Uncategorized

6 Types of Bolt Failure and How to Prevent It

Designing fasteners into your application requires a complete analysis of the joints to make sure nothing was missed. Understanding the types of failures that can occur will help with this analysis. Read on to see how to prevent bolt failure.

Failure Type of Failure Solution
  Overloading (stretching)

– Make sure that the appropriate material and grade was used

– Make sure your design is well understood and that the bolts are not overstressed

  Fatigue

– Make sure that the appropriate material and grade was used

– Make sure your design is well understood and that the bolts are not overstressed

– Make sure that the fasteners are well-tightened

  Galling

– Use a lubricant

– Avoid fastener misalignment

– Avoid high speed installation – keep installation speeds low

– Avoid rough surface – smooth finishes

[1] Shearing

– Make sure to re-evaluate tightening strategy

– Threaded section in shear plane – use shank instead

[2] Galvanic corrosion

– Avoid use of dissimilar metals

– Prevent moisture entrapment

– The fastener should be the cathode (more noble)

  Hydrogen Embrittlement

– Eliminate susceptible alloys, hydrogen, stress (service or residual)

– Use non-electrolytic platings

– Increase bake times

– Risk is never eliminated

 

For further information on these or other types of bolt failure, please feel free to contact us at ProvenProductivity@bossard.com

 

Fadi Saliby
Technical Sales Director
FSaliby@bossard.com

 


[1] http://www.learneasy.info/MDME/MEMmods/MEM30006A/Bolted_Joints/Bolted_Joints.html

[2] https://en.wikipedia.org/wiki/Corrosion

September 15, 2017
Read More

What Your Bolts are Telling You

When identifying nuts and bolts, there is some information that can be gathered about them simply by looking at the part. Here is a quick overview of how to identify nuts and bolts by taking a quick glance.

Metric bolts have the class marked on the bolt and the nut. This is identified by two numbers separated with a decimal point. This is an easy way to determine that the bolt thread is metric. A line below the property class is used to indicate if boron was used in the manufacturing of the base material. Common classes are 8.8, 10.9, and 12.9.

Inch bolts are identified by lines on the head of the bolt. If there are no lines but a head marking, that is a Grade 2 bolt. Grade 2 is soft and not heat treated. Three equally spaced lines 120 degrees apart are used with a Grade 5 bolt. Six equally spaced lines are used to identify a Grade 8 bolt. The use of boron steel is identified by equally spacing the identification marks over 180 degrees on the bolt head face.

Metric nuts are identified with a number. This number should be the same as the first number on the bolt. Inch nuts have multiple ways to be identified depending on the standard produced to. If not explicitly stated (number on the face for the grade of the material) the nut will be identified by lines. Grade 2, non-heat treated parts, will have either no line or one line on one of the faces of the nut. Grade 5 nuts will be marked with two lines that are 120 degrees apart. Grade 8 parts will have two lines that are roughly 30 degrees apart (similar grades have 3 equally spaced lines; these are manufactured to alternative standards).

Many times, the bolt head will have a manufacturer’s mark as well. The manufacturer’s mark can be very telling when processing issues with problem parts. If the head markings between two parts are different, they would not be from the same manufacturer, let alone the same manufacturing lot.

More information about identifying nuts and bolts can be found on Bossard’s Website or by contacting us at ProvenProductivity@bossard.com with any questions.

 

Brandon Bouska
Application Engineer
bbouska@bossard.com

September 08, 2017
Read More

Quick Guide to Serrated Flange Nuts

A hex flange nut with serration is a nut that is formed with an enlarged circular base that flairs out from the bottom of the nut. When the nut is torqued into place, the bearing surface of the serrated base displaces the material of the mating surface. This forms a locking effect which resists vibrational loosening.

Because of the seated surface, the nut will require a greater amount of torque to loosen adding to the locking feature. The flanged surface will span an oversized or a poorly aligned hole and provides a more uniform bearing stress to clamp force ratio than a finished hex nut.

Serrated hex flange nuts are available in Grade 5 and Grade 8.

When using serrated hex flange nuts, the bearing surface will be damaged as the serrations dig, which can cause concerns for engineers. A painted surface will become chipped exposing the material. This may cause accelerated corrosion of the assembled parts.

Contact us at ProvenProductivity@bossard.com for more information on multi-functional fasteners.

Joe Stephan
Application Engineer
jstephan@bossard.com

September 01, 2017
Read More

4 Essential Steps to Choosing a Fastener Finish

Choosing a Fastener Finish

When choosing a fastener finish, there are many factors to be considered. But if you follow these four essential steps, finding the perfect fastener finish becomes much easier.

  1. Corrosion resistance
    The necessary corrosion resistance depends on the operating environment of your product. Is the product protected from the elements, or is it exposed to moisture or weather changes? Industrial or agricultural environments where dirt, debris, or chemicals encounter fasteners can also be a factor.

