Sheaves and drums
The bending of a wire rope around a sheave causes slight movements between the strands and wires of the rope. Any change in the load also results in slight movements. The free and unobstructed movement of the wires and strands is crucial for the performance of the rope. This is in fact what gives the rope its flexibility and why it is very important that the radius of the groove is correct. If the groove radius is too small, the rope will be squeezed between the sides of the groove. Extremely high pressure will prevent the movement of the strands and wires. If the groove radius is too large the contact area between the rope and the groove will be very small, resulting again in high pressure. In both cases the rope's condition will deteriorate very rapidly, in some cases to such extent that the rope must be discarded after just a couple of weeks.
Since the diameter tolerance for wire ropes is always above the nominal diameter, the groove radius will also be above the nominal. In most cases the groove radius is 3% larger than ½ the rope diameter.
It is important to keep in mind that during use, not only the rope but also the sheaves and drum are subject to wear. When sheaves wear, the groove radius normally becomes smaller because it adjusts to the diameter of a worn wire rope. If the rope is replaced on the same sheave, the groove might be too small.
Before replacing ropes on an existing installation, always check the groove radius with a proper gauge and replace or re-groove if necessary.
Drum or sheave to rope diameter ratio:
The bending radius of a wire rope over a sheave or drum is critical for obtaining optimum service life from the rope. There is no practical formula to calculate the expected service life because it depends on many different variables (load spectrum, acceleration, speed, vibration, sheave and drum diameters, etc) but it is a known fact that the D/d ratio (Drum or sheave to rope diameter) is very important. The larger this ratio is the longer will be the service life of the rope. In most cases it is not possible to use a very large sheave or drum for practical reasons (cost, weight, space required, etc.).
The following table specifies the minimum recommended D/d ratios for different rope constructions. This table applies to general applications in which the rope speed is less than 1 m/s (3.3 ft/s). For higher speeds the ratio should be increased by 5% for every 0.5 m/s (1.65 ft/s).
This table doesn't apply to cranes and hoists. For those there are standards, specifications and design rules that take into account the load spectrum, speeds, hazards, etc. and set the D/d ratio accordingly.
Any deflection of the wire rope over a sheave of 15° or more must be considered as a full deflection and the D/d from the table applies. If the deflection is less than 15° it is possible to use a roller or sheave with a smaller diameter but never less than 1 rope lay length for a grooved sheave or 1½ lay lengths for a smooth roller.
In multiple sheave applications it is very important to avoid changing the direction in which the rope bends between sheaves. The rope should always wrap around the sheaves in the same direction. Reverse bending may shorten the service life of the rope by up to 50%.