Author Archives Bossard

How to Choose the Best Thread Protrusion Length

Length of Thread Protrusion

How many threads should protrude through a nut in an optimal joint?

The generally accepted answer is 1 to 3 threads. Most externally threaded fastener blanks are manufactured with a header point prior to thread rolling, which leaves the first 1 to 2 threads undersized for ease of assembly. To ensure full load carrying capability for a nut and bolt combination, this rule makes sure we have fully formed threads throughout the entire thickness of the nut.

One notable exception to this is for nuts with a locking feature at the top – or toplock nuts. This includes all nylon insert nuts. For these, it is best to have a minimum of 3 threads protruding through the nut to ensure that the locking feature is engaged on a fully formed external thread. Anything less could compromise the locking affect.

Is there a rule for the maximum number of threads protruding through a nut?

Too many threads is a waste of material, adds unnecessary weight and can be a hazard or cause interference with other components. However, functionally there is no downside to having too much thread protrusion.

When selecting fastener lengths, be conscious of the standard length increments. Metric fasteners are generally available in 5mm length increments up to 70mm and 10mm increments beyond this. Inch fasteners have similar standards. When choosing your fastener lengths, it is best to select the shortest fastener that will consistently give you 1-3 threads protruding through the nut.

For questions, please contact our Bossard engineering team at ProvenProductivity@Bossard.com.

Doug Jones
Applications Engineer
djones@bossard.com

June 01, 2018
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How to Improve Your Thread Forming Screw Design

Design Recommendations for Thread Forming Screws

Thread forming screws per DIN 7500 produce a chip-free, gage correct metric internal thread into ductile metals up to 160HV hardness. Thread forming screws may be reused up to 20 times, but care must be taken when re-starting screws to avoid cross threading. They are not intended for use in brittle metals such as gray cast iron; however, they can be used in non-ferrous metals and light metals, as well as steel.

Screw Hole Preparation

Hole preparation and hole size are key to trouble free assemblies with thread forming screws. In general, the material thickness should be 1xd minimum (for example, a 6mm plate for a M6 screw). Punching, drilling or laser cutting are common methods for creating through holes for thread forming screws. Different hole sizes may be required for each method based on the break-out of a punched hole, or the size of the heat affected zone of a laser cut hole, which makes the surface harder and more difficult to thread. Different material composition and thickness will also require different hole sizes.

Good design practice requires a countersunk hole as thread forming will slightly raise the surface of the mating part without the chamfer. This can cause mating parts to not sit flush against the tapped component creating a gap.

Assembly Torque

Recommended assembly torque is 80% of the breaking torque of the screw, and half way between the driving torque and the breaking torque. Drive torque and strip/break torque is recommended to select the optimum hole size for your application.

Check out www.bossard.com for more recommendations for thread forming screws, or contact us directly at ProvenProductivity@Bossard.com to perform a drive/strip torque test for your application.

Doug Jones
Applications Engineer
djones@bossard.com

May 25, 2018
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Quick Guide: Austenitic Stainless Steel Fasteners

Stainless Steel Fasteners

Are you thinking about using stainless steel fasteners in your next application but aren’t sure if it’s the right fit? Read on to see if it’s the right fit for your product.

Austenitic Stainless Steel

Austenitic stainless steel is by far the most common material used to manufacture stainless steel fasteners. It has a chromium content between 15 and 20 percent and a nickel content between 5 and 19 percent. It offers the highest corrosion resistance than martensitic stainless steel and ferritic stainless steel. It is also considered to be non-magnetic. The tensile strength may vary between 72,000 psi and 115,000 psi. Austenitic stainless steel is not heat treatable however the strength of this material may be improved by cold working or strain hardening.

18-8

The material that is commonly referred to as 18-8, is a type of austenitic stainless steel that contains approximately 18% chromium and 8% nickel. Austenitic stainless steel includes AISI grades 302, 303, 304, 304L, 316, 32, 347, & 348.

For more information about stainless steel fasteners and how you can use them in your applications, contact us at ProvenProductivity@Bossard.com.

May 18, 2018
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What You Need to Know About Thread Engagement in Blind Holes

Thread Engagement in Blind Holes

Nuts are designed with a specific proof load strength and thickness that when paired with the proper grade or property class of bolt, the nut will always be stronger. This is good joint design, but what precautions should be taken when designing without nuts and into blind threaded holes?