 

  1. Friction control
    Friction control is often overlooked when choosing a fastener finish, but it is a key component. If you don’t know the friction of your fastener finish, then you don’t know how much torque to apply to the joint to achieve your desired clamp load. Using torque values from a chart can be dangerous and lead to premature joint failure.

 

  1. Current regulations
    Recent regulations on chemicals such as hexavalent chromium have also dramatically changed the composition of fastener finishes over the past five to ten years. Your industry may not require compliance to RoHS or WEEE regulations, but there is a good chance that your fastener finish is different than it was ten years ago. Know the difference and how it affects your end product.

 

  1. Cost
    Finally, cost is always a factor. A lot of designer finishes exist out there to address all of your concerns, but you will pay for the technology. Educate yourself on the actual needs of your design and what is available. Then, you should be able to arrive at the proper finish for your product.

Contact us at ProvenProductivity@bossard.com for more information on fastener finishes.

 

Doug Jones
Applications Engineer
djones@bossard.com

August 25, 2017
Read More

Why Do Bolts Break?

Have you ever broken a bolt? How do you determine why the bolt failed?

One of the most common types of failure is overloading. All bolts have a maximum load that they can bear before they begin to yield, and generally this load is applied in the form of torque. If friction is lower than expected, the bolts may yield before reaching the prescribed torque. When a bolt yields, it will stretch, causing a “necking down” in the threaded area of the clamping zone that is not engaged into the mating threads. Assemblers can usually feel the bolt stretching as it will take many more rotations of the wrench before either breaking or stalling the wrench. If the bolt breaks, you will see an obvious reduction in surface area at the break where the bolt has necked down.

If a bolt breaks after it has been assembled, there a couple of failure modes that should be considered.

How does fatigue failure occur? Fatigue failure happens when the bolts have not been tightened properly, or have loosened up during its service life. If enough force is acting on the loosened joint during use of the product, bending stresses can weaken the fastener, eventually causing it to fail. This can normally be diagnosed by a fastener expert by close examination of the broken fastener and the mating components.

A third, less common type of failure is caused by hydrogen embrittlement. This type of failure is considered a delayed failure and will always happen after assembly. The hydrogen embrittlement time to failure is typically within 48 hours. The break will almost always be directly under the head of the fastener and not in the threads. The head may break off completely, or it may simply crack enough to relieve clamp load, and remain attached. Either way, the joint has failed and is not safe. This type of failure, while not common, almost always occurs in very high strength fasteners, or case hardened fasteners that are electroplated.

Contact us at ProvenProductivity@bossard.com for more information on failure analysis of bolted joints.

 

Doug Jones
Applications Engineer
djones@bossard.com

 

August 18, 2017
Read More

Vibrational Loosening

What is vibration, and how does it contribute to the loosening of fasteners? The answer may seem obvious, but understanding the mechanics of vibrational loosening can help us take steps to prevent it.

Imagine a block of wood on a ramp. The angle of the ramp is low enough that the block of wood does not slide down the ramp. Now, we repeatedly wrap on the ramp with a hammer, not too hard, but enough to make the block of wood jump a bit and slide down the ramp. This is like how vibration causes threads to rotate loose, or “down the ramp”. When vibration occurs, it briefly, but repeatedly, lessens the pressure between the block and the ramp (or thread flanks) and the block naturally slides down the ramp.

Now, if we use a heavier block of wood it takes more vibration to cause the block to slide down the ramp. This is similar to adding more clamp load into the joint. But, given enough amplitude and frequency of the vibration, we still get the same result – the block sliding down the ramp, or rotational loosening.

Lastly, if we clamp the block of wood to the ramp, so that there is pressure on the top of the block, it is not allowed to bounce and lose friction on the bottom side, and it does not slide down the ramp. So, how can we simulate this condition in a threaded joint? Your normal nut and bolt joint will always have thread tolerances to ease assembly, creating gaps on the back side of the threads. However, if we can use thread forming screws, which make their own threads into the mating part during assembly, we have no thread gaps.

Thread forming screws, while not the solution for every joint, work very well in situations where vibrational loosening is a high risk.

Contact us at ProvenProductivity@bossard.com for more information on solutions for vibrational loosening.

Doug Jones
Applications Engineer
djones@bossard.com

August 11, 2017
Read More

Tool Accuracy and Effects on Clamp Load

There are many options on the market for tightening hardware. Some of the common tools used on this spectrum are torque wrenches, impact guns, impulse guns, and DC drivers. Each of these have an accuracy generally associated with them (see table below). What does this accuracy mean with respect to the hardware and more importantly, how does this affect the end user?

