Types of Cores
Selecting the proper core type is a critical factor when specifying wire rope. Wire ropes are generally manufactured with either a fiber core (FC) or an independent wire rope core (IWRC).
- Fiber Core (FC)The fiber core is made of natural fibers or polypropylene, offering greater elasticity than a steel wire core. This flexibility can help absorb shock loads and reduce internal wear, making fiber cores suitable for applications where bending and flexibility are essential.
- Independent Wire Rope Core (IWRC)The IWRC is typically a separate 7×7 wire rope at the center of the main rope. Compared to a fiber core, a steel core:
- Increases rope strength by approximately 7%
- Increases weight by around 10%
- Provides substantial support to the outer strands, enhancing rope durability during operation
- Resists crushing and heat, improving overall rope longevity
Design Factor
The Design Factor is the ratio between the rope’s minimum breaking load (MBL) and its rated capacity (RC), indicating the percentage of the rope’s ultimate strength at which it operates. This factor accounts for normal wear and potential stresses in various applications.
Example:
If a rope has a maximum operating load of 10,000 lbs and an ultimate strength of 50,000 lbs, the design factor is 5, meaning the rope operates at 20% of its ultimate strength. Choosing the appropriate design factor requires evaluating the specific conditions of each installation.
Fleet Angle
The fleet angle is the angle between the rope running to or from the extreme left or right of the drum and a line drawn from the center of the sheave perpendicular to the drum axis.
- For smooth drums, the recommended maximum fleet angle is 1.5°
- For grooved drums, it is 2°
Exceeding these limits can cause poor rope winding or rubbing against drum flanges. Conversely, too small a fleet angle may result in the rope piling up against the flange head. Before installing wire rope on a drum, always verify the fleet angle and correct any improper conditions.
Sheave Alignment
Proper alignment of sheaves is crucial to ensure smooth rope operation and longevity. The main sheave should be aligned with the center of the hoisting drum. Misalignment can lead to rapid deterioration of both the rope and sheave flanges. For high-speed applications, sheaves should also be dynamically balanced.
Note:
Wire ropes are subject to failure if abused, misused, or overloaded. Regular inspection, proper maintenance, and adherence to industry standards (including OSHA guidelines) are essential for safe operation.
