Machinery's Handbook, 31st Edition
Synchronous Belts
2607
Synchronous Belts Synchronous Belts ANSI/RMA IP-24.— Synchronous belts are also known as timing or positive-drive belts. These belts have evenly spaced teeth on their surfaces, which mesh with teeth on pulleys or sprockets to produce a positive, no-slip transmission of power. Such designs should not be confused with molded notched V-belts, which transmit power by means of the wedging action of the V-shape. Synchronous belts are used where driven shaft speeds must be synchronized to the rotation of the driver shaft and to eliminate the noise and maintenance problems of chain drives. Standard Timing Belts: Conventional trapezoidal, or rectangular tooth, timing belts come in six cross sections, which relate to the pitch of the belt. Pitch is the distance from center to center of the teeth. The six basic cross sections or pitches are MXL (mini extra light), XL (extra light), L (light), H (heavy), XH (extra heavy), and XXH (double extra heavy) (Fig. 12 ). Belts are specified by pitch length, cross section (pitch), and width. Double-sided timing belts have identical teeth on both sides of the belt and are used where synchronization is required from each belt face. They are available in XL, L, and H cross sections. Size Designations: Synchronous belt sizes are identified by a standard number. The first digits specify the belt length to 0.1 inch (2.54 mm) followed by the belt section (pitch) designation. The digits following the belt section designation represent the nominal belt width times 100. For example, an L section belt 30.000 inches pitch length and 0.75 inch in width would be specified as a 300L075 synchronous belt.
0.080 ” (2/25 ” ) pitch mini extra light (MXL)
2/25 ”
0.200 ” (1/5 ” ) pitch extra light (XL)
1/5 ”
0.375 ” (3/8 ” ) pitch light (L)
3/8 ”
0.500 ” (1/2 ” ) pitch heavy (H)
1/2 ”
0.875 ” (7/8 ” ) pitch extra heavy (XH)
7/8 ”
1.250 ” (1-1/4 ” ) pitch double extra heavy (XXH)
1-1/4 ”
Fig. 12. Standard Synchronous Belt Sections The RMA nomenclature for double-sided belts is the same as for single-sided belts with the addition of the prefix “D” in front of the belt section. However, some manufacturers use their own designation system for double-sided belts. Standard Sections: Belt sections are specified in terms of pitch. Table 40 gives the Stan dard Belt Sections and their corresponding pitches.
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