Unit3 Gear

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Unit3 Gear

  1. 1. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/3/1 he he k k lic lic GEAR C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c UNIT 3 GEAR OBJECTIVES General Objective : To understand the technology of gears manufacturing Specific Objectives : At the end of the unit you will be able to: Ø Know the methods of gear manufacturing Ø Know the methods of direct and simple indexing Ø Apply direct and simple indexing when cutting gears on a milling machine. Ø Apply various formula to calculate gear-tooth dimensions.
  2. 2. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/3/2 he he k k lic lic GEAR C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c INPUT 3.0. GEAR MANUFACTURING Gears can be manufactured by casting, forging, extrusion, drawing, thread rolling, powder metallurgy, and blanking sheet metal (for making thin gears such as those used in watches and small clocks). Nonmetallic gears can be made by injection molding and casting. Gears may be as small as those used in watches or a large as 9 m in diameter. The dimensional accuracy and surface finish required for gear teeth depend on its intended use. Poor gear-tooth quality contributes to inefficient energy transmission and noise and adversely affects the gear’s frictional and wear characteristics. Submarines gears, for examples, have to be of extremely high quality so as to reduce noise levels, helping the submarine avoid detection. There two basic gear manufacturing methods which involve the machining of a wrought or cast gear blank: form cutting and generating. 3.1. FORM CUTTING In form cutting, the cutting tool is similar to a form-milling cutter made in the shape of the space between the gear teeth (Fig. 3.1). The gear-
  3. 3. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/3/3 he he k k lic lic GEAR C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c tooth shape is produced by cutting the gear blank around its periphery. The cutter travels axially along the length of the gear tooth at the appropriate depth to produce the gear tooth profile. After each tooth is cut, the cutter is withdrawn, the gear blank is rotated (indexed), and the cutter proceeds to cut another tooth. The process continues until all teeth are cut. Each cutter is designed to cut a range of number of teeth. The precision of the form cut tooth profile depends on the accuracy of the cutter and on the machine and its stiffness. Although inefficient, form cutting can be done on milling machines, with the cutter mounted on an arbor and the gear blank mounted in a dividing head. Form cutter Gear blank Figure 3.1. Producing gear teeth on a blank by form cutting Because the cutter has a fixed geometry, form cutting can only be used to produce gear teeth that have constant width, that is, on spur or helical gears but not on bevel gears. Internal gears and gear teeth on straight surfaces, such as in rack and pinion, are form cut with a shaped cutter, using a machine similar to a shaper. Broaching can also be used to produce gear teeth and is particularly applicable to internal teeth. The process is rapid and produces fine surface finish with high dimensional accuracy. However, because broaches are
  4. 4. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/3/4 he he k k lic lic GEAR C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c expensive and a separate broach is required for each gear size, this method is suitable almost exclusively for high-quantity production. Gear teeth may be cut on special machines with a single-point cutting tool that is guided by a template in the shape of the gear tooth profile. As the template can be made much larger than the gear tooth, dimensional accuracy is improved. Form cutting is relatively a simple process and can be used for cutting gear teeth with various profiles, however, it is a slow operation, and some types of machines require skilled labor. Consequently, it is suitable only for low-quantity production. Machines with semiautomatic features can be used economically for form cutting on a limited production basis. 3.2. GEAR GENERATING The cutting tool used in gear generating may be one of the following: 3.2.1. A pinion-shaped cutter 3.2.2. A rack-shaped straight cutter 3.2.3. A hob 3.2.1. The pinion-shaped cutter can be considered as one of gears in a conjugate pair and the other as the gear blank (Fig 3.2); it is used on machines called gear shapers (Fig 3.3). The cutter has an axis parallel to that of the gear blank and rotates slowly with the blank at the same pitch-circle velocity in an axial reciprocating motion. A train of gears provides the required relative motion between the cutter shaft and the gear-blank shaft.
