Safety wire
Encyclopedia
Safety wire or lockwire is common in the aircraft
Aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...
and racing
Racing
A sport race is a competition of speed, against an objective criterion, usually a clock or to a specific point. The competitors in a race try to complete a given task in the shortest amount of time...
industries as an extra precaution to keep vital fastener
Fastener
A fastener is a hardware device that mechanically joins or affixes two or more objects together.Fasteners can also be used to close a container such as a bag, a box, or an envelope; or they may involve keeping together the sides of an opening of flexible material, attaching a lid to a container,...
s from unintentionally loosening and parts from falling off due to vibration or other forces. The use of safety wire is a type of positive locking device
Positive locking device
A positive locking device is a device used in conjunction with a fastener in order to positively lock the fastener. This means that the fastener can not work loose from vibrations...
. It also allows rapid and easy visual inspection of fasteners to ensure that they have been tightened. Safety wiring of fasteners is often a requirement to pass technical safety inspections in motorsport racing. Safety wire itself is available in multiple gauge
American wire gauge
American wire gauge , also known as the Brown & Sharpe wire gauge, is a standardized wire gauge system used since 1857 predominantly in the United States and Canada for the diameters of round, solid, nonferrous, electrically conducting wire...
s and different materials, depending on the application. In consumer aircraft and racing applications, stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
wire is used, most commonly in .032" diameter, although other gauges are used for specific circumstances. Typically, safety wire is threaded through a hole drilled into a fastener or part, then twisted and anchored to a second fastener or part, then twisted again, usually with the aid of safety wire pliers.
Principle
There are a few techniques for different applications. Non-proprietary safety wire systems all work on the same principle, which is that the tension of the wire on the fastener prevents it from working itself loose due to vibration or other forces. The wire itself maintains tension and remains in place by being twisted around itself and attached to the fastener to be secured on one end and an anchor point (which could be another fastener) on the other end. Since safety wire is made of a malleable alloyAlloy
An alloy is a mixture or metallic solid solution composed of two or more elements. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may or may not be homogeneous in distribution, depending on thermal history...
, it retains its shape after being bent, rather than springing back to its original shape. This property allows it to remain locked around an object, such as when it is passed through a small hole on a fastener, looped back upon itself, and then twisted. The same process is then repeated around the anchor point, which could be another fastener. Since it remains twisted instead of unraveling, it acts as a fixed loop and will not back out without considerable force (greater than the stresses which it is intended to counter) being applied.
Mousing
Mousing is the application of safety wire, called mousing wire in this use, to secure a threaded clevis pinClevis pin
A clevis fastener is a three piece fastener system consisting of a clevis, clevis pin, and tang. The clevis is a U-shaped piece that has holes at the end of the prongs to accept the clevis pin. The clevis pin is similar to a bolt, but is only partially threaded or unthreaded with a cross-hole for a...
to a shackle
Shackle
A shackle, also known as a gyve, is a U-shaped piece of metal secured with a clevis pin or bolt across the opening, or a hinged metal loop secured with a quick-release locking pin mechanism...
. This is done by passing a couple of turns of mousing wire through the hole provided for this purpose in the unthreaded end of the clevis pin and around the body of the shackle's hoop. Alternatively, some threaded shackles are provided with a hole through the threaded end of the pin beyond where it emerges from the threaded hole. A cotter pin or a couple of loops of mousing wire through this hole serves the same purpose and secures the shackle in a closed position. Nylon zip-ties are also commonly used in applications where the shackle must be secured, but easy removal is required.
