Author Archives Bossard

Tips for Choosing Fastener Material

Fastener Design Tips part 5

By far the workhorse material of the fastening world is carbon or alloy steel, often plated or coated to resist corrosion in service. But, is this material suitable for all designs? What about cast iron, stainless steel, aluminum or even carbon fiber? If we make poor choices on material or fastener finishes, we can be setting our joints up to fail. Let’s take a brief look at each of the aforementioned materials:

Joint Materials Recommended Fastener Material Recommended Fastener Finish Comments
Plain steel Plain carbon or alloy steel Plain finish
Cast iron Plain carbon or alloy steel Plain finish
Aluminum Aluminum alloy or stainless None Zinc flake coated plain carbon steel may also be used
Copper Copper None

 

Stainless is suitable
Stainless steel Stainless steel None
Zinc die cast Plain carbon steel Zinc plated
Carbon fiber Stainless steel

It is ideal to match the fastener material as closely as possible to the joint material not only to guard against galvanic corrosion, but also match the thermal expansion and contraction of the materials as they cycle through different temperatures.

There are also specialty fastener materials designed specifically for high temperature service.

 

For more information on which fastener material is right for your design, check out www.bossard.com or contact us at ProvenProductivity@bossard.com.

Doug Jones
Applications Engineer
djones@bossard.com

January 18, 2019
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What You Need to Know About Grades and Property Classes

Fastener Design Tips part 4

Whether designing in inch or metric fasteners, you need to be aware of the grade (inch) or property class (metric) of your fasteners. Below is a matrix that helps you compare the difference between the two:

matrix

 

 

 

 

When designing with nuts and bolts, grades and property classes of hardware should match. However, higher strength nuts may be used with lower strength bolts. The nut must always be stronger than the bolt for safe and proper joint design, so using a lower strength nut with a higher strength bolt is asking for trouble.Typically, as strength increases, so does price. It is also important to note that stronger is not always better. High strength fasteners tend to be less flexible and often require better tightening control to take advantage of their strength.

Finally, strength of washers is often overlooked. The purpose of using a flat washer under a hex head bolt or nut is to spread the load and increase the surface pressure limit of the bearing surface. If non-hardened washers are used with hardened bolts, the hex head can dig into the face of the washer, allowing the clamp load to settle and the joint to loosen.

 

For more information on fastener grades and property classes, visit www.bossard.com or contact us at ProvenProductivity@bossard.com.

Doug Jones
Applications Engineer
djones@bossard.com

January 11, 2019
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7 Factors to Keep in Mind When Selecting a Fastener Finish

Fastener Design Tips part 3

Many engineers will put great thought into selecting the proper type of fastener for their design but overlook the importance of the finish. When selecting the best finish for your fasteners, it is wise to consider the following factors:

  1. Safety – The incorrect finish could contribute to a failure of the fastened joint
  2. Corrosion protection – What is the expected service life of the part and the service condition?
  3. Resistance to handling damage – How do nicks and scrapes from handling and wrenching affect the finish?
  4. Criticality of the joint – Will the assembly fail if the joint comes loose? What are the consequences of an assembly failure?
  5. Functionality – Will the finish prevent my fasteners from assembling due to thread or recess fill?
  6. Availability – Is the finish readily available?
  7. Cost – Is the finish cost effective for my assembly?

There are many exotic coatings that have been developed for specific applications, with more being produced every day. Some of the most commonly available finishes are:

  • Electrodeposited Zinc (“commercial” zinc)
  • Electrodeposited Zinc Nickel
  • Mechanical Zinc
  • Zinc Flake
  • Hot Dip Galvanized
  • Epoxy Electrocoat

 

For more information on fastener finishes, visit our website at www.bossard.com, or contact us at ProvenProductivity@bossard.com.

Doug Jones
Applications Engineer
djones@bossard.com

January 04, 2019
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Why You Should Pay Attention to Fastener Head Styles

Fastener Design Tips part 2

Choosing the proper fastener head style often boils down to either the engineer’s preference, what’s currently popular, or what’s needed for a very specific (non-standard) use. All of these situations can be dangerous and costly.

