Fastener Technology

This is the first of a five-part series about locking methods for fasteners – the first post will cover lock washers and how to use lock washers.

For joints subject to vibration and cyclical loading, maintaining clamp load and/or keeping the joint tight is one of the biggest fastener problems engineers face. As stated in previous blogs, the best joint design to ensure against loosening is a hard joint with a clamping range of five times the screw diameter, tightened to proper clamp load. When this is not feasible, using a locking fastener method is necessary.

First, let’s look at locking washers:

1. Split Lock Washers/Helical Lock Washers

Available at every hardware store for pennies a pound, split washers have very limited usefulness as a locking fastener, especially with grade 5 or 8 fasteners (metric property class 8.8 or 10.9).

Made of very hard spring steel with a small bearing surface area, the spring rate they produce (the amount of force to flatten) is far below the optimal clamp force of a grade 5 or higher bolted joint. This washer’s use should be limited to grade 2 fasteners, small machine screws, or metric property class 5.8 and lower.

2. Ribbed Lock Washers

These conical washers are made to work with higher strength fasteners. The first washer pictured has ribs on the convex side which creates friction between the bearing surface of the fastener (nut or bolt) and works well with grade 5 (metric 8.8) fasteners, but not grade 8 (metric 10.9). The higher strength fasteners have a higher surface hardness, and the ribs will not engage with the bearing surface, limiting their locking ability.

The second ribbed washer pictured above works well with grade 8 (metric 10.9) fasteners. The more aggressive, higher hardness ribs on both surfaces will engage with the harder material to create friction locking.

3. Wedge Ramp Locking Washers

This style of locking washer comes as a pair of washers glued together. The outer surfaces have ribs that are hard enough to work with grade 8 (metric 10.9) fasteners. The wedge ramps between the two washers lock together during tightening, but when loosening the joint, the washers rotate against one another, creating slightly more clamp load as they overcome the angle of the ramp.

It is important to note that any of these locking washers should NOT be used with flat washers, and if used with a bolt and nut joint, they should be on both ends. Rotational loosening caused by vibration will occur at the bearing surface with the least amount of friction, so flat washers should be avoided.

It’s also important to note that because these washers increase bearing friction, more torque may be needed to achieve your desired clamp load. Some washer manufacturers publish recommended torque values, but for critical joints, a joint study is recommended to arrive at your optimal setting torque.

There are many other styles of locking washers, but the ones above will get you through most situations. For more information on locking fasteners email us at ProvenProductivity@bossard.com.

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Dowel pins are great for aligning, but in a world where light weighting is becoming more and more pivotal, there may be some alternatives to the standard hardened and ground dowel pin.

The first option to review is if a fully ground and hardened dowel pin is necessary. Can a similar result be achieved with a pin that does not require as much processing and hole preparation?

Looking at a simpler way to align mating components may not only be a way to reduce weight in the completed assembly, but also a way to reduce total cost. The alternative to a reamed hole with a dowel pin is to use a counter bored hole with a hollow dowel pin which a bolt or screw can pass through. This is a very effective way to combine functions of the drilled hole required for the dowel pin and the fastener required for clamping the parts together.

Hollow Dowel Pins & Split Dowel Pins

Hollow dowel pins work for many applications and have some benefits associated with them. They can be used to reduce the tolerance required on the mating hole as well as reduce overall weight. Hollow dowel pins can also come with a split in them. This split offers some advantages over seamless hollow dowel pins. Split dowel pins are easier to manufacture than seamless hollow dowels and can be used with a tolerance zone that isn’t as tight as standard dowel pins.

Another great alternative for plastic parts is barbed pins. These allow the material flow around the barbs to lock in place. If a pin needs to be locked into metal, this can be done with a knurled pin or a grooved pin. Both of these pins can act as a pivot and can also be used for locating purposes.

For more information about dowel pins and how they can be used in your application, contact us at ProvenProductivity@bossard.com.

Brandon Bouska
Application Engineer
bbouska@bossard.com

When designing joints with washers, it is important to understand and use the correct washer for the job. Below are a few tips for making a good selection:

1. When using flat washers, match washer hardness with bolt and/or nut hardness to avoid joint settling.

• 140 HV ≤ 8
• 200 HV ≤ 8
• 300 HV ≤ 9

2. Split lock washers are not intended for use with hardened bolts/nuts, only with property class ≤ 8.

3. Toothed lock washers are designed to permit electric current flow between components, and do not provide much of a locking feature. They are also only used for low property class ≤ 8.

4. Specialty washers exist for high strength joints, for example:

• Rip-Lok – designed for property class 8.8
• Conical spring washer with serrations on top

• Schnorr washer – designed for property class 10.9
• Small OD conical spring washer with serrations on both sides

For more details on selecting the correct washers for your application, checkout www.bossard.com or send us an email at ProvenProductivity@bossard.com.