Use
Safety wire installation is a fairly simple task, but needs to be done properly to ensure proper security for the fastener in question. First, a length of wire is cut off of a spool. The length of the piece of wire to be used is determined by the distance from the location of the fastener to be secured to another fixed location, which will act as an anchoring point for the safety wire. The wire should be cut to a length that is slightly longer than twice that distance. The next step is to take the removable fastener, such as a bolt, drill a hole through a part of it that will be accessible even when it is fastened (such as the head of the bolt), fasten it in the desired location, and then thread one end of the safety wire through the hole and pull it back upon itself, such that the two ends of the wire are now equal in length and form a loop through the fastener. The wire is then grabbed a short distance before the loose ends (with enough wire left so that it can be secured around the anchor point) with the tool of choice and pulled taut. Using safety wire pliers or the twisting tool of choice (simple pliersPliers
Pliers are a hand tool used to hold objects firmly, for bending, or physical compression. Generally, pliers consist of a pair of metal first-class levers joined at a fulcrum positioned closer to one end of the levers, creating short jaws on one side of the fulcrum, and longer handles on the other...
can be used effectively, if not efficiently), the wire is twisted to the optimal number of twists per inch. The wire is then released from the tool, with the two loose ends remaining adjacent to each other. One of those ends is then threaded through a hole on the anchor point, and pulled through and brought together with the other loose end, such that a loop is formed around the anchor object and the wire between the anchor object and the original fastener is twisted and barely taut. With the two loose ends now on the opposite side of the anchor point (with one end going through the anchor and one going around it), the twisting process is repeated on the loose ends. When finished, the loose end is cut off with a pair of diagonal cutters or wire cutters (many safety wire pliers have a wire cutting feature), such that enough twists are left to prevent the wire from unraveling.
Safety wire is not reusable so when it is removed it is cut.
Proper techniques
When using the most common gauge of safety wire, which is .032, guidance for installation can be found in several publically available sources. When using .041 or .040, optimal performance is achieved by twisting the wire 5 to 7 times per inch. On the smallest wire .020, 8 to 10 twists per inch is optimal.Equally important to the size of wire and number of twists, is the direction the safety wire is run from the fastener to the anchor point. The safety wire should be threaded through the object fastener such that it creates tension in the opposite direction of the fastener's removal. For example, if a standard automotive bolt in the U.S. is being secured, then the safety wire when installed should put tension on the bolt in a clockwise direction, since that is the direction that the bolt turns to tighten.
When drilling a fastener, the choice of where to drill it depends on the type of fastener and to what it will be wired. The alternative to drilling holes in fasteners is to use safety wire tabs (see Safety Wire Tabs section below), or to purchase pre-drilled fasteners.
Equipment
Safety wire
Safety wire is commonly .020, .025, .032 or .041 inches in diameter, but .021 and .051 are also available, and usually made of stainless steel, but is also available in monelMonel
Monel is a trademark of Special Metals Corporation for a series of nickel alloys, primarily composed of nickel and copper, with some iron and other trace elements. Monel was created by David H. Browne, chief metallurgist for International Nickel Co...
and inconel
Inconel
Inconel is a registered trademark of Special Metals Corporation that refers to a family of austenitic nickel-chromium-based superalloys. Inconel alloys are typically used in high temperature applications. It is often referred to in English as "Inco"...
alloys for high temperature applications and copper for break-away applications. For consumer applications, it is typically sold in 1 pound spools enclosed in a small cardboard or plastic canister.
Safety wire pliers
Safety wire pliers come in three common sizes: 6, 9, or 12 inch(also called 7, 10, 13 inch, respectively, for manufacturers who choose to include the knob at the end of the pliers, which is used to actuate the twisting of the pliers, in the measurement). Safety wire pliers work as follows: The technician grabs the two loose ends of the wire to be twisted in the end of the pliers; he then locks the handles of the pliers closed, which secures the loosed wire ends in the jaws; he then grabs the knob at the end the pliers, releases the main body of the pliers, and pulls back on the knob which causes the pliers to twist. The wire then twists. When the wire has been twisted sufficiently, the technician holds the pliers, releases the knob, and squeezes the handles of the pliers to release the locking mechanism, which loosens the jaws, thereby releasing the twisted wires. He then proceeds to the next fastener in the series or uses the plier's built-in wire cutters to trim off the excess wires. The final step is the push down the sharp end of the secured safety wire to eliminate any hazard from the sharp edge left from the excess being cut off.In addition to built-in wire cutters, some safety wire pliers feature an angled or diagonal tip to their jaws which allows easier grabbing of wire. The angled tip is also useful for pushing down the sharp clipped end of the secured safety wire. Some models have a feature that catches the excess wire that is cut off. This consists of a pair of rubber pillows located in the depression aft of the cutting blades.
