INTRODUCTION
Gears are used extensively for transmission of power. They find application in : Automobiles, gear boxes , oil engines, machine tools, industrial machinery, agricultural machinery, geared motors etc. To meet the strenuous service conditions the gears should hace: robust construction, reliable performance , high efficiency,economy and long life. Also, the gears should be fatigue free and free from high stresses to avoid their frequent failures. The gear drives should be free from noise, chatter and should ensure high load carrying capacity at constant velocity ratio. To meet all the above conditions, the gear manufacture has become a highly specialized firld. Below,we shall discuss the various materials and manufacturing processes to produce gears.
MATERIALS USED IN GEAR MANUFACTURING PROCESS
The various materials used for gears include a wide variety of cast irons, non ferrous metal & non-material materials the selection of the gear material depends upon:-
Type of Service
Peripheral speed
Degree of accuracy required
Method of manufacture
Required dimensions & weight of the drive
Allowable stress
Shock resistance
Wear resistance
1) Cast iron is popular due to its good wearing properties, excellent mach inability & Ease of producing complicated shapes by the casting method. It is suitable where large gears of complicated shapes are needed
2) Steel is sufficiently strong & highly resistant to wear by abrasion.
3) Cast steelis used where application for industrial gears where high toughness combined with strength.
4) Plain Carbon steels find application for industrial gears where high toughness combined with high strenght.
5) Alloy steels are used where high tooth strength & low tooth wear are required.
6) Aluminium is used where low inertia of rotating mass is desired.
7) Gears made of non-Metallic materials give noiseless operation at high peripheral speeds.
GEAR MANUFACTURE BY CASTING METHOD
Gears can be produced by the various casting method processes. Sand casting is economical and can take up large size and module, but the gears have rough surfaces and are inaccurate dimensionally. These geras are used in machinery where operating speed is low and where noise and accuracy of motion can be tolerated, for example, farm machinery and some hand operated devices. Send Casting is suitable for one off or small batches. Large quantities of small gears are made by 'Die-Casting. These gears are fairly accurate and need little finishing. However the materials used are low melting ones, such as alloys of zinc, aluminium and copper so, there gears are suitable for light duty applications only (light loads at moderate speeds), for example, gears used in toys. Gears made by 'Investment Casting' may be accurate with good surface finish. These can be made of strong materials to withstand heavy loads. Moderate- size gears are currently being steel cast in metal moulds to produce performs which are later forged to size. Light gears of thermoplastics are made by 'Injection Moulding'. This method is suitable for large volume production. However, gear tooth accuracy is not high and initial tool cost is high. These gears find use in instruments, household appliances etc.
These gear boxes are made by resin sand casting
Method : Sand Casting, Resin Sand Casting
Material : Gray Iron and Ductile Iron
Casting Iron in sand mold
GEAR GENERATING PROCESS
Gear Hobbing:
Hobbing is the process of generating gear teeth by means of a rotating cutter called a hob. It is a continues indexing process in which both the cutting fool & wthe gear blork piece rotate in a constant relationship while the hob is being fed into work. For in route gears, the hob has essentially straight sides at a given pressure angle. The hob and the gear blank are connected by means of proper change gears. The ratio of hob & blank speed is such during one revolution of the hob, the blank turns through as many teeth. The teeth of hob cut into the work piece in Successive order& each in a slightly different position.Each hob tooth cuts its own profile depending on the shape of cutter, but the accumulation on the shape of cuts produces a curved form of the gear teeth, thus the name generating process. One rotation of the work completes the cutting up to certain depth.
GEAR SHAPING ( THE FELLOWS PROCESS)
In gear shapers, the cutters reciprocate rapidly. The teeth are cut by the reciprocating motion of cutter. The cutter can either be 'rack-type cutter' or a rotary pinion type cutter'.
Gear Cutting by milling:
-Disc type cutter
For cutting a gear on a milling machine, the gear lank is mounted on am arbor which is supported b/w a dead centre & a lieu centre in the in dering head. The cutter is mounted in the arbor of the cutter must be aligned exactly vertically with the centre line of the indexing head spindle. The table of machine is moved upward until the cutter clears the gear blank. The table is then moved upwards by an amount Equal to the full depth of the gear tooth. The vertical movement may be less if the gear is to be cut in two or more passes. After this, the longitudinal feed of the table is engaged. The gear blank moves under the rotating cutter & a tooth space is cut. After this, the movement of the table is reversed so that the cutter again clears the gear blank. The gear blank is then indexed to the next position for cutting the second tooth space. This procedure is repeated until all the teeth have been milled.
There is a flat circular disc type cutter and the plane of rotation of the cutter is radialwith respect to the blank.
-End mill cutter
In this method the cutter rotates about am axis which is set racially with respect to the blank. The cutting edge tie on a surface of revolution, so that any axial cross-section of the cutter corresponds to the shape required for the space between two adjacent teeth on the finished wheel. The milling machine used in this method is vertically milling machine.
The End mill cutter is mounted on the milling machine spindle through a chuck.
1) The this type of cutter is used to cut big spur gear of cutter is Employed for the manufactureof pinion of large pitch.
2) This method is very slow since only one tooth is cut at a time. To overcome these drawbacks, ''multiple tools shaping cutter head'' is used to cut all the tooth spaces of the gear at the same time.
Gear shaping
BEVEL GEAR GENERATING
The teeth of bevel gears constantly change in form, from the large to the small. There are to common types of bevel:-
1) Straight Bevel- gear generator:
For generating straight- bevel gears, the rolling motions of two pitch cones are employed motions of two pitch cones are employed instead of pitch cylinder.
In this method, two reciprocating tools which work on top & bottom sides of a tooth & are carried on the machine cradle. The cradle & work roll up together with the gear blank at the top of roll, when a tooth has been completely generated, the work is withdrawn from the tool and the machine inclined, while the cradle is rolled down to the starting position.The operating cycle is repeated automatically until all the teeth in the gear have been cut.
The advantages of this process are that a previous roughening cut is not necessary, thus saucing one handling of the blank, longer cutter life, improved quality of gear and less set-up time.
2) Spiral Bevel-Gear Generator:-
In this method, a rotating circular cutter generates spiral teeth that are curved & oblique proper tooth profile shapes are obtained by relative motion in the machine between work cutter. The machine has adjustment by which both spiral- bevel gears & hypoid gears can be generated.
Spiral bevel Gears have an advantage have on advantage over staright bevel gear is that teet are Engage with one another gradually by eliminating any noise & shock in their operation.
Bevel gear cutting Before: Gear Blank
After : Bevel Gear
