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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
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Why are my zinc plated screws stretching and breaking?

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

July 07, 2017
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Specialty Screw Threads for Direct Assembly

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

June 30, 2017
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Loose Fasteners: Causes and Solutions

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

June 23, 2017
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Fastener Lead Times

“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

June 16, 2017
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Fastener Cost Drivers

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

June 09, 2017
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