Why are Lightweight slewing rings, (bearings) rapidly becoming the first choice for design engineers in demanding motion control applications, for example in the aerospace and defense industries? Quite simply, where absolute accuracy, optimum mechanical performance and good corrosion resistance is essential, lightweight slewing rings are proven in the most challenging applications. Karl Brundell, MD of bearing specialists Carter Manufacturing, examines the reasons why in more detail.
Sometimes the numbers tell the story. For example, we have recently seen one of our lightweight, heavy duty aluminum slewing bearings specified for a turret mounting in a defense application. The turret diameter measures over 2 meters yet has a run-out measured in microns. Put simply, a slewing ring or bearing allows design teams to replace paired bearings with a single item, which reduces parts count, weight, complexity and cost. Typical applications which Carter have been involved with include; propulsion systems and radar gimbal bearings for Spacecraft, CT scanners, cardio vascular machines for Medical applications, helicopter seat mounts, weapon systems for Aerospace and other precision military systems including robots, gun turrets, and surveillance systems.
Innovative Design = Exceptional Accuracy
Whilst there are many slewing rings available for less demanding motion control applications, the choices for high technology applications are limited. Examples of these include AFV’s (armored fighting vehicles), satellites, ultra-high vacuum environments or military specification tracking systems requiring significantly higher levels of accuracy, weight saving and corrosion resistance. With the aluminium slewing ring mentioned previously, exceptional accuracy has been achieved by employing a revolutionary split thin section bearing design. It has a split inner and outer ring allowing the bearing preload to be determined by the shaft and housing fits, so clearance or preload settings can be set in the design stage of the application. This means a substantial reduction in costs by avoiding the need to request special preloads to be set into the bearing on factory assembly, while stainless steel components in parts of the design, optimizes corrosion resistance.
A slewing bearing is designed so that the rolling elements create a reactive movement within the bearing when under load, this opposes the existent overrunning (or overturning) loads which are present in the motion control application.
This is critical to designers of military land, sea and aerial equipment where smaller size makes vessels, vehicles and UAV’s more difficult to see or track in operational use. Another advantage of using lightweight slew bearings is in applications where it’s necessary to have an associated drive system or gear ring in place. For these projects, slew rings can be specified with the necessary machining (for example, incorporating a gear ring) already in place.
This added engineering is possible on the majority of slew bearings in most sizes and load ratings helping to achieve important savings in initial cost and also removes unnecessary complexity. The slewing bearings supplied by Carter to defense and aerospace contractors feature gear ring, or teeth, which can be incorporated on either the inner or outer race (or even both) as required by the selected drive or pick-up system.
Design strength, precision and durability
The most commonly specified slewing bearings are plain, flanged, or geared which are mounted onto the required faces using holes in both the inner and outer ring for absolute design strength, precision and durability. These are important design features, especially for high-accuracy applications such as radars, target trackers or AFV turrets. Alternative mounting methods can also be utilized, depending on the lateral or longitudinal design load parameters that have been set.
The range of slew bearings available for use in high-tech applications in the defense, aerospace, cryogenic or nuclear industries provides different internal and external styles. Once the forces are predicted for each application, engineers can choose from a range of options. For example; a single or double ball design, a roller design (or double roller as required for single direction use), cross rollers, (suitable for dual-direction uses), triple rollers, or ball/roller/needle combinations with either split design or wire-race inserts. This enables Carter to tailor a slew bearing response to a specific application, whether it’s designed to carry a 200gm scanner rotating at 200deg/sec, or a 12 tonne telescope that takes 20 hours to complete full rotation.
Material choice and the importance of Sealing and Lubrication
The materials choices available for ultra-high vacuum applications include aluminium which is ideal for weight-critical applications requiring the combination of being ultra-compact, yet require significant strength. Whilst design engineers might be focused on delivering drawings that match the required specifications, it’s sometimes left to others to consider another important aspect of bearing performance that applies to slew bearings, just as much as any other type of bearing – and that is sealing and lubrication.
Slew bearings used in aggressive environments, on spacecraft, ultra-high vacuum or extremes of temperature applications need special seals and lubrication whereas for general manufacturing applications there are many standard options to choose from. However, the more fascinating science and manufacturing projects has resulted in the development of bespoke seals and innovative lubricants that have proved to be genuine game-changers. Also, the extensive range of defense and aerospace applications where lightweight slew rings are now being specified, has highlighted the benefits of Carter’s in-house and in-demand CAD/CAM,FEA facility.
Accuracy, longevity, lightness and corrosion resistance
Accuracy, longevity, lightness and corrosion resistance are vital and Carters facilities allow designers to simulate their bearing needs and capture the essential data required helping them to choose the best materials and bearing design choices in advance of producing working prototypes. The essential parameters are; rotational speed, flexing loads, platform rigidity and stiffness, vertical play, mounting options, corrosion protection, ingress protection, and smoothness of rotation. To cater for such a diverse range of applications slew rings can be specified with rolling elements made from a variety of base materials including ceramics, chromium steel and stainless steels.