A wire rope is only as good as the inspection and maintenance program behind it, especially on a high-duty-cycle crane in a demanding environment, says Richard Phillips, mechanical engineer at Casper, Phillips & Associates Inc.

An important component on virtually every crane is wire rope. Thus, avoiding rope replacement, downtime, and safety risks, should be a priority for all crane manufacturers, suppliers, owners, and users.

[Photo above: Clamping forces at rope terminations adds additional stress that can create weak points.]

There are many widely accepted best practices that can prolong the life of a rope, including thorough inspection, maintenance, and lubrication. Unfortunately, these practices are not always given the required attention.

In general, there are two types of rope inspection, with different requirements:

• Each work shift: Work shift inspections are broader and more general. They are concerned with discovering gross damage, such as dog legs, birdcaging, or other obvious signs of rope damage.
• Periodic: This type of inspection is more in-depth and requires keeping permanent records as mandated by the Occupational Safety and Health Administration (OSHA) and other regulatory agencies, and as recommended by American Society of Mechanical Engineers (ASME). Periodic inspection requires more stringent attention to specific details. These inspections should record observations and dimensions, such as diameter, lay measurements, broken wire counts, evidence of rope core failure, abuse, and other types of wear.

A Long Process

Make no mistake, inspecting a wire rope is time-consuming. The entire length must be examined visually, and the longer the rope, the more monotonous the task becomes; monotony can lead to lapses in concentration. Yet, rope inspection must be taken seriously and never overlooked.

Effective inspection requires good light, sharp vision, and, in many cases, the ability to physically touch the rope. Even then, not all defects are visible, and access to all areas of potential damage can be difficult. Wire breaks at end terminations may be hidden by mechanical splices, such as thimbles, spelter sockets, or wedge sockets. Damage to the core may be concealed by outer strands, requiring special techniques to examine the rope’s center.

Abrasion is a major concern. It occurs when wires and strands rub against each other during operation, leading to strand failure and other issues. Abrasion happens every time a wire rope changes direction while in motion, such as moving around a sheave or wound onto a drum. Harsh operating environments exacerbate this, with contaminants like dirt, grit, and grime increasing abrasion and further reducing rope life. On drums with multiple layers of rope, strand-to-strand nicking can develop, a type of abrasion that accelerates wear. There are established criteria for inspecting nicking and determining when rope replacement is necessary. In general, as abrasion increases, bending fatigue resistance decreases. To mitigate this, drum and sheave diameters should be larger relative to the rope, reducing bending stress on highly abrasive ropes.  Replacing a damaged wire rope, meanwhile, is far easier than replacing a broken one.

Changing the Drum Diameter is Impractical

Design considerations are a separate facet of wire rope maintenance. A poorly designed reeving system will not achieve satisfactory rope life, even with the best possible maintenance practice. The smaller the bend radius, the quicker the rope wears out. This means if drum or sheave diameters are too small rope bending fatigue will be accelerated.

Reality is, it is difficult to change fundamental crane features such as the drum diameter. The motors spin the drums at a given speed so changing the drum diameter would change the hoist speed with respect to the motor speed. It will also change the torque the motor needs to supply to pull the rope.

Changing drum diameters will also impact the control system’s ability to keep track of hook elevation. It is common to have a shaft encoder on the drum that counts the turns. If the drum diameter changes so will the amount of rope it pulls in or puts out per turn, so the control system would need to be reprogrammed. Solutions involving changing the drum diameter will not be practical once the crane is built.

Reverse bending accelerates fatigue, distorts the rope’s internal structure, and dramatically shortens its service life, which is why rope ideally should not be bent in the opposite direction of its sheaves.

Sheaves, which are grooved wheels that the rope runs over, should also be central to wire rope inspection. Misaligned sheaves, worn grooves, or frozen rollers can shorten rope life dramatically. Undersized and worn grooves can deform the rope over time. There are simple tools to measure groove wear. One big visual indication is striation patterns on the groove surface. The wire rope is generally harder than the contact surface on the drum or sheave. For high contact stresses the surface area of the sheave or drum can displace, leaving a contact pattern that resembles the wire rope strands. Hardening the drum or sheave contact surface greatly reduces the risk of this type of wear and significantly increases the useful life of the component.

The Importance of Lubrication

Lubrication is a crucial, if often misunderstood, facet of the wire rope discussion. During manufacturing, wire ropes are thoroughly lubricated, but this lubrication is only intended to protect the rope until it is put into service. Until then, it should be stored in a covered area protected from rain and sun.

If wire ropes are not subsequently and properly lubricated, the corrosion and internal abrasion referenced above can occur. In high tension operations, combining normal bending and reverse bending (where it can’t be avoided), the internal strands of a wire rope can experience significant friction when corrosion and / or poor lubrication is present. This leads to internal wear and a shorter service life.

There are many different lubrication techniques. Most crane owners use painting or swabbing, which relies on the technique of the technician applying the lubricant and the type of lubricant used. There are also automated lubrication systems, such as a continuous bath, dripping, and pressure spray methods. The automated systems can be more economical as they continually apply lubricant while the rope is in operation.

Before applying lubrication, accumulations of abrasive materials should be removed from the rope. It is important that the core of the rope receives proper lubrication. After removing contaminants, the clean state of the rope makes for a convenient time to inspect the conditions of the rope wires. Lubricant should be reapplied with considerable pressure to ensure the core is properly lubricated.

In one recent case study, corrosion was observed to be present on several ropes. Lubrication had been applied to the ropes through both roller and sprayer applications, but no mention was made of cleaning the ropes off prior to lubrication. It did not appear that the lubrication had been able to penetrate past the outer surface, which is critical for proper rope lubrication. Although the spray and roller lubrication methods are mentioned in maintenance manuals, we recommended using a pressurized lubrication system be installed on cranes where extending the rope life is beneficial to the owner/operator.

In conclusion, inspection and maintenance, with close attention to lubrication and sheaves, are essential to the avoidance of rope replacement, downtime, and safety risks. Through daily and periodic inspections, the requirements of OSHA and ASME can be met, and safe crane operations can continue long after it would be possible if such best practices are neglected.

CP&A offers a wide variety of services, including specification, design, manufacturing review, modification, and accident investigation. Visit www.casperphillips.com to learn more.