Gears are machine elements that transmit motion by means of successively engaging teeth. The gear teeth act like small levers.There are two profiles of gears :
1.Involute
2.Cycloidal
What is involute gear?All the gear teeth have top flat portion and two side curves. The side curves for the involute gears are in the form of involute curve of a circle.Involute curve of a circle can be generated by the locus of an end point of an imaginary taut string unwounding from the circle.
What is cycloidal gear?Cycloid is a curve generated by locus of any point on a circle which is rolling around another circle. If the second circle rolls outside the first circle then the generated curve is called epicycloid and if it rolls inside the first circle then the generated curve will be hypocycloid. The gear whose teeth profile is made up of cycloidal curves is called cycloidal gear. Each tooth profile will be combination of epicycloid and hypocycloid curves.
DIFFERENCE BETWEEN INVOLUTE AND CYCLOIDAL GEAR PROFILE :
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S.No
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Involute Tooth Profile
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Cycloidal Tooth Profile
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1
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Being the angle made by the common tangent of
base circles with a common tangent to pitch circles at pitch point, the
pressure angle remains constant throughout the engagement. This ensures
smooth running of the gears.
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Pressure angle varies continuously; being zero at
the pitch point and maximum at the point of engagement and disengagement.
This causes continuous variation in power component and also in bearing load.
The running is less smooth.
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2
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Involute tooth profile consists of a single (involute) curve and the
track cutter used for regenerating the profile has straight teeth. The rack
cutter is cheaper and the method of manufacture is simpler. This leads to
reduction in the cost of manufacture of involute teeth.
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The cycloidal tooth profile consists of two curves (epicycloid and
hypocycloid). The method of manufacture is more involved and leads to
costlier gear tooth.
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3
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Perhaps the most desirable feature of involute
teeth is that a small variation in centre distance does not change the
velocity ratio. Thus distance between shafts need not necessarily be
maintained exact as per design specifications. This gives great flexibility
during assembly and larger tolerances may be permitted.
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Exact centre distance is necessary to transmit
constant velocity ratio.
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4
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Since involute curve doesn’t exist within base circle, interference is
always possible if base circle radius is larger than dedendum circle radius.
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Since outside the pitch (directing) circle epicycloidal curve exists
and inside it the hypocycloidal curve exists, cycloidal curve can exist
everywhere on tooth profile and no interference exists.
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5
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The radius of curvature of involute curve, near
the base circle, is quite small and contact stresses are likely to be very
high. The tooth profile in flank portion is almost radial. Both the factors
together produce a tooth weaker in flank region compared to cycloidal tooth.
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Cycloidal curve (hypocycloidal in particular)
produces a spreading flank and, for this reason, cycloidal tooth is
stronger .
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6
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In involute tooth profile gears, convex surface of pinion tooth comes
in contact with convex portion of gear tooth and this leads to more wear.
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In cycloidal tooth profile epicycloidal shape of face of gear tooth
comes in contact with hypocloidal flank portion of pinion tooth. Thus a
convex flank has a contact with concave face which results in lesser wear.
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Involute tooth profile is almost used generally everywhere and given preference over cycloidal tooth profile or any other gear tooth form due to following advantages:
• Involute gears can handle centre sifts of the gears better. This provides assembly flexibility.
• Involute gear produces lesser noise than cycloidal gears.
• Manufacturing of accurate involute gear teeth is easy.