The main things you need to be concerned with when designing joints with tapped holes are material strength and thread engagement. Generally, you are stuck with a specific material, so the one variable you can change is the depth of thread engagement. Below is a rough guide take from IFI’s “Mechanical Fastening and Joining” handbook by Bengt Blendulf:

Thread Engagement Chart

Tapped Material 8.8/grade 5 10.9/grade 8 12.9/alloy
Steel, hardened 0.8-0.9d 0.9-1.0d 1.0-1.3d
Steel, medium carbon 0.9-1.0d 1.0-1.2d 1.2-1.5d
Steel, low carbon 1.0-1.2d 1.2-1.4d
Cast iron (grey) 1.0-1.2d 1.2-1.4d
Light alloys 1.3-1.6d

For more information on thread engagement, check out our technical section at www.bossard.com and try out our thread engagement calculator which is also available as an iPhone app, or contact us at ProvenProductivity@Bossard.com.

Doug Jones
Applications Engineer
Email: djones@bossard.com

May 11, 2018
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Why Your Fastener is Loose and How to Fix It

Keeping Fastened Joints Tight

Do you have a fastened joint that keeps coming loose? Before proposing a solution, we first need to understand why it’s coming loose.

Reasons for a Fastened Joint Being Loose

  1. Is there visible damage to the surfaces under the head of the bolt or nut? If so, you may have an embedment issue where the surface of the clamped material is not hard enough to support the load of the joint. The best way to address this is to increase the surface area of the fastener by using hardened washers or flanged hardware.
  2. If embedment is not your issue, but you have loose hardware, then rotational loosening is occurring. Some things to check for:
  • What is the clamping length of your joint? 5 times the diameter of the bolt is recommended to ensure proper bolt stretch.
  • Is the proper clamp load being applied? See your hardware supplier to help with bolted joint calculations or a joint study to determine the proper torque to achieve your desired clamp load.

Fixing the Loose Fastened Joint

 Sometimes it is not possible to employ the 5 times the diameter rule, and we still have loosening. Several strategies may be used to help combat this issue.

  1. Thread locking
    • Thread forming screws
    • Pre-applied locking adhesive
    • Liquid thread lockers applied during assembly
    • Lock nuts
  2. Locking at the bearing surface
    • Serrated hardware – make sure to use serrated nuts AND bolts
    • Locking washers
      • Rip-Lock™ – good for up to and including 8.8 property class
      • Ribbed lock washers – good for 10.9
      • Nord-Lock® – good for up to and including 12.9 property class

For more information on keeping joints tight, check out www.bossard.com or contact our engineering department at ProvenProductivity@bossard.com.

Doug Jones
Applications Engineer
Email: djones@bossard.com

May 04, 2018
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All About Rationalization and How It Can Save Your Company Money

Fastener Cost Savings

Rationalization: the action of reorganizing a process or system to make it more logical and consistent.

When we talk to customers about saving them money as a supplier of fasteners, the term “rationalization” always comes up. So how does the definition above apply to saving money just by looking at fasteners?

There are a couple of ways to approach the process:

BOM

The first is to look at one product that is being produced and start with the BOM (bill of materials) for that item. Often the BOM is missing some key descriptors, which we need to identify first by reviewing samples or prints to make sure we have a complete description. Once we have good, complete descriptions, then we can filter the list by size, grade and/or finish and look for similar parts. Next, we ask the question, “Do you need three M8 nuts, or can you get by with one that works for all applications?” By going through this exercise, we can normally eliminate several part numbers which ultimately saves the customer money.

Large Scale Approach

The other way to approach the process is to look at the entire factory, assuming that multiple items are being produced. By looking at the larger scale, we can eliminate more parts and come up with more cost savings. This only works well if we see the entire fastener BOM, so if there is more than one fastener supplier, we need access to all items.

Suggestions from a rationalization exercise can always yield cost savings, at least in theory. However, the reality is that making changes to existing products also has a cost which may exceed the savings. So what is the value of rationalization?

By going through the exercise with existing products, a strategy can be developed to use going forward on future builds. If possible, make a “first choice” list using one drive style, one finish and one grade of fastener that will work in most applications. Publish this list and teach engineers to use it as much as possible. This will help keep the number of special parts to a minimum.

Rationalization can yield some good cost savings ideas, but for the best results, get us involved in the design phase of your engineering project at ProvenProductivity@Bossard.com.

Doug Jones
Applications Engineer
Email: djones@bossard.com

April 27, 2018
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Want to Convert to Metric Fasteners?

Converting to Metric Fasteners

Many companies that have been designing with inch (imperial) fasteners for years are very hesitant to switch over to metric. Why would anyone ever use metric hardware? Where we have seen the most interest is from companies who are expanding into global markets. Most countries other than the US are using metric fasteners and for them, imperial fasteners are a mystery.

Metric is simple once you get the hang of it, and some things even make more sense, such as the marking of property classes (grades) on nuts and bolts. The metric system uses numbers stamped into the head or face of the nut rather than symbols that we use for inch hardware. For bolts, the first number indicates the strength in MPa while the second number tells us the percentage of yield strength.

For example:

Property class 10.9 – 10 indicates a tensile strength of 1000MPa, 9 indicates that the yield strength is 90% of tensile.

