Timing Belts - Cross & Morse - #5

Text version of the page

table.main {} tr.row {} td.cell {} div.block {} div.paragraph {} .font0 { font:4.00pt "Lucida Sans Unicode", sans-serif; } .font1 { font:6.00pt "Lucida Sans Unicode", sans-serif; } .font2 { font:9.00pt "Lucida Sans Unicode", sans-serif; } .font3 { font:10.00pt "Lucida Sans Unicode", sans-serif; } .font4 { font:6.00pt "Palatino Linotype", serif; } .font5 { font:7.00pt "Palatino Linotype", serif; } .font6 { font:8.00pt "Palatino Linotype", serif; } .font7 { font:9.00pt "Palatino Linotype", serif; } .font8 { font:10.00pt "Palatino Linotype", serif; } .font9 { font:12.00pt "Palatino Linotype", serif; } .font10 { font:25.00pt "Palatino Linotype", serif; } .font11 { font:6.00pt "Trebuchet MS", sans-serif; } .font12 { font:8.00pt "Trebuchet MS", sans-serif; } .font13 { font:10.00pt "Trebuchet MS", sans-serif; } Timing Belt Drive Selection In order to select a Timing Belt Drive it is first necessary to compile together all the relevant design parameters, to include:- a. Type of driver, shaft speed, and power to be transmitted. b. Type of driven machinery, shaft speed or drive ratio. c. Approximate shaft centre distance required. d. Number of hours daily, drive will be used, and any special operating conditions (temperature, abrasive dust, etc.). e. Shaft diameters and any space restrictions affecting pulley diameters or widths. With this information a suitable drive can be selected by the selection procedure in conjuction with the following guidelines: a. Power transmission capabilities are always related to the smallest pulley in the system, regardless of whether it is the driving or driven pulley. b. Where there is a choice of pulley combination for a given drive consideration should be given to the following:- 1. Larger pulleys reduce amount of belt flexing and therefore improve belt life. 2. Larger pulleys often enable use of narrower belts. 3. Large pulleys can be more expensive. c. There must be at least one flanged pulley in the drive, and where centre distance is more than 8 times the diameter of the smaller pulley, both pulleys should be flanged. d. If shaft centre adjustment is inadequate to correctly tension the belt, an idler pulley will be required. It is preffered to run idlers on the back of the belt, when a ground back belt should be used. e. Cast Iron pulleys must not be used on drives where belt speeds exceed 30 metres per second. Selection Procedure 1. Drive Ratio Where not known this can be obtained by dividing the speed of the faster shaft by the speed of the slower shaft. Note if the driven shaft is faster this is a Speed Increasing Drive. 2. Calculate the Design Power The design Power Pd is determined by multiplying the transmitted motor power P by the application factors f1, f2, f3 and f4 as applicable. Pd = P(fl + f3- f4) f2 Application Factor f1 - Service Factor- relates the type of driver and driven equipment to the daily useage, refer Table 1. Application Factor il - Speed Increasing Drives - refer Table 2 for factor relevent to respective speed increase ratio. Application Factor i3 - If an idler is used add 0.2. Application Factor i4 - If machinery only used intermittently or seasonally deduct 0.2 from service factor. 3. Select Belt Pitch Applying the calculated Design Power and the speed of the smallest pulley to the graphs on pages 5-6 to select suitable belt pitch for the application. Using this pitch of belt should provide a well proportioned drive, but where space limitations apply, another pitch of belt may be required. Both Metric (HTD) and Classical Belt Drives are available for selection. Generally HTD Belts provide a more compact, quieter drive, and are preffered for new applications, however classical belts offer a wider selection of drive ratios with std. pulleys, and for one-off drives often a lower cost. 4. Pulley Selection Refer to Standard Drive Ratios - Table 3 and select a suitable combination of pulleys to provide the correct drive ratio. For economic and availability reasons it is preferable to use pulleys of maximum 80 teeth, and to minimise belt fatigue a minimum of 20 Teeth (28 Teeth 14M drives). From the appropiate pulley dimension tables confirm that pulley sizes selected are available and will accommodate shaft diameters and not exceed space limitations. 5. Determine Belt Length and Center Distance Having selected belt size (pitch), numbers of teeth in pulleys, and knowing approximate shaft centres one can select belt length and calculate actual centre distance. a. Determine Belt Length For drives with pulleys of equal numbers of teeth. Calculate Number Teeth in Belt Nc = 2. Ao + Z1 p For drives with pulleys of dissimilar numbers of teeth. Calulate Number Teeth in Belt Nc = 2Ao + (Z1++Z2) + 2.533p (Z2 - Z1)2 p 2 100Ao 7) Where Ao = Approximimate Centre Distance mm p = Belt Pitch mm Z1 = Number Teeth Small Pulley Z2 = Number Teeth Large Pulley Note: Nc must always be greater than 0.9 (Zl + Z2) Refer to standard Belt Tables (pages 12-13 Metric Drives, page 30-31 Classical) and select nearest belt length to numbers of teeth calculated. b. Determine Actual Shaft Centres The actual centre distance A can then be determined from the following formula where Na is number of teeth in belt selected. A =p/4 (Na - Zl + Z2 + /(Na - Zl + Z2V-2.027 (Z2-Zi)2 2 V 2 10 ) This will provide a reasonably accurate result but for fixed centre drives please contact Cross & Morse Engineering. 6. Factors to correct for Teeth in Mesh and Belt Length a. Teeth in mesh factor f5 Applicable only on drives with pulleys less than 18 teeth or drive ratio greater than 3: 1. For a belt to transmit full power a minimum of 6 teeth must be in mesh on each pulley. The number of teeth in mesh can determined from the following formula: Number Teeth in Mesh (TIM)= Z1 [0.5 - (Z2- Z1)p] 18.85A The Design Power Pd must be multiplied by factor f5 taken from table 2a. b. Belt Length Correction factor - i6 To allow for variation in rates of loading a belt length factor f6 is applied to the Design Power Pd for final selection power Ps. The factors, provided in table 4, only apply to HTD drives, for Classical timing belt drives f6 = 1. Thus:- Selection Power Ps = Pd .is.ie 7. Belt Width Selection Having determined Selection Power Ps, and knowing the size of small pulley and relative shaft speed use the Rating Tables on pages 7-9 to determine Rated Power Pr. which is where appropriate columns for pulley size and shaft speed intersect in table for selected belt size. If a column for the actual shaft speed is not available use the next lower speed available, and if shaft speed is below 100r.p.m. use column for 100 r.p.m., but multiply power read off by actual shaft speed divided by 100. A belt width factor Wf can then be determined by dividing the Selection Power Ps by the Rated Power Pr. Wf = Ps Pr From the table immediately above the relevant Power Rating Table select the belt width which has a width factor equalor greater than the value calculated for Wf. X O 7 9 0 Û @ o o en en 3 o 8. Confirmation of Drive n D Refer to pulley tables 14-21 (HTD Drives) or 29-33 (Classical Belts) to confirm that pulley sizes (numbers of teeth and width) are available, and capable of accommodating shaft sizes, and that at least one pulley is of flanged construction. If belt speed exceeds 30M/S ensure no cast iron pulleys used; also pulleys should be balanced. Ensure that calculated centre A can be accomodated with adjustment to correctly tension belt on assembly, and to enable belt to be fitted over pulley flanges if applicable, refer o to page 10. 3