SkySentry Tether Sheaves
SkySentry provides a variety of “Sheaves,” or pulleys, to help channel the tether from the winches to the aerostats. A sheave is quite important for a number of important reasons. First, when placed the proper distance from the winch, the tether will naturally level wind on the winch drum. With fiber optic tethers, this natural leveling helps avoid side stresses on the tether which could deteriorate the fiber optic strands inside. Second, a sheave helps cushion shocks to the tether by providing some angular reduction of possible turbulent loads the tether may experience when connected directly to the winch drum. Finally, a properly placed sheave helps with “housekeeping,” wherein the tether avoids entanglements with objects in the flight operations area.
Two major categories of Sheaves are available
Small bend radius sheaves work well for synthetic tethers. Synthetic tethers easily tolerate short bends, as long as the sheave has a slippery surface. SkySentry’s arch sheaves enable the aerostat to fly in any direction from the sheave, have no sharp edges for cutting the tether, and cannot be routed in reverse—a common problem we encounter with conventional pulleys during new crewmember training.
Large radius sheaves are best suited for tethers with fiber optic cables or other bend radius constraints, such as composite materials, expected loading, and desired service life. Many variables impact the design of these sheaves; hence, SkySentry produces its own sheaves customized for each system. Typically for SkySentry’s Tactically Expedient Aerostats, we strive to design the whole system so that no more than 20% of the Rated Breaking Strength is pulled on the tether. Following these rules, we are comfortable with sheave diameter approximately 25 times the diameter of the tether. For example, a 0.40 inch diameter tether with copper lines and fiber optics would match a sheave drum diameter of 10 inches. Of course, higher working loads would demand larger bend radius drums to enhance the working life of the tether.
These sheaves also have mounting provisions for Fiber Optic Rotating Joints, to allow seamless transfer of data from payloads on the aerostat to Ground Control Stations.
We strive to keep all of our sheaves as simple as possible, with holes placed around the mounting frame for ground anchors or bolts on a mooring platform.
Please let us know your sheave requirements. Production typically only takes a couple weeks.