Push Pull Cable Controls
A push pull cable control is used for the transmission of multi directional precision control of mechanical motion. The many varieties of push pull cables or controls are adaptable to a wide range of applications and...
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This article takes an in depth look at wire rope.
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Wire rope is a collection of metal strands that have been twisted and wound to form the shape of a helix with the purpose of supporting and lifting heavy loads and performing tasks that are too rigorous for standard wire. On shipping docks, rigging, and load bearing equipment, wire rope is attached to swivels, shackles, or hooks to lift a load in a controlled, even, and efficient manner.
The uses for wire rope include adding support to suspension bridges, lifting elevators, and serving as additional reinforcement for towers. The design of wire rope, with its multiple strands wrapped around a stable core, provides strength, flexibility, and ease of handling for applications that have bending stress.
Individual designs of wire rope involve different materials, wire, and strand configurations as a means for supporting and assisting in the completion of lifting or supportive applications.
The term wire rope encompasses a wide range of mechanical tools that are made to perform heavy and extreme lifting jobs. Wire rope is a complicated and complex tool with multiple moving parts capable of moving in unison. A 6 by 25 wire rope has 150 outer strands that move as one in an intricate pattern supported by a flexible core.
An essential part of the design of wire rope is the required clearance between the strands to give each stand the freedom to move and adjust when the rope bends. It is this unique feature that differentiates wire rope from solid wire and other forms of cable.
The three basic components of wire rope are the wire, strand, and core, which are wound together to form the rope.
The basic element of wire rope is wire that is used to configure, shape, and form the rope. Typically, steel, stainless steel, and galvanized wires are the first choice with aluminum, nickel alloy, bronze, copper, and titanium being second possibilities. The choice of wire is dependent on the type of work the wire is going to be used to perform with strength, flexibility, and abrasion resistance being the major determining factors.
Galvanized steel is coated with zinc as a protection against corrosion and is an affordable alternative to stainless steel.
Stainless steel wire rope has all of the basic qualities of galvanized and general wire rope with the added benefits of corrosion and rust resistance; this makes it the ideal choice for harsh and stressful conditions.
Steel wire rope is classified as general purpose wire rope and comes in a wide variety of sizes, diameters, and strengths. It is the most common type of wire rope and is used for several industrial, manufacturing, and construction applications.
Before going further into the discussion of how wire rope is made, it is important to understand the numbers used to describe each type. All wire ropes have a core around which wires are wound. The various styles of cores vary according to the construction and design of the requirements of the wire rope that is being produced.
Wire rope is classified by the number of strands it has as well as the number of wires in each strand. The most common classification is a seven wire rope that has one strand in the center and six around its circumference. This type of wire rope is lightweight with a very simple construction. The majority of wire ropes are more complex and intricate with multiple intertwining strands and wires.
What must be understood about wire rope is that it has a complicated configuration. It is actually wires wrapped around wires to form bundles that are wrapped around other bundles. In the case of a seven wire wire rope, the core has bundles of wires wound around it; this can be seen in the image below.
The first step in wire rope creation is the production of wire strands where wires are wound around a single core wire. The number of wires included in the strand is dependent on the specified strength, flexibility, and size requirements of the rope. Once the strand is completed, it is straightened before being moved to wire rope construction.
Like wire ropes, strands have different patterns; patterns are the arrangements of the wires and their diameters. Though most strands have a core, there are strand patterns that have three or four wires without a core that are referred to as centerless strands. The design of each strand pattern is meant to enhance the strength of the wire rope and improve its performance.
All of the wires of a centerless strand are of equal size and are wound together to form a helical shape.
The single layer strand is one step up from the centerless pattern and has a single wire core with multiple wires wound around the circumference.
Each of the wires has the same diameter as the centerless pattern.
For a multiple layer strand, the layers of wire are placed over one another in successive order. The placement of the wires on top of each other must be such that they fit smoothly and evenly.
In the Seale pattern, smaller diameter wires are wrapped around the core. The outer layer has the same number of wires but with larger diameters.
