Slewing Rings Offer Critical Design & Manufacturing Advantages

Why are Lightweight slewing rings, (bearings) rapidly becoming the first choice for design engineers in demanding motion control applications, such as those occurring in the aerospace and defense industries?  Quite simply, where absolute accuracy, optimum mechanical performance and good corrosion resistance is essential, lightweight slewing rings have proven ability in the most challenging and hi-tech applications. Karl Brundell, MD of Abingdon based precision bearing specialists Carter Manufacturing, examines the reasons why in more detail.

Sometimes it’s the numbers that tell the story. For example, we have recently seen one of our lightweight, heavy duty aluminum slewing bearings specified for turret mounting in a defense related application. The turret diameter measures over 1 meter 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 slewing bearing applications in which Carter have been involved with include; propulsion systems and radar gimbal bearings for Spacecraft, CT scanners and cardio vascular machines for Medical applications, helicopter seat mounts, weapon systems for Aerospace and other precision military systems including robots, gun turrets, camera and surveillance systems.

Innovative Design = Exceptional Accuracy

Whilst there are many slewing rings available for less demanding motion control applications, the choices for use in high technology applications are limited. Examples of these include AFV's (armored fighting vehicles), satellites, ultra-high vacuum environments or military specification tracking systems which 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 which enables the clearance or preload settings to be set in the design stage of the application. The important benefits of this are, a substantial reduction in costs by avoiding the need to request special preloads to be set into the bearing on factory assembly and as we use stainless steel components in parts of the design, corrosion resistance is significantly improved.

A slewing bearing is designed so that the rolling elements create a reactive movement within the  bearing when it is under load, which opposes the existent overrunning (or overturning) loads which are present in the motion control application. This has proved critical to designers of military land, sea and aerial equipment where smaller size makes vessels, vehicles and UAV's are 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 can be carried out on the majority of slew bearings in most sizes and load ratings and has proved pivotal in achieving savings, not just in initial cost but also by removing unnecessary complexity. The slewing bearings supplied by Carter Manufacturing 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

Although available in a variety of types and styles, the most commonly specified slewing bearings are either plain, flanged, or geared. In most applications, 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. These are all important design features, especially for high-accuracy applications such as radars, target trackers or AFV turrets. Alternative mounting methods can also be utilised if required, dependent upon the lateral or longitudinal design load parameters that have been set or are expected. As 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 defense, aerospace, cryogenic or nuclear industries (as examples) provides a variety of different internal and external styles. When 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 means that the possibilities are considerable and allows Carter 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.

Material choice and the importance of Sealing and Lubrication

There are some interesting materials choices available for ultra-high vacuum applications including aluminium, which can be specified 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.

For example, slew bearings used in aggressive environments, on spacecraft, ultra-high vacuum or extremes of temperature applications need special seals and lubrication. For general manufacturing applications there are many standard options to choose from, but for high-end uses in some of the areas described, Carter have been involved in some fascinating science and manufacturing projects. This 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, aerospace and other high-tech applications where lightweight slew rings are now being specified for, has highlighted the benefits of Carter’s in-house and in-demand CAD/CAM,FEA facility and advanced modelling programs.

Accuracy, longevity, lightness and corrosion resistance

Most of Carter’s projects involve 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. The essential parameters to be established when we first discuss an application include rotational speed, flexing loads, platform rigidity and stiffness, vertical play, mounting options, corrosion protection, ingress protection, and smoothness of rotation. Also, there is the need to achieve the customer-specific declared design performance objectives, most of which usually involve hostile environments of some sort. Establishing the overturning moment load is clearly paramount as are the other key bearing design aspects such as, physical size in both diameter and thickness, load capability and materials selection.

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 mounted directly too many special order slew rings.

More at:  https://www.carterbearings.co.uk/bearings/lightweight-slewing-rings

Tel: +44 (0) 1865 821 720   email: sales@carterbearings.co.uk     ENDS.

Contact: Karl Brundell, Carter Manufacturing Ltd, Unit 7 Isis Court, Abingdon Business Park,

Oxon OX14 1DZ.