The typically large variation in accelerations within a mechanism can cause significant
oscillations in the torque required to drive it at a constant or near constant speed. The
peak torques needed may be so high as to require an overly large motor to deliver them.
However, the average torque over the cycle, due mainly to losses and external work done,
may often be much smaller than the peak torque. We would like to provide some means
to smooth out these oscillations in torque during the cycle. This will allow us to size the
motor to deliver the average torque rather than the peak torque. One convenient and relatively
inexpensive means to this end is the addition of a flywheel to the system.
TORQUE VARIATION Figure 11-8 shows the variation in the input torque for a
crank-rocker fourbar linkage over one full revolution of the drive crank. It is running at
a constant angular velocity of 50 rad/sec. The torque varies a great deal within one cycle
of the mechanism, going from a positive peak of 341.7 Ib-in to a negative peak of
-166.41b-in. The average value of this torque over the cycle is only 70.21b-in, being
due to the external work done plus losses. This linkage has only a 12-lb external force
applied to link 3 at the CG and a 25 Ib-in external torque applied to link 4. These small
external loads cannot account for the large variation in input torque required to maintain
constant crank speed. What then is the explanation? The large variations in torque are
evidence of the kinetic energy that is stored in the links as they move. We can think of
the positive pulses of torque as representing energy delivered by the driver (motor) and
stored temporarily in the moving links, and the negative pulses of torque as energy attempting
to return from the links to the driver. Unfortunately most motors are designed
to deliver energy but not to take it back. Thus the "returned energy" has no place to go.
Figure 11-9 shows the speed torque characteristic of a permanent magnet (PM) DC
electric motor. Other types of motors will have differently shaped functions that relate
motor speed to torque as shown in Figure 2-32 and 2-33 (pp. 62-63), but all drivers
The typically large variation in accelerations within a mechanism can cause significantoscillations in the torque required to drive it at a constant or near constant speed. Thepeak torques needed may be so high as to require an overly large motor to deliver them.However, the average torque over the cycle, due mainly to losses and external work done,may often be much smaller than the peak torque. We would like to provide some meansto smooth out these oscillations in torque during the cycle. This will allow us to size themotor to deliver the average torque rather than the peak torque. One convenient and relativelyinexpensive means to this end is the addition of a flywheel to the system.TORQUE VARIATION Figure 11-8 shows the variation in the input torque for acrank-rocker fourbar linkage over one full revolution of the drive crank. It is running ata constant angular velocity of 50 rad/sec. The torque varies a great deal within one cycleof the mechanism, going from a positive peak of 341.7 Ib-in to a negative peak of-166.41b-in. The average value of this torque over the cycle is only 70.21b-in, beingdue to the external work done plus losses. This linkage has only a 12-lb external forceapplied to link 3 at the CG and a 25 Ib-in external torque applied to link 4. These smallexternal loads cannot account for the large variation in input torque required to maintainconstant crank speed. What then is the explanation? The large variations in torque areevidence of the kinetic energy that is stored in the links as they move. We can think ofthe positive pulses of torque as representing energy delivered by the driver (motor) andstored temporarily in the moving links, and the negative pulses of torque as energy attemptingto return from the links to the driver. Unfortunately most motors are designedto deliver energy but not to take it back. Thus the "returned energy" has no place to go.Figure 11-9 shows the speed torque characteristic of a permanent magnet (PM) DCelectric motor. Other types of motors will have differently shaped functions that relatemotor speed to torque as shown in Figure 2-32 and 2-33 (pp. 62-63), but all drivers
การแปล กรุณารอสักครู่..
![](//thimg.ilovetranslation.com/pic/loading_3.gif?v=b9814dd30c1d7c59_8619)