Internal Recess Drives

For internal recess drives, the most commonly available styles for smaller machine screws (1/4″ or M6 and smaller) are slotted, cross recess (Philips), cross recess (Pozidriv), Torx and Torx Plus. Internal hex drives are common in what we refer to as socket head screws, but they are intended more for tooling than for use in production. The matrix below should give you some points to consider when choosing an internal drive screw:

Slotted Philips Pozidriv Torx Torx Plus
No tool required* BEST NR NR NR NR
Consumer assembly OK BEST NR** OK NR**
Production hand assemble NR NR OK OK OK
Production power assemble NR NR OK OK BEST
Tool life POOR POOR OK OK BEST
Torque transfer POOR POOR OK BETTER BEST

*No tool required: In some designs, such as access doors on equipment, tools may not be readily available, so anything from a coin to a car key may be used to rotate the slotted head and gain access.

**NR: Pozidriv and Torx Plus drives are easily confused with Philips and Torx, even though they require different drivers. For consumer assembly, it is recommended to steer away from these.

Internal Recessed Drive Screws

For internal recess drive screws, the head style is imperative to consider. Many options exist, but not all are readily available. The most common head style is the pan head and it is available in nearly every size up to 1/4″ or M6. Less common styles are round head, cheese head, fillister head, hex head and hex washer head.

Larger Screws

For larger screws, hex head, hex flange head and hex socket head are the most common styles available. Hex socket heads are generally only used when there is a clearance issue where a socket will not fit on the head to drive it. Hex flange screws are generally more expensive than hex heads, but with their larger bearing surface, they can often eliminate the need for washers which can ultimately lead to a cost savings. Hex heads may be used without washers when bearing up against a very hard surface with a properly sized hole, but in many cases the surface pressure limit of the mating part is not high enough to support the load without adding hardened washers or a flange.

 

For more information on choosing the correct head style for your next design, check out our website at www.bossard.com or contact us at ProvenProductivity@bossard.com.

Doug Jones
Applications Engineer
djones@bossard.com

December 28, 2018
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How to Design Soft Joints the Right Way

Fastener Design Tips part 1

Product engineers seldom receive training on fasteners, so some common mistakes are made in the design phase. The next five blogs will focus on some of these common mistakes.

What are Soft Joints?

A joint is considered “soft” when one or more of the materials being joined together is softer or weaker than the fasteners used to secure the joint. This may result in low preload which could lead to loosening or fatigue failure depending on the outside forces acting on the joint. Some examples of soft joints are gasket joints and plastics.

Gasket Joints

These are soft joints which sandwich a soft gasket material between two surfaces to seal against leaking. They must not be over tightened, or the gasket could fail. Over time, these gaskets can break down and begin to leak or allow the fasteners to loosen. An alternative method to help prevent failure of this joint is to design a step or groove in the gasket surface which allows the gasket material to compress and seal. This provides the hard surfaces to contact one another and achieve a higher clamp load, once again making a hard joint.

Plastics

Many designs now incorporate plastics, which can be tricky to assemble without cracking. When attaching plastics to metal, the use of a shoulder screw or some type of compression limiter that prevents too much stress on the plastic material can be incorporated. Another alternative is threading specialty screws such as Delta PT® screws directly into the plastic. With the proper hole size and design, this method has been proven to be highly effective and economical.

 

For more information on soft joints, or for help with the assembly of your next great design, visit our website www.bossard.com or contact us at ProvenProductivity@bossard.com.

Doug Jones
Applications Engineer
djones@bossard.com

December 21, 2018
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Locking Fastener Methods to Secure Your Applications: Additional

Locking part 5

Part five of our series on fastener locking methods will focus on some alternative locking methods not previously covered.

