Biomedical Engineering Reference
In-Depth Information
maximum acceleration and, hence, the force required to move the follower (Ham et al.,
1958).
Cam mechanisms are fairly common in biomechatronic applications. They can be
used to generate specific motion profiles for prosthetic limbs or to drive pump diaphragms
in pulsatile artificial hearts.
3.6.3 Gears and Belt Drives
Gears and belt drives provide a means of matching the characteristics of a motor to the
load using circular elements of different diameters. Considering the case where all of the
power from the motor is transmitted to the load, the available load torque,
τ
L
, (Nm) is
τ
L
=
ω
m
r
L
r
m
τ
m
ω
L
τ
m
=
(3.63)
where
ω
m
(rad/s) is the speed of the motor,
ω
L
(rad/s) is the speed of the load, and
τ
m
(Nm) is the motor torque.
The gear (or belt drive) ratio
ω
L
/
ω
m
is determined by the ratio of the two pitch circle
radii
r
m
/
r
L
, which in turn determines the load speed as a function of the motor speed.
The reciprocal of this gives the load torque as a function of the motor torque, therefore
decreasing the output speed results in increasing available torqueand vice versa.
In Figure 3-67, where
r
1
40 mm is the pitch circle radius of the driven element and
the load is driven by the element with radius
r
2
=
=
20 mm, the output speed will be double
that of the input and the available torque will be halved.
FIGURE 3-67
Torque and speed
matching methods.
(a) Spur gears. (b)
Belt drives.
