shock loading wire rope supplier

Shock loading can occur in any situation where the load on the crane suddenly increases. The crane and accessories are designed to take up the weight of loads gradually and steadily. They are not designed to withstand sudden increases or decreases in the apparent weight of the load.  Some examples of how shock loading can occur are shown below.

Operators and equipment owners should be aware of the causes and potential dangers of shock loading. Because the equipment is being used in a way that it is not designed for, shock loading can lead to damaging the equipment, the facility, or injuring personnel. Understanding the causes of shock loading will help to prepare operators to safely and accurately operate the equipment.

Skilled operators are a company"s first defense against shock loading. Lifting and lowering should always be done in slow speed until all slack has been taken out of the wire rope and any below the hook devices. Additionally, operators should be aware of their surroundings, making sure that the load they are lifting is not likely to snag on other pieces of machinery or the building itself. When lifting or lowering the load, operators should be careful to make sure the load is not bouncing as they operate the hoist. Additionally, operators should ensure that their loads are secure and well balanced.

Beyond operator training and best practices, features such as the HoistMonitor®assist operators in preventing shock loads. The HoistMonitor ensures that starting and stopping is initiated in slow speed, which helps prevent a jumping motion of the load. Sudden load supervision, also a standard feature of the HoistMonitor, prevents the hoist from continuing the hoisting motion when a load increase is suddenly detected, like if the load snagged on another item.

When overloads and shock loads occur in a rigging operation, the results can be deadly. A failure of gear or equipment can take place at the time the over/shock load happens or in many cases weeks, months or years later.

Most of us are familiar with the statistics used in rigging books and charts on the affects of shock loading. When a load of “X” pounds is allowed to free-fall or is popped off the ground, it introduces a load to the lifting device which can be two times or more its static weight. This compounding of weight takes its toll on the load’s internal and external structure, rigging attachment points, all rigging hardware, slings, hoist hook, running ropes, drum and entire hoisting system whether overhead or mobile crane.

A typical method of shock loading results from turning or flopping a load over from one plane to another. (Actual case) A coal-fired steam plant uses pulverizer journal assemblies to crush the coal into a fine talc - like powder for burning. The journals are awkward and difficult to handle with no available lifting lugs. After a journal is pulled from service and it has received maintenance, it is transported back to the pulverizer unit. A bridge crane picks up the journal from its vertical carrying cradle and sets the base on the floor. The crane then trolleys to pull or “flop” the journal over to a 45 degree angle. A special sling assembly is then used to hoist the journal into the pulverizer cavity.

During the “trolley and flop” movement, the slamming of the journal arms into their chain slings sends dust and dirt flying off the overhead bridge crane. How much weight in real pounds was introduced to the crane? Has anything happened to the crane’s structure? Does anyone suspect a broken weld, metal fatigue fracture or that damage has possibly occurred to the hoist system or wire rope? What if this happens twice a month for four years? Your imagination can provide many unwelcome answers to these questions.

Have you ever heard an employee say, “We were only lifting 2 tons on our 5 ton bridge crane and the whole thing came down on top of us!” Was it the 2 ton lift that caused the accident? Certainly not! It was the four years of repeated abuse, shock loading and structural damage which turned a fine bridge crane into a life threatening bucket of bolts.

If you have these situations in your operation, do everything possible to develop alternative rigging methods. Make a comprehensive inspection of all hoisting and rigging components. Using the proper procedures for each type of equipment perform load tests and make another inspection to ensure reliability. (Always check with the equipment manufacturer for testing procedures and limitations.)

Even with experienced crane operators, it can be challenging to lift a load without incurring stress to the crane, the load or the building’s structure. That’s because, before a load lifts off the ground, the rigging gear is loose and any upward hoisting would first move the rigging gear, not the load. Once the rigging gear is taut and the load is engaged, however, the hoist must be operated very slowly so as not to jerk the load into the air. Excess speed during the critical time of lifting the load off the ground can “shock” the crane system, causing high stress.

Konecranes has the answer: Shock Load Protection. With Shock Load Protection, the hoist drive monitors the load. If it is picked up too fast, the hoisting speed is automatically reduced until the load is in the air. This protects the crane, lifting load and the whole building from extra stress. This, in turn, provides lower maintenance costs for the crane and maximizes cycle times by reducing hoisting speed only during the critical moment of lift off.