Safety wire pliers also vary in the direction of their rotation when the twisting knob is pulled, with some pliers rotating clockwise
Clockwise
Circular motion can occur in two possible directions. A clockwise motion is one that proceeds in the same direction as a clock's hands: from the top to the right, then down and then to the left, and back to the top...
, some rotating counter-clockwise (often called Right-Hand Twist and Left-Hand or Reversing Twist), and yet others have a selector function to allow the user to twist the wire in either direction without switching tools. The pliers can also be purchased with the option of the twisting knob returning to its position automatically or manually.
Safety wire twisters
There is another type of basic safety wire twister which is similar to a standard screwdriver, except that the tip has a small grasping mechanism to hold the ends of the wire while the technician turns the handle to twist the wire. The advantage to this tool is its long and thin design, which can access hard-to-reach areas where one's hands or pliers do not fit. It is commonly referred to as a "pignose" due to its snout-like appearance.
Safety wire tabs
Safety wire tabs are washers that are used to secure fasteners by transferring the force of the safety wire to the head of the fastener to be secured. They are installed just like any other washer, after which the sides of the tab are bent up to make contact with the sides of the head of the fastener. One side of the tab is longer than the other with a small hole at the top, through which safety wire is threaded. Once the Safety Wire is properly installed, the sides of the tab transfer the force of the safety wire to the fastener, as though the fastener itself had been drilled and had the safety wire run though it.The advantage of safety wire tabs is that the fastener to be secured does not need to be drilled, which can be advantageous for fasteners that should not or cannot be drilled because of size or damage concerns. They can also be useful when a fastener needs to be replaced, the replacement is not already drilled, and circumstances do not afford the time or tools to properly prepare the replacement fastener. The disadvantages are that it adds extra distance between the head of the fastener and the surface to which it is to be secured, and it is not as secure as securing the wire directly to the fastener itself as the tab could be a point of failure if it somehow unbends or the hole breaks (which is more likely than the hole in a drilled fastener failing due to the thinness and malleability of the material from which it is made).
Pre-drilled fasteners
For certain applications where safety wiring is common, fasteners come pre-drilled with holes to accept safety wire. When wiring something that did not come with pre-drilled fasteners stock, however, the more cost effective way (as opposed to replacing all stock fasteners with pre-drilled ones of the same type) is often to drill the stock fasteners.Drilling jigs
Because the use of twisted safety wire to secure fasteners requires the fasteners to be drilled, tool makers offer jigs to help technicians drill the fasteners to be secured. Although pre-drilled fasteners can be obtained, most fasteners to be secured start out never having been intended to be secured (e.g., a production motorcycle which was built for the street but which has been converted in to a race-bike). Such fasteners need to be drilled. Drilling them is often difficult as, due to their small size and irregular shape, securing them properly and applying a drill effectively can be trying. As a result, technicians often break drill bits or damage the fastener when the bit slides off position. A useful tool is a drill press, because it allows the technician to apply the force of the drill bit directly to the fastener being drilled and eliminates lateral movement; but even with a press the fastener needs to be secured to prevent it from sliding out from beneath the bit. Even though drill presses ease the process, a press isn't always available, such as at a race event; and even with a press, the problem of securing the fastener still exists. To solve those problems, jigs are available which are designed to securely hold the fastener and provide a guide-channel for a drill bitDrill bit
Drill bits are cutting tools used to create cylindrical holes. Bits are held in a tool called a drill, which rotates them and provides torque and axial force to create the hole. Specialized bits are also available for non-cylindrical-shaped holes....
