1、BRITISH STANDARD CONFIRMED AUGUST 1990 BS 978-1:1968 Incorporating Amendment No. 1 Specification for Fine pitch gears Part 1: Involute Spur and Helical Gears UDC 621.833.1BS 978-1:1968 This British Standard, having been approved by the MechanicalEngineering IndustryStandards Committeeand endorsed by
2、 theChairman of the EngineeringDivisional Council,was published under theauthority of the General Council on 31 January 1968 BSI 10-1999 First published October 1941 First revision January 1952 Second revision November 1962 Third revision January 1968 The following BSI references relate to the work
3、on this standard: Committee reference MEE/17, MEE/17/4 Draft for comment 66/14936 ISBN 580 00028 1 Co-operating organizations The Mechanical Engineering Industry Standards Committee, under whose supervision this British Standard was prepared, consists of representatives from the following Government
4、 departments and scientific and industrial organizations: The Government department and scientific and industrial organizations marked with an asterisk in the above list, together with the following, were directly represented on the committee entrusted with the preparation of this standard: Associat
5、ed Offices Technical Committee Institution of Civil Engineers Association of Consulting Engineers* Institution of Gas Engineers Association of Mining Electrical and Institution of Heating and Ventilating Mechanical Engineers* Engineers Board of Trade Institution of Mechanical Engineers British Chemi
6、cal Plant Manufacturers Institution of Mechanical Engineers Association (Automobile Division) British Compressed Air Society Institution of Production Engineers* British Electrical and Allied Manufacturers Locomotive and Allied Manufacturers Association* Association of Great Britain British Gear Man
7、ufacturers Association* London Transport Board British Internal Combustion Engine Machine Tool Trades Association* Manufacturers Association Ministry of Defence British Iron and Steel Federation Ministry of Labour (H.M. Factory Inspectorate) British Mechanical Engineering Federation Ministry of Powe
8、r British Pump Manufacturers Association Ministry of Public Building and Works Crown Agents for Oversea Governments and Ministry of Technology National Administrations Engineering Laboratory* Electricity Council, the Central Electricity Ministry of Transport Generating Board and the Area Boards Nati
9、onal Coal Board in England and Wales National Physical Laboratory (Ministry of Engineering Equipment Users Association Technology) Gas Council Radio Industry Council Institute of Marine Engineers* Royal Institute of British Architects British Horological Institute Ministry of Technology Electronic E
10、ngineering Association Navy and Vickers Gearing Research Institute of British Foundrymen Association Institution of Engineering Inspection North-East Coast Institution of Engineers Institution of Engineers and Shipbuilders in and Shipbuilders Scotland Society of Motor Manufacturers and Traders Milli
11、ng Cutter and Reamer Association Ltd. Ministry of Defence, Army Department University of Sheffield Ministry of Defence, Navy Department Manufacturers of fine pitch gears Amendments issued since publication Amd. No. Date of issue Comments 3305 May 1980 Indicated by a sideline in the marginBS 978-1:19
12、68 BSI 10-1999 i Contents Page Co-operating organizations Inside front cover Foreword iii 1 General 1.1 Scope 1 1.2 Terminology and notation 1 1.3 Preferred range of normal diametral pitches 2 1.4 Basic rack tooth profile 2 2 Design details 2.1 Outside diameter and centre distance 3 2.2 Modification
13、 of addendum and centre distance 3 3 Accuracy of gear blanks and gears 3.1 General 13 3.2 Dual flank composite testing 14 3.3 Limits of tolerance on blanks and gear elements 15 4 Limits and tolerances of assembly 4.1 Centre distance 17 4.2 Depth of engagement 18 5 Information to be given on drawings
14、 Appendix A Centre distance and addendum modification 21 Appendix B Examples of calculations 23 Appendix C Comparable diametral pitches and metric modules 29 Appendix D Single flank composite testing: Limits of tolerance 29 Appendix E Calculation of measuring pin dimensions for external and internal
15、spur gears 29 Figure 1 Basic rack tooth profile for unit normal diametral pitch 2 Figure 2 Basic rack tooth profile for unit normal diametral pitch with backlashallowance 2 Figure 3 Limiting values of addendum modification coefficient 5 Figure 4 Minimum permissible addendum modification coefficient
16、for helicalpinions (! n= 20 ) 6 Figure 5 Tolerance zone for profile error 15 Figure 6 Example of information to be given on drawings 20 Figure 7 Chart for normal backlash when x 1+ x 2= y 28 Table 1 Normal diametral pitch P n(inch units) 2 Table 2 Data for spur pinions of unit diametral pitch having
17、 fewer than 17 teeth 4 Table 3 Centre distances for spur gear pairs of unit diametral pitch for theconditions given in clause2.2.4 7 Table 4 Forces appropriate to the pitch suitable for brass and steel productgears 14 Table 5 Limits of tolerance 14 Table 6 Limits of tolerance on gear blanks 15 Table
18、 7 Limits of tolerance on tooth profile 16 Table 8 Limits of tolerance on pitch 16 Table 9 Limits of tolerance on tooth alignment 16BS 978-1:1968 ii BSI 10-1999 Page Table 10 Limits of tolerance on radial run-out of teeth 17 Table 11 Limits of tolerance on tooth thickness 17 Table 12 Limits of toler
19、ance on centre distance of mounting 17 Table 13 Minimum depth of engagement for spur gear pairs with 28 to 33 teeth (z 2+ z 1 ) 18 Table 14 Minimum depth of engagement for spur gear pairs with more than33 teeth (z 2+ z 1 ) 18BS978-1:1968 BSI 10-1999 iii Foreword This standard makes reference to the