  5. 5. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/3/5 he he k k lic lic GEAR C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Gear cutter Base circle Pitch circle Gear blank Base circle Figure 3.2. Gear generating in a gear shaper using a pinion-shaped cutter Cutter spindle Gear teeth Spacer Pinion-shape cutter Gear blank Figure 3.3.. Gear generating with a pinion-shaped gear cutter Cutting may take place at either the down stroke or the upstroke of the machine. Because the clearance required for cutter travel is small, such as flanges (Fig 3.3). The process can be used for low-quantity as well as high-quantity production. 3.2.2. On a rack shaper, the generating tool is a segment of a rack (Fig.3.4) which reciprocates parallel to the axis of the gear blank. Because it is not practical to have more than 6 to 12 teeth on a rack cutter, the cutter must be disengaged at suitable intervals and returned to the starting point; the gear blank remain fixed.
  6. 6. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/3/6 he he k k lic lic GEAR C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Figure 3.4. Gear generating with rack-shaped cutter 3.2.3. A gear-cutting hob (Fig. 3.5) is basically a worm, or screw, made into a gear-generating tool by machining a series of longitudinal slots or gashes into it to form the cutting teeth. When hobbing a spur gear, the angle between the hob and gear blank axes is 90o minus the lead angle at the hob threads. All motions in hobbing are rotary, the hob and gear blank rotate continuously, much as two gears meshing until all teeth are cut. Top view Gear blank Hob Gear blank Figure 3.5. View of gear cutting with a hob
  7. 7. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/3/7 he he k k lic lic GEAR C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Hobs are available with one, two, or three threads. If the hob has a single thread and the gear is to have 40 teeth, for example, the hob and gear spindle must be geared together so that the hob makes 40 revolutions while the gear blank makes one revolution. Similarly, if a double-threaded hob is used, the hob would make 20 revolutions to the gear blank’s one revolution. In addition, the hob must be fed parallel to the gear axis for a distance greater than the face width of the gear tooth (Fig. 3.5) in order to produce straight teeth on spur gears. The same hobs and machines can be used to cut helical gears by tilting the axis of the hob spindle. Because it produces a variety of gears rapidly and with good dimensional accuracy, gear hobbing is used extensively in industry. Although the process is suitable for low-quantity production, it is most economical for medium to high-quantity production. Gear–generating machines can also produce spiral-bevel and hypoid gears. Like most other machine tools, modern gear-generating machines are computer controlled. Multi axes computer-controlled machines are capable of generating many types and sizes of gears using indexable milling cutters. 3.3. CUTTING BEVEL GEARS Straight bevel gears are generally roughed out in one cut with a form cutter on machines that index automatically. The gear is then finished to the proper shape on a gear generator. The generating method is analogous to the rack-generating method already described. The cutters reciprocate across the face of the bevel gear as does the tool on a shaper (Fig 3.6).
  8. 8. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/3/8 he he k k lic lic GEAR C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Gear blank Cutter Figure 3.6. Cutting a straight bevel gear blank with two cutter The machines for spiral bevel gears operate on essentially the same principle. The spiral cutter is basically a face-milling cutter that has a number of straight-sided cutting blades protruding from its periphery ( Fig.3.7 ). Cutter Gear blank Figure 3.7. Cutting a spiral bevel gear with a single cutter
  9. 9. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/3/9 he he k k lic lic GEAR C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c 3.4 GEAR-FINISHING PROCESSES As produced by any of the process described, the surface finish and dimensional accuracy of gear teeth may not be sufficiently accurate for certain applications. Moreover, the gears may be noisy or their mechanical properties, such as fatigue life, may not be sufficiently high. Several finishing processes are available to improve the surface quality of gears. The choice of process is dictated by the method of gear manufacture and whether the gears have been hardened by heat treatment. Heat treating can cause distortion of parts. Consequently, for precise gear-tooth profile, heat-treated gears should be subjected to appropriate finishing operations. 3.4.1. Shaving The gear shaving process involves a cutter, made in the exact shape of the finished tooth profile, which removes small amounts of metal from the gear teeth. The cutter teeth are slotted or gashed at several points along its width, making the process similar to fine broaching. The motion of the cutter is reciprocating. Shaving and burnishing can only be performed on gears with a hardness of 40 HRC or lower. Although the tools are expensive and special machines are necessary, shaving is rapid and is the most commonly used process for gear finishing. It produces gear teeth with improved surface finish and improved accuracy of tooth profile. Shaved gears may subsequently be heat treated and then ground for improved hardness, wear resistance, and accurate tooth profile.
