cycles per revolution of the input shaft. This fluctuation, which is accompanied by corresponding angular accelerations, increases with the operating angle and can be as much as 5 of peak angular velocity (in the case of a 30° operating angle). In selecting a joint the effect of these fluctuations on static torque, inertia torque and system performance needs to be kept in mind.
The nonuniformity of the
transmission can be eliminated by using two appropriately phased universal joints in
series, as shown in Figure 2. In such cases the velocity variation induced by one joint
can be made to cancel that of the other, thereby transmitting a constant (1:1)
angular velocity ratio between shafts. The angular velocity Fluctuation of the
intermediate shaft, however, cannot be avoided.
Two universal joints in series also permit coupling of two
laterally displaced shafts (single Cardan joints are limited to intersecting shafts).
Single Cardan joints have the following advantages:
• Low side thrust on bearings.
• Large angular displacements are possible.
• High torsional stiffness.
• High torque capacity.
They have the following disadvantages:
• Velocity and acceleration fluctuation increases with operating angle.
• Lubrication is required to reduce wear.
• Shafts must lie in precisely the same plane.
• Backlash difficult to control.
4.0 KINEMATICS
For a uniformly rotating input shaft, the
output shaft angular-velocity and angular acceleration undergo two cycles per revolution
of the input shaft. The angular displacement of the out’ put shaft does not precisely
follow that of the input shaft but leads or lags, again with two cycles per revolution.
The angular-velocity variation as a function of
operating angle is illustrated in Figure 3.
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