The market for military unmanned aerial vehicles (UAV), also commonly referred to as unmanned aerial systems (UAS), has seen meteoric growth over the past decade. These systems are now considered a key part of defense strategy. As a critical element of defense, UAV and UAS systems place a high priority on reliability. Ensuring that reliability means that certain critical factors—including friction, leakage, and life of seals—must be addressed in the design of the UAV. These are becoming even more important as systems continue to shrink, placing a greater emphasis on the need to avoid friction in the sealing of the gimbals, or pods, that cover and protect sensitive onboard electronics.
Manufacturers are beginning to recognize that achieving a low dynamic coefficient of friction is a key driver of gimbal design in UAVs. Particularly in the design of small systems, seals are close to the “ragged edge” of frictional cut-offs, so manufacturers are now designing their systems with the seal in mind. The system must be designed around those cut-offs in order to achieve the desired friction level.
Often, O-rings are specified for use in gimbal design. But because these seals are made of rubber, they can produce stiction, or static friction. An attractive alternative to the problems presented by the O-ring is a spring-energized seal, of the type pioneered by us here at Bal Seal Engineering, Inc. With this kind of seal, polymers can be combined to achieve consistently high lubricity. Together with a geometry that reduces the amount of torque necessary to move a gimbal without sacrificing sealability, the low-friction material formulation can reduce or eliminate stiction force and any resulting jitter during target tracking.
Because the seal is preventing friction, it is also preventing wear. It is important to note that sealing ability and long life are inversely proportional, because in order to achieve a better seal, designers must typically create more surface contact, which will increase friction and increase wear.
In gimbal applications, there is an area in which the desired balance of low friction, wear and leakage rate converges. In order to arrive at this “sweet spot,” it is important for engineers to partner with a seal supplier that understands the interplay between application requirements, seal material and design. To create our low-friction spring-energized seal for gimbal applications, we have combined a filled PTFE material with our Canted Coil Spring technology and designed the cross section to accommodate light, medium or heavy spring load. We have also engineered variability into the flare on the energized lip seal, giving it more surface contact for sealing or less contact for lower friction.
UAVs and their gimbals are critical to the shift in defense systems. These systems must operate at an optimal level, which means that certain critical characteristics—including friction, leakage, and life of seals—cannot be an afterthought. They must be an integral part of the UAV design.
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