Biomedical Engineering Reference
In-Depth Information
FIGURE 3-72
Translation screw
operation.
In a toothed belt, the sheaves have axial grooves that engage teeth on the belt. The
belts usually include a number of thin steel cables that carry tension under load, which
permits a light drive to operate at high speed (up to 100 m/s). These drives are compact,
light, quiet, and low maintenance. However, they are more sensitive to misalignment than
flat or V belts (Black and Adams, 1968).
3.6.4 Translation Screw Devices
Translation screws are commonly used compact devices to convert rotary motion produced
by a motor into linear motion against large forces. In most translation screws, the screw
rotates in its bearings, and the nut moves axially, as shown in Figure 3-72. However, in
some devices the nut rotates while the screw moves axially with no rotation.
Consider that the nut of a screw is moved against an axial load by the rotation of the
screw, illustrated in Figure 3-73. The load on the nut will be transferred to the screw as
a distributed load on the surface of the threads in contact. To facilitate analysis, it can be
assumed that the load is concentrated at point
o
on the mean circumference,
π
d
(m), of
the thread.
Under static conditions the direction of the load on the thread must be normal to the
thread surface, along the vector
ao
as shown in Figure 3-73. As the screw rotates so that
the nut is moved against an external force,
Q
(N), the line of action
ao
will be rotated
through the angle of friction,
(rad), to
bo
.
For equilibrium of forces, the component of
bo
parallel to the axis of the screw is
φ
Q
=
bo
cos
(α
+
φ)
(3.65)
where
α
(rad) is the lead angle of the screw, and
φ
(rad) is the friction angle.
The component of
bo
at right angles to the axis of the screw is
F
=
bo
sin
(α
+
φ)
(3.66)
FIGURE 3-73
Forces on a square
threaded translation
screw.
