1、SMPTE RP*?4 92 83574Ul1 O000552 485 W SMPTE RECOMMENDED PRACTICE 16-mm Sprocket Design RP 74-1992 Revision of RP 74-1977 1 Scope This practice provides dimensions and specifications for the design of sprockets used with 16-mm motion- picture raw stock or processed film. 2 Dimensions and specificatio
2、ns 2.1 The teeth shall be equally spaced at an index angle of 360/N0 where N is the number of teeth. A suitable tolerance for the index angle is f 1 minute of arc for sprockets having 8 to 17 teeth, 30 seconds of arc for sprockets having 18 to 34 teeth, and t.20 seconds of arc for sprockets having 3
3、5 to 64 teeth. 2.2 The root diameter is computed from the equation: P D= NX -T where N is the number of teeth, P is the sprocket pitch, and T is the film thickness. The different root diameters in tables la and 1 b were derived using a value for T of 0.15 mm (0.006 in). If optimum working conditions
4、 are desired with film materials of different thickness, the root diameter values in tables la and 1 b should be recomputed. 2.3 Figure 1 shows that either the entering or leaving film path may fall within the limits speci- fied by radii RI and Re. If the film path is convex with respect to the spro
5、cket surface (curvature away from the sprocket surface), a minimum value of 4.762 mm (0.1875 in) for RI is recom- mended. This is an arbitrary choice, but seems appropriate for 16-mm equipment. Page 1 of 8 pages Figure 1 - Sprocket The shape of the film path as the film leaves the root of the sprock
6、et is determined by film stiffness, curl, set, and tension as well as by the shape and location of rollers or guides. For the specified tooth shape, the film will be allowed to back-slip over the root circle a maximum distance of 0.066 mm (0.0026 in) for each tooth, measured at the pitch line (film
7、thickness assumed to be 0.15 mm 0.006 in), by the time the contact point between film and tooth has reached the assumed maximum work- ing height of 0.66 mm (0.026 in), measured radially from the root circle. 2.4 The drive sprocket is most sensitive to tooth shape because the tooth action may take pl
8、ace over the entire working height and is, therefore, used to derive the desired shape. For the usual drive sprocket, the pitch is greater than the film pitch which causes the film to slip backward in Copyright O 1992 by the SOCIETY OF MOTION PICTURE AND TELEVISION ENGINEERS 595 W. Hartsdale Ave., W
9、hite Plains, NY 10607 (914) 761-1100 Approved September 18,1992 SMPTE RP*74 92 8357401 0000553 311 I RP 74-1 992 relation to the sprocket travel. The direction of the friction force between the film and the root surface is such as to assist the feed or driving action. 2.4.1 Of the total 0.066-mm (0.
10、0026-in) accom- modation provided at each tooth for film slippage, approximately 0.01 3 mm (0.0005 in) is allocated to the combined tolerance of perfora- tion pitch and sprocket tooth pitch (shorter-than- average perforation pitch combined with longer-than-average tooth pitch). An additional 0.010 m
11、m (0.0004 in) is allocated for distortion of the perforation edge under light load (less than 0.56 N 2 oz-force between the perforation edge and the tooth for acetate film of 0.15 mm 0.006 in thickness). The remaining 0.043 mm (0.001 7 mm) approximates 0.6% film shrinkage. It should be noted that an
12、other combination of greater load and lower shrinkage could fall within the same allowable maximum of 0.053 (0.0021 in). The user is cautioned against high loading because of possible destructive film fatigue and wear, film distortion away from the base circle, and malfunction. The selected val- ues