Shock Load Protection is designed for smooth load pickups and works to prevent shocks to the load and the crane, extending the lifetime of the crane’s steel structure and mechanical parts. Shock Load Prevention is a feature of Konecranes Variable Frequency Drives for hoist control, and it works to eliminate shock loads automatically. With this automated feature, the operator can focus on controlling the load, monitoring his or her environment and ensuring that the load remains secure. Without the operator needing to purposely slow down operation as the hoist is raised, the crane can operate efficiently, speeding up operation while decreasing the mechanical wear and tear on the overhead crane.

Shock Load Protection is available for overhead cranes with, or with the capability of having, Variable Frequency Drives. Contact a Konecranes Representative to see if Shock Load Protection or any of our other Smart Features, can help your business.

Before we address shock loading let us take the time to understand the difference between static and dynamic forces. Static force is stationary. Static force usually refers to an object not in motion. Whereas dynamic force refers to an object that has unequal forces acting upon it. Rapid acceleration in lifting and rapid deceleration in lowering of a small or large load can result in what is often referred to as dynamic load.

Remember Newton"s second law? F = ma Force equals mass times acceleration. Acceleration refers to a change in the rate of velocity. Assuming we are referring to the same mass (size of the object), we see that the force exerted on an object is proportional to the acceleration it is given. This is the basis of the phenomena known as shock loading.

Generally speaking shock force or shock loading occurs when an operator takes up sling slack rapidly or suddenly releases the load creating a sudden jerk. Both rapid acceleration and deceleration of a load can create a shock force that far exceeds the working load limit of the wire rope. Always remember that the sudden release of a load can cause internal and external damage to a wire rope. Why is the safe working load limit of rigging slings and crane lines significantly lower than their minimum breaking strength? A safety factor must always exist. Remember that minimum breaking strengths are stated for static, straight lifts or pulls.

The four pictures in this post clearly illustrate what shock force or shock loading will do to a wire rope. Note the one strand has become unraveled from the other strands in forming the wire rope. Look closer and you will note broken wires at different points of the strand. Remember a series of individual wires make up each strand on a wire rope.

Avoid shock loading to nylon or wire rope slings when beginning your lift. The crane or hoist should be engaged slowly until the load is suspended. The speed in which you lift or lower the load should be increased or decreased gradually. Sudden starts or stops place a heavy load on the slings and load line, up to 50 times the actual weight. Once any sling has been shock loaded it must be removed from service.

Manufacturers identify their classes of wire ropes beginning with two numbers such as 6x25. The "6" means that there are 6 strands or larger wires making up the wire rope and the second number "25" means that there are 25 smaller wires laid around each other to make up each strand. The other wires in some wire ropes are called filler wire. Wire fatigue resistance will increase as the number of wires per strand increases.

Known as the industry workhorse of wire ropes, the 6x25 Filler Wire maintains a good balance between resistance to abrasion and fatigue resistance. When both abrasion resistance and fatigue resistance are  required, the 6x26 Warrington Seale is a better alternative.

Keep the wire rope lubricated so that rust and dirt will not weaken it  by acting as an abrasive on the rope as it spools through the sheaves and drums. Lubrication of the rope  allows individual wires to move and work together so that all the wires carry the load instead of just a few. Weather and other exposures can also remove

Natural and synthetic fiber rope offerings have expanded over the past years with the introduction of new materials such as Kevlar and other advancements that can provide a higher strength and a better

Recommended working loads will vary according to the rope size, type and manufacturer. Twisted rope is oftentimes has the working load listed between 10% to 15% of the tensile strength and braided rope is between 15% to 20%. Therefore, a braided rope with a listed tensile strength of 12,500 pounds might only have a safe working load of 1,850 pounds. Ropes have different qualities and you should carefully assess your rigging requirements. In example, Kevlar has a considerably higher breaking strength than most available products,

It is necessary to check with your cordage vendor to obtain the safe working load of your rope prior to purchasing it for any rigging or lifting purpose!

Many factors, including rope usage, load conditions and weather exposure affect the rope’s working load capabilities. You should inspect your rope daily for concentrated wear. It must be free of frayed strands and broken yarns, cuts and abrasions, burns and discoloration. If there is excessive soiling or paint buildup, place it out of service. Check for chemical or heat damage and ultraviolet deterioration. This type of degradation is indicated by discoloration and the presence of splinters and slivers on the rope surface. Do not use wire rope or V-belt sheaves for synthetic rope as the rope will be pinched inside.

To understand shock forces you must know that there exists static (not in motion) forces, and dynamic (in motion) forces. In the real world we rarely have just simply static forces occurring during lifts. Even small amounts of speeding up or slowing down of the load result in dynamic loads. The very act of even slow lifting often results in some forces caused by movement such as swinging and drifting.