(with either a hand drill or a press) so that the technician can easily and directly apply force from the drill to the fastener without having it slip off or breaking the bit.
Proprietary methods
Safety cable is an alternative to safety wire. Safety cable is defined as a group of strands right-hand helically twisted without a core. This eliminates the need for twisting during installation, as is required with safety wire. Several companies manufacture safety cable, and it is becoming an industry standard due to easier control of critical inputs and reduced installation time. Installation and quality requirements of safety cable are governed by SAE AS4536.One safety cable manufacturer Daniels Manufacturing Corporation of Orlando
Orlando, Florida
Orlando is a city in the central region of the U.S. state of Florida. It is the county seat of Orange County, and the center of the Greater Orlando metropolitan area. According to the 2010 US Census, the city had a population of 238,300, making Orlando the 79th largest city in the United States...
, Florida
Florida
Florida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...
. Daniels manufactures a product called Safe-T-Cable, which they advertise as, "The complete fastener retention system."
The Daniels system works by providing pre-cut lengths of safety wire that have a large cap on one end. The cable is threaded through a hole on the fastener to be secured, which is large enough to accommodate the wire but too small for the cap on the other end of the cable to pass through it. After the other end of the wire is passed through the anchor point, the technician takes an extra end cap and the special tool that is available from Daniels and crimps the cap on to that end of the wire. Again, the end cap is too large to pass through the hole that the wire passed through, and thus the cable is secured.
Advantages and disadvantages
Although many systems purport to be more efficient than installing traditional safety wire, an advantageous by-product of the twisting method of installing safety wire is that it leaves a highly visible and easily inspectable indication that the fasteners in question are in fact properly secured.In addition, safety wire twisting is a standard, non-proprietary technique, and tools and materials can be easily found, cheaply purchased, and mixed with other brands while still working properly (provided of course that all components are used properly and with the proper types of complimentary components and tools, if not brands).
The primary disadvantage of traditional safety wire is the time it requires to install properly when securing fasteners, although technicians who use it often can implement it fairly quickly. It also leaves behind waste products when ends are clipped off or when it is cut off of secured fasteners that need to be removed during maintenance, resulting in sharp metal bits that can easily damage soft materials or injure skin. However, the amount of waste product is relatively small, it is non-toxic, and the hazard can be mitigated altogether if technicians properly dispose of any waste product. When clipping off ends, the ends can go flying off which makes their recovery difficult and can cause injury to anyone in the immediate vicinity, such as the technician or an assistant; however, this can also be easily mitigated by using extra care or by using safety wire pliers that have a special insert that is designed to catch clipped off ends. Another disadvantage is that since the manual skill required to implement traditional safety wire is easily learned, the techniques required to maximize the retentive force of safety wire (e.g., in which direction the retentive force should be exerted, the direction of twist, proper angles for securing multiple fasteners, proper twists per inch, which type of wire to use, etc.) are often ignored by non-formally trained technicians (e.g., hobbyists) who use safety wire for their projects.
Alternatives
There are also other systems of fastener retention that do not rely on safety wire at all, such as lock washers, thread-locking fluidThread-locking fluid
Thread-locking fluid is a thin, single-component adhesive, applied to the threads of fasteners such as screws and bolts to prevent loosening, leakage, and corrosion. Typically, thread-locking fluids are methacrylate-based, and cure anaerobically...
and cotter pin
Cotter pin
A split pin, also known in U.S. usage as a cotter pin or cotter key, is a metal fastener with two tines that are bent during installation, similar to a staple or rivet. Typically made of thick wire with a half-circular cross section, split pins come in multiple sizes and types.The British...
s, all of which accomplish the same objective as safety wire, which is to prevent vibration or other forces from causing the undesired loosening of a mechanical fastener.