15-01-2013, 01:52 PM
INVOLUTE SPUR GEARS
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INTRODUCTION
In lecture 2 various terminologies used in describing a gear and a gear pair in mesh was
dealt in detail. For convenience, the gear nomenclature is reproduced in Spur gear nomenclature
Machine Design II Prof. K.Gopinath & Prof. M.M.Mayuram
Indian Institute of Technology Madras
observed here that by varying the various parameters, it is possible to get infinite varieties
of gears. In practice if we use such large varieties of gears, then the manufacture, quality
check, replacement in the case of failure all become more complicated. To overcome this,
standard gear systems are evolved and these are dealt in detail here. Earlier the definition
of interference in gears was illustrated by many figures and methods of avoiding
interference were dealt in detail. For easy recollection of interference, refer to
where the interference portion of the teeth is shown in red. Interference in gears
STANDARD TOOTH SYSTEMS FOR SPUR GEARS
To reduce the varieties of gears to a manageable numbers, standards are evolved.
Standard makes it easy for design, production, quality assurance, replacement etc. Three
commonly used pressure angles are 14.5o, 20o and 25o pressure angle systems as shown
in Fig. 3.3. In this, one can have full depth gears or stronger stub tooth gears. In Standard
tooth system for metric gears, addendum: a =1m, dedendum: b= 1.25m where as the for
the stub tooth gears, addendum a = 0.8m and dedendum: b= 1.0m. The shorter tooth
makes it stronger and its load carrying capacity increases. It also helps in avoiding
interference in certain cases.
PROFILE SHIFTED GEARS
Interference in smaller number of tooth pinions can be avoided by having unequal
addendum, longer addendum for the pinion and shorter for the gear as seen in the earlier
figure. These are called profile shifted gears or non-standard gears (Fig. 3.4). AGMA
defines the addendum modification coefficient as x1 and x2 which sums up to zero, being
equal in magnitude and opposite in sign.
WHAT PROFILE SHIFTING DOES TO GEARS
1. The pinion becomes thicker at the base and thus stronger
2. The gear tooth correspondingly weakens since full depth gear tooth is stronger than full
depth pinion tooth, thus equalizes the strength.
3. The unequal addendum tooth forms increase the sliding velocity at the tooth tip.
4. Consequently tooth surface stresses increase.
5. The friction losses in the gear mesh also increase at high sliding velocities.
WHEN PROFILE SHIFTING IS USED
1. When the interference is to be avoided
2. When predetermined centre distance has to be attained
3. To increase the strength at the root and flank of the teeth.
4. To improve sliding and contact relation.
5. To shift the beginning of the effective profile away from the base circle.