However, the force we are talking about it the one that occurs rapidly as opposed to slow dynamic forces. Shock force is more commonly referred to as shock load. This derives from the fact that engineers routinely refer to forces occurring in and on structural members as loads. In rigging a load is an object to be lifted or flown from one point to another point, hence the use of the phrase shock force in this article.

Shock forces can occur for any number of reasons, most notably; an operator taking up sling slack with a sudden jerk, the rapid acceleration or deceleration of the load, failure of fair leads or sheave guides to prevent the rolling out of a slack line.

The magnitude of a shock force can be many times that of the weight of the load be lifted. This is why the safe working load of rigging equipment is substantially lower than the minimum breaking strength. Minimum breaking strengths are stated for static, straight pulls. A factor of safety must therefore exist.

The amount of force created in a shock situation is dependent on, among other things, the weight of the load and the distance of travel. The exact determination can be quite complicated because the value of the load"s stopping distance is based on the amount of elastic stretch.

In order to precisely calculate the stopping distance we would need to know the exact composition of the wire rope, the equivalent cross sectional area, and the apparent modulus of elasticity of the wire rope composite, and then use a complicated formula to calculate the exact amount of elastic stretch*.

Many manufacturer"s websites state that there exists no practical method to estimate shock force. Gelrum** provides an example of a 75 foot long (L) 1⁄4 inch diameter galvanized cable sling subjected to a shock force by the sudden dropping of a 500 pound load 6 inches. The resulting shock force is 2,296 pounds, a value over 4 times the weight of the load!

*A free applet, or automated calculator, for wire rope elastic stretch is located online at:http://www.macwhyte.com/Technical/Metallic-Area-Elastic-Stretch-Calculator

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Wire rope is extremely sturdy and can be used in many different applications. In order to withstand harsh conditions, wire rope has basic guidelines of inspection it must meet. Continue reading to find out the guidelines of inspection for wire rope.

Abrasion damage is usually caused by the rope making contact with an abrasive surface. It can also be caused by simply passing over the drum and sheaves during regular, continued use. To minimize this risk, all components should be in proper working condition and be of appropriate diameter for the rope. Badly worn sheaves or drums will cause serious damage to a new rope and will greatly diminish the integrity of the rope quickly.

Corrosion is hard to assess but is more problematic than abrasion. Corrosion is usually the result of the lack of lubrication. It will most likely take place internally before there are any apparent signs on the rope’s surface. One telltale sign of corrosion is a slight discoloration, which is generally the result of rusting. This discoloration indicates a need for lubrication which should be dealt with as soon as possible. Failure to attend to this situation will lead to severe corrosion which will cause premature fatigue failures in the wires and strands. If this occurs, the rope will need to be removed immediately.

Diameter reduction is an extremely serious deterioration factor and can occur for several reasons. The most common reasons for diameter reduction are excessive abrasion of the outside wires, loss of core diameter/support, internal or external corrosion damage, or inner wire failure.

Examining and documenting a new rope’s actual diameter when under normal load conditions is critical. During the life of the rope, the actual diameter of the rope should be regularly measured at the same location under similar loading conditions. If this protocol is followed correctly, it should divulge a routine rope characteristic—after an initial reduction, the overall diameter will stabilize, then gradually decrease in diameter during the course of the rope’s life. This occurrence is completely natural, but if diameter reduction is confined to a single area or happens quickly, the inspector must quickly identify the source of the diameter loss and make the necessary changes if possible. Otherwise, the rope should be replaced as soon as possible.

Crushing or flattening of wire rope strands can happen for many reasons. These issues usually arise on multilayer spooling conditions but can also develop just by using the wrong wire rope for the specific application. Incorrect installation is the most common cause of premature crushing/flattening. Quite often, failure to secure a tight first layer, which is known as the foundation, will cause loose or “gappy” conditions in the wire rope which will result in accelerated deterioration. Failure to appropriately break-in the new rope, or even worse, to have no break-in protocol whatsoever, will also result in poor spooling conditions. The inspector must understand how to correctly inspect the wire rope, in addition to knowing how that rope was initially installed.

Another potential cause for the replacement of the rope is shock loading (also known as bird-caging). Shock loading is caused by the abrupt release of tension on the wire rope and its rebound culminating from being overloaded. The damage that ensues can never be amended and the rope needs to be replaced immediately.

There are several different instances that might result in high stranding. Some of these instances include the inability to correctly seize the rope prior to installation or the inability to maintain seizing during wedge socket installation. Sometimes wavy rope occurs due to kinks or very tight grooving issues. Another possible problem arises from introducing torque or twist into a new rope during poor installation methods. In this situation, the inspector must assess the continued use of the rope or conduct inspections more often.

