Author Archives Bossard

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

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

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

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.

2. 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.

3. 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.

4. 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

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

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