  10. 10. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/3/10 he he k k lic lic GEAR C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c 3.4.2. Burnishing The surface finish of gear teeth can also be improved by burnishing. Introduced in the 1960s, burnishing is basically a surface plastic-deformation process using a special hardened gear-shaped burnishing die that subjects the tooth surfaces to a surface rolling action (gear rolling). Cold working of tooth surfaces improves the surface finish and induces surface compressive residual stresses on the gear teeth, thus their fatigue life. However, burnishing does not significantly improve gear-tooth accuracy. 3.4.3. Grinding, honing and lapping For the highest dimensional accuracy, tooth spacing and form, and surface finish, gear teeth may subsequently be ground, honed, and lapped. Specially-dressed grinding wheels are used for either forming or generating gear-tooth surfaces. There are several types of grinders of gears, with the single index form grinder being the most commonly available. In form grinding, the shape of the grinding wheel is identical to that of the tooth spacing (Fig. 3.8) The honing tool is plastic gear impregnated with fine abrasive particles. The process is faster than grinding and is used to improve surface finish. To further improve the surface finish, ground gear teeth are lapped using abrasive compounds with either a gear-shaped lapping tool (made of cast iron or bronze) or a pair of mating gears that are run together. Although production rates are lower and costs are higher, these finishing operations are particularly suitable for producing hardened gears of very high quality, long life, and quiet operation.
  11. 11. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/3/11 he he k k lic lic GEAR C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Grinding wheel Gear Position: 15o or 0o Position: 0o Figure 3.8. Grinding by generating with two wheels 3.5. METRIC GEARS AND GEAR CUTTING Countries which have been using a metric system of measurement usually use the module system of gearing. The module (M) of a gear equals the pitch diameter (PD) divided by the number of teeth (N), or M = only, N whereas the DP of a gear is the ratio of N to the PD, or DP = . The DP of PD a gear is the ratio of the number of teeth per inch diameter, whereas M is an actual dimension. Most of the term used in DP gears remains the same for module gears; however, the method of calculating the dimensions has changed in some instances. Table 3.2. gives necessary rules and formulas for metric spur gears.
  12. 12. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/3/12 he he k k lic lic GEAR C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c 3.6. METRIC MODUL GEAR CUTTERS The most common metric gear cutters are available in moduls ranging from 0.5 to 10 mm. However metric modul gear cutters are available in sizes up to 75 mm. Any metric modul size is available a set of eight cutters, number from #1 to #8. The range of each cutter is the reverse of that of a DP cutter. For instance, a #1 metric modul cutter will cut from 12 to 13 teeth; a #8 DP cutter will cut from 135 teeth to a rack. Table 3.1. shows the cutters available and the range of each cutter in the set. Table 3.1 Metric module gear cutter Milling Cutter Numbers Module size (mm) Cutter No. For Cutting 0.50 3.50 0.75 3.75 1 12 – 13 teeth 1.00 4.00 2 14 – 16 teeth 1.25 4.50 3 17 – 20 teeth 1.50 5.00 4 21 – 25 teeth 1.75 5.50 5 26 – 34 teeth 2.00 6.00 6 35 – 54 teeth 2.25 6.50 7 55 – 134 teeth 2.50 7.00 8 135 teeth to rack 2.75 8.00 3.00 9.00 3.25 10.00