There are a lot of guidelines for troubleshooting wire rope. At Silver State Wire Rope and Rigging, Inc., we take these guidelines seriously, and so should you. All of our products are tested and guaranteed to be the best fit for your specific needs. We can also help you with your troubleshooting needs. Contact us today!

Examine slings for wear, fatigue, crushed or broken wires, kinking, ballooning or "bird-caging", heat damage, etc. Check both before and after using slings to detect any damage or defects. See Hoist wire rope for more inspection tips.

Northern Strands manufactures wire rope slings rated up to 36,000 lbs and sells round synthetic slings that are rated up to 140,000 lb capacity. This capacity is the Working Load Limit of the sling, which is the maximum amount of weight or force that the sling"s user is allowed to put on the sling. Note: These slings do not break at the working load limit. These slings are designed with a safety factor of 5:1. This means that 5 times as much force as the working load limit has to be applied to the sling before it potentially fails. This means the wire rope slings have a Breaking Strength of up to 180,000 lbs and the round synthetic slings can withhold up to 700,000 lbs.

Uneven loading - Slings are made up of either wires or fibers that must all share the weight of the load evenly. If any situation arises where the sling is bent or wrapped around an object, there is potential that some of the wires or fibers will be taking on a greater share of the load than others.

Workers involved in hoisting and rigging must exercise care when selecting and using slings. The selection of slings should be based upon the size and type of the load, and the environmental conditions of the workplace. Slings should be visually inspected before each use to ensure their effectiveness. Improper use of hoisting equipment, including slings, may result in overloading, excessive speed (e.g., taking up slack with a sudden jerk, shock loading), or sudden acceleration or deceleration of equipment.

Alloy steel chains are strong and able to adapt to the shape of the load. Care should be taken when using chain slings because sudden shocks will damage them. This may result in sling failure and possible injury to workers or damage to the load.

Wire rope is composed of individual wires that have been twisted to form strands. Strands are then twisted to form a wire rope. When wire rope has a fiber core, it is usually more flexible but less resistant to environmental damage. Conversely, wire rope with a core that is made of a wire rope strand tends to have greater strength and is more resistant to heat damage.

When selecting a wire rope sling to give the best service, there are four characteristics to consider: strength, ability to withstand fatigue (e.g., to bend without distortion), ability to withstand abrasive wear, and ability to withstand abuse.

Strength – Strength of wire rope is a function of its size (e.g., diameter of the rope), grade, and construction, and must be sufficient to accommodate the maximum applied load.

Fatigue (Bending without Failure) – Fatigue failure of wire rope is caused by the development of small cracks during small radius bends. The best means for preventing fatigue failure of wire rope slings is to use blocking or padding to increase the bend radius.

Abrasive Wear – The ability of wire rope to withstand abrasion is determined by the size and number of the individual wires used to make up the rope. Smaller wires bend more readily and offer greater flexibility, but are less able to withstand abrasion. Larger wires are less flexible, but withstand abrasion better.

Abuse – Misuse or abuse of wire rope slings will result in their failure long before any other factor. Abuse can lead to serious structural damage, resulting in kinks or bird caging. (In bird caging, the wire rope strands are forcibly untwisted and become spread outwards.) To prevent injuries to workers and prolong the life of the sling, strictly adhered to safe and proper use of wire rope slings.

Wire rope slings must be visually inspected before use. Slings with excessive broken wires, severe corrosion, localized wear, damage to end-fittings (e.g., hooks, rings, links, or collars), or damage to the rope structure (e.g., kinks, bird caging, distortion) must be removed from service and discarded.

Fiber rope and synthetic web slings are used primarily for temporary work, such as construction or painting, and are the best choice for use on expensive loads, highly finished or fragile parts, and delicate equipment.

Fiber rope slings deteriorate on contact with acids and caustics and, therefore, must not be used around these substances. Fiber rope slings that exhibit cuts, gouges, worn surface areas, brittle or discolored fibers, melting, or charring must be discarded. A buildup of powder-like sawdust on the inside of a fiber rope indicates excessive internal wear and that the sling is unsafe. Finally, if the rope fibers separate easily when scratched with a fingernail, it indicates that the sling has suffered some kind of chemical damage and should be discarded.

Shock Absorbency - Regardless of the construction material, shock loading (e.g., excessive speed, rapid acceleration or deceleration) of slings should be minimized. However, it should be noted that synthetic web slings can absorb heavy shocks without damage.