Rail

By Mark Thurber, CEO, Maelstrom Chemical Technologies LLC

Fastening systems have been an engineering necessity dating back to when mankind first attempted construction. Joining two separate elements to work as one is the most fundamental concept of creation.

Since this modest beginning, designers have searched for a means of keeping these elements secure, durable and reliable until removal is required. Though this seems to be the goal most sought after, any weekend mechanic will swear that the opposite seems more often to be the rule, rather than the exception.

Fastener enhancement products traditionally encompassed only the most basic of notions including cotter pins and deformed components for locking, and messy greases and anti-seize for lubricants. In the recent decades, higher tech adhesives, sealants and coatings have come into play, taking strides in the reliability and multi-tasking of fastener systems. Today, there are a variety of new, highly specialised product choices and wider utilisation opportunities for use on components, far beyond that of fasteners.

Fastener locking devices
Adhesives as locking elements can be most broadly subdivided into two general
categories:

• Pre-applied: Applied to the fastener en-masse by fastener coating services, long before the intended installation.

• Bottled: Applied to the joint at the time of assembly.

Pre-applied products do not impart any direct performance properties that cannot be found in a bottle, however, the advantage is realised in the pre-application ‘delivery system’ itself. Time saved by pre-application of adhesives (and the resulting direct, economic pay-back) plus the assurance that an adhesive not inadvertently missing from a joint, are indispensable benefits to mass assembly.

User applied, point of assembly materials
Modern fastener retaining took its biggest leap forward with the development of bottled, liquid thread locking adhesive. Anaerobic adhesives, as they are sometimes called, are ‘triggered’ (or initiate reaction) by the absence of oxygen contact; hence the name ‘anaerobic’.  American Sealants Company first offered anaerobics for the sealing of flanges in the 1950s.

A full spectrum of choices of these types of products are now available for the user - low strength, high strength, removable, permanent, high temperature, penetrating, high viscosity, low viscosity, thixotropic, fast cure, slow cure, low friction, red, green, yellow, blue and purple; just to begin the list. Regardless of grade, whether permanently retaining a stud in an engine block or installing a serviceable pipe plug in a refrigerator, they are all based on the same chemistry.

Recent innovations have come to this field that include a variety of non-anaerobic thread locking materials offered by STS/Tectorius®. This includes a variety of user applied materials that mimic the hard to get pre-applied products (previously only available through professional service centres in large volume). In addition to easy to use packaging, many are friendly to special substrates, temperature sensitive platings and plastics, where anaerobic adhesives should never be used.

In addition to threaded fasteners, many of these new breed products are finding use in untraditional types of applications on splines, pulleys and moving assemblies offering a ‘cushioning effect’ to mitigate squeak and rattle for NVH (noise, vibration and harshness) improvement.

Pre-applied thread locking adhesives
Pre-applied adhesives began being widely accepted in the early 1970s. Prior to this, adhesives were not highly looked upon, and considered less dependable than the
more tangible, mechanical methods. 3M® Corp introduced its highly popular Scotch-Grip® brand, offering several grades of microencapsulated, epoxy-based adhesives. Loctite® Corp also introduced its line of microencapsulated, acrylic-based adhesives, known as Dri-Loc®, during the same approximate time frame. Even though other products are also available on the market, each fall into one of two chemical groups -epoxy or acrylic technology.

Epoxy type
Epoxy technology for use on threaded fasteners was initially pursued by 3M Corporation. In the most elementary terms, epoxy based adhesives used for fasteners are quite similar to two-part epoxy kits that can be found in any hardware store. Typical fully cured epoxies are the most durable, chemical resistant, versatile and tough adhesives known. However, delivery of these components to the right place, in correct proportion and preventing the reaction from beginning until the right moment, is an enormous hurdle to overcome.

3M took a unique approach to this delivery and proportion dilemma. A technology, formerly used only in pharmaceuticals and printing, called ‘microencapsulation’ held the key to the problems of component separation. Microencapsulation involves encasing a material within an impermeable microscopic shell; not unlike a miniature egg. Microcapsules are frequently used in consumer products such as ‘scratch and sniff’ swatches, perfume samples, and ‘carbonless’ copy paper. Only close examination with a microscope will reveal clusters of clear spheres dispersed throughout the thread deposit.

