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Ultimate Guide on Tolerances for Fasteners & Assemblies

One of the keys to understanding the metric system is the understanding of its tolerance system. There is simply no manufacturing method that enables production to perform to exact dimensions. In the inch system, tolerance is mainly done by giving minimums and maximums.

The ISO tolerance system was created by the International Standard Organization in 1948. Originally it was developed for fits and is still mainly used in this manner. It is based on IT Grades which are base tolerances from which all the individual tolerances are derived.

The system is composed of a letter and number base.

The letter indicates the location of the tolerance (whether plus or minus, how much plus, how much minus).

The number indicates the tolerance range. The bigger the number, the bigger the spread between the minimum and the maximum, and therefore the larger the tolerance range.

Download the Bossard Technical section for basic tolerances and tolerance fields, or email ProvenProductivity@bossard.com with any questions regarding tolerances.

Joe Stephan
Application Engineer

Bossard

November 17, 2017

Fastener Technology

What is meant by the term “standard fasteners”? Generally, it refers to fasteners that are stocked on the shelf rather than made to order, which translates into shorter lead times. Often, looking at a catalog will give you a pretty good idea of what is available, but below are some general guidelines and a few exceptions to note.

Standard fasteners lengths follow a pattern. For short fasteners, typically lengths under 20mm are available in 8, 10, 12, 16 and 20mm. Lengths from 20mm to 70mm are available in 5mm increments, while lengths over 70mm skip to 10mm increments. For example, you can get an M10 x 65 or M10 x 70, but the next available length is M10 x 80.

Width Across Flats (WAF) for hex products can be a bit confusing. DIN standard hex head parts are generally stocked more than ISO standard hex heads. This can be significant in diameters M8, M10 and M12. DIN parts are 1mm larger WAF for these sizes, requiring a larger wrench to drive them. Other diameters share the same WAF for both standards. Many distributors consider DIN and ISO hex heads as equivalent even though the heads may be a slightly different size.

Metric socket head products are not readily available in all property classes. Socket head cap screws are offered in 8.8 and 12.9, but not in 10.9. Flat socket head cap screws and button socket head cap screws are available in 10.9, but not in 8.8 or 12.9. Exceptions can be made with special orders, but these are not considered standard.

Doug Jones
Applications Engineer
djones@bossard.com

Bossard

November 10, 2017

Fastener Technology

When selecting fasteners that are to be assembled together, it is important to consider their strength compatibility. The nut should always be stronger than the bolt, so when using higher strength bolts, such as metric class 10.9, make sure to use the correct, corresponding nut. In this case, a class 10 nut would be correct. Stronger nuts may be used with lower strength bolts without any problems.

Strength of washers should also be considered, but are often overlooked. Using non-heat treated washers with heat treated hex head cap screws can cause joint settling as the relatively small bearing surface of the cap screw can embed itself into the soft washer over time, causing a loss of clamp load.

Standard metric flat washers have class designations that can be paired with heat treated bolts. Below is a compatibility chart that will help you make the right choice.

Check out the “Technical Resources” section at www.bossard.com for more information on selecting the proper fasteners, or reach out to us at ProvenProductivity@bossard.com with any questions.

Doug Jones
Applications Engineer
djones@bossard.com

Bossard

November 03, 2017

Fastener Technology

How Important is Process Control for Fasteners

Many fastener manufacturers are faced with several different customer requirements or international standards. This leads manufacturers to create sampling plans to detect nonconforming product during mass production or final inspection. There are also several different AQL (Acceptance Quality Limit) levels according to batch or lot sizes. Do you think relying on final inspection sampling is the best idea? Even if they choose various samples from multiple bins of the same batch?

Fastener manufacturers are no different than any other industry. Speed, delivery, and quality are key, and most companies have been driving lean methodologies and efficiency tools into manufacturing processes. But does this have a negative impact on quality? Maybe, but most fastener manufacturers do still maintain minimum sampling plans that are fairly aligned with some international standards for mass production.

So what is a good detection method to minimize nonconforming product during the manufacturing process? Contact us through ProvenProductivity@bossard.com, and see what quality processes we encourage Bossard manufacturers to use to ensure quality fasteners for our customers.

Tony Peters
Quality Manager

Bossard

October 27, 2017

Uncategorized

All design engineers are faced with questions like “What torque is the correct torque?” and “How do I know my assemblers are achieving the correct torque?”. When Bossard engineers hear these questions we typically respond by asking “What clamp load are you trying to achieve?”. When asked, engineers usually reply with confused expressions, or the unveiling of a torque chart from engineering manuals or colleagues. There is no doubt time and effort went into these torque charts, but how does the testing to create the chart relate to the application the engineer is working on?

Bossard’s biggest piece of advice is to test, test, test! The approach that yields the most success and maximizes the strength of fasteners is to test the application until failure. Until you know the point that the current fastener fails, you are throwing darts at your target clamp load. In some instances, friction varies from lot to lot and can affect the clamp load enough resulting in loose fasteners. Testing until failure helps you achieve a baseline average of total fastener strength. This baseline encompasses friction and is incontestable. The only thing left for the engineer to decide is the percentage of the baseline that is needed for the application. Typically, 75% of the failure torque is used for assembly but there are times in critical applications that fasteners are needed to be pushed further.
When investigating current torque strategies at your company don’t be afraid of torque charts. They can often be used as a great starting point. When a clamp load needs to be met to hold something together, challenge the tightening strategy and test it to prove its validity. You will never regret testing the actual application.

