for Vehicles does not allow serrated hardware but many electrical components seem to be migrating towards a lower cost to replace the two with a single serrated nut or replacing the flat with a star washer and it causes hot spots as seen in infrared pictures of a connection compared to the flat washer. I am searching for better answers in this area.

    Like

  6. Thanks for the interesting article about fasteners. I actually didn’t know that the number-letter combo indicates what the thread class of a fastener is. Definitely going to keep this in mind for future projects, especially since it seems like each one has it’s own benefits to them. Plus, it would be cool to test out these different types and see how differently they feel.

    Like

  7. Here is something really trick for locking threads. It is called the Saper-Lock. It uses a coil spring which is contained within the nut and wraps into the threads. Threading into the nut springs the coils open to allow for the threads to make up. As you try to unthread the bolt, the spring wraps tighter around the minor diameter of the bolt not allowing rotation (sort of like Chinese handcuffs). The more torque you put into the bolt, the tighter it gets. These are used in very high vibration environments such as train stations. To release the bolt, one leg of the spring is expanded with the socket to relieve the tension and allows the bolt to release. Very clever Germans. Google Saper-Lock.

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  8. This week I was working with an 84 year old (!!!) construction worker as we tightened down some column anchor bolts using nuts and lock washers. He imparted some “old timer wisdom” to me that lock washers work by biting into the fastening nut and metal plate to prevent loosening. I always appreciate insight provided by senior citizens as I’ve found that they do have a lot of wisdom. Being a still curious about things 71 year old, I decided to google that lock washer theory and ended up on your site. Who knew there could be so much information and controversy about fasteners!! Have bookmarked your site for future reference. Thanks!

    Like

  9. I have found lots of references that seem very legitimate proclaim that the split lock washer is useless or worse. I work in aerospace and they are common, so I had a hard time believing it was still used based on tradition and the momentum of wrong information. So I did some research and found a very small section of Bickford’s Handbook discussing these parts. Bickford is an expert on fasteners and he has made his career studying them. His handbook is one of the most used ME references for fasteners. He refers to a study published in a journal where it was determined that they are beneficial when used appropriately.

    The pitfall all of the readily available resources fall into is assume the design intent behind the split lock washer was only to flatten the washer where the cut ends are on the same plane. The study indicates this was not the case. When load applied approaches the yield limit for the bolt, the washer twists or rolls. This is due to the trapezoidal cross section of the washer. The spring constant is much higher in this mode, and it effectively adds a significant length to the bolt. It is well documented that this has a beneficial effect on preventing loosening.

    The following is just conjecture on my part, I have not found anything solid to back it up. But I believe these were first produced prior to 1900. I could not find anybody credited with the invention. They were cheap to manufacture, and were copied and used without really understanding how they worked. I think the helical shape of the lockwasher is a result of the method of manufacture, not a requirement of the design. This was further complicated by the metric DIN standards omitting the trapezoidal shape in DIN 127. Note that ASME, MS, and NAS standards keep the trapezoidal cross-section. Studies were performed like some of the ones you cite that show they are ineffective long after the design intent was forgotten. A theoretical analysis where one does not go past the force required to flatten the washer make it appear insignificant.

    The study discussed by Bickford implies that to get the benefit from the lockwasher you must reach a certain pre-load. So the base material has to be able to withstand that load. Also the most benefit would be seen with designs where a critical joint does not have the space for a long shank bolt or screw. So they are not the best and for every application. But they are not useless either.

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  10. I always thought split washers were used to make sure there is tension on the threads during temperature cycles. A normal bolt/nut would expand when hot, and be completely loose if it expanded enough that the nut no longer pressed against the surface.  The spring effect makes sure there is friction even if it changes size, keeping the nut from rotating.

    Might be worth while to try some temperature cycling experiments. One bolting cold, and one bolting hot. And then put them into a fridge and into a warm room, back and forth. This would emulate a car warming up and cooling off in the winter.

    Thanks for the interesting article!

    Like

  11. Looking to replace a lock washer, washer, 5/16-18 bolt combination with a 5/16-18 serrated flange bolt. Would the serrated flange bolt require the same torque spec? Thanks

    Like

  12. An exception to #6 (number of threads required) should be noted in the case of steel bolts into plastic material. The steel bolt doesn’t stretch nearly as much as the plastic under load. Additional threads will provide additional strength as the force can be distributed across more than 6 threads.

    Liked by 1 person

  13. This is great info for when you don’t want a joint to move, but what if you need movement? Bolts and washers are frequently used as rotating joints in many devices. For example, I found this page while searching for the correct order to reinstall washers and rubber spacers in a guitar wah-wah pedal. In this application what is desired is smooth, squeak-free movement with just enough friction to hold the pedal in a steady position when not in use. Any tips on this category of bolted joint?

    Like

    • Sorry for the late response, I had to sit on this one for a while. I think the answer depends entirely on how much thrust load the retaining nuts will subjected to. I googled a guitar wah-wah pedal, and in something like that there is likely almost no load seeking to push the nut off. I would suggest a nylock nut for something like this because its clean & simple and can still be removed.
      But your question got me thinking about higher load applications, like hinges for tilting solar panels on my college solar car. In cases like that I would want to throw in a bronze washer, or maybe even a thrust bearing.

      Like

  14. I have 1997 Dodge Ram 2500 with a 5.9 commins diesel engine and an automaic transmission. I bought the truck with a cracked flexplate. When I went to check the torque on the Torque converter/ flexplate for the 35 pounds of torque, 3 bolts were good and 3 were very loose. I torqued them all to spec and the vibration/noise stopped for 3 miles then the flexplate broke completely into an inner and outer ring.
    .
    Replaced the flexplate and drove 10 miles and I hear a rattle again. I did NOT use blue loctite because I wanted to see if all noise disappeared, which it did for 10 miles. I plan on replacing all the bolts and blue locktighting them.

    After an extensive internet search it appears this engine habitually loosened the torque converter bolts. I did NOT find any solutions, some manufacturers even called for the use of red loctite.. So I was wondering if a distorted thread one time use bolt would be better to hold torque or a serrated washer type bolt? It uses a 3/8 24 thread 5/8 inch long #8 hardened bolt.
    .
    Any ideas since these bolts have severe vibration and rotation?

    Like

    • If they are that bad, perhaps drill the heads and wire them in place? Or a second plate that could mechanically hold the bolt heads so they can’t turn, rather like 6 spanners, one for each bolt, welded together to hold them all?

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      • Thanks for the input. Unfortunately these bolts are inserted through an inspection hole half way up on the 5.9 Cummins engine, making anything more than finding the hole by feel and inserting the bolt very difficult.

        Like

  15. Hello, i have a problem with some fasteners, and i hope you can help me.
    I have a flange with a lot of fasteners and when i try to check the torque with a torque wrench,nut and bolt spin toguether like one body, but when i lock the nut, the torque wrench clicks at the established torque value.
    It is ok? Has the bolt lost his preload? Has the friction coeficcient increased by corroson or foreign particles?

    I hope you can clear my doubts.
    Thank you.

    Like

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