13、 of Ri, working height and maximum pitch difference, in turn, determine the values of XT which have been computed and tabulated. 2.4.2 For a concave film path with respect to the sprocket surface (curvature toward the sprocket radius center), the limiting radius, Re, has been computed for the same X
14、T and the same accom- modation of 0.066 mm (0.0026 in), assuming a parabolic schedule (displacement function pro- portionate to the square of time) of reduction versus time (see reference I). These values of R2 are set forth in tables 1 a and 1 b. For those exiting film paths from drive sprockets co
15、rre- sponding to larger values of RI and R2, including a straight tangent path, the accommodation of 0.066 mm (0.0026 in) for film slippage takes place in less than 0.66 mm (0.026 in) of the working height (or more accommodation results at the same height). Conversely, the slowest accommodation corr
16、esponds to the exiting path defined by the minimum values of R2. Therefore, the R2 value is recommended where maximum uniformity of motion is desired. 2.5 The pitch of the usual holdback sprocket should be equal to or less than the pitch of the film. The shortest film pitch is assumed to be 7.536 mm
17、 (0.2967 in) corresponding to 0.8% shrinkage of long-pitch film (7.620 mm 0.300 in). (This value is chosen rather than the 0.6% used for the tooth shape to avoid inadvertent interference at entering teeth.) The user may again exercise control by correct choice of the root diameter if a change is war
18、ranted. The fric- tion between the film and the root surface, as well as against guides, assists in film holdback. The tooth shape for a holdback sprocket has little control over the pitch differential accommodation, as the load shifts rather abruptly from the disengaging restraining tooth to the ro
19、ot of the following tooth. The tooth shape specified will ensure clearance at the entering position. If a holdback sprocket is to provide the best possible uniformity of motion, the design must be developed with care (see reference 1). 2.6 The pitch of combination sprockets, 7.600 mm (0.2992 in), sh
20、ould correspond to the pitch of film with 0.3% shrinkage. This value is closer to the feed sprocket pitch than to the holdback sprocket pitch to prevent the film from riding high on the teeth or being damaged by guides at the entering path when used for driving action with the sprocket pitch shorter
21、 than the film pitch. Entering guides may be needed for good hold- back action when the sprocket pitch is longer than the film pitch. 2.7 The desired tooth shape can be generated by a hob corresponding to the basic rack speci- fied by KH and BH in table 2 and figure 4. If the first hob covers the ra
22、nge of N from 8 to 17 inclusively and the second hob 18 to 64 inclu- sively, no deviations from the ideal tooth shape greater than 0.005 mm (0.0002 in) will occur. 2.8 The tooth width at the base, dimension W, allows ample material for rounding off the tip while preserving the 0.66 mm (0.026 in) or
23、more of working height. The value chosen does not limit the angle of wrap on the sprocket as a wider tooth would. If the wrap length is defined as one-half the sum of the number of pitch lengths in the arc of engagement, E, and the number of pitch lengths in the arc of contact, C (figure I), then th
