pressure angle is an important parameter in gear design.
because of this pressure angle only power is transmitted.
ideal pressure angle is zero. sice no radial seperating
force will be acting for zero pressure angle. now a days we
use 20 degree presure angle
Pressure angle is an important factor in design of spur
gear.If the pressure angle is high it results in a noisy
operation.
self locking means worm and worm gear gets locked to itself
preventing further power transmission.
self locking means there is "no back drive" between worm
and worm wheel.It says we can transmit power from worm to
worm wheel only and vice versa is impossible.
SELF LOCKING
self locking is the phenomena of pressure angle and lead angle.
in worm gears during operation in reverse direction, the
pressure angle exceeds the lead angle and thus lock the gear
mechanism known as self locking
the condition in which worm gear doesnot turn the worm is
called self-locking.it depends on the lead angle and
coefficient of friction between them.if the lead angle is
less than coefficient of friction then self-locking occurs
pressure angle is the angle between the line of action and
the tangent drawn to the pitch surface at the pitch
point.when the gears mesh each other,the power transmission
is only due to the contact force at the pitch point.if the
pressure angle is high then one component of this force will
act towards the bearings and thus bearing force increases
which result in wear.and the another component of this force
transmits the power.
*Pressure angle is the angle at which the driving gear
applies the normal force on the driven gear.The tangential
component of this normal force (Fncos(alpha))is what
contributes to the torque transmission and the radial
component(Fnsin(alpha)) only loads the shaft. Higher the
pressure angle lower the component that contributes to torque.
* Normally the driving member in a worm gear drive is the
worm. That is, it is used for speed reduction. The wheel
does not drive the worm. Motion is from worm to wheel.Hence
it is termed as self locking. It is a safety feature as in
cases of sudden power failure, the load being lifted will
not fall down as in the case of a hoist.However the wheel,
under certain conditions of lead angle,can drive the worm
worm gears become reversible.
Pressure angle is the angle between tangent of pitch line
normal to that pitch line. if pressure angle will be less
then failure of gear occurs.
Self locking means meshing of gear after same time of
interval that is contcat ration of each gear will be after
same time of period
pressure angle in gear set is defined as the angle between
the line of action (common normal) and the line
perpendicular to the line of center.pressure angle rotated
in the direction of driven gear.
Significance:
increase of pressure angle
1-reduction in the danger of undercutting and interference
2-reduction of slipping speed
3-increase rigidity of toothing
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