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This material is copyright - all rights reserved. Reproduced under licence from The Steel Construction Institute on 12/2/2007 To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ Steel Designers' Manual - 6th Edition (2003) Bearings 845 able. In these instances the engineer has the option of designing the units using available literature (see references to Chapter 28) and perhaps incorporating standard bearings from a manufacturer as components of a completed assembly or alternatively engaging a recognized manufacturer to design and produce the item as a special bearing. For straightforward applications such as may be required on a short single-span bridge, it may be worthwhile investigating the relative costs of a simple fabricated bearing compared with the equivalent proprietary unit. Bearings (particularly ‘special’ bearings) can prove to be a large item of expenditure in a structure and an estimate of the costs involved should be made early in the design stage. (10) Summary of design requirements. Before selecting a particular bearing it is suggested that a summary of all relevant parameters is prepared. This can then be used if necessary for submission to the bearing manufacturers for examination and recommendations as to particular bearing types. A typical format for such a sheet is given in Table 9 of BS 5400: Section 9.1. 2 28.2.1.2 Materials Generally materials fall into three groups: (1) those able to withstand high localized contact pressures e.g. steel. (2) those able to withstand lower contact pressures but having a low coefficient of friction; these slide easily in a direction perpendicular to the direction of the pressure and thus accommodate translational movement, e.g. polytetrafluoroethylene (PTFE). (3) those able to withstand contact pressure and also to accommodate translational or rotational movements by deformation of the material (e.g. elastomers). Certain of these materials may be confined within a steel cylinder in order to increase their compressive resistance. (a) Mild or high-yield steel The coefficient of friction of steel on steel is of the order of 0.3 to 0.5, unless continuously lubricated; in order to provide for movement alternative arrangements are usually necessary. Traditionally this has been through the use of single or multiple rollers or knuckles. Rollers will permit translation in one direction and, if a single roller is used, rotation about an axis perpendicular to that direction. Knuckles permit rotation about one axis only. Rotation in two directions may be achieved using spherical-shaped bearing surfaces. The allowable pressures between surfaces for steel on steel contact depend upon the radii of the two surfaces and the hardness and ultimate tensile strength of the material used. BS 5400: Section 9.1 2 gives expressions for design load effects in such
This material is copyright - all rights reserved. Reproduced under licence from The Steel Construction Institute on 12/2/2007 To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/ Steel Designers' Manual - 6th Edition (2003) 846 Bearings and joints cases. As load-carrying requirements increase, the use of steels with greater hardness is dictated. This can be achieved by use of high-grade alloy steels of various compositions. For design purposes, Table 2 of BS 5400: Section 9.1 2 gives indicative values of coefficients of friction of between 0.01 and 0.05 for steel roller bearings. (b) Stainless steel Stainless steel is frequently used in strip or plate form to provide a smooth path for sliding surfaces. It is important to utilize a material for the sliding surface which will not deteriorate and adversely affect the coefficient of friction assumed for design of the structure.A typical arrangement is a polished austenitic stainless steel surface sliding against dimpled PTFE. (c) Polytetrafluoroethylene (PTFE) PTFE has good chemical resistance and very low coefficients of static and dynamic friction. Unfortunately, pure PTFE has a low compressive strength, high thermal expansion and very low thermal conductivity.As a consequence it is frequently used in conjunction with ‘filler’ materials which improve these detrimental effects without significantly affecting the coefficient of friction. The coefficient of friction varies with the bearing stress acting upon it. BS 5400: Section 9.1 2 gives the relationship shown in Fig. 28.1 for continuously lubricated pure PTFE sliding on stainless steel. Lubrication of the pure PTFE is commonly achieved by means of silicone grease confined in dimples which are rolled on to the surface of the material. References 2 and 3 give further guidance on the restrictions on shape, thickness and containment on the PTFE and stainless steel components. In preliminary design and assessment of forces on structures using PTFE sliding coefficient of friction 0.08 0.06 0.04 0.02 note! (i) if pure ptfe used unlubricated, use 2 x coefficient of friction shown (ii) if filled ptfe used, use 4 x values I I I I I I 5 10 15 20 25 30 bearing stress (N/mm2) Fig. 28.1 Coefficient of friction for continuously lubricated pure PTFE
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