The Warrington pattern is like the multiple layer pattern with one variation. Like the multiple layer pattern, the inner wires and the core are the same and have the same diameter. The difference is in the outer layer, which has wires of alternating sizes of large and small with larger diameter wires laying in the valleys of the inner wires.
All of the wires of a filler pattern are the same size. What makes this pattern unique is the insertion of small wires in the valleys of the inner wires to fill the gap between the inner and outer layer.
The flattened strand pattern is also known as the triangular strand, which can be triangular or oval. Three round wires form the core. The outer flattened surface has a greater sectional metallic area; this makes this pattern stronger and longer lasting.
The core of a wire rope runs through the center of the rope and can be composed of a variety of materials, which include synthetic fibers, natural fibers, a single strand, or another wire rope. The core supports the wound strands, helps maintain their position, is an effective lubricant carrier, and provides support.
Wire ropes with fiber cores are restricted to light loads and are not used in severe, harsh, or stressful conditions. Polypropylene and nylon are types of synthetic fiber cores and can be used in conditions where there is exposure to chemicals.
Cores made of wire are classified as independent wire cores. The core of a wire rope with a wire core is actually a wire rope with another wire rope serving as the core, as can be seen in the diagram below. These types of wire ropes are used where the rope will be exposed to exceptional resistance and crushing.
A strand, or wire strand core, is exactly like the rest of the strands of the wire rope with wires of the same diameter and size as the other strands.
The choice of core and creation of the strands are the simplest yet most essential parts of wire rope construction. Wire rope lays, the method used to wind the strands, is more complex and involves several choices.
Lay is a term used to describe three of the main characteristics of wire rope: direction, relationship, and linear distance. The strands can be wrapped around the core going right or left. Right or left refers to the direction of the strands wrapped around the core and the wires within the strands. The linear distance is how far a strand moves when it is making a revolution around the core.
Types of lays
In a regular lay, the wires and strands spiral in opposite directions. With a right hand regular lay, the wires spiral to the left and the strands to the right. In the left hand regular lay, the wires spiral to the right and the strands to the left. This type of lay is easy to handle but wears out quickly because the crown wires are in contact with the bearing surface.
In the Lang, or Albert, lay, the wires and strands spiral in the same direction with right hand lay being the most common. The wires in a Lang lay appear to run parallel to the center line of the rope. The difficulty with Lang lay wire ropes is handling since they tend to kink, twist, and crush.
An alternate lay is a combination of regular lays and lang lays; this is only used for specialized applications.
The diagram below gives a complete overview of each of the steps found in the manufacture and assembly of wire rope or wire cable.
Wire rope is an exceptionally strong tool that has been configured and designed to withstand the abuse and mistreatment of long use. In most applications, wire rope has to endure extreme stress and strain. It is for these reasons that coatings have been developed to protect wire rope from abrasions, corrosion, UV rays, and harmful and damaging chemicals.
Three main types of coatings are used to protect wire rope: polyvinyl chloride (PVC), polypropylene, and nylon. Of the three types, PVC is the most popular.
PVC is popular because it is multifunctional, extremely flexible, and general purpose as well as low cost. It has an operating temperature between -30° F (-35° C) and 180° F (80° C) with a hardness of 90 on the durometer.
In cases where there are severe and hazardous working conditions, polypropylene is the recommended choice since it is capable of protecting wire rope against corrosion and chemical leaching. Additionally, it is resistant to impact damage and abrasion. Polypropylene is a tough, rigid, and crystalline thermoplastic that is made from a propene monomer and is resilient as well as inexpensive.
Nylon is exceptionally abrasion resistant, which makes it ideal for use in cold environments. It is not as flexible as PVC but has excellent protection against corrosion and impact. It has excellent chemical resistance at temperatures between -65° F (-54° C) and 230° F (110° C) and is available in a wide assortment of colors, or it can be transparent.
Braided wires are electrical conductors made up of small wires that are braided together to form a round tubular braid. The braiding and configuration of braided wire makes them very sturdy such that they do not break when flexed or bent. Braided wires are widely used as conductors, are commonly made from copper due to copper's exceptional conductivity, and can be bare or coated depending on the application.