Wrench Type Accuracy
Impact Gun

50%

Impact Gun (with Torque Bar)

20%

Hand Torque Wrench

20%

Driver with Mechanical Clutch

10%

DC Driver

5%

Let’s use an example to show the importance of tool accuracy with respect to the final clamp load of the assembly. A standard M8 bolt, class 8.8, is to be tightened to its full clamp load with three different tools and accuracies of 10%, 20%, and 50%. All variables are maintained to be the same over all these examples (coefficient of friction is assumed to be between .09 and .14). The minimum clamp load, which should be used to design the assembly, would be 3.8 kN. The maximum recommended clamp load regardless of tool accuracy that could be achieved is 18.7 kN (roughly 75% of yield strength). That same M8 tightened with a tool that offers 20% accuracy now has a minimum clamp load of 7.7 kN. When tightened by a tool that offers 10% accuracy, the bolt now has a minimum clamp load of 9.4 kN. The mean of each of these three is 11.3 kN, 13.2 kN, and 14.1 kN. In short, this means that by changing your tool’s accuracy, your hardware can consistently provide a more consistent clamp load and a higher designed clamp load for the same bolt.

When designing hardware assemblies when a torque study has not been completed, standard rule of thumb is to assume tool accuracy of 20%. There is an advantage to knowing an assembly tool’s accuracy as you get the most out of the hardware being used. To calculate clamp load with tool accuracy, head over to www.Bossard.com/application-engineering/fastener-expert-tools for a selection of calculators to aid in the design of hardware assemblies.

Brandon Bouska
Application Engineer
bbouska@bossard.com

August 04, 2017
Read More

The Importance of Friction in Your Bolted Joint

One of the major topics we have been discussing is coefficient of friction and the importance of understanding how it can affect your bolted joint. Not all fastener coatings have the same frictional characteristics; some can have a lubricated top coat and some have a non-lubricated top coat (higher friction value). So how does this affect your bolted joint? In most cases if you were to switch to a part that has a lubricated top coat from one without it, you would see bolts stretching and breaking. The cause is that the coefficient of friction has decreased which increased your clamp load at the previous torque value resulting in a failed joint.

The reason it is important to understand this relationship is because it can increase manufacturing costs or even create warranty claims because the changes in friction have a direct relationship to the clamp load created in the joint.

At Bossard we have the capability to perform joint testing so we can help you better understand coefficient of friction and the torque tension relationship in your joints. We can perform testing both onsite at your facility or at one of our engineering design centers. Contact us at ProvenProductivity@bossard.com for more information.

 

Jon Dabney
Application Engineer
Jdabney@bossard.com

July 28, 2017
Read More

Hole Size Matters!

Daddy to the rescue, the savior of broken toys! Well, that’s what my initial thought was before I had to go buy my daughter a new toy because I chose the wrong screw to replace the unusable one. My daughter’s favorite toy would always come loose from the plastic joint due to the repeated fix and the wear and tear from a 6-year-old. Who would have known there is an art in fastening plastic joints with a screw? Did you know the size of your pilot hole, in relation to the diameter of the boss is important? Too large of a hole will not have enough thread engagement, and too small of a hole will cause stress fractures to the boss. The material of boss will have an effect on the size of the pilot hole to have proper seating of the screw. There are also advantages of choosing the right screw for the job. You want to have a screw that has low driving torque and high stripping torque. Having great vibration resistance, especially when dealing with 6-year-old playing habits is very important. Click Here to find the right screw to fit the hole.

What will it take for you to be the hero again and be the super parent that you are? That’s right, hole recognition and proper design. The experts at Bossard can help you design and choose the right hole sizes for the easy fix. Saving one toy at a time is what you do best! Contact us at ProvenProductivity@bossard.com with any questions.

 

John Syharath
Technical Sales
jsyharath@bossard.com

July 21, 2017
Read More

Fastener Materials

Fasteners come in a variety of materials.  The choice of materials will be decided by the application and environment.  The most popular material options are steel and stainless steel. Other options include alloys based on aluminum, copper, titanium, nickel, cobalt and plastics. Important parameters that will help choose the right material are spelled out in the table below.

Parameters Application examples

Mechanical
(strength, ductility, toughness, fatigue)

Steel fasteners come in several grades – make sure that all static and dynamic loads are identified when choosing the right strength.

Corrosion resistance
(galvanic corrosion and stress corrosion cracking)

Several stainless steels and other nickel, cobalt and titanium based alloys have very good corrosion resistance properties.

Beware of galvanic corrosion when designing with different metals!

Temperature resistance
(high temperature oxidation and creep)

Nickel based alloys are used where high corrosion and heat resistance is needed.

e.g., aircraft and land-based gas turbine engines.

Magnetic permeability Some stainless steels can be obtained with a very low magnetic permeability which is required in applications like magnetic resonance imaging (MRI) scanners.
Weight saving

Some titanium grades show a 40% weight saving compared to steel with equal strength.

e.g., automotive applications.

 

For more information on fastener materials, contact us at ProvenProductivity@bossard.com.

 

Fadi Saliby
Technical Sales Director
FSaliby@bossard.com

July 14, 2017
Read More