Lightweight slewing rings, or bearings, are rapidly becoming the first choice for design engineers in the aerospace and defence sectors, where absolute accuracy, light weight, improvements in mechanical performance and good corrosion resistance are demanded. Mike Page of Abingdon-based Carter Bearings examines the case for them.
Sometimes it is the numbers that tell the story. For example, we have recently seen one of our lightweight, heavy duty aluminium slewing bearings specified for turret mounting in a defence-related application: it is more than 1 metre across yet has a run-out measured in microns. In a nutshell, a slewing ring or bearing allows design teams to replace paired bearings with a single item, thus reducing parts count, weight, complexity and cost.
Whilst there are many slewing rings readily available for run-of-the-mill machine or factory floor operations, the choices for use in high technology applications such as AFV’s (armoured fighting vehicles), satellites, ultra-high vacuum environments or military specification tracking systems (to name but three examples which we have much experience of) are limited. Applications like these demand significantly higher levels of accuracy, weight saving and corrosion resistance. In the case of the aluminium slewing ring mentioned previously, this exceptional accuracy has been achieved by employing what we believe is a revolutionary split thin section bearing design. It has a split inner and outer ring which allows the bearing preload to be determined by the shaft and housing fits. This enables the clearance or preload settings to be set in the design stage of the application. The advantage obtained is a sometimes substantial reduction in costs by avoiding the need to request special preloads to be set into the bearing on factory assembly, and because we also utilise stainless steel components in parts of the design, corrosion resistance is good too.
A slewing bearing is designed so that the rolling elements create a reactive moment within the bearing when it is under load: a moment which opposes the existent overrunning (or overturning) loads which are present in the given application. Apart from the advantages already mentioned, the use of a slewing ring or bearing can also significantly reduce the physical size of the relevant part on a project. This has proved especially useful for 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 we have helped designers realise through the use of lightweight slew bearings is in applications where it is 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 can be done with almost all slew bearings in most sizes and load ratings and has proved pivotal in achieving savings not just in cost but also by removing unnecessary complexity too. Usefully on the slewing bearings we supply to defence and aerospace contractors, the gear ring, or teeth, can be incorporated on either the inner or outer race (or indeed both) as required by the selected drive or pick-up system.
Design strength, precision and durability
Although available in a variety of types and styles, the most commonly specified slewing bearings are either plain, flanged or geared. In a typical application, the slewing bearing is mounted to the required faces using holes in both the inner and outer ring to deliver absolute design strength, precision and durability: all vital attributes and especially so in high-accuracy applications such as radars, target trackers or AFV turrets for example. Alternative mounting methods can also be embraced should they be required, dependent upon the lateral or longitudinal design load parameters that have been set or are expected. Because the uses for slewing bearings are so varied, so are both the geometry and internal raceway designs within the bearings themselves.
The range of slew bearings available for use in high-tech applications in the defence, aerospace, cryogenic or nuclear industries (as examples) provides a variety of different internal and external styles. When we look at the forces predicted to be present in a given application, we can choose either 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. The possibilities are considerable and allow us to tailor a slew bearing response to a specific application, whether it is designed to carry a 200gm scanner rotating at 200deg/sec or a 12 tonne telescope that takes 20 hours to complete a full rotation. There are also some interesting materials choices available too, with special materials for use in ultra-high vacuum applications and others (for example aluminium) which can be specified for applications that are weight-critical yet require significant strength.
It is worth noting that whilst design engineers might be focused on delivering drawings that match the required specifications, it is sometimes left to others to consider another aspect of bearing technology that applies to slew bearings just as much as any other type, and that is sealing and lubrication. For example, slew bearings which are used in aggressive environments, or spacecraft, ultra-high vacuum or extremes of temperature need special seals and lubrication. For run-of-the-mill shop floor applications there are many standard options readily at hand, but for high-end uses in some of the areas described, we have been involved in some fascinating science and manufacturing which has resulted in the bringing to market of some bespoke seals and innovative lubricants that have proved to be genuine game-changers. It is also worth noting that because of the type of defence, aerospace and other high-tech applications which lightweight slew rings are now being specified for, the use by customers of our unique in-house CAD/CAM, FEA and advanced modelling programmes has increased notably.
Accuracy, longevity, lightness and corrosion resistance
Most of the projects on which we work together with customers involves very high-specification bearings with relatively low production runs. Accuracy, longevity, lightness and corrosion resistance are seen as vital attributes and our facilities allow designers to simulate their bearing needs, capture the essential data they require, and work with us to refine the optimum materials and bearing design choices in advance of producing working prototypes. Typical essential parameters to be established when we first discuss an application include rotational speed and flexing loads, platform rigidity and stiffness, vertical play, mounting options, corrosion protection, ingress protection, and smoothness of rotation in addition to achieving the customer-specific declared design performance objectives: most of which usually involve hostile environments of some sort. Establishing the overturning moment load is clearly a paramount consideration from which other key bearing design aspects such as physical size in both diameter and thickness, load capability and materials selection et al must come.
To cater for a diverse range of customer applications, which includes so many things from spacecraft to surveillance cameras, and helicopter seating to hybrid vehicles, slew rings can be specified with rolling elements made from a variety of base materials including ceramics, chromium steel and stainless steels. A range of ‘external’ mounting options is also offered to suit different designs and applications, so in addition to a substantial choice of standard products, slewing rings are available that allow special ancillaries to be attached. For example, monitoring or measuring equipment such as rotary encoders, sensors, position indicators, counters or vibration analysis senders can be readily mounted directly to many special order slew rings.
Typical slewing bearing applications:
- Spacecraft – propulsion systems, radar gimbal bearings
- Medical – CT scanners, cardio vascular machines
- Aerospace – helicopter seat mounts, weapons systems
- Precision machinery – robots, gun turrets, camera and surveillance systems
For more information go to www.carterbearings.co.uk.