Braided wire can be round and tubular or flat. Round tubular braids fit in most spaces where flat braided wire will not. Flat braided wire begins as round braided wire which is flattened on a capstan. They are exceptionally strong and designed for medical and aircraft applications.
Metals used to make wire rope are various grades of stainless steel, bright steel, and galvanized steel. Though the majority of wire rope manufacturers use these three metals, other metals such as copper, aluminum, bronze, and monel are also used on a limited basis.
The most important aspect of wire rope is the wire and the metal from which it is made. The strength and resilience of wire rope is highly dependent on the quality of metal used to make it, and these are essential factors to be considered when purchasing it.
Bright steel wire does not have a coating and is rotation resistant, (designed to not rotate when lifting a load). It is drawn from hot rolled rods that are put through a die to match its specific dimensional tolerances, mechanical properties, and finish. Bright wire is used as a single line in conditions that require a rope that will resist cabling.
Galvanized steel has a zinc coating for corrosion resistance and has the same strength and durability as bright steel. Environmental conditions determine the use of galvanized steel. In mildly severe and slightly harsh conditions, galvanized steel wire is an economical replacement for stainless steel.
In the manufacturing process, galvanized wire goes through the process of galvanization, a method of coating steel wire with a protective and rust resistant metal. Galvanized wire is exceptionally strong, rust resistant, and flexible enough to meet the needs of a variety of applications.
Stainless steel does not have the same strength and endurance as bright steel or galvanized steel but has the many benefits commonly associated with stainless steel, such as resistance to stains, wear, rust, and corrosion. More expensive than the other two metals, stainless steel has the added benefit of lasting longer and providing exceptional performance.
Wire rope made from copper is mostly used for electrical applications due to its exceptional electrical characteristics. The benefits of copper wire rope are its durability, flexibility, and resilience compared to standard copper wire. The strength of copper wire rope is seen in its use in applications where there are vibrations and shaking.
The wire rope lubrication process begins during its fabrication and continues during its use. Lubrication of wire rope is designed to lower the amount of friction it endures and provide corrosion protection. Continued lubrication increases the lifespan of wire rope by preventing it from drying up, rusting, and breaking.
The types of lubricants for wire rope are penetrating or coating with coatings covering and sealing the outside of the rope. Penetrating lubricants go deep into the rope and seep into the core where they evaporate to form a thick coating or film.
The application of the lubricant is dependent on the type of core. Fiber cores absorb the lubricant and serve as a reservoir that retains the lubricant for an extended period of time. With metal cores, the lubricant is applied as the wire is twisted into strands to give complete saturation and coverage of the wires.
Petrolatum compounds are translucent and provide excellent corrosion and water resistance. They tend to drip off at high temperatures but keep their consistency in cold conditions. Petrolatum is a mixture of hydrocarbons from the distillation of petroleum that belong to the methane family of hydrocarbons. It can be used in semi-solid or liquid form and forms a jelly in its semi-solid form.
Asphaltic compounds are a mineral based oil combined with bitumen to create a tacky, high viscosity lubricant with an undiluted viscosity. As a lubricant, asphaltic compounds create an oil film that separates the mating surfaces and are applied as a spray. Once applied, the meshing of surfaces causes the solvent to flash; this leaves a viscous coating of lubricant.
There are several types of greases that are used as wire rope lubricating agents and are made up of oil, a thickener, and additives. The essential components are the base oil and additives, which influence the behavior of the grease. The thickener holds the base oil and additives together. The amount of base oil in a grease is between 70% and 95% with an additive of 10%.
The additive in grease enhances the positive properties of the oil and suppresses the negative properties. Common additives are oxidation and rust inhibitors as well as pressure, wear, and friction reducing agents.
Greases used for wire rope are a soft semifluid and create a coat that partially penetrates the rope when applied with a pressure lubricator.
Of the many choices for lubricants, vegetable oil is the easiest to use and penetrates the deepest. The design of the additives for vegetable oils gives them the necessary qualities required to penetrate deep into a wire rope. The exceptional penetration provides protection against wear and corrosion. Since vegetable oil is a fluid, it helps in washing the wire rope to remove external abrasive contaminants.
