16-04-2012, 05:08 PM
AN OVERVIEW OF GEAR MANUFACTURING PROCESSES
AN OVERVIEW OF GEAR MANUFACTURING PROCESSES.pdf (Size: 2.41 MB / Downloads: 412)
GEARS IN AUTOMOBILE TRANSMISSIONS
A gear box transmits the engine power to the driving wheels with the help of different gearing systems. Different gear combinations are used to give the smooth running, the lower fuel consumption, and the optimum driving comfort. Generally, passenger car transmissions are provided with 4-5 forward speeds and one reverse speed. In front wheel models, hypoid gears have been replaced by helical gears. Fig. 4.1 shows a typical transaxle of front drive model. Involute splines, both external and internal, are also widely used on various shafts and hubs for slide meshings in transmission system. Bevel gear and pinion are still used in differential of automobiles. However, parallel axes spur and helical gears are the main gears in automotive transmission. Manufacturing of gears presents a demanding challenge for metallurgists in heat treatment, for supervisors in machining and gear cutting, and for quality engineers in keeping the quality to the required standards.
Fig. 4.1 A Typical Transaxle of a Modern Passenger Car
Gear manufacturing process dynamics are undergoing a major breakthrough in last two decades. Solutions being sought are not corrective but preventative. Normally, either soft gear process dynamic or hard gear process dynamic is being aimed. Objective is to cut the number of operations or machines through which a work gear needs to pass to attain the final specifications of dimensions and tooth form quality.
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In soft gear process dynamic, the gear teeth are generated by gear hobbing or shaping depending on the component design constraints. Soft finishing of gear teeth is carried out by gear shaving, rolling or grinding to attain the gear quality grade. Even after the heat treatment deterioration, the quality specification remains well within the desired final specification to meet product final performance requirements such as noise, etc.
In hard gear process dynamic, hobbed and/or shaped, or warm forged/rolled gears after heat treatment undergo final finishing operation, such as hard finishing, honing, or grinding. Overall economy becomes the deciding factor for selection of the process dynamic.
GEAR QUALITY AND MANUFACTURING PROCESSES
The functional necessity of a gear pair defines the limits of the deviations of all gear specifications. Gear quality refers to these permissible limits of deviations. Gear quality grades are standardised for different normal module/DP ranges and different ranges of reference diameters in AGMA, DIN, JIS and other standards. AGMA provides 8 grades from 15 to 8, where the higher grade number indicates the better gear accuracy. In DIN and JIS, a lower grade number means better gear accuracy.
Manufacturing processes used to produce finished gear specifications have certain capability limitations. Machine, work fixture, cutter, arbor, machined blanks, and also the cutting parameters add some amount of errors to different gear elements. Stages of manufacturing processes are to be accordingly decided. Fig. 4.2 gives a guideline for the capability of different manufacturing processes in terms of achievable quality grade requirements.
Fig. 4.2 Process Capability of Different Gear Manufacturing Processes
RAW MATERIALS FOR TRANSMISSION GEARS
Gears are generally designed for a finite life. Alloy steels are most favoured gear material. Case hardening steels provide the ideal features required for gear material. For gear teeth,
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the surface is to be hard with soft and tough core to provide wear and fatigue resistance. Case hardening steels do have varying chemical composition, and are named accordingly, e.g. Chrome Steel, Low molybdenum steel, Chrome molybdenum steel, Nickel-chrome-molybdenum steel.
Basic requirements of good gear materials may be summarised as follows:
1. Well controlled hardenability, that helps in getting consistent and predictable result after heat treatment. Hardenability is the property of a steel that determines the depth and distribution of the hardness induced by quenching.
2. Least non-metallic inclusions especially oxides that generally present machining difficulties.
3. Good formability for better forge die life and consistency of forge quality.
4. High and consistent machinability.
5. Low and stabilised quenching distortion.
6. No grain growth during present practice of high temperature carburising, which can cause higher quenching distortion and lower toughness.
During recent years significant progress has been made in production of steels ideally required for gear. Gear steels are being developed to have totally controlled hardenability reducing distortion or making it accurately predictable and repeatable. With improved steel making processes, chemical compositions are being established to reduce inter-granular oxidation. Toughness and fatigue strength are getting improved dramatically. All these are through the improved steel manufacturing technology - especially the development of secondary refining (vacuum degassing and ladle refining applying arc heating) and related techniques.
BASIC FORMING PROCESSES
Hot forging is most commonly used for gears. Maximum and highly uniform density is ensured by complete filling of forging die. During forging or upsetting, material grain is made to flow at right angle to the direction of the stress on gear teeth in actual dynamic loading. Uniform grain flow also reduces distortion during heat treatment. Generally shaft gears are upset. Even roll forging is used for cluster gears for high productivity. Cold/warm formings are high production though capital intensive methods used presently to produce gear blanks with much better dimensional control and about 20% material saving. Parts are formed without flash or mismatch. Draft angles are held to 1/2 degree on long parts and concentricity upto about 1 mm.
A good forging is a necessity. With faulty forging, no amount of excellence of design and care in manufacturing of gears from the best available material can ensure production of good quality gears. Machinability, ultimate strength, final quenching distortion, and surface finish will all be affected by the forging practices.
New cold forging methods produce a neat finished gear profile combining forming with rolling. Differential gears of automotive transmission are being commercially produced with neat tooth forms. Even, the gear teeth of spiral bevel gears are reported to be formed by plastic deformation of induction heated bevel gear blank using tooth rolling tool. The process produces a very high tooth finish, and results in a lot of material saving. On a larger gear, depending on application, a finishing operation of hobbing or grinding may be necessary with
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a material stock removal of 0.4 mm-0.8 mm on tooth flanks. Cold rolling is already practiced for high speed production of splines and serrations with many built-in advantages.
GEAR BLANK MACHINING
Quality of gear manufacturing starts with blank machining. Accuracy in blank machining is a necessity for attaining the desired quality standard of finished gears. According to shape, the gears are called round gears and shaft gears.
For round gears, the dimensional and/or inter-related tolerances that must be closely controlled are as follows :