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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?

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

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

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

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.

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

Fadi Saliby
Technical Sales Director
FSaliby@bossard.com

For years, zinc electroplating has been the standard finish for fasteners, and hexavalent chromium was used over the zinc to protect against corrosion of the parts. With Restriction of Hazardous Substances, also known as RoHS, gaining traction in the United States, many platers are eliminating hexavalent chromium in favor of trivalent, which meets the restrictions for now, but also has some unwanted side effects.

Trivalent chromate is not self-healing, like hexavalent, so handling damage can degrade the corrosion resistance quickly if something is not added to the finish. Often what is added is some form of sealer, which helps with corrosion resistance but will change the friction coefficient of the joint, in many cases making it lower, especially when high hour corrosion resistance is desired.

So, back to the original question in the title: why are my zinc plated screws stretching and breaking? The easy answer is “lower friction”. The question you should be asking is, what is the friction coefficient of my electroplating? If you do not specify, it’s almost certain that it has changed in the last five years, and as a result, you are getting a different clamp load in your bolted joints.

For more information on how changes to your finish can affect your clamp load, contact us at ProvenProductivity@bossard.com.

Doug Jones
Applications Engineer
djones@bossard.com

Not all screws thread into nuts. There are many specialty type screws that are designed to thread directly into untapped holes in different types of material.

Wood screws have been around for years, but standard wood screws often don’t work well in particle board or MDF (medium density fiberboard). Standard wood screws have a 60° flank angle, and when threaded into a drilled hole, have to move quite a bit of material to make room for that thread. Specialty screws with a narrower flank angle and a taller thread put less stress on the wood and provide greater pull out force without damaging the material.

Similar specialty screws exist for soft thermoplastics, having very sharp, narrow angled threads that can be successfully assembled into molded or drilled holes or into bosses. Harder thermoset plastics may require a cutting slot on the end of a screw to remove some of the material, acting like a thread tap.

Spaced thread tapping screws work well for sheet metal, but for thicker steel, thread rolling screws with a finer pitch are required. Thread rolling screws have the same pitch and shape as regular screws, and are sometimes hard to tell apart from each other. Thread rolling screws often have a triangular shape to the point and the body of the thread, which you can feel if you roll the threads between your fingers. These screw threads are also generally case hardened to make them harder than the steel that they are threading into.

Similar screws with special thread geometry exist for threading directly into lightweight metals such as aluminum and magnesium.

For more information on specialty screw threads, contact us at ProvenProductivity@bossard.com.

Doug Jones
Applications Engineer
djones@bossard.com

A common headache for engineers and consumers alike is fasteners coming loose and causing problems. So, what are some of the causes of loosening?

Under tightening, which results in low clamp load, is one main cause of loosening fasteners. If a joint does not have enough clamp load to keep the joined parts from slipping against one another, rotational loosening or fatigue failure may occur. It’s important to follow manufacturer’s recommendations for proper tightening.

Using fasteners with a small bearing surface area to join softer materials can lead to embedment of the head into the material. Over time this can cause a loss in clamp load resulting in loose joints. It’s important to understand surface pressure limits of screws and mating materials, and match them accordingly to avoid embedment. Flanged head fasteners or hardened washers are a good way to spread the load out and avoid this problem. It should be noted that many flat washers are not hard enough to support high-strength fastener loads, so choose your washers accordingly!

Vibration is another cause of loose fasteners. In a nut and bolt joint, a good design will actually stretch the bolt slightly to create a rubber band-like effect, which helps to keep the fastener tight. A good rule of thumb for design engineers is to use a clamping length of 5 times the bolt’s diameter to ensure good stretch when torqued properly. For joints subjected to vibration which cannot use this rule, then serrations, locking patches, or specialty lock washers may be needed.

For more information on how to keep fasteners tight, contact us at ProvenProductivity@bossard.com.

Doug Jones
Applications Engineer
djones@bossard.com

“It’s just nuts and bolts, why does it take 26 weeks to get my parts?” Lead times for fasteners can be frustrating and hard to understand when I can often go to the hardware store and get something that will work. So why does it take so long to get parts?

Standard off-the-shelf parts don’t require much lead time. Depending on the order quantity and the available inventory, parts can normally ship anywhere from one day to three weeks from the receipt of an order. For high volume standard parts or special parts, the lead time can increase substantially.

High volume standards may not be in stock, so a manufacturer will have to make them to order. Fastener manufacturers schedule their work to keep their machines busy and running as efficiently as possible, so when they receive an order to make a certain size, it may not fit into their schedule for several weeks. The same is true for specialty parts. Normal lead times for domestic manufacturers is 12 to 14 weeks on a first-time order. For overseas suppliers, transit time is added which can easily reach 24 to 26 weeks. Expediting an order is often possible, but break in fees and/or increased shipping fees may apply which raise the cost of the parts.

The best way to combat long lead times is to give plenty of notice for first-time orders, and then provide accurate forecasting for future orders.

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

Doug Jones
Applications Engineer
djones@bossard.com

Why do some fasteners cost so much more than others? This is a question many purchasing professionals and engineers struggle to understand. Many factors contribute to the cost of a fastener.

Fasteners which conform to a standard and are readily available will always come with a lower price tag. Any slight change that deviates from the standard, whether it’s thread length, a special finish, or a slightly different dimension will require costly modifications or parts that need to be made special to order. This can easily double or triple your part cost.

Volume is another key factor to better pricing. Generally, there are three prices for a part depending on if they are considered low, medium or high volume. In general for screws from M5 – M12 (1/4″ – 1/2″) high volume is greater than 250,000 pieces, low volume is under 1000 pieces and medium volume is in between. Many manufacturers also have MOQs or minimum order quantities for non-standard parts which are based on weight. It is not uncommon to see MOQs of 2,000 pounds on special fasteners.

Fastener finish, or the plating or coating of the fastener, is often what forces us to purchase non-standard fasteners. Most standard, off-the-shelf fasteners will come with commercial zinc plating, which may be fine for interior applications which never see moisture. However, if your application requires more corrosion resistance, then the finish becomes a key factor and you are no longer buying standard fasteners. Many specialty finishes exist to meet the customer’s needs, but without high volume requirements, they can be quite costly.

For more information on the fastener cost drivers, contact us at ProvenProductivity@bossard.com.

Doug Jones
Applications Engineer
djones@bossard.com