Property class 8.8 – 8 indicates a tensile strength of 800MPa, 8 indicates that the yield strength is 80% of tensile.

Metric Fasteners Chart

A quick comparison of grades and property classes is shown below:

Grade (inch) Property Class (metric)
5 8.8
8 10.9
Alloy Steel 12.9

If you are considering a conversion to metric hardware, contact Bossard at ProvenProductivity@Bossard.com, or check our technical resources at www.bossard.com including an inch to metric calculator that is also available as an iPhone app.

Doug Jones
Applications Engineer
Email: djones@bossard.com

April 20, 2018
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3 Types of Joint Studies

Joint Study

The purpose of a joint study is to understand the forces acting on the assembly during tightening. A joint study may be necessary if you have a critical joint or if you are having warranty claims that can be linked back to joint failure.  When is a joint considered critical? If the failure of the joint may cause injury or have a serious monetary impact, it should be considered critical.

Typical Types of Joint Studies

1.Torque Tension Analysis

This study is used to make sure that your prescribed torque is achieving your intended clamp load. Low clamp load may lead to loosening through embedment, rotational loosening and/or fatigue which all can result in failure of the joint. High clamp load may yield the fasteners or the mating joint components, lowering their clamping force which can also result in failure of the joint.

2.Drive & Strip Torque Analysis for Thread Forming Screws

Performance of thread forming screws in both metal and plastic are greatly dependent on the hole size and preparation. The correct hole size should offer a good balance between low drive torque and high strip torque which can be determined through testing. These types of joints aren’t often as sensitive to the amount of clamp load they retain, but choosing the proper hole size and assembly torque will greatly affect the joint’s performance.

3.Vibration Analysis

Joints subjected to vibrational forces may experience loosening and eventual failure if not designed properly. Different types of fasteners and locking features are often utilized to address vibrational loosening. Performing a vibration study helps to select the proper hardware for your specific situation.

To learn more about joint studies and to talk to an engineer about your project, contact us through ProvenProductivity@bossard.com.

 

Doug Jones
Applications Engineer
djones@bossard.com

April 13, 2018
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What’s the Difference Between Kits and Assemblies?

Kits & Assemblies

The difference between a kit and an assembly is quite simple:

  1. An assembly is taking two or more parts and combining them together to form one ultimate part.
  2. A kit is combining two or more components and either bagging or boxing them together. Typically, this gives the end user the ability to easily assemble the parts without confusion.

Kitting and assembling components is a task that typically involves multiple different items, and can take up a lot of time for both your employees and your company. Every item has its mate and place in different applications and if it isn’t done properly, it can potentially cause a butterfly effect. The result will be failures due to incorrect assembly, loss of production due to having to stop, and having to locate the proper part because it wasn’t kitted correctly.

Kits and assemblies need to be done right. It takes patience, attention to detail, and are extremely time consuming. If you feel you have a kit or assembly opportunity that you need assistance with, please contact Bossard at ProvenProductivity@bossard.com.

 

Eric Barfels
Technical Sales
Ebarfels@bossard.com

April 06, 2018
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Fine Threads vs. Coarse Threads: Which One is Right for Your Application?

Threads

Coarse threads have become the standard for most fastener applications, but when should fine threads be considered?

Fine Thread Strengths

Fine threads are technically stronger under static loading, because they have a larger minor diameter which translates into a larger cross-sectional area As. If using fine threads to increase strength, it is important to make sure that the mating thread – the nut or tapped hole – can support the additional load. This may require a thicker nut or more thread engagement in a tapped hole. Remember the cardinal rule that the nut member must always be stronger than the bolt!

Fatigue Resistance of Fine Threads

In joints with high cyclical loading, fatigue failure becomes a concern. Repeated cyclical stress can create cracks which typically occur in the first loaded thread of the joint. Studies have shown that fine threads increase the percentage of load on the first thread, which can lead to a shorter fatigue life. In this scenario, fine threads should not be a first choice.

Fine Thread Vibration Resistance

In the past, vibration resistance was thought to be a benefit of fine threads. The smaller helix angle, at least in theory, slows down the loosening process. A tradeoff of this benefit is the slower assembly time of fine threads; the smaller helix angle requires more angle of rotation to advance, slowing down the assembly process. A better solution to vibrational loosening can often be found in some sort of locking mechanism which can be recommended by your fastener source.

In conclusion, fine threads should not be used for load bearing joints unless there is a very specific reason and testing is done to validate the joint. Some exceptions could be hard to tap materials or thin wall materials. For non-load bearing joints that require adjustment, fine threads may be your best option.

For more help on choosing the proper fasteners for your project, contact us through ProvenProductivity@bossard.com.

 

Doug Jones
Applications Engineer
djones@bossard.com

March 30, 2018
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