1、BRITISH STANDARD CONFIRMED AUGUST 1990 BS 978-3:1952 Incorporating Amendment No. 1 Gears for instruments and clockwork mechanisms Part 3: Bevel gears UDC 621.833:681-2BS978-3:1952 This British Standard, having been approved by the Mechanical Engineering Industry Standards Committee and endorsed by t
2、he Chairman of the Engineering Divisional Council, was publishedunder the authority ofthe General Council on 24January1952 BSI 12-1999 First published October 1941 First revision January 1952 ISBN 0 580 34085 6 Co-operating organizations The Mechanical Engineering Industry Standards Committee under
3、whose supervision this British Standard was prepared consists of representatives from the following Government departments and scientific and industrial organizations: Admiralty* Institute of Petroleum Air Ministry Institution of Civil Engineers Associated Offices Technical Committee Institution of
4、Gas Engineers Association of Consulting Engineers Institution of Heating and Ventilating (Incorporated)* Engineers British Chemical Plant Manufacturers Institution of Mechanical Engineers* Association Institution of Mechanical Engineers British Compressed Air Society (Automobile Division) British El
5、ectrical and Allied Manufacturers Institution of Production Engineers* Association* Locomotive Manufacturers Association British Engineers Association* Machine Tool Trades Association* British Internal Combustion Engine Ministry of Fuel and Power Manufacturers Association Ministry of Labour and Nati
6、onal Service British Iron and Steel Federation (Factory Department) British Railways, The Railway Executive* Ministry of Supply* Crown Agents for the Colonies Ministry of Transport Department of Scientific and Industrial Ministry of Works Research Office of the High Commissioner for India Engineerin
7、g Equipment Users Association War Office Institute of Marine Engineers The Government departments and the 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: Assoc
8、iation of Mining Electrical and National Association of Drop Forgers Mechanical Engineers and Stampers British Clock and Watch Manufacturers National Association of Lift Makers Association National Federation of Engineers Tool British Gear Manufacturers Association Manufacturers British Horological
9、Institute National Physical Laboratory Institution of Engineering Inspection North East Coast Institution of Engineers Manufacturers of Instrument Gears and Shipbuilders Milling Cutter and Reamer Trade Association Society of Motor Manufacturers and Traders Amendments issued since publication Amd. No
10、. Date Comments 2408 March 1956 Indicated by a sideline in the marginBS978-3:1952 BSI 12-1999 i Contents Page Co-operating organizations Inside front cover Foreword ii Section 1. General 1 Scope 1 2 Terminology and notation 1 Section 2. Form of gear tooth 3 Form of gear tooth 2 4 Tooth spiral 3 5 Ad
11、dendum and dedendum 3 6 Caliper settings 3 Section 3. Permissible errors and tolerances 7 Pitch errors 4 8 Tooth thickness tolerance 4 Appendix Notation 5 Chart 1 Interference factor (Q) for bevel gears 6 Figure 1 Straight bevel gear 1 Figure 2 Right-hand bevel gear 1 Figure 3 Left-hand spiral bevel
12、 gear 1 Figure 4 Left-hand spiral bevel gear 2 Figure 5 Basic rack tooth shape (for unit normal pitch) 2BS978-3:1952 ii BSI 12-1999 Foreword This standard makes reference to the following British Standards: BS 545, Bevel gears (machine cut). BS 721, Machine cut gears. C. Worm gearing. BS 2519, Gloss
13、ary of terms and notation used with gearing. This British Standard supersedes BS978, “Gears for clockwork mechanisms,” which was published in1941 as a war emergency measure to assist in the reduction of types and sizes of both gears and cutters. As a result of experience gained in its use it became
14、evident that a revision was necessary. It has been considered desirable to divide the standard into four separate parts. These are: Part 1: Involute spur, helical and crossed helical gears; Part 2: Cycloidal type spur gears; Part 3: Bevel gears; Part 4 Worm gears. The preparation of Part4 will be un
15、dertaken when the revision of BS721, “Machine cut gears. C. Worm gearing” has been completed. Suitable hobs and cutters will be dealt with in a later British Standard. It should be noted that this standard is similar to BS545, “Bevel gears (machine cut)”, that standard having been used as the basis
16、for the preparation of this one. A British Standard 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
17、of pages This document comprises a front cover, an inside front cover, pages i and ii, pages1 to 6 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.BS978-3:1952 BSI
18、12-1999 1 Section 1. General 1 Scope This part applies to machine cut conical gears, of the classes described below, connecting intersecting shafts and having pitches from20to80DP. The teeth shall have a normal pressure angle of20 at the pitch cone and they may be straight or curved (seeFigure 1 toF
19、igure 4). The standard provides for ClassA gears for use where a high degree of accuracy is required, and ClassB gears for other applications. For integrating electricity meters the particular form of gear herein specified is not necessary and it is recognised that other existing forms of gear will
