Unit3 Gear

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  • 1.F T ra n sf o F T ra n sf o PD rm PD rm Y YYY er er ABB ABB y ybubu 2.0 2.0toto re re J3103/3/1 he hekk lic lic GEAR C C wom wom w w w.w.A B B Y Y.c A B B Y Y.cUNIT 3 GEAROBJECTIVES General Objective : To understand the technology of gears manufacturingSpecific Objectives : At the end of the unit you will be able to:Know the methods of gear manufacturing Know the methods of direct and simple indexing Apply direct and simple indexing when cuttinggears on a milling machine. Apply various formula to calculate gear-toothdimensions.

2. F T ra n sf oF T ra n sf o PD rmPD rm YYY Y erer ABBABB yybu bu 2.02.0to to rereJ3103/3/2 hehek k liclic GEAR CC womwom ww w. w.A B B Y Y.cA B B Y Y.cINPUT 3.0. GEAR MANUFACTURINGGears 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 gears 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 CUTTINGIn 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. F T ra n sf o F T ra n sf o PD rm PD rm Y YYY er er ABB ABB y ybubu 2.0 2.0toto re re J3103/3/3 he hekk lic lic GEAR C C wom wom 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 After each tooth is cut, the cutter is withdrawn, the gear blank is rotated (indexed), and the cutter proceeds to cut another tooth.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 cutterGearblank Figure 3.1. Producing gear teeth on a blank by form cuttingBecause 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. F T ra n sf oF T ra n sf o PD rmPD rm YYY Y erer ABBABB yybu bu 2.02.0to to rereJ3103/3/4 hehek k liclic GEAR CC womwom ww w. w.A B B Y Y.cA 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 GENERATINGThe 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. F T ra n sf o F T ra n sf o PD rm PD rm Y YYY er er ABB ABB y ybubu 2.0 2.0toto re re J3103/3/5 he hekk lic lic GEAR C C wom wom 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 circleFigure 3.2. Gear generating in a gear shaper using a pinion-shaped cutterCutter spindle Gear teethSpacer Pinion-shapecutter Gear blank Figure 3.3.. Gear generating with a pinion-shaped gear cutter Cutting may take place at either the down stroke or theupstroke of the machine. Because the clearance required for cuttertravel is small, such as flanges (Fig 3.3). The process can be usedfor low-quantity as well as high-quantity production.3.2.2.On a rack shaper, the generating tool is a segment of arack (Fig.3.4) which reciprocates parallel to the axis of the gearblank. Because it is not practical to have more than 6 to 12 teethon a rack cutter, the cutter must be disengaged at suitable intervalsand returned to the starting point; the gear blank remain fixed. 6. F T ra n sf oF T ra n sf o PD rmPD rm YYY Y erer ABBABB yybu bu 2.02.0to to rereJ3103/3/6 hehek k liclic GEAR CC womwom ww w. w.A B B Y Y.cA 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 longitudinalslots or gashes into it to form the cutting teeth. When hobbing a spurgear, the angle between the hob and gear blank axes is 90o minus thelead angle at the hob threads. All motions in hobbing are rotary, thehob and gear blank rotate continuously, much as two gears meshinguntil all teeth are cut. Top view Gear blank HobGearblankFigure 3.5. View of gear cutting with a hob 7. F T ra n sf o F T ra n sf o PD rm PD rm Y YYY er er ABB ABB y ybubu 2.0 2.0toto re re J3103/3/7 he hekk lic lic GEAR C C wom wom w w w.w.A B B Y Y.c A B B Y Y.cHobs are available with one, two, or three threads. If the hobhas a single thread and the gear is to have 40 teeth, for example, thehob and gear spindle must be geared together so that the hob makes 40revolutions while the gear blank makes one revolution. Similarly, if adouble-threaded hob is used, the hob would make 20 revolutions to thegear blanks one revolution.In addition, the hob must be fed parallel to the gear axis for adistance greater than the face width of the gear tooth (Fig. 3.5) inorder to produce straight teeth on spur gears. The same hobs andmachines can be used to cut helical gears by tilting the axis of the hobspindle.Because it produces a variety of gears rapidly and with gooddimensional accuracy, gear hobbing is used extensively in industry.Although the process is suitable for low-quantity production, it is mosteconomical for medium to high-quantity production.Geargenerating machines can also produce spiral-bevel andhypoid gears. Like most other machine tools, modern gear-generatingmachines are computer controlled. Multi axes computer-controlledmachines are capable of generating many types and sizes of gearsusing indexable milling cutters. 3.3. CUTTING BEVEL GEARSStraight 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. F T ra n sf oF T ra n sf o PD rmPD rm YYY Y erer ABBABB yybu bu 2.02.0to to rereJ3103/3/8 hehek k liclic GEAR CC womwom ww w. w.A B B Y Y.cA B B Y Y.cGearblankCutter Figure 3.6. Cutting a straight bevel gear blank with two cutterThe machines for spiral bevel gears operate on essentially thesame principle. The spiral cutter is basically a face-milling cutter thathas a number of straight-sided cutting blades protruding from itsperiphery ( Fig.3.7 ).CutterGear blank Figure 3.7. Cutting a spiral bevel gear with a single cutter 9. F T ra n sf o F T ra n sf o PD rm PD rm Y YYY er er ABB ABB y ybubu 2.0 2.0toto re re J3103/3/9 he hekk lic lic GEAR C C wom wom w w w.w.A B B Y Y.c A B B Y Y.c 3.4 GEAR-FINISHING PROCESSESAs 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. ShavingThe gear shaving process involves a cutter, made in the exactshape of the finished tooth profile, which removes small amounts ofmetal from the gear teeth. The cutter teeth are slotted or gashed atseveral points along its width, making the process similar to finebroaching. The motion of the cutter is reciprocating. Shaving andburnishing can only be performed on gears with a hardness of 40 HRCor lower.Although