While the concept of an overhead crane wire rope might seem simple enough, there is far more than meets the eye when it comes to selecting the correct wire rope for your application. It is critical to understand the difference between a general purpose wire rope, rotation-resistant wire rope and compacted steel wire rope, etc.
Anatomy of Wire Rope
A wire rope consists of multiple strands of wire laid twisted around a core which is either fiber core (FC), that provides additional elasticity to the rope, or an independent wire rope core (IWRC) that has less stretch and more strength. The size of a wire rope refers to its diameter, and the greater the diameter, the greater the tensile strength of the rope.
Also, while the term cable and rope may be used interchangeably, the diameter differentiates between them. Rope refers to diameters larger than ⅜” while cable refers to diameters less than ⅜”.
Because of its tensile strength, a wire rope can be used to transmit force, motion, and energy.
The more wires there are in a strand of wire rope of a given diameter, the more flexible that rope will be. One of the key aspects of the usefulness of rope, wire or otherwise, is its ability to bend and wrap around pulleys and sheaves. If a rope cannot bend, it will not be as useful.
When a rope does bend, the wires and strands that make up the rope slide on top of each other. Because of this, wire ropes that will experience significant bending will need to be lubricated. In addition, the actual wires experience bending stress which can, over time, lead to fatigue failure. If the stresses are higher than the yield point of the wires, the wire rope may experience a permanent deformation.
Rope Class
Each strand in a wire rope is comprised of multiple wires (usually 7, 19, or 37). A rope with 7 strands of 19 wires would be referred to as a 7×19 wire rope. This type of designation serves as the class of the wire. These numbers may be followed by a pair of letters that provide information about how the outside layer of the wire rope is constructed.
Lay of Wire Rope
The lay of a rope refers to how the wire strands are twisted. A regular lay wire rope has the wires twisted one way and the strands twisted in the opposite direction. A lang lay, on the other hand, has the wires and strands wrapped in the same direction. A regular lay is going to be less likely to kink or untwist compared to a lang lay, but a lang lay is going to have better abrasion resistance and flexibility. Lay can also be designated as right or left: a right lay wraps around the core in a clockwise direction, while a left lay wraps around in a counter-clockwise direction.
Grade of Wire Rope
The grade of the rope identifies the grade of steel used in the wire. Some of the more common grades include…
- IPS, improved plow steel
- EIPS, extra improved plow steel that is about approximately 10% stronger than IPS
- EEIPS, extra improved plow steel that is approximately 10% stronger than the EIPS
- GIPS, galvanized improved plow steel
- DGEIP, drawn galvanized improved plow steel (drawn wires typically have a higher break load than GIPS)
Wire Rope Finish
The finish of a wire rope refers to the coating (if any) applied to the strands. If a rope is described as bright finish, it is made from high carbon steel wires that have no finish or protective coating applied to them. A galvanized finish has a coating of zinc added to enhance corrosion protection. Another finish option is a zinc aluminum coating added to wire for 3x better corrosion protection than a galvanized finish.
How Wire Rope Works
Because of its tensile strength, a wire rope can be used to transmit force, motion, and energy.
The more wires there are in a strand of wire rope of a given diameter, the more flexible that rope will be. One of the key aspects of the usefulness of rope, wire or otherwise, is its ability to bend and wrap around pulleys and sheaves. If a rope cannot bend, it will not be as useful.
When a rope does bend, the wires and strands that make up the rope slide on top of each other. Because of this, wire ropes that will experience significant bending will need to be lubricated. In addition, the actual wires experience bending stress which can, over time, lead to fatigue failure. If the stresses are higher than the yield point of the wires, the wire rope may experience a permanent deformation.
Choosing the Right Wire Rope
There are several key wire rope characteristics that must be considered when choosing the right one for your application. One of the most important characteristics of wire rope is its strength which is typically reported as its minimum breaking force (MBF). However–and this is very important–a wire rope should never be used at its MBF.
A stranded rope should also have an application-appropriate crushing resistance. The crushing resistance refers to the rope’s ability to resist having its cross-section, stands, wires, or core deformed by outside forces. This is similar to the ability of a rope to resist deformation in the form of wire rope peening, where the outside wires of the rope appear to have been hammered.
Fatigue resistance is another key aspect of choosing the right wire rope and particularly applies to ropes that experience bending stresses. The longer a wire rope is exposed to bending stresses, the less strength it is going to have. A rope with a higher fatigue resistance is going to have a longer useful life. The more wires in a rope, the better its fatigue resistance. However, the size of the wire rope also plays a key part: ropes should not be bent over drums or sheaves that force the rope to bend sharply.
When a load is placed on a rope, the wires will try to straighten out to accommodate a load, generating a torque that can cause the rope to spin. Some need to be rotation resistant wire rope to prevent the load from rotating when applied to the rope. A similar characteristic is the stability of the rope, which refers to the ability of the rope to spool smoothly or avoid tangling.
Another key wire rope characteristic is its resistance to metal loss, which usually affects the outside wires of the rope and occurs through abrasion. As the rope rubs against other hard surfaces, metal can be worn away from the outer wires.
Conclusion
Overhead crane wire ropes are used in many different industries, including mining, oil & gas, marine, steel, logging, ski lifts, and elevators. It is vitally important that you find the right wire rope for your application, whether you need a rotation-resistant wire rope for a critical petrochemical application or a general-purpose wire rope for a winch. If you are in the market for stranded core wire rope, contact the experts at Hi-Speed Industrial Service.