20、continue to be used. 2 Terminology and notation The terms and notation shall be as defined inBS2519 “Glossary of terms and notation used with gearing.” For the convenience of users an abbreviated list of symbols is given in theAppendix. Figure 1 Straight bevel gear Figure 2 Right-hand spiral bevel g
21、ear Figure 3 Left-hand spiral bevel gearBS978-3:1952 2 BSI 12-1999 Section 2. Form of gear tooth 3 Form of gear tooth The normal section of the British Standard basic rack tooth for bevel gears shall be as shown inFigure 5, and corresponds to the developed section of the outer ends of the teeth of t
22、he crown gear. The tooth flank is straight, with20 pressure angle, except that a slight easing of the tip is permissible. The amount of this easing shall not exceed that shown inFigure 5. A cutting tool of form counterpart to a single flank and root fillet of the basic rack may be used to generate s
23、ingle flanks and root fillets of bevel gear teeth of British Standard form. In general, neither the amount of the tip easing, nor the fillet radius on the bevel gear tooth will be a constant multiple of the pitch at all distances from the apex. The root of the tooth shall normally lie on a cone whic
24、h has its apex coincident with the pitch cone apex, but to suit methods of manufacture, it may be made to lie on another cone which is co-axial with the gear but whose apex is not coincident with the pitch cone apex. The tip cone angle shall be made so that the tip of the tooth lies parallel to the
25、root of the mating gear, thus producing a constant bottom clearance along the tooth. Figure 4 Left-hand spiral bevel gear Figure 5 Basic rack tooth shape (for unit normal pitch)BS978-3:1952 BSI 12-1999 3 It is recommended that the face width should not normally exceed either one-third of the cone di
26、stance or three times the pitch. 4 Tooth spiral The form of tooth spiral is not standardized as it is dependent on the type of cutter and the method of cutting employed. For this reason the spiral angle at different positions along the face is not specified. 5 Addendum and dedendum The addendum shal
27、l be determined by: where k p , the addendum coefficient for the pinion is equal to: orwhichever is the greater andk w =k p The value of Q is obtained fromChart 1 attached. NOTE 1If the shafts are at right angles: The dedendum of the tooth shall be equal to the difference between the addendum as det
28、ermined above and the whole depth of tooth. Normallyk w =k p . Where the number of teeth in the two gears is small, i.e.,whent sec p +T sec wis less than Q, interference or undercutting will be present and the use of such gears should be avoided. If t is less than the addendum and dedendum shall eac
29、h be reduced by0.40.0344t sec 2 2sec p(for unit normal pitch) unless this quantity exceeds0.065, in which case the gears are outside the scope of this standard. Example. To find the addendum of a pair of spiral bevel gears connecting perpendicular shafts and having the following particulars: T = 45;
30、 t = 10; n =20 ; 2 =30 ; P n =50 tan p =and so p =12 32 k pis equal to the greater of the values: i.e.,k =0.38. Addendum of pinion=(1+0.38)=0.0276in. Addendum of wheel=(10.38)=0.0124in. 6 Caliper settings Caliper settings may be obtained as follows: Then: Caliper thickness setting for wheel,G Calipe
31、r thickness setting for pinion, g Caliper height setting for wheel,H Caliper height setting for pinion, h 10 45 - 1 50 - 1 50 -BS978-3:1952 4 BSI 12-1999 Section 3. Permissible errors and tolerances 7 Pitch errors The pitch errors shall not exceed values in accordance with the formula given below, e
32、xpressed in thousandths of an inch. L a= Length in inches of arc of pitch circle over which the error is measured; this shall not exceed half the circumference. NOTEThe results of the measurements are expressed with reference to the working axis of rotation and the permissible errors given include t
33、he effects of eccentricity. 8 Tooth thickness tolerance The thickness of the gear teeth shall, in general, be less than the nominal thickness (G for the wheel or g for the pinion). The tolerances shall be in accordance with the formula given below, expressed in thousandths of an inch. Class of gear
34、Maximum permissible pitch error A B Class of gear Tolerance A BBS978-3:1952 BSI 12-1999 5 Appendix Notation A Wheel addendum. a Pinion addendum. C Cone distance. D Wheel pitch diameter. d Pinion pitch diameter. F Face width. G Caliper thickness setting for wheel. g Caliper thickness setting for pini
35、on. H Caliper height setting for wheel. h Caliper height setting for pinion. k Addendum coefficient P Diametral pitch. P n Normal diametral pitch. P t Transverse pitch or circular pitch. P n Normal pitch. Q Interference factor. T Number of teeth in wheel. t Number of teeth in pinion. (gamma) Pitch a
36、ngle. 2(sigma) Spiral angle at pitch circle. n(psi) Normal pressure angle.BS978-3:1952 6 BSI 12-1999 Chart 1 Interference factor (Q) for bevel gears7 blankBS 978-3:1952 BSI 389 Chiswick High Road London W4 4AL BSIBritishStandardsInstitution BSI is the independent national body responsible for prepar
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