MECHANICAL COMPONENTS FEATURE The benefits of being thin


When applications require maximum performance despite space or weight restrictions, serious consideration should be given to thin section bearings. Richard Burgess, Les Miller and David VanLangevelde from Kaydon Bearings Division, explain why


Kaydon bearings are available in the UK from R.A. Rodriguez, www.rarodriguez.co.uk


W


ith thin section bearings, instead of the cross-section increasing with


the bore size, it starts relatively small and stays constant. This produces larger ratios of diameter to radial section that lead to space and weight savings of up to 85%. However, as thin section bearings can do the same job as bigger, heavier, bearings, they are used in countless applications.


BEARING DESIGNS In general, thin section bearings have a radial cross-section that is less than one-fourth their bore diameter and a radial cross-section less than twice the diameter of the rolling-element. Nevertheless, their load capacity is more than adequate for many applications. A variety of standard cross-sections are


available. Kaydon Bearings Division’s Reali-Slim line, for example, is in cross sections from 3/6in to 1in square; with bore diameters from 20mm to 1m, although custom bore diameters can be as large as 1.3m. The Ultra-Slim line are 2.5mm wide with 3mm cross-sections and bores from 35 to 170mm. A number of designs are available: Radial contact bearings – intended


primarily for pure radial loads, but with deep grooves can accept some thrust in either direction. If an increased diametral clearance is specified, the contact angle under axial load increases to give greater thrust capacity. Angular contact bearings – these can support radial and unidirectional thrust


Four-point contact bearings can support any combination of radial, thrust and moment loading


load; and are usually mounted as opposed pairs as they need a thrust load to establish the proper contact angle between ball and races. Four-point contact bearings – can


support any combination of radial, thrust and moment loading. One can often do the job of two angular-contact bearings.


SEPARATORS Radial contact and four-point contact ball bearings typically have snap-in, continuous ring separators, while angular contact bearings use circular-pocket, continuous ring separators. Although brass, low-carbon steel and glass-fibre reinforced nylon are standard materials for these, for high-speed applications or those requiring corrosion resistance, stainless steel or phenolic laminate separators are often used. An alternative when more balls than


usual are needed is formed wire separators or segmental cages; but in low speed positioning applications when torque uniformity is a major requirement, ball separation can be accomplished with helical compression springs. Furthermore, thin section bearings


perform best when the rolling elements are clean and lubricated. This can be accomplished with a seal or non-contacting shield.


CONSIDERATIONS Thin section bearings can be used where bearing cross-section can be specified early in the design process. As well as their weight and space savings, these offer benefits in applications requiring high stiffness, accurate positioning at low shaft speed or a combination of radial, thrust and moment loads. Designers are able to reduce housing sizes without changing shaft size, and use larger shafts to increase stiffness and/or provide space for other components. Large-diameter tubular shafts –


Conventional rotating kingpost configurations that use two bearings and a long shaft can be replaced with more conventional designs using a thin section bearing. Their proportions allow the use


/ DESIGNSOLUTIONS


of large-diameter hollow shafts where large-bore conventional bearings may not fit; and such shafts provide a cavity to route electrical wiring, hydraulic lines, etc. Standardisation – Thin section bearings


provide opportunities for component standardisation when product lines are manufactured in various sizes based on shaft diameter or power requirements. Speed and stiffness – Maximum speeds


tend to be lower than for conventional rolling element bearings because the rings are more flexible; so the bearing support structure must compensate for this. Distortion and deflection – The


flexibility of thin section bearings does not affect their performance, as long as the bearings are properly mounted and the races are uniformally supported. Low rolling element deflections – For


a given bore size, thin section bearings have more rolling elements than conventional bearings, which decreases deflection at the ball-to-raceway contact. When combined with proper constraint, this stiffness is helpful in large diameter bearing applications where limited bearing deflection at maximum load is desirable.


THE RIGHT FIT Changes in external fit can impose significant bearing loads that increase running torque and decrease life. To minimise this, the runout tolerances specified for the housing and shaft should be identical to those for the bearing. In addition, all tolerances – for shaft and housing-bore concentricity, shaft and housing shoulders, and corner squareness of all mounting surfaces – should be specified with care. Since thin section bearings are often


mounted in lightweight structures with thermal expansion rates different from that of bearing steel, attention should be given to temperature differentials between shaft and housing. Steel liners are another way to control fit, and special diametral clearances can be provided to allow for changes in internal clearances. It is a good idea to consult with the


bearing manufacturer when a change in fit creates an interference greater than or equal to the bearing diametral clearance, or a looseness greater than twice the diameter tolerance of the bearing race.


Kaydon Bearings www.kaydonbearings.com Enter 210


DESIGN SOLUTIONS | MARCH 2014 17


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52