Serrated Flange Nuts/Bolts

Serrated flange nuts and bolts use serrations on the bearing surface to create higher friction and prevent loosening, primarily on sheet metal joints. Care must be taken to NOT use any washers with this hardware and to ensure serrations on both the nut and bolt to prevent any rotation. When designed correctly, joints using serrated hardware perform very well, but corrosion can be a concern if the hardware is installed after paint as some material can be removed during assembly and especially if removal is required.

serrated flange nuts and bolts

 

 

 

 

 

 

Double Nuts

Use of a jam nut (or thin nut) to “double nut” a joint has been around for many years, and can be a very effective method of locking a threaded joint. The assembly method and the use of two nuts for each joint may not be the most efficient, and many people do not install them correctly. When using a jam nut, the thinner nut goes on first, and the standard nut gets tightened to full torque on top, while holding the jam nut in place. This can make it tricky to induce the proper pre-load on the joint which is part of the reason this method is seldom used any more.

double nuts

 

 

 

 

 

 

Mechanical Thread Locking

Some manufacturers have come up with special thread geometry to create a mechanical locking in either the internal or external threads. Different from locking nuts, which starts with a standard thread, and deforms or damages it to create friction, these specialty threads are rolled or tapped into the fastener when the threads are initially formed, which provides greater consistency and a more predictable clamp load.

Tab Washers

Tab washers have multiple tabs which are bent both in opposing directions after assembly to lock the nut or bolt head into the mating surface.

tab washers

 

 

 

 

 

 

Castle Nuts

Castle nuts or slotted nuts have slots cut or formed across the flats to allow for a cotter pin to be passed through a cross drilled hole in the mating male threaded component. Often used on highly critical joints such as front wheel bearing assemblies on automobiles, this positive locking feature is a guaranteed, but costly way to secure a bolted joint.

castle nuts

 

 

 

 

 

Safety Wire

Generally used in aircraft, high performance automotive, or other high critical applications, these bolts and/or nuts will have holes drilled through the heads or flats. Safety wire can then be passed through, twisted and attached to neighboring fasteners to prevent rotational loosening.

safety wire

 

 

 

 

 

 

Questions? Want to learn more about locking fastener applications? Contact us at ProvenProductivity@bossard.com.

For more shopping options click here.

Doug Jones
Applications Engineer
djones@bossard.com

December 14, 2018
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Locking Fastener Methods to Secure Your Application: Lock Nuts

Locking part 4

Part four of our series on fastener locking methods will delve into lock nuts, also referred to as prevailing torque nuts because they are not “free spinning” but require a prevailing torque to assemble them.

Many different styles of locking nuts have been developed over the years, but we will focus on two general categories: all metal lock nuts and nylon insert lock nuts.

All Metal Lock Nuts

All metal lock nuts start their life as free spinning nuts but pass through either a press or punch machine which deforms part of the thread to create friction during assembly. Classified as either top lock or side lock, it is a good idea to know the difference and determine the best style for your application.

Top Lock

Top lock nuts have a thread deformation at one end of the nut, meaning assembly can only take place in one direction. The most common type of top lock nut (often referred to as a stover lock nut) has a cone shape on top, making it easy to identify the top of the nut for directional assembly.

stover lock nut

 

 

 

 

 

Side Lock

Two-way lock nuts may be assembled in either direction, which allows them to be used in automated assembly. These nuts can be identified by the punch marks on the flats. It is common to have either one punch mark, or two punch marks on opposing flats.

two way lock nut

 

 

 

Nylon Insert Nuts

Nylon insert nuts use a non-threaded nylon ring which gets crimped into the top of the nut at the end of the manufacturing process. As the nut is assembled onto a bolt, threads form into the nylon ring at the top of the nut, creating resistance.

nylon insert nuts

 

 

 

 

Each style of nut has its advantages and disadvantages. All metal lock nuts can be used at higher temperatures than nylon insert nuts, but nylon insert nuts tend to have more consistent clamp load from lot to lot. All metal lock nuts typically require a wax on top of the normal zinc plated finish to keep from galling, which can result in some unpredictable clamp loads in critical joints.

 

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

For more shopping options click here.

Doug Jones
Applications Engineer
djones@bossard.com

December 07, 2018
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Locking Fastener Methods to Secure Your Application: Nylon Patches & Plugs

Locking part 3

Part three of our series on fastener locking methods will focus on nylon patches and plugs.

Nylon Patches

Nylon patches have some similarities to adhesive patches as they are both applied prior to arriving at the customer and they cannot be forgotten, but this is where the similarity ends.

