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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.

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.

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.

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.

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Doug Jones
Applications Engineer
djones@bossard.com

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.

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

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

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