24、e wrap length may be as high as 6314 pitch lengths without producing interference at the entering teeth of a drive sprocket if the film shrinkage does not exceed 0.8%. Page 2 of 8 pages SMPTE RP*ir 92 m 835irOL 0000554 258 m RP 74-1 992 Table la - Sprocket dimensions in millimeters N RDD RDC RDH K B
25、 R2 XT 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 60 19.252 21.678 24.1 03 26,529 28.954 31.380 33.805 36.231 38.656 41 .O82 43.507 45.933 48.358 50.784 53.209 55.635 58.061 62.912 67.763 72.614 77.465 82.31 6 87.1 67 92.01 8 96.869 1 01.720 106.
26、571 11 1.422 116.273 121.124 125.975 130.826 135.677 145.379 19.201 21.620 24.040 26.459 28.878 31.297 33.71 6 36.135 38.554 40.974 43.393 45.81 2 48.231 50.650 53.069 55.489 57.908 62.746 67.584 72.423 77.261 82.099 86.938 91.776 96.61 4 1 o1 -453 106.291 111.129 11 5.967 120.806 125.644 130.482 13
27、5.321 144.997 19.097 21.503 23.909 26.315 28.721 31 .I27 33.533 35.940 38.346 40.752 43.158 45.564 47.970 50.376 52.782 55.188 57.595 62.407 67.219 72.031 76.843 81.656 86.468 91.280 96.092 100.904 105.717 1 10.529 11 5.341 I 20.1 53 124.965 129.778 134.590 144.214 1.709 1.750 1.809 1.875 1.939 2.00
28、1 2.061 2.120 2.177 2.233 2.288 2.342 2.395 2.446 2.497 2.547 2.597 2.695 2.790 2.883 2.974 3.064 3.153 3.241 3.326 3.41 2 3.496 3.579 3.661 3.743 3.825 3.904 3.982 4.139 0.1 11 0.121 0.1 34 0.1 49 0.164 0.178 0.192 0.206 0.21 9 0.233 0.245 0.258 0.271 0.283 0.296 0.308 0.320 0.343 0.367 0.390 0.41
29、2 0.434 0.456 0.478 0.499 0.520 0.541 0.562 0.582 0.603 0.623 0.642 0.662 0.702 14.726 15.933 17.137 18.387 19.697 21 .O37 22.392 23.769 25.157 26.559 27.970 29.374 30.801 32.214 33.646 35.082 36.522 39.420 42.328 45.254 48.206 51 .I 66 54.1 62 57.1 58 60.1 67 63.207 66.258 69.337 72.423 75.537 78.6
30、79 81.822 84.964 91.351 0.2478 0.2410 0.2355 0.2309 0.2270 0.2237 0.2209 0.21 84 0.21 62 0.21 42 0.2124 0.21 O9 0.2094 0.2082 0.2070 0.2059 0.2049 0.2031 0.2016 0.2003 0.1 991 0.1 981 0.1 971 0.1 963 0.1 956 0.1949 0.1 943 0.1 937 0.1932 0.1927 0.1 922 0.1918 0.1915 0.1908 64 155.081 154.674 153.839
31、 4.292 0.740 97.81 1 0.1902 N - Number of teeth RDD - Root diameter +0.03 -0.00 of drive sprocket of 7.620 pitch RDC - Root diameter +0.03 -0.00 of combination sprocket of 7.600 pitch RDH - Root diameter +O.OO -0.03 of holdback sprocket of 7.559 pitch K - Circular arc radius for tooth shape, +O.OO -
32、0.05 B - Radial distance of arc center inside root circle, +0.013 -0.000 R2 - Minimum radius of film path concave to sprocket XT - Offset of tooth at working height RI - Minimum film path radius convex to sprocket, 4.762 Tooth working height - 0.660 Maximum pitch difference - 0.066 Film tliickness -
33、 0.1 52 Other thickness - Root diameter = N*pitch/x - thickness Page 3 of 8 pages SMPTE RP*74 92 8357401 0000555 194 RP 74-1 992 Table 1 b - Sprocket dimensions in inches N RDD RDC RDH K I3 R2 XT 8 9 IO 11 12 t3 14 15 16 17 18 19 20 21 22 23 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 60 64 0
34、.7580 0.8535 0.9489 1 .O444 1 .I 399 1.2354 1.3309 1.4264 1.5219 1.61 74 1.71 29 1 .a084 1.9039 1.9994 2.0948 2.1904 2.2859 2.4769 2.6678 2.8588 3.0498 3.2408 3.4318 3.6228 3.8137 4.0047 4.1957 4.3867 4.5777 4.7687 4.9596 5.1506 5.3416 5.7236 0.7559 0.851 2 0.9465 1 .O41 7 1.1369 1.2322 1.3274 1.422
35、6 1.51 79 1.6i31 1.7084 1 .a036 1.8989 1.9941 2.0893 2.1846 2.2798 2.4703 2.6608 2.851 3 3.041 8 3.2322 3.4228 3.61 32 3.8037 3.9942 4.1 847 4.3752 4.5656 4.7561 4.9466 5.1371 5.3276 5.7085 0.7519 0.8466 0.941 3 1 .O360 1.1307 1.2255 5.3202 1.41 50 1.5097 1.6044 7.6991 1.7939 1.8886 1.9833 2.0780 2.