Wire rope is widely used in machines, structures, and varied lifting applications. Its type, size, and requirements are determined by how it will be used. Regardless of its use, wire rope guarantees exceptional strength and provides high quality and excellent performance.
The lifting of heavy loads for centuries involved the use of hemp rope or chains, neither of which was a guaranteed or substantial method. Early in the 18th Century, between 1824 and 1838, Wilhelm Albert, a German mining engineer, combined the twisting of hemp and strength of chains to create today‘s wire rope.
Since its beginnings, wire rope has undergone fantastic changes leading to the many varieties found in industry today.
The most common use of wire rope is as a part of a crane hoist wherein it is attached to the hook of the hoist and wrapped around a grooved drum. The tensile strength and durability of wire rope makes an ideal tool for lifting and keeping loads secure. Though it is used in several industries, it is very popular for production environments wherein materials need to be lifted quickly and efficiently.
In addition to its many lifting applications, the strength and stability of wire rope is useful in other applications, especially in the aerospace industry. Pedals, levers, and connectors in the cockpit of an aircraft are connected with wire rope. The wires provide for the passage of power between systems and mechanisms; this allows control of the aircraft. Wire rope is used to control propeller pitch, cowl flaps, and the throttle. It also assists in lowering and minimizing vibrations.
Tires are reinforced with wire rope to increase their durability and strength. All automotive production environments make use of wire ropes for supplying materials, moving heaving loads, and positioning equipment. Wire rope can be found in the production of steering wheels, cables, exhausts, springs, sunroofs, doors, and seating components.
As surprising as it may seem, the place that wire rope has the greatest use is in the home, where its strength, long life, endurance, and resilience provide guaranteed protection and performance. The main reason wire ropes are so popular for home use is cost.
Inexpensive, easy to obtain, easy to install, and easy to maintain, wire ropes provide an additional method for performing home repairs and structural support. Their excellent flexibility and sturdiness combined with their invisibility has made wire rope an ideal solution to several home maintenance issues. It is used to support staircases, fences, decks, and hang plants.
The search and production of crude oil has relied on wire ropes for centuries to lift drill bits, insert shafts, and support oil rigs on land and the water. When equipment, machinery, and tools have to be lowered into the depths of the earth and sea, wire ropes are the tool that the oil industry relies on to do the job.
Many of the tasks of oil production require tools that are capable of enduring severe and harsh conditions. Wire ropes have to withstand enormous pressure, extraordinary stress, and a wide range of temperatures. The use of wire rope includes maintaining oil rig stability and moorings for offshore rigs.
Wire rope has long been a standard component for the transportation industry, from the cable cars of San Francisco to the lift chairs for ski resorts. For many years, cable cars have relied on heavy duty cables (wire ropes) to be pulled by a central motor from multiple locations. It is a method of transportation that has existed for centuries.
In Europe, funiculars use cables that hang from a support to move cars up and down a mountain with cables moving in opposite directions. The word funicular is from the French word funiculaire, meaning railway by cable. The terms wire rope and cable are used interchangeably when discussed by professionals. The first part of funicular, or funiculaire, is from the Latin word "funis," meaning rope.
The major use for wire ropes in the food and beverage industries is as a means for lifting and moving heavy loads. Wine barrels and containers full of ingredients are lifted and placed through use of cranes and wire ropes. They are also part of conveyor systems that move products from one station to another.
From the beginnings of amusement rides up to the present, wire ropes have been an essential part of attraction construction and safety. They pull cars on roller coasters, hold cabins that swing, and move carriages through haunted houses. The main concern of amusement parks is safety. The strength, stability, and guaranteed performance of wire ropes ensures that people who attend amusement parks will have a good time and stay safe.
The rigging used to complete the stunts in modern movies depends on wire rope for safety. Much like in amusement rides, wire ropes protect performers from injury and harm as they hang above a scene or carry out an impossible move.
The live theater industry uses wire ropes to raise and lower curtains, support overhead rigging, and hold backdrops and scenery pieces. During a production, rapid and efficient movement is a necessity that is facilitated by the use of wire ropes.
