When considering sheaves or pulleys for a shade structure it is important to consider the profile of the sheave as sheaves intended for steel cable differ from those that are ideal for fibrous rope.
As the shade industry continues to move towards high-performance synthetic ropes that have none of the traditional weaknesses of organic fiber—and are easier and lighter to work with than steel wire ropes—it might be tempting to switch out the rope in your design without changing the sheave. However, this can damage your system and reduce the lifespan or safety of the constituent parts.
Avoid this mistake and assemble an optimal product by considering the effect sheave size and shape will have on your new rope.
Grooves are Important
For certain fabric structure applications, loads can be far lower than the maximum tensile strength of the ropes you’re using, which may make a faster, smaller, and more lightweight, system preferable. However, don’t let this fool you into believing any sheave will do. Improperly matched equipment can accelerate degradation at any working load. Also, wind loads on structures made from fabric can be extreme.
In most cases, the size and shape of the sheave groove will be much more impactful than the overall diameter of the sheave. An improper groove can damage ropes and cables much more quickly because it could create problematic heat, friction, or pinching in the system.
As a result, you’ll need to select a sheave with a groove of the proper width and shape for your rope.
In general, the groove of a sheave should be at least 5% wider than the rope or cable that passes through it, but not more than 10%.
While it should always be wider than the rope, too much clearance will cause the rope to flatten under stress as it tries to form to the shape of the groove. This can cause premature wear on the groove, degrade the fiber in the rope, and reduce system stability and tensile strength.
More likely, if you’re transitioning from steel cable to rope, you’ll find that the groove is too small for a rope that is lighter weight even when thicker than the old cable. Adding tension to a rope that is too thick to be properly cradled by the sheave will deform it down into the narrower groove. With too little clearance, the groove will pinch the rope, introducing damaging friction and abrasion that will cause heat and slow down the system.
The two primary groove shapes are the traditional U-groove and a modern V-groove.
Grooves with a V-shaped opening, typically of 35 to 70 degrees, are not rounded to cradle the tubular shape of a typical wire or fiber rope. This prevents the rope from contacting the bottom of the groove—only the walls (the flanges) press on the rope, pinching it when tension pulls it towards the base of the groove. These two smaller points of contact increase pressure and friction between the rope and the sheave, adding traction or grip that some applications may desire (to prevent slippage or power a shaft connected to the sheave). However, V grooves also lead to fatigue, pinching, and damage to fibrous rope from friction.
Wire tends to be more compatible with V-grooves than fiber, due to steel’s rigidity. The V shape can keep a steel cable from sliding or rolling in the sheave groove. Soft fiber ropes will deform into the V shape and suffer associated stresses. Steel cables deform less, but can abrade the groove over time or become lodged too deep between the sheave’s V-shaped flanges if the angle is too narrow.
Synthetic fiber ropes are best suited to U-grooves that support 135 or 150 degrees of the rope diameter, preventing unwanted deforming, twisting, or flattening. In a U shaped depression, a soft fiber rope won’t roll or slide as easily as wire because pressure conforms it to the groove’s shape. This U-groove supports the fiber rope’s structure more evenly, which balances wear, maintains its tubular shape, and increases strength.
A final consideration when switching from steel cable to fibrous rope in shade systems or tension fabric structures is damage to the equipment from steel abrasion.
Even if your sheaves are the appropriate diameter and have suitable grooves for your new ropes, the previous use of steel cable may have scored or marked the grooves. These burrs may scratch and scrape fibrous ropes in harmful ways that could be avoided with new sheaves.
Also look out for broken sheave flanges, which enable wire and fibrous ropes to jump the sheave and become badly cut or sheared.
Plenty of Options
Check out our selection of sheaves that have been load-tested in the marine industry to handle the aggressive winds on your fabric structure, in a variety of sizes and materials to suit your rope and your needs.