Thin-section bearings can provide design engineers with significant advantages when space is tight, tolerances are extreme and weight-saving is also a defining attribute. Here, Carter Manufacturing’s Mike Page explains what thin-section bearings are and what they can do.
Thin-section bearings offer design engineers a unique combination of advantages in a single hard-wearing and lightweight package. As the name implies, they are significantly thinner in section than ‘ordinary’ rotating bearings and possess attributes that make them very different from ordinary radial bearings. One of the most important (and useful) of these is that whereas most radial bearings are designed so that the bearing thickness increases proportionately as the bore size gets bigger, in the case of thin-section bearings, the thickness remains fixed even though the bore size increases. Using the Silverthin range as a benchmark example, here we see the cross-sectional measurement fixed in 12 different thicknesses ranging from 4.75mm to 25.4mm, with bore sizes available from 25.4mm right up to more than 1000mm, providing a useful matrix of options to designers.
Generally, thin-section bearings are specified for use in critical applications and environments where space is tight, weight must be minimised and absolute precision must be guaranteed. For example, radar platforms, robotic arms, medical scanners, gimbal mounts and missile tracking systems all make extensive use of thin-section bearings. In these types of application, thin-section bearings have helped to reduce the total cost in a system by allowing for design efficiency over standard bearing sizes, with the added advantage of using just a single bearing which shows little change in its weight even as its diameter increases. Speaking of weight saving, the advantages offered by thin-section bearings in this respect can be substantial. For example, on a bearing with a bore of 100mm, a thin-section bearing might weigh 1kg less than a standard option: and on a larger bearing with a bore of around 900mm the weight saving could be in the region of 400kg. Serious numbers. When designing with thin-section bearings, engineers should always remain aware that they can be affected by the tolerance, roundness, and expansion and contraction rates of the housing and shafts used to mount them. The range of temperatures in which thin-section bearings will typically operate is –52degC to +120degC.
Thin-section bearings made by Silverthin can be manufactured in a variety of materials, with 52100 vacuum-gassed Chrome steel and 440C martensitic stainless steel being amongst the most popularly specified, although 17-4PH and M50 tool steel can also be specified. As with many other types of bearing, thin-section bearings can also be specified with a variety of specialist coatings if required; for example, thin-dense chrome which helps to extend service life, delivers even smoother running and assists in combating corrosion. In the Silverthin case, the company has its own coating which increases Rockwell hardness from a typical 58-62 up to a new high of 71. As a result, material thus coated provides even better corrosion resistance than standard 440C steel can deliver. As an added advantage for applications demanding increased durability, seals manufactured from Buna-N rubber/metal shields are also available. Designers can choose between three definitive types of thin-section bearing, dependent upon their application requirements, with each of them delivering a different set of advantages.
Radial contact by the balls may be preferred for some uses, whereas the alternatives of either angular or four-point contact might be preferred for others. Likewise, designers can make choices regarding ball types (including ceramic balls) together with different cage designs, alternative materials options and a variety of lubrication choices. On some thin-section bearings, special seals and/or retainers can be manufactured too. The combination of thin-section size preferences, the various materials choices, finishing processes and roller-contact types and materials are all vital elements in early discussions on the sort of projects in which these bearings excel.
As with all bearings, lubrication is an essential element that should be considered in advance, and OEMs invariably choose one of the three lubricant types which are most commonly used: oil, grease, and dry film or surface treatment. Oil normally provides better lubrication because it covers critical surfaces more thoroughly and dissipates heat more rapidly, whereas grease is clearly more easily retained, allowing the use of simplified bearing housings and seals. It should be noted that in addition to ‘standard’ lubricating oils and greases, thin-section bearing manufacturers such as Silverthin can also offer vacuum-compatible oils and greases, as well as dry film lubricants alluded to previously, including graphite, tungsten disulphide, silver, lead, PTFE and molybdenum disulphide to assure lengthy and trouble-free thin-section bearing service life.
Just a moment – coping with undesirable loads
Unwanted moment loads can be the design engineer’s enemy, and in the combating of unwanted axial or radial loads, thin-section bearings have been proven to excel, partly because they are designed from the outset to cope with it. Project designs which are expected to generate significant moment loads but which also have critical space and weight-saving requirements (to say nothing of the need for cost reduction) often see the maximum benefit from the use of a single thin-section bearing rather than paired standard bearings. This is because the thin section design delivers inherent resistance to moment load, so the lifespan of a critical bearing can be enhanced if a thin-section type is deemed suitable.
Overall, designers have more than 300 sizes and types of thin-section bearings to choose from. To help designers ensure that the correct types and versions of bearings are specified, those available from Silverthin come in three different designs, each of which offers its own distinct advantages. The C-Type is a “Radial Contact” type of conventional thin-section bearing design. It has a single row of balls which allows for Conrad-type assembly, widely recognised for its enhanced capabilities in resisting radial loads whilst withstanding moderate axial and reversing loads. Overall, this type is the most popularly specified type of thin-section bearing. An alternative thin-section bearing design is what is generically known as the A-Type, where A represents the “Angular” contact type of bearing. This thin-section bearing features a reduced shoulder on one side of the inner or outer race ball path to assist the bearing in accommodating both radial and single-direction thrust loads. The A-Type of bearing requires an application of thrust to establish the appropriate race and ball contact angle, and is commonly used in a pair for applications requiring axial loads in one direction, or perhaps for reversing axial loads. They provide the appropriate stability, load capacity and repeatability needed in applications running at higher speeds than the C-Type of bearing, and are capable of coping with high rpm and temperatures in continuous operation. Speed and load capability is based on dimensions, and Carter Bearings offers users the facility of its own predictive simulation software that allows engineers access to critical speed, load and temperature data for calculations at the design stage.
A further type of thin-section design is also available. The X-Type is a “4-Point Contact” bearing designed for use where a high level of rigidity, i.e. resistance to moment arm deflection or flexing, is required. This type of thin-section bearing can accept radial and thrust loads in either direction and has proven to be effective in applications where very high moment loads are present. X-Type bearings are designed with “gothic arch”-shaped raceways which create 4 contact points between each ball and the raceway. Preloading can provide additional precision and rigidity of movement if desired, and in some applications a single X-Type bearing may be used replace two standard thin-section bearings to deliver the joint advantage of the required performance coupled with a very low installed deck height. It should be noted, however, that X-Type thin-section bearings are not always the best option when used in low-torque or high-speed applications.
Further information about thin-section bearings can be found at www.carterbearings.co.uk.