Hourglass Slewing Drive Best for Mobile Application
Mobile applications represent many unique and challenging load scenarios for effectively handling and rotating the payload. These applications include man-lifts, cranes, utility trucks, sewer equipment, tire hands, oil tools, forestry fallers, steering, and military vehicles, among others.
These crucial applications often experience multi-planar loads simultaneously. Axial loads move to separate the slewing raceways from each other, while radial loads act to move the ring gear into or away from the driving worm shaft. The overturning moment load acts to tilt the raceway and accompanying ring gear, putting the system in flex. Due to the mobile nature of the applications, there is often not a super solid foundation for support. Designing to maximize performance while minimizing weight is important to marketplace evolution and ultimate competitiveness for Original Equipment Manufacturers. Therefore the housings and ensuing drives cannot just be enlarged endlessly to stiffen the unit.
Standard gearsets are termed non-enveloping, such as a straight worm or pinion driving a standard helical or spur gear. Enveloping gearsets are manufactured with a curvature in one or more of the components. The curvature allows more of the driving and driven gear to be in contact with each other, thereby reducing surface stress and increasing power capability of the gear drive.
In enveloping gearsets the gear can be enveloped around the worm (making a crescent moon shaped gear), or the worm can be enveloped around the gear (making an hourglass shaped worm).
Through extensive testing it can be seen that the loadings from mobile applications flex an enveloped (crescent) shaped gear, such that the curve does not ride on the worm centerline as in theory, but instead it bends such that just the tip of the moon curved gear contacts the worm. This works as an ice pick, digging out the worm shaft quickly to destruction as it rotates.
An enveloped (hourglass shaped) worm shaft however maintains its centers between the taper roller bearings of the housing and has no such movement or contact issues against a standard helical gear. The helical gear does flex under load (as do all gears including an enveloped gear), but because the faces of the helical gear are straight, it rides up and down the thread of the worm shaft maintaining an identical tooth contact area and thus performance no matter its location.
From the results of these decades of testing Kinematics manufactures its gearsets for the mobile equipment industry using and hourglass (enveloped) worm shaft to drive a standard helical gear, and the results have been magnificent.