It's been said that man's invention of nails, rivets, screws, and other basic fasteners helped pave the road from the Stone Age to the Space Age. If that is true, then fastener loosening has provided quite a few of the speed bumps and pot holes on that road.
Keeping fasteners tight, particularly threaded fasteners, seems like a simple task, but the moving nature of the machinery they are used on is what makes them so troublesome. How nails and rivets work is fairly well known. But because the physics of the threaded fastener is not as well understood, it tends to cause the most problems.
Causes of loosening
Threaded fasteners are employed primarily to clamp objects together using tension. Rotary force or torque imposed on the fastener provides that tension. Problems occur when this clamping load deteriorates.
About 85 percent of the torque and effort of tightening a bolt is absorbed by the friction in the threads and under the head. Only 15 percent produces clamping load. Therefore, high torque may be absorbed by high friction and not produce tension. Torque is not the most precise method of controlling clamping load, although it is the most common. When bolt and nut manufacturing is closely controlled, the tension produced in a bolt for a given torque varies up or down by 15 percent.
Although it is always the first suspect in any case of lost clamp, vibration, as commonly perceived and observed, is not capable of bolt loosening by itself. If vibration is violent enough to cause shifting of the threads, then it will cause loosening, and in only 50 to 100 cycles. However, vibration that violent is usually perceived as shock, shudder, or impact. Toward the end of the loosening cycle, common vibration can and will rattle the fastener loose. This is why it often takes full blame for loosening.
The actual cause of loosening is side-sliding or shifting of the threads. The empty space between the threads of a nut and bolt leaves room for movement that leads to self-loosening and loss of clamping force. The friction in the threads and under the head of the bolt is reduced to zero when the clamped parts and threads slide sideways to the bolt axis.
Each time this happens, the bolt can unwind by itself. The loosening process of a non-locking fastener starts with the first motion. It normally takes less than 100 side motions to completely loosen a bolt.
This shifting can occur any time the side force exceeds the friction between surfaces, as produced by the clamping load. There are three common causes of shifting:
Any one or combination of these conditions can occur from shipping trauma, extreme heat or cold, or just plain abuse. The effects are cumulative and self-accelerating. As these affect clamping load, there is increased probability that side-sliding will occur.
Various methods and devices have been employed over the years to reduce or prevent loss of clamping load in threaded fasteners.
The earliest attempts involved the use of lock wires and split pins in conjunction with nuts and bolts with holes drilled in them. Although effective, these measures had some serious drawbacks. Each fastener had to be the correct length, and the holes had to be aligned on each individual bolt. Consider the difficulty and time required using this method to assemble numerous parts requiring many threaded fasteners.
As fastener manufacturing skills improved, more complex methods of threadlocking were developed. Two of the most common mechanical methods of threadlocking are thread distortion and the use of washers. Although these methods can be effective for short-term threadlocking, anaerobic threadlockers can provide short-term, long-term, and even permanent tightening when necessary.
The first chemical threadlockers, developed by Loctite, eliminated many of the design faults and shortcomings of threaded fasteners. Chemical threadlockers are anaerobic liquids that cure to a tough, solid state when activated by a combination of contact with metal, and a lack of air. The resulting cured material is a thermoset plastic that cannot be liquefied by heating, and resists most solvents.
The purpose of threadlockers is to lock and sometimes seal threaded components without changing fastener characteristics or altering torque-tension relationships. In addition, chemical fasteners offer a number of other advantages over mechanical tightening methods:
Selecting the right threadlocker There are several key factors to consider when choosing a threadlocking compound:
The chemical resistance properties of threadlocking compounds vary between different grades. The most popular anaerobic products will generally resist water, natural or synthetic lubricating oils, fuels, organic solvents, and refrigerants.
Like most organic materials, threadlockers lose strength at elevated temperatures. Most show significant strength retention at temperatures up to 300 F (150 C). Hot-strength formulations can increase this working temperature to 450 F (230 C) for those applications where it is considered necessary.
The most common myth about liquid threadlockers is that once they are cured, they cannot be removed. In fact, all threadlocked fasteners can be removed. Different grades of threadlocker can be used depending on the task. Fasteners secured with low- and medium-strength grades can be removed with common hand tools. Those secured with high-strength grades can be removed by applying heat for a specified time.
Threadlockers are not just for specialized uses, either. They perform effectively on fasteners and threaded assemblies of any type and size, in any kind of equipment. MT
Information supplied by Robert A. Valitsky, a manager of technical communications at Loctite Corp.'s North American Engineering Center, 1001 Trout Brook Crossing, Rocky Hill, CT 06067; (860) 571-5416; Internet www.loctite.com