Acrylic type
The acrylic types of adhesives were a venture in making standard forms of bottled thread lockers fit into the pre-applied mold. Loctite Corp was on the forefront of this concept with their Dri-Loc product line. Followers in the manufacture of similar types of products are STS/Tectorius® with their very wide collection found in the Tec-Bond™ adhesives family. One largely accepted misconception involving these products is the use of the term ‘anaerobic’ meaning ‘absence of oxygen’. This is often used to describe bottled thread lockers, as the absence of oxygen in the tight confines between the fastener and the mating part will cause the adhesive to cure. This is not the same reaction mechanism taking place in the pre-applied thread lockers. Most of the pre-applied thread lockers based on acrylic functionality, are initiated in a distantly similar fashion to the microencapsulated technology previously discussed. The active ingredients to this style of adhesive are similar materials one would use for fibreglass auto or boat repair, even more distantly to the adhesives used for windshield repair and rear view mirror mounting. Unlike the epoxy system, the main initiator is encapsulated, rather than the matrix forming resin. Once all components are present in an active state, their reaction will proceed regardless of the presence or absence of oxygen.

This type of system offers several positive benefits; fast development of retaining torque being the most prominent. Some level of removal torque may be detected as early as 10 minutes after installation. Other notable characteristics include the water-based application medium, and an ability to achieve high breakaway torque.

Underhead sealing
Ever increasing performance requirements and quality standards have made sealing and cushioning under the head of a fastener a very important option for engineers and fastener suppliers. Under head seals, in their most basic forms, protect the fastener hole against passage of fluids. Of course, the prospect of water intrusion causing immediate damage is the most obvious vulnerability. The additional, long term perspective for prevention of corrosion in a joint, weighs heavily in favour of adding this element to an assembly.

Supplementary features beyond simple sealing and/or making a fastener ‘multi-task’ can be compelling and innovative approaches to joint design. Thoughtful selection of under head coatings can provide a variety of less obvious, but greatly beneficial enhancements to an assembly. Tectorius Tec-U-Seal™ family of products includes sealant that will not only seal, but also cushion the assembly to reduce noise from vibration. Furthermore, such products will prevent scratching of paint or plated surfaces during installation, which would otherwise compromise corrosion protection.

Other offered features can include lubrication and temporary retention. Pre-applied Tec-U-Seal can replace Mylar® washers as a durable and lubricious isolator for pivot pins in hinges, actuators and movable sub-assemblies, to reduce squeaks and increase durability. Yet other grades of Tec-U-Seal are formulated with a pressure sensitive surface so that fasteners can be pressed into a receiving hole, holding themselves in position prior to permanent installation. It is easy to see how this simple feature could streamline assembly in both a factory environment as well as in consumer level construction projects, such as outdoor storage building construction.

Components surface sealing and lubrication
Mobile phones, personal electronic devices and use of high tech engineering materials have brought with them great need for adhesive, coatings and sealant performance. With the adoption of a smartphone being a required part of everyone’s life from grade school forward, it is taken for granted that the materials contained in, and facilities where these items are made, must also be as high tech as the final articles themselves. Many electronics require a clean room environment when they are assembled, and components not only within the device, but also within the assembly room cannot emit even the tiniest particles at any point during their lifetime. Without proper qualification of materials and sub-components, any item with a visual display or lens could be clouded by vapor deposits, electronics would short circuit from conductive chips, and memory media damaged by particles.

This is a monumental challenge when components are made from filled plastics and metals, where the slightest frictional contact can produce a loose burr, chip or fiber particle. STS/Tectorius offers a wide choice of products for surface sealing and enhancement, specifically designed to mitigate particle and outgassing from any number of substrates that are being used in highly sensitive environments.

Similarly, new high tech engineering materials such as carbon fibre and specialised alloys can offer unexpected galvanic corrosion challenges, not previously experienced with steel. Fasteners coated with specialised dielectric materials, prevent situations from arising that might not be detected for months or years.

Chemical coatings and component enhancement materials are often considered an afterthought or perhaps a ‘Band-Aid’ to nagging assembly issues. By simply scratching the surface of this topic, one can see that the technology involved in a coating can far exceed that of the component itself and indispensable to the very viability of the final assembly.