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Doug Jones
Applications Engineer
djones@bossard.com

How do I know if I have the right installation force? Did I set it right? Do I have the right tool? These are some of the most valid questions when installing and using rivet nuts. Knowing the mating material, grip range, hole size, and clamp force will aid in getting the right fit for the joint in your application. Securing a proper clamp load will prevent from loosening and/or rotational loss. Understanding the creep and material relaxation is also important when using this joint design.

Finding the Correct Fit

Examples of Rivet Nut Applications

There is a lot to consider when choosing the right rivet nut. There are many choices for body styles, material, and coating finishes. Let the experts at Bossard help you narrow down your search and get you on the right path to a secure joint. Let us help you save money and avoid costly warranties or claims. Contact us at ProvenProductivity@bossard.com to learn more.

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John Syharath
Technical Sales
jsyharath@bossard.com

If assembly requires applying liquid thread locker to any threads, pre-applied thread locking patches should be investigated.  Pre-applied thread locking patches could potentially be an untapped area for both time savings and cost savings.

Thread Locking Materials

Pre-applied thread locking materials work in a couple different ways. A nylon thread locker pushes work by forcing metal to metal contact between the threads on the opposite side of the applied area. Because of the way this type of thread locker works, coating all 360° of the threads is not advised.  This type of patch can be reused if care is taken. A thread locking patch works like a two-part epoxy. When installation occurs, the two parts mix together and after the two parts set, the area between the threads are filled. This type of thread locker can only be used once and if the joint needs to be serviced, a bolt with a new patch must be used.

Filling the thread tolerance area between the threads will help resolve some vibrational loosening that might be occurring.

There are many features of pre-applied thread locking patches including the amount of locking material applied to the threads is more consistent, parts can be processed in large lot quantities, and the pre-applied thread locking patch usually has a long shelf life.

Please feel free to contact Bossard to help answer any questions that may arise.

For more information on thread locking patches, check out www.bossard.com or contact us directly at ProvenProductivity@bossard.com.

Brandon Bouska
Application Engineer
bbouska@bossard.com

Now more than ever consumers are demanding vehicles that are ecologically friendly, as well as fuel efficient, so they can save more at the pump. To meet this demand, automotive engineers are faced with the task of designing capable engines to fit these new requirements. Often, that means making the engines on all vehicles, from small compact cars to large trucks, smaller and lighter.

To reduce engine weight and size, the components must be tightly condensed together, giving the engine parts little wiggle room. The necessary ports and passages used to deliver oil and all the other mandatory engine operations must be carefully integrated into the remaining available space, resulting in thin port walls.

The Challenge of Thin Port Walls

Thin port walls are quickly becoming more common, but they aren’t built to withstand the usual methods of older style plugs, such as cup plugs or threaded screws. These older plug styles can vary greatly in their results, causing a larger fluctuation in process performance—and with thin port walls and tightly packed engine components, even a small discrepancy during the installation process can cause a disaster.

The process must be precise and accurate every time to prevent damage or leakage, as many times ports are in a dangerous proximity to bearings or moving parts. Engineers have consequently been relying on low stress expander seals to attain reliable, precision sealing.

Benefits of Low Force Seals

Low force seals, such as SFC KOENIG EXPANDER® plugs, cause less damage to the port wall and components, having minimal effect on even the most complex engine geometries. All the advantages of low force seals are listed here below:

• Less negative effects on engine components
• Prevent cracking of base
• Avoid expansion of port into nearby components
• Reduce the risk of leaks and wear
• Less effect on overall system performance
• Comes with its own installation equipment to ensure needed precision; gives the installer more control

Unlike cup plugs, threaded plugs and the like, low force seals provide leak-proof security and work well when dealing with extremely sensitive applications. Their reliability is even more critical when dealing with ports located deep within the maze of a tightly packed engine. Due to the little to no margin-for-error in engines with thin port walls, the strict tolerances and reliable installation methods of low force seals are essential to the successful performance of these modern machines.

For more information about SFC KOENIG EXPANDER® plugs and their other products, contact us at ProvenProductivity@bossard.com.

Another consideration when building a fastener strategy is drive styles, especially in smaller screws with internal recess drives. If no fastener strategy is enforced, many customers may have several different styles in their bill of materials, creating a proliferation of similar parts. Some of the more common drive styles are shown below:

Slotted

• A slotted head style is not recommended for use in high volume or automated assembly. Sometimes this drive is still the best option for access panels that need to be serviced in the field using simple, readily available tools.

Phillips

• This is a common drive for most head styles and okay for higher volume assembly. One disadvantage is that the tapered side walls of the drive require significant down force during tightening or removal to prevent the tool from slipping and damaging the drive. Once the drive is damaged, it can be hard to remove.