  13. 13. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/3/13 he he k k lic lic GEAR C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Table 3.2. Formula for calculating metric gear To Obtain Knowing Rule Formula Addendum (A) Normal Module Addendum equals module A=M Module Multiply module by p CP = M x 3.1416 Pitch diameter 3.1416 Multiply pitch diameter by p and number of CP = M x and divide by number of teeth N Circular pitch (CP) teeth Outside diameter Multiply outside diameter by OD x 3.1416 and number of p and divide by number of CP = teeth teeth minus 2 N-2 Divide 90 by number of teeth. Module and Find the sine of this angle and 90 CT = PD x sin outside diameter multiply by the pitch N diameter. Chordal thickness (CT) Multiply module by p and M x 3.1416 Module CT = divide by 2 2 CP Circular pitch Divide circle pitch by 2 CT = 2 Clearance (CL) Module Multiply module 0.166 mm CL = M x 0.166 Dedendum (D) Module Multiply module 1.166 mm D = M x 1.166 Pitch diameter PD Divide pitch diameter by the and number of M= module N teeth CP Module (M) Circular pitch Divide circular pitch by p M= 3.1416 Outside diameter OD Divide outside diameter by and number of M= number of teeth N+2 teeth Pitch diameter Divide pitch diameter by the PD N= and module module M Number of teeth (N) Multiply pitch diameter by p Pitch diameter PD x 3.1416 and divide product by N= and circular pitch CP circular pitch Number of teeth Add 2 to the number of teeth OD = (N + 2) x M Outside diameter and module and multiply sum of module (OD) Pitch diameter Add 2 modules to pitch OD = PD + 2M and module diameter Module and Multiply module by number of PD = M x N number of teeth teeth Outside diameter Subtract 2 modules from PD = OD – 2M and module outside diameter Pitch diameter (PD) Multiply number of teeth by Number of teeth N x OD outside diameter and divide and outside PD = product by number of teeth N+2 diameter plus 2 Whole depth (WD) Module Multiply module by 2.166 mm WD = M x 2.166 Centre-to-centre Divide the sum of the pitch PD1 + PD 2 Pitch diameters CD = distance(CD) diameters by 2 2
  14. 14. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/3/14 he he k k lic lic GEAR C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c Example 1. 1. A spur gear has PD of 60mm and 20 teeth. Calculate: (a) Modul (b) Circular Pitch (c) Addendum (d) Outside diameter (e) Dedendum (f) Whole depth (g). Cutter number Solutions: (a) Modul = PD/N = 60/20 = 3 mm (b) CP =M×p = 3 × 3.1416 = 9.425 mm (c) Addendum = Modul = 3 mm (d). Outside diameter = ( N + 2 ) × M = 22 × 3 = 66 mm (e). Dedendum = M × 1.666 = 3 × 1.666 = 4.998 mm
  15. 15. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/3/15 he he k k lic lic GEAR C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c (f) Working depth = Modul × 2.166 = 3 × 2.166 = 6.498 mm (g). Cutter number ( see Table 3.2 ) = 3
  16. 16. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/3/16 he he k k lic lic GEAR C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c ACTIVITY 3A 3.1. Two identical gears in mesh have a CD of 120 mm. Each gear has 24 teeth. Calculate; (a) Pitch diameter (b) Modul (c) Outside diameter (d) Whole depth (e) Circular pitch (f) Chordal thickness 3.2. Name 3 methods of gear generating.
  17. 17. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/3/17 he he k k lic lic GEAR C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c PD FEEDBACK ON ACTIVITY 3A N 2xCD 3.1.(a)PD = ( equal gears ) 2 2x120 = 2 240 = 2 = 120 mm PD (b) M = N 120 = 24 = 5 (c) OD = (N + 2 ) x M = 26 x 5 = 130 mm (d) WD = M x 2.166 = 5 x 2.166 = 10.83 mm
  18. 18. F T ra n sf o F T ra n sf o PD rm PD rm Y Y Y Y er er ABB ABB y y bu bu 2.0 2.0 to to re re J3103/3/18 he he k k lic lic GEAR C C w om w om w w w. w. A B B Y Y.c A B B Y Y.c (e) CP = MxP = 5 x 3.1416 = 15.708 mm MxÕ (f) CT = 2 5x3.1416 = 2 7.85 mm 3.2. 1. Pinion- shaped cutter 1. Rack-shaped straight cutter 2. A hob

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