20、following British Standards: BS 978, Fine pitch gears. BS 978-2, Cycloidal type gears. BS 978-PD 3376, Addendum No. 1 to BS 978-2, Double circular arc type gears. BS 978-3, Bevel gears. BS 978-5, Hobs and cutters. First published in1952 and revised in1962, this British Standard has been further revi
21、sed under the authority of the Mechanical Engineering Industry Standards Committee. In the1962 edition, dual flank composite testing was introduced in the form of an appendix to the specification. As this method of test became more widespread, users of the standard were becoming confused between the
22、 application of the composite tolerances and application of the individual elemental tolerances. The prime reason for revision, therefore, has been to give dual flank testing the importance it now deserves by placing it in the mandatory part of the standard as the main accuracy testing procedure. Al
23、so care has been taken to ensure that the user is not misled by attempting to reconcile composite errors with elemental errors. It is necessary to retain the elemental tolerances because these will continue to be used in some fields as acceptance criteria and because they are essential as the basis
24、for manufacturing gears and cutting tools 1) . Recognizing also that single flank testing is used reference to this is made in an appendix. Opportunity has been taken to effect improvements in presentation and in the examples of calculations, and the terminology and notation is in accordance with th
25、at agreed within the International Organization for Standardization (ISO). This standard is a companion work to BS978, “Fine pitch gears”, Part2,“Cycloidal type gears” (including addendum PD3376, “Double circular arc type gears”), Part 3, “Bevel gears”, and Part 5, “Hobs and cutters”. A British Stan
26、dard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligations. Summary of pages This document comprises a front cover,
27、 an inside front cover, pages i to iv, pages 1 to 29 and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover. 1) BS978, “Fine pitch gears”, Part5, “Hobs and cutters”.iv bla
28、nkBS 978-1:1968 BSI 10-1999 1 1 General 1.1 Scope This British Standard relates to involute spur, helical and crossed helical gears, having diametral pitches finer than20, in four accuracy grades. Examples of application are: Class A. Astronomical apparatus and scientific instruments; colour printin
29、g machinery; control systems (when multiple trains of gears are involved this grade would generally be used for low speed components where the effect of positional accuracy is important). Class B. Navigational instruments; high speed components of control systems where quietness and smooth running a
30、re essential; machine tool speed control; photographic instruments. Class C. General purpose instruments; accounting and calculating machinery; counters; clockwork mechanisms. Class D. Textile machinery; switch operating gear; clockwork mechanisms where accuracy is not the primary consideration. It
31、is recommended that all spur, helical and crossed helical gears for use in horology should be produced to this British Standard with the possible exception of gears for weight or spring-driven mechanisms in which the pinion is the driven member and has12 or fewer teeth. In such cases it may be prefe
32、rable to use gears conforming to BS978-2 2) . 1.2 Terminology and notation As the first step towards achieving international understanding of gears terminology, the terms and definitions of Draft ISO Recommendation No.888 3) , “International vocabulary of gears”, have been applied, together with cor
33、responding notation derived from Draft ISO Recommendation No.889, “International gear notation, symbols for geometrical data” 3) . The notation used is as follows: 2) BS 978, “Fine pitch gears”, Part2, “Cycloidal type spur gears”. 3) To be incorporated in BS2519, “Glossary of terms for toothed geari
34、ng”. a Centre distance a v Virtual centre distance d Reference circle diameter d 1 Reference circle diameter: pinion d 2 Reference circle diameter: wheel d a Tip diameter d a1 Tip diameter: pinion d a2 Tip diameter: wheel d R Measuring pin diameter d b Base circle diameter j n Normal backlash j a Co
35、efficient for calculation of backlash l Length of arc P n Normal diametral pitch s Arc tooth thickness s 1 Arc tooth thickness: pinion s 2 Arc tooth thickness: wheel x Addendum modification coefficient x 1 Addendum modification coefficient: pinion x 2 Addendum modification coefficient: wheel %x 1 Se
36、condary addendum modification coefficient: pinion %x 2 Secondary addendum modification coefficient: wheel y Centre distance modification coefficientBS 978-1:1968 2 BSI 10-1999 1.3 Preferred range of normal diametral pitches The pitch shall be selected from the following preferred range. Table 1 Norm
37、al diametral pitch P n(inch units) 1.4 Basic rack tooth profile The shape and proportions of the basic rack tooth for spur helical and crossed helical gears shall be as shown either inFigure 1 orFigure 2. In the case of helical and crossed helical gears, the shape and proportions of the tooth are on
38、 a section at right angles to the pitch cylinder helix. Figure 1 shows the basic rack usually employed where it is desired to have minimum backlash, as in servomechanisms. z Number of teeth z 1 Number of teeth: pinion z 2 Number of teeth: wheel z v Virtual number of teeth ! n Normal pressure angle !