Have more questions about clamp load and proper torque? Reach out to Bossard at ProvenProductivity@bossard.com to receive personalized advice from one of our Application Engineers!

Ben Oostdik
Application Engineer
boostdik@bossard.com

Bossard

October 20, 2017

Torque and Galling

When doing research for your bolted joint it is important to understand the coefficient of friction of the finish of the fastener. There are many different options for coatings that will give you different ranges in values. Typically, those values are tested to a certain specification. Whether it’s an OEM specification, or per the ISO 16047 standard, each one can have minor differences (if you have questions on standards reach out to a Bossard engineer). So what do those values actually mean and how are they relevant to your joint?

Most of the testing that is done to validate the values of the coating is done on M10 surrogate bolts with a standard hex head cap screw. They are tested in lab conditions with either a plain finish washer or nut that is cleaned and degreased or with different variations done in a lab setting. The cause for concern is using those values in your joint. The values can change depending on your bearing surface and your mating threads (material, surface condition, lubrication, bearing surface area, etc.). As stated in previous Proven Productivity blog posts, it’s important to understand the relationship so you prevent costly errors down the road.

That is why we at Bossard always recommend doing bolted joint testing. We have the capability to perform joint testing so we can help you better understand the coefficient of friction and the torque tension relationship in your joints. We can perform testing onsite at your facility or you can visit one of our engineering design centers.

Jon Dabney
Application Engineer
Jdabney@bossard.com

Bossard

October 13, 2017

Fastener Technology

Take a minute to think of some of the most amazing mechanical marvels we have today: electric vehicles, exo-skeletons, cell phones, wind turbines, and many more. What do most of them have in common? They are all held together with some type of fasteners.

Now think about your product. What happens if the fasteners fail during use? At best, the customer is not happy and may look at other brands for their next purchase. At worst, damage to property or personal injury could result.

So, why are fasteners always the last thing we think about in our design? It’s true that assembly is the last step in the product design process, but fastener selection should be carefully considered in the early product design stages to ensure the best and safest product. Early fastener selection is also associated with time and cost savings making your process more lean and efficient.

Not sure what is the best choice? Have questions about what’s readily available or what’s new in fastener technology? Contact your fastening solutions provider to help you select the best hardware for your project. Don’t wait until two weeks before production starts!

Contact ProvenProductivity@bossard.com for information about the latest and greatest hardware innovations, and get help with your next project!

Doug Jones
Applications Engineer
djones@bossard.com

Bossard

October 06, 2017

Fastener Technology

Standardization in the fastener industry is a necessity. Without fastener standards, there would be inconsistency and inefficiency. Because of fastener standards, engineers and consumers alike know exactly what to expect.

Because of the consistency that comes along with standards, international business and trade becomes much easier. Companies can purchase products from around the world and can rest easy knowing that the product will fit in their application. There are many organizations that create fastener standards; one of these organizations is the European Committee for Standardization.

In 1991, the European Committee for Standardization, also known as CEN, began working on the standardization of the fastener industry intended to be applicable throughout Europe. International Organization for Standardization (ISO) standards are adopted as European (EN) standards wherever possible. However, new EN standards are established when the ISO standards are not deemed suitable.

German Institute for Standardization (DIN) standards are being replaced by EN or ISO standards. In the future, DIN standards will apply only to products for which no ISO or EN standard exists.

DIN EN ISO plus a number (e.g. DIN EN ISO 4027) would indicate that a combination of all three standards are acceptable.

DIN ISO plus a number (e.g. DIN ISO 7049) indicates an ISO standard that is an adopted unchanged DIN standard.

Standards can sometimes be confusing. If you have any questions about fastener standardization, let us know by reaching out to us at ProvenProductivity@bossard.com.

Joe Stephan
Application Engineering
jstephan@bossard.com

Bossard

September 29, 2017

Quality Practices

You’ve spent months on your design doing careful analysis and selecting the right materials. You are finally getting close to production – but wait, you haven’t selected a fastener supplier yet. You need mostly standard fasteners, except for those few specialty print parts. It’s just fasteners; how hard can it be?

The next step is to turn over your fastener list to the buyer and have them find the cheapest hardware they can negotiate to save a few pennies on the product. After all, it doesn’t matter where they come from. All fasteners are the same, right?

This scenario may be a bit harsh, but is all too often true. If manufacturers are making standard fasteners, they should be using the same fastener standards. Unfortunately, some manufacturers take short cuts to keep costs down in this highly competitive market. Some low-cost providers may make perfectly good fasteners 90% of the time, but is 90% good enough for your design? Assembly issues, failures in the field, warranty claims costs, and poor customer satisfaction when a new widget breaks or doesn’t perform as advertised can be very expensive and time consuming.

My advice is this: know where your fasteners are coming from. Here are some questions you can ask yourself before selecting a supplier:

Does the manufacturer have good process controls?
Are they tracking reject rates both internally and externally?
Are they subcontracting fastener finishes or heat treatment?
Do the subcontractors have good process controls?

Doug Jones
Applications Engineer
djones@bossard.com

Bossard

September 22, 2017

Fastener Technology

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