Pozidriv

• Similar to Phillips, a Pozidriv head style has vertical side walls in the drive which have less tendency to “cam-out” when installing or removing. This drive typically extends tool life in high volume assembly but does require a Pozidriv driver bit which can be confused for a Phillips drive bit. Pozidriv heads can be identified by the four tick marks around the cross (see above). Driver bits are marked with a “pz” while Phillips are marked with a “ph”. 

Torx, Torx Plus or Hexalobular

• These drives have been around for several years and are more readily available in metric than inch hardware. Torx and hexalobular are very similar and use the same driver, while Torx Plus has slightly different geometry and offers longer tooling life in high volume assembly. Torx driver bits are marked with “T” while Torx plus are marked with “IP”.

Socket

• This is an internal hex drive which uses a hex key, sometimes also referred to as an Allen key. It is often used in larger diameter bolts which have limited space for a standard socket as a driver. In smaller screws, the internal hex may become damaged as tooling wears, which can make them difficult to remove.

For more information on the availability of these drive styles, and which one best fits your needs, check out www.bossard.com or contact us directly at ProvenProductivity@bossard.com.

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Doug Jones
Applications Engineer
djones@bossard.com

Another key component to a fastening strategy is managing or eliminating high risk fasteners. What are high risk fasteners? High risk fasteners are those which are subject to delayed failure, also known as hydrogen embrittlement (HE).

High-Risk Parts

For a part to be considered high risk, it must have three overlapping elements:

1. A hardness (core or case) in excess of Rockwell C 38
2. Subjected to processing, which induces hydrogen gas, such as electroplating or acid cleaning
3. Assembled in a manner which sustains high tensile or bending stress

Common high-risk fasteners are listed below:

• Electroplated
  • Property class 12.9 screws
  • Alloy steel grade socket head cap screws
  • Case hardened thread rolling screws
  • Retaining rings
  • Spring pins & clips
  • Conical washers – by themselves or assembled to screws of any grade/class (SEMs screws)

Any of the above listed fasteners can be managed by selecting a finish other than electroplating which does not induce hydrogen along with using mechanical cleaning methods.

For more information on high risk fasteners, check out www.bossard.com or contact us directly at ProvenProductivity@bossard.com.

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Doug Jones
Applications Engineer
djones@bossard.com

As mentioned in part one of this series, many manufacturers treat fasteners as an afterthought, but having a clearly defined fastener strategy can offer many benefits in the long term.

Another key point to consider when building a fastener strategy is joint criticality.

Joint Criticality

Looking at each joint and asking yourself, “What happens if this joint comes loose or fails completely?” is a good way to help guide your strategy. Your levels of response may look something like this:

• Level 1 – Product may cease to function, but can be easily repaired by consumer – generally not a warranty claim
• Level 2 – Product may fail but warranty claim is unlikely
• Level 3 – Product may fail and warranty claim is likely
• Level 4 – Product may fail and injure consumer

Levels 3 and 4 should help guide your strategy in friction control, locking features, and method of assembly. These joints should use controlled tightening methods and tools along with finishes having specific friction ranges engineered into them. If you are unable to incorporate the five times the diameter clamping range into the joint, locking features may also need to be incorporated to help keep things from loosening.

Warranty issues may be worth investigating when building a fastener strategy as well. If loosening or corrosion problems are prevalent, changes to the strategy may help address these claims in the future.

For help with critical joints, or warranty claims, check us out at www.bossard.com or contact us directly at ProvenProductivity@bossard.com.

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Doug Jones
Applications Engineer
djones@bossard.com

For many manufacturers, fasteners are somewhat of an afterthought, but having a clearly defined fastener strategy can offer many long-term benefits.

A few key points to consider when building a fastener strategy are as follows:

Metric or Inch System

Many manufacturers in the United States still primarily use inch fasteners for obvious reasons, but if there are any plans to expand into foreign markets, metric fasteners will be a preferred choice. It may seem like a scary proposition to switch, but the right fastener supplier can help!

Finish Requirements

Understanding the life cycle of your product, the environment it will be operating in, and which fastener finishes will satisfy your needs is important. Selecting specific finishes and building them into your strategy can also help prevent part proliferation. It’s also important to understand regulations that may be associated with certain finishes, such as RoHS and REACH. Even if your industry is not bound by these regulations, they may still affect the product you are getting whether you know it or not. If you have been using what you think is the same zinc electroplating for ten years, it’s almost certain that you are getting a different finish now than was being applied originally. You should be aware of things such as reduced corrosion protection resulting from material handling, and different coefficients of friction which could require different assembly torques.

Property Class or Grade

If your bill of materials contains multiple fasteners of the same size and configuration, but with a different grade/property class, you may want to consider consolidating to one part that will work for all strength requirements.

Keep a lookout for future blogs on this topic, and as always, check us out at www.bossard.com or contact us directly at ProvenProductivity@bossard.com to see how we can help you build your fastener strategy.

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Doug Jones
Applications Engineer
djones@bossard.com