39、 nw Normal pressure angle, operating ! t Transverse pressure angle ! tw Transverse pressure angle, operating “ Helix angle at reference cylinder “ b Helix angle at base cylinder a BS 978, “Fine pitch gears”, Part 5, “Hobs and cutters”. 32 48 100 160 24 36 64 120 180 28 40 80 140 200 Figure 1 Basic r
40、ack tooth profile for unit normal diametral pitch Figure 2 Basic rack tooth profile for unit normal diametral pitch with backlash allowanceBS 978-1:1968 BSI 10-1999 3 Figure 2 is used where it is desired to have backlash to avoid any risk of jamming and where the load is usually uni-directional, for
41、 example, in clockwork mechanisms. When non-topping hobs are used it is permissible to use a hob conforming toFigure 1 basic rack and to cut at greater depth than normal to obtain the required tooth thickness. For internal gears (Figure 1 andFigure 2 basic racks) the addendum shall normally be0.850
42、instead of1.000. In cases when the tips of the teeth have a radius of0.25 or greater (up to0.3max. as shown) the1.000 addendum may be applied. 2 Design details 2.1 Outside diameter and centre distance For the normal case when both z 1sec 3 “ and z 2sec 3 “ are greater than17 the dimensions are as fo
43、llows: 2.2 Modification of addendum and centre distance 2.2.1 For an explanation of the principles followed in connection with addendum modification seeAppendix A. 2.2.2 Where no limitations are imposed, the value of x for any gear whose virtual number of teeth z vis less than17 is given by the form
44、ulae: This gives values in accordance with the lower limiting line XX inFigure 3 and a small and normally negligible amount of undercutting is thereby permitted. The addendum modification coefficient for helical pinions from6 to16 teeth and helix angles up to45 is shown inFigure 4. 2.2.3 When the pi
45、nion is modified as above and there are no other limitations, the wheel is unmodified and its dimensions are obtained from the formulae in2.1; but the dimensions of the pinion and centre distance are as follows: Parallel axis external gear pairs designed as above have backlash at this centre distanc
46、e. (SeeAppendix A.) (1) (2) (3) (4) (5) (For spur gears, sec“ = 1) (SeeTable 2 andFigure 3 andFigure 4) (6) where z v= z sec“ sec 2 “ b. z sec 3 “ (7) (8) (Figure 1 type gears) (9) (Figure 2 type gears) (10) (11) x 1 z vsin 2 2 - n =BS 978-1:1968 4 BSI 10-1999 Table 2 Data for spur pinions of unit d
47、iametral pitch having fewer than 17 teeth 2.2.4 Enlarged spur pinions with minimum backlash. Spur pinions enlarged according toTable 2 may be engaged with wheels in which x 2is zero, at centre distance derived fromTable 3. Gears derived fromFigure 1 basic rack will have no backlash at this centre di
48、stance. A small and negligible reduction of tip-to-root clearance is obtained at this centre distance. Table 2 is not applicable to helical gears, for which the formulae in Appendix A are required. Alternatively, spur pinions enlarged according toTable 2 or helical pinions enlarged according toFigur
49、e 4 may be engaged with wheels in which x 2= x 1 , in which case the centre distance is as obtained in2.1, and Such gears derived fromFigure 1 basic rack will have zero backlash at nominal dimensions. No. of teeth z 1 Addendum modification coefficient x 1 Tip diameter d a1 Increase in circular tooth thickness at standard pitch dia. Recommended minimum number of teeth in mating wheel z 2 8 a 9 a 10 0.5321 0.4736 0.4151 11.0642 11.9472 12.8302 0.3874 0.3448 0.3022 26 25 24 11 12 13 0.3566