Nylon patches are a hard, nylon material that is applied as a powder, and then melted into the threads with heat. The nylon material is typically NOT a 360° patch, but only applied on one side of the threads. The material is soft enough to form threads into it during assembly, but hard enough to create additional friction on the opposing side of the patch. This patch has some re-usability as it does not cure like an adhesive after assembly, but remains hard. Most manufacturers claim some locking effectiveness up to 3 to 5 times of complete dis-assembly and re-assembly. Applications which require minute adjustments without complete dis-assembly can benefit from this style of patch.

nylon patches

 

 

 

 

 

 

 

Nylon Plugs

Nylon material can also be added to the threads in the form of plugs or strips.

nylon plugs

nylon plugs 2

 

 

 

 

 

 

This method of application requires machining of the screws and pressing in the nylon plug or strip. While slightly more expensive to manufacture, some customers still prefer plugs over patches and believe they achieve improved performance with this style.

Some notable advantages of nylon over adhesive are a longer shelf life under ideal storage conditions and the possibility to re-use.

 

For more information on nylon locking features, or to help you decide which locking method is right for your application, check out www.bossard.com or contact us at ProvenProductivity@bossard.com.

For more shopping options click here.

Doug Jones
Applications Engineer
djones@bossard.com

November 30, 2018
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Locking Fastener Methods to Secure Your Application: Adhesives

Locking part 2

For part two of our series on fastener locking methods, the focus will be on adhesives. In general, there are two ways to apply locking adhesives to a fastener joint. Liquid adhesive such as Loctite® can be applied to the threads at the time of assembly, or an adhesive patch can be pre-applied to the threads at the time of processing, prior to shipping to the customer.

Liquid Adhesives

The application of liquid adhesives can be done with a squeeze bottle, with a brush, or even rubbed on the threads with a glue stick applicator. One of the challenges with these types of application is getting the right amount on the threads to create the desired locking effect without wasting material. Some manufacturers of liquid thread locker have even developed precise metering guns to deliver the same amount of material for each application.

Liquid adhesives

 

 

 

 

 

 

 

 

 

 

 

 

 

Manufacturers of thread locking material will give recommendations on how much material and which delivery system makes the most sense for your application, but how does thread locking adhesive work?

The common thinking is that thread locking adhesives simply act as a glue, bonding the male and female threads together so that they cannot rotate loose. Although this is true to some extent, the real benefit to locking adhesives is that they harden after assembly and fill any gaps between the threads. Rotational loosening occurs whenever outside forces (such as vibration) acting on the joint cause a loss of friction in the threads and bearing surface. If the force and the frequency are high enough, air gaps in the threads allow for the loss of friction. Eliminating the air gaps with thread locking adhesive is a very effective way to prevent rotational loosening.

Pre-Applied Adhesives

Pre-applied adhesives prevent rotational loosening in the same manner as described above, but they are applied at the manufacturer as a patch. The adhesive material is microencapsulated so that it will not dry until the fasteners are assembled, crushing the microcapsules and releasing the curing agent.

Some advantages of pre-applied adhesives include:

  • No waste from over-application
  • Adhesive is in the same location each time
  • Cannot be forgotten by the assembler
  • No mess

pre-applied adhesives

 

 

 

 

A disadvantage of pre-applied adhesives is that they have a limited shelf life which needs to be monitored. If parts have been sitting for too long before assembly, the patch material may dry out and be less effective. Most pre-applied adhesives have a shelf life of 1 – 4 years.

Shop Tape Adhesives

For more information on which thread locking adhesive is right for your application, visit us at www.bossard.com or contact us at ProvenProductivity@bossard.com.

Doug Jones
Applications Engineer
djones@bossard.com

November 23, 2018
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Locking Fastener Methods to Secure Your Application: Lock Washers

Locking part 1

This is the first of a five-part series about locking methods for fasteners – the first product we discuss is 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, these washers have a 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

Ribbed lock washers Ribbed lock washer 2

 

 

 

 

 

 

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

Wedge ramp locking washers

 

 

 

 

 

 

This style of locking washer comes as a pair of washers glued together. The outer surfaces have ribs which 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, visit us at www.bossard.com or email us at ProvenProductivity@bossard.com.

To shop latches, hinges, locks and accessories, click here.

Doug Jones
Applications Engineer
Email: djones@bossard.com

November 16, 2018
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