L491 48254 1744 yoo at lg ‘Spur Gears (Gear Rack (Worm Gears Bevel Gears )(_Miter Gears (Internal Gears (Screw Gears Home SELECTING stock GEARS GEAR CATALOG GEAR DRAWING GEAR KNowLepce HOW TO BUY KNOW ABOUT PARAMETERS THAT DETERMINE GEAR SHAPES OP. > Gearknowledge, > The fist step of mechanism desion using gears. > Know about parameters that determine gear shapes Understanding various technical terms Is essential n designing gears. Inthis chapter, we exon the terms anc their meanings of gear parameters. 1. Gear shapes In general tooth profiles with ivolute curves are used for shaping gears. (Figure 41) “Involute curve” is the curve drawn by the end ofthe thread which Is belng unwound from aeylinder under tension ‘The characteristics of the Invalite curves are Meshes properly even when the distance between shafts has a slight error Easy to form correct tooth profile Rotates smoothly since the gears mesh whilerling on a curve Involute Curve ha Figure 4-1 Involute curve 2. Types of design parameters necessary for gear designParameters needed for gear design are shown below. (Figure 42) 2. Reference circle 3. Base ckcle 4 Boot citcle 5.Tipdiameter 6 Reference diameter 7. Base diameter 8. Root diameter 9. Addendum 40. Dedendum 11. Tooth depth 12 Facewidth 18. Centerline 14, Center distance 15 Reference pitch 16. Fitch point 17. interference point 18 Backlash 19. Length of path of contact {ine of action) Figure 4-2 Parameters necessary for gear design 3. Basic parameters necessary for gear design 1. Module “The modules the size ofa tooth in mm. ‘As the module cannot be measured directly witha caliper o a scale, novice designers may be perplexed by the module at first when designing a gear. “The module i the numeric value obtained by dividing the pitch clameter by the number of teeth and is expressed by the formula below m=d/z ‘m: module, pitch diameter, 2: number of teethThe value ofthe module is determined by the standard numbers, “The standard number isthe numeric value to be commonly used when manufacturing industrial products. According to JIS 3 1701-2017, the standard values ofa module for involute gears are set as shawn below. Use of modules on | series are preferred, (Table 4) “Table 4-1 Module values that are commonly used (JS 8 1701-2017) Iseres [01 02 03 04 05 06 08 1125 15 225345 6 8 10 12 16 20 25 WA) 80 T DIS O05 O35 OAS OSS OF OFS OF TIS 13S 17S 2 DS aS AS SS GD TS we series _| 45 “The module is changed depending on PV values ‘The PV value is obtained by multiplying the load pressure P by the slip speed V. “Thetefor, the module needs to be set lager as the load applied tothe gear and its rotational speed increases. On the other hand, the module can be set smaller ag the loa¢ appled to the gear and its rotatlonal speed decreases. It must be very dificult to choose te size ofthe module when designing gears forthe fist time. Designers must proceed with the design workby adjusting various parameter values including module as well as examining size and strength “There are two ways to temporarily determine the module value. 1, Determine the module value temporarily empirically Understanding modules of gears used in various products helps to determine the module value ©) Wrist watches .. approx. 0.05-0.2 Printers and copiers in office. approx. 0.61.0 Racks and pinions for ear steering. aporox.1.75-2.5 “Transmission for ear. approx. 1.53.0 2. Determine the module value temporarily from a catalog of commercial standard gears Catalogs have allowable torque values by material or module, You can referto the values to determine the mocule temporally, 2. Pressure angle “The pressure angles the angle at which the tooth nclnes against the normal line tothe pitch line. (Figure 4-3) General, a pressure angle of 20" is used. However, pressure angles of 14.5" or 17.5 are also sometimes used, The larger the pressure angles, the ddedendum becomes widor and strength improves, 1. Addendum 2. Tooth surface 3. Dedencum 4, Normal line tothe pitch line 5 Pressure angle 6. Pitch 7.Piteb ine Figure 4-3 Pressure angle (standard basie rack tooth profile) 3. Number of teeth You need to