Ropeblock Cast Sheaves engineered for durability
In any type of crane system, a wire rope being bent over a sheave will lead to rope fatigue. Recent studies show that this particular type of fatigue is even more severe when the wire rope is in a twisted state. In such cases, the most commonly used rotation resistant ropes may see their longevity reduced by up to 70 percent. At Ropeblock, we took it upon ourselves to address this issue by developing a unique type of sheave (Ropeblock DryLube sheave).
One of the major causes of wire rope twist in crane applications is friction between the wire rope and the sheave. This friction, in turn, is primarily caused by the angle of attack of the wire rope on the sheave groove (a phenomenon also known as fleet angles). Under tension, the end of a wire rope will rotate when free (e.g. with a swivel). However, such rotation is not possible when the wire rope is fixed directly to the crane. The additional twist resulting from fleet angles then generates extra torsion. When the accumulated twist in the crane system is released between the tip of the crane and the lower block, block rotation may eventually ensue.
CHOOSING THE OPTIMUM MATERIAL
The degree of friction in crane systems largely depends on the type of sheave used. Plastic sheaves, especially, are known to cause above-average friction, hence, more twist. Consequently, after meticulous R&D, Ropeblock introduced a unique type of cast sheave. It ensures optimum interaction with the wire rope, resulting in significant friction reduction and record amounts of Cycle Bends Over Sheave (CBOS) in any application.
The secret of the Ropeblock cast sheave is dry graphite lubrication, which works in a way similar to bronze bearings with graphite nodes. By making the wire rope slide rather than twisting it, Ropeblock DryLube sheaves substantially lower the friction coefficient. Once the rope and groove are properly run in, wear is reduced to practically zero. What’s more, improved sheave ductility provides better resistance against impact or unintended blows against objects, while the material grain makes sheaves more crack-resistant. As an added benefit when operating in extreme circumstances, Ropeblock DryLube sheaves have a better than average low temperature performance, allowing service down to even -40°C/-40°F.
Research in a lab environment proved that by minimizing wire rope twist Ropeblock DryLube sheaves make wire rope last up to 40 percent longer when compared to the same size and geometry plastic sheaves. When compared to regular steel sheaves of the same size and geometry, Ropeblock sheaves perform better by 25 percent. All in all, Ropeblock DryLube sheaves make crane systems safer, improve uptime and optimize cost effectiveness. They come in hundreds of different bearing and D/d sizes, ranging from wire rope 10 mm to 76 mm and beyond, and are accepted by all major classification societies.