36、1728 2.2675 2.4570 2.6464 2.8359 3.0253 3.2148 3.4043 3.5937 3.7831 3.9726 4.1621 4.351 5 4.541 O 4.7304 4.91 99 5.1094 5.2988 5.6777 0.0673 0.0689 0.0712 0.0738 0.0763 0.0788 0.081 1 0.0835 0.0857 0.0879 0.0901 0.0922 0.0943 0.0963 0.0983 0.1003 0.1022 0.1061 O. 1 098 0.1 I35 0.1 171 O. 1206 0.1241
37、 0.1276 0.1309 0.1 343 0.1 376 0.1409 0.1441 0.1474 0.1 506 0.1 537 0.1 568 0.1 630 0.0044 0.0048 0.0053 0.0059 0.0065 0.0070 0.0076 0.0081 0.0086 0.0092 0.0096 0.0102 0.01 07 0.01 11 0.01 17 0.0121 0.0126 0.0135 0.0144 0.01 54 0.01 62 0.0171 0.01 80 0.01 88 0.0196 0.0205 0.0213 0.0221 0.0229 0.0237
38、 0.0245 0.0253 0.0261 0.0276 0.5798 0.6273 0.6747 0.7239 0.7755 0.8282 0.881 6 0.9358 0.9904 5 .O456 1.1012 1.t565 1.21 26 1.2683 1.3246 1.381 2 1.4379 1.5520 1.6665 1.781 7 1 .a979 2.0544 2.1324 2.2503 2.3688 2.4885 2.6086 2.7298 2.851 3 2.9739 3.0976 3.221 3 3.3450 3.5965 0.00976 0.00949 0.00927 0
39、.00909 0.00894 0.00881 0.00870 0.00860 0.00851 0.00843 0.00836 0.00830 0.00824 0.00820 0.0081 5 0.0081 1 0.00807 0.00800 0.00794 0.00789 0.00784 0.00780 0.00776 0.00773 0.00770 0.00767 0.00765 0.00763 0.00761 0.00759 0.00757 0.00755 0.00754 0.00751 6.1056 6.0895 6.0567 0.1 690 0.0291 3.8508 0.00749
40、N - Number of teeth RDD - Root diameter +0.001 -0.000 of drive sprocket of 0.3000 pitch RDC - Root diameter +0.001 -0.000 of combination sprocket of 0.2992 pitch RDH - Root diameter +O.OOO -0.001 of holdback sprocket of 0.2976 pitch K - Circular arc radius for tooth shape, +O.OOO -0.002 B - Radial d
41、istance of arc center inside root circle, +0.0005 -0.0000 R2 - Minimum radius of film path concave to sprocket XT - Offset of tooth at working height Ri - Minimum film path radius convex to sprocket, 0.1875 Tooth working height - 0.0260 Maximum pitch difference - 0.0026 Filmthicknecc - 0.0060 Other
42、thickness - Root diameter = N*pitch/n - thickness Page 4 of 8 pages RP 74-1992 13.3G +O -0.05 13.03 f0.05 -0 (0513 +0.002 -01 2.90 +O -0.03c (0.114 +O -0.OPI1 2.36 f0.03 fO.093 ?O.OOH 1.24 t0.03 I 1. (0.049 tO.0011 LOCATION OF FILM REFERENCE EDGE 0.10 k 0.03 - 10.004 5zo.001 t NOTE - Dimensions in m
43、illimeters, inches in pareniheses. Figure 2a - Recommended lateral profile for all equipment t4.73 +O -0.05 0.51 MIN. LOCATION OF FILM REFERENCE ELIGE *LANO OPTIONAL Page 5 of 8 pages NOTE - Dimensions in millimeters, inches in parentheses. Figure 2b - Alternate lateral profile for silent equipment
44、only _ SMPTE RP*74 92 August 1941. 2 Chandler, J.S.; Lyman, D.F.; and Martin, L.R. Proposals for 16-mm and 8-mm sprocket standards. Journal of the SMPE 48(6):483-520; June 1947. be obtained (several teeth are usually engaged simulta- neously). The minimum value of the radius (concave toward the spro
45、cket) defining the exiting film path for minimum flutter or maximum smoothness has been designated as R2 and is listed in tables 1 a and 1 b for each value of N (see reference 1). Computing the values of R2 would hardly be possible without an electronic computer because a method of successive approx
46、imations must be used. The limiting radius, R2, of the film leaving the drive sprocket defines the shape of the tooth face. A carefully modified epicycloid best fits this ideal curve. It is far simpler to specify and use the specifications if the curve of the tooth face is a circular arc with radius
47、 and center given. On investigation, it was found that errors would be sufficiently small to make the circular- arc specification practical. From the data for the tooth face as derived in computing R2, a point on the face was selected at one third the working tooth height. Using the position of this
48、 point with the established root and tip positions, the radius and its center were computed for each sprocket. Comparing the positions of points along the sprocket face as defined by the circular arc to those as defined by the ideal curve derived in computing Rz, the maximum deviations at other than
49、 the three fixed points were in the order of 0.005 mm (0.0002 in). The arc specification is convenient and lends itself to small- quantity production of sprockets with a single formed cutter and indexing means. For large-quantity production, the use of hobs is more economical. Many sprockets have been produced using involute shapes of some specified pressure angle. The slope of the resultant tooth at the root is unde- sirably reduced, and the tooth shape is poorer for steadi- ness and flutter. The use of the circular arc, as specified by K and B in tables 1 a and 1