determine the numberof teeth ofa par of meshing gears to caleulate the speed ratio as explained nthe eecond article, Be careful about ‘minimum numberof teeth (except for worm). “The numberof teth of the pinion needs tobe reduced as much as possible to enlarge speed reduction or increase rato. However, the tp of the rack type ‘tool may overlap with the dedendum andthe tip may gauge the dedendum if te number of teeths below a certain valu. (Figure 4-4)"Dedendum looks fragile and could break when the numberof teoth i 10. Figure 4-4 Tooth tapered toward dedendum due to undercut ‘This is called “interference of tooth” and the tooth tapered toward the dedendum is called “undercut”, (Figure 4-5) Undercut Figure 4-5 Undercut ‘The limit ofthe numberof teeth to prevent the undercut is given by the following formula, When pressure angle a= 20, the imiting numbers 17 Z=2/sin’a However, itdoes not mean the gear with undercut is useless. Generally the practical minimum number ofteth is 14 To prevent undercut, you can use “shit” (changing distance between the gear cutting tool and the geat). Due tothe limitation ofthe minimum numberof teeth, the diameter ofthe pinion cannot be extremely small, Therefore, if you want to increase the reduction fall the meshing gear becomes large, and you need to take into account the space required in your design. 4. Twist direction Gears with righthand teeth have the teeth inclined tothe rght relative tothe gear shaft when you hold the shaft upright, while gears with lefthand teeth have the teeth inclined tothe left. As for racks and worms, the gears with righthand teeth have the teeth rse tothe right when you hol the shaft upright hile the gears with lefthand teth rise tothe left pair af helical gears or epral bevel gears needs to have the same module and pressure angle but in addition, you need to pay attention to the twist ‘ection, {As for teeth of helical gears, spiral bevel gears and racks, the twist direction of gears should be opposite to each other to mesh, (Figure 4-6) “Two helical gears which have the teth incline tothe same direction never mesh, so you need to prepare two helical gears whose twist directions are opposite Righthand teeth /Lefthand teeth Pinion: lefchand teeth, Rack right-hand teethPinion righthand teeth, Rack: left-hand teeth Oo Righthand teth /Lefthand teeth Figure 4-6 Gears whose twist directions are the opposite (nthe ather han, twist directions of gear teeth of screw gears or worm gears used for skew shafts need tobe the same to mesh (Figure 47) Rightshand teeth / Leftthand teeth Right-hand teeth / Left-hand teeth fini thy Right-hand teeth / Leftthand teeth Right-hand teeth / Left-hand teeth a) Screw gear b) Worm gear Figure 4-7 Gears whose twist directions are the same “Meshing twist directions vary according to the gear types * 5. Torsion angle ‘The tooth is inclined relative tothe axis ofthe cylinder, ‘This inclination amount is called “torsion angle. (Figure 4-8) The screw gears shown in Figure 47 have the torsion angle of 45" ‘You can decice the torsion angle of meshing two gears freely, but note that the trust ciection (axa direction) component becomes larger as the torsion angle inereases, and the machine efficiency deteriorates as a result, Below 25" is desirable for general helical ears to avo too much thrust.Torsion angle LUN Figure 4-8 Torsion angle of helical gear We discussed parameters necessary for gear design and their meanings. Next we wll exalain types of gear material (Tobe continued.) “tlustration: KAOSUN DISCLAIMER “The purpose of writing this article was to educate the readers withthe elementary level of gear technology. We hope thatthe actual design and manufacturing of gears and machinery utlzing gears are done with sufficient technical and specialized considerations under the use’ ful responsislty We disavow any labilty and will not compensate for any ctect or indirect damages caused by the gears designed by the users who read this article, Kohara Gear Industry Co.,Ltd Nakacho Kawaguchi-shi Saitama-ken TEL: +81 48 254 1744 FAX: +81 ‘opytight© 2021 Kohara Gear industry Co,Ltd. All Rights Reserved.