GB/T 28250-2012The types and dimensions of timing belt mold hobs (English PDF)

2 Normative references

The following documents are indispensable for the application of this document. For dated references only the edition cited applies; for undated references the latest edition, including any amendments, applies.

The documents listed are GB/T 6132, Interchange dimensions of milling cutters and milling cutter arbors; GB/T 11616, Dimensions of synchronous belts; and GB/T 28251, Belt pulley hobs and belt mold hobs - Technical specifications.

3 Types and dimensions

3.1 The type and the dimensions of the hob are as laid down in Figure 1 and Table 1. Figure 1 shows the hob in section and in end view, with the outside diameter de, the overall length L, the bore diameter D, the dimension a at each end of the toothed part and the flutes marked Z.

3.2 The hob is used for machining belt molds with a straight-sided tooth profile.

3.3 The hob is made single start and right hand, with chip flutes that are straight and parallel to its axis.

3.4 The rake angle of the hob is 0°.

Table 1, in millimetres, has seven columns: pitch Pb, belt type, de, L, D, a and number of flutes Z. It gives five sizes: pitch 5.080 for belt type XL, with de 50, L 50 and D 22; pitch 9.525 for type L, with de 63, L 71 and D 27; pitch 12.700 for type H, with de 80, L 90 and D 32; pitch 22.225 for type XH, with de 118, L 125 and D 40; and pitch 31.750 for type XXH, with de 150, L 170 and D 50. The value of a is 5 for all five sizes, its cell being merged over the whole table. The number of flutes Z is 14 for the first four sizes and 12 for the largest.

3.5 The axial tooth form dimensions of the hob are as laid down in Annex A, and the calculated dimensions of the hob are as laid down in Annex B.

3.6 The dimensions and deviations of the keyway are as laid down in GB/T 6132.

3.7 Designation example: a belt mold hob of pitch Pb = 12.700 mm is designated belt mold hob 12.700 GB/T 28250-2012.

3.8 The technical requirements for the hob are as laid down in GB/T 28251.

A Annex A (normative) Tooth form dimensions of the belt mold hob

A.1 states that the axial tooth form dimensions of the hob are as laid down in Figure A.1 and Table A.1. Figure A.1 shows one tooth of the profile as a curve symmetrical about the Y axis, with the points A, B, C, D, E and F marked in that order from the crest outwards on each side, on an X and Y coordinate system whose origin A is at the crest.

Table A.1, in millimetres, gives for each pitch the range of numbers of teeth that can be machined and then, for the segments AB, BC, CD, DE and EF of the profile, a list of paired X and Y coordinates. The number of coordinate pairs differs from segment to segment and from pitch to pitch, and the individual coordinate values are not reproduced here. The ranges of numbers of teeth are 30 to 130 for pitch 5.080; 33 to 160 for pitch 9.525; 48 to 340 for pitch 12.700; 50 to 200 for pitch 22.225; and 56 to 144 for pitch 31.750. The table runs over two pages, the second headed Table A.1 (continued).