Over the years that we have been manufacturing sheaves here at Titan Machine Corp., we've often been asked to weigh in on the appropriate geometries for your sheave’s rope grooves. While there are a myriad of factors that ultimately determine the necessary amount of traction, we’ll try and summarize the tradeoffs of the most common groove types.
The U Groove
The most basic type of groove is the U groove. The structure of a U groove is simple: just a half-moon-shaped profile that uniformly supports the entirety of the cable as it makes contact with the sheave. In the oldest traction applications, this groove was used on winding drums. Today, we still see these grooves used in traction applications on double wrapped equipment and most frequently when simply being used for deflectors or other non-traction sheaves.
The U Groove provides the least amount of traction, but also distributes rope pressure evenly across the entire contacting portion of the cable, minimizing rope wear.
The V Groove
The V Groove is the polar opposite of the U Groove. It is shaped just as the name implies; a V. Because the walls of the V groove are flat and taper inward, they concentrate rope pressure on a theoretical single tangent point on the rope where there’s contact between the cable and the groove sidewall. This creates tremendous traction on the cable by pinching harder as weight increases but can tend to deform the rope into a triangular shape over time. As the angle of the V gets smaller the potential traction increases, as does the pressure on the ropes.
The V Groove provides the most traction. However, because of the concentrated rope pressure, it will wear cables sooner than other groove geometries and it’s possible that the elevator system will not be able to break traction in either direction particularly when pulling the car up leading to safety considerations.
The V Groove is only used on traction sheaves, not other applications.
The Undercut U Groove
The undercut U Groove is a compromise between the U Groove and the V Groove. There have been iterations of the V groove such as the so-called “progressive groove”, but with modern machining practices and precise engineering calculation and designs, today we most often produce what is called an undercut U groove. Essentially a U groove with a short section of the groove bottom’s radius plunge machined to produce a relief underneath the U.
This geometry supports a great deal of the rope and thereby reduces rope pressure and still allows for the undercut’s corners, that the cable sits on top of, to “bite” the cable and provide traction. The size of the undercut is identified by the arc of the rope's circumference that is covers. With a 90° undercut, the angle between two lines drawn from the rope center to each corner of the undercut will equal 90°. The undercut angle or width also depends on the rope diameter. The larger the undercut, the greater the potential traction and rope pressure.
By combining the benefits of the U groove with those of the V groove, the undercut U groove is also often called a “safety groove” because while it can provide the necessary traction to hoist the car or counterweight in either direction, the groove also inherently is designed to break traction when overwhelming force is applied as would be the case in the event a machine inadvertently continued driving upward with the car in the overhead.
When selecting a groove profile, there are several considerations and these are just a few:
Speed, capacity, and travel distance of the elevator
Angle of wrap the cable will have around the traction sheave
Traction cable design (regular vs. lang lay, EHS cable, Hardness, etc.)
At Titan, we’ve worked on the largest variety of sheaves imaginable. Whether directly duplicating an existing sheave, or engineering a replacement for a sisal rope-lined winding drum (yes, we’ve worked on those too), Titan has the expertise and equipment to get your project underway and complete on time and under budget.
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