Wire rope is a tool that we tend to envision as indestructible, unable to succumb to any form of damage. Though it is exceptionally sturdy and strong as well as capable of enduring constant use, it is just as susceptible to breakdown as any other tool.
To avoid serious harm and damage, wire ropes should be scheduled for regular inspections. There are situations that can damage or break a wire rope; these should be understood prior to the problem arising.
Guide rollers have the potential to damage and cause abrasions on wire rope if they become rough and uneven. Of the various elements of a crane and lift, guide rollers have the greatest contact with the mechanism‘s wire rope. Regular inspection of guide rollers will ensure they are not damaging the rope or causing abrasions.
Bending is normally a regular part of wire rope usage; this occurs repetitively as the rope passes through a sheave. As a wire rope traverses the sheave, it is continually bent and develops cracks or breaks. The cracking and breaking are exacerbated by movement on and off the groove of the drum. Normally, the breakage happens on the surface and is visible. Once it appears, it accelerates to the core of the rope.
A bird cage is caused by a sudden release of tension and a rebound of the rope. This type of break requires that the rope be replaced since the place of the break will not return to its normal condition.
Localized wear is part of the contact with the sheave and is difficult to detect unless the rope is closely inspected.
In essence, drum crushing is misuse of a wire rope and is caused by using a small drum for heavy and high loads in multiple winding conditions.
Wire ropes are multi-layered; this makes them flexible and torque balanced. The layering inside and outside creates flexibility and wear resistance. Relative motion between the wires causes wear over time, which leads to internal breakage. The detection of these breaks can be indicated by an electromagnetic inspection that calculates the diameter of the rope.
Kinked wire rope is caused by pulling a loop on a slack line during installation or operation; this causes a distortion in the strands and wires. This is a serious condition that necessitates rope replacement.
Corrosion damage is the most difficult cause of wire rope damage to identify, which makes it the most dangerous. The main reason for corrosion is poor lubrication that can be seen in the pitted surface of the rope.
The types of damage and problems listed here are only a small portion of the problems that can be caused if a wire rope is not regularly lubricated and inspected. Various regulatory agencies require that wire ropes be inspected weekly or monthly and provide a list of factors to examine.
As with any type of heavy duty equipment, wire rope is required to adhere to a set of regulations or standards that monitor and control its use for safety and quality reasons. The two organizations that provide guidelines for wire rope use are the American Society of Mechanical Engineers (ASME) and the Occupational Safety and Health Administration (OSHA).
ASME is a professional association that provides guidelines to promote the engineering profession. OSHA is a government agency whose purpose is to protect workers and ensure their safety.
All wire rope manufacturers and users closely follow the standards and guidelines established by OSHA and ASME. In the majority of cases, they will identify the specific standards they are following in regard to their products.
OSHA‘s regulations regarding wire rope fall under sections 1910, 1915, and 1926, with the majority of the stipulations listed in 1926 under material handling, storage, use, and disposal.
Covered in 1926 are:
Guidelines for wire rope inspection are stipulated in section B30.30 Ropes Standard.
ASME 30-1.8.1 (b) states Frequent Inspections
"Running rope in service shall be visually inspected daily, unless a qualified person determines it should be performed more frequently. The visual inspection shall consist of observation of all rope that can reasonably be expected to be in use during the day‘s operations. The inspector should focus on discovering gross damage that may be an immediate hazard."
In regard to ASME guidelines under B30.30, inspectors should look for:
ASME 30-1.8.1 (c) states Periodic Inspections
"The inspection frequency shall be based on such factors as rope life on the particular installation or similar installations, severity of environment, percentage of capacity lifts, frequency rates of operation, and exposure to shock loads. Inspections need not be at equal calendar intervals and should be more frequent as the rope approaches the end of its useful life. Close visual inspection of the entire rope length shall be made to evaluate inspection and removal criteria."
In regard to ASME guidelines under B30.30, inspectors should look for:
Repetitive Wear Sections
Known Wear Areas Based on Previous Experience with the Machine Being Inspected
Locations Where Rope Vibrations are Damped
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