Medical Design Briefs: 2021 Technology Leaders - 52

TECHNOLOGY LEADERS Motors/Motion Control
Design Strategies for Converting
Rotary Motion into Linear Motion
W
hen it comes to motion for
handheld and miniature
medical device designs, a
standard electrical mini
motor may seem like a good starting
point. But while a miniature motor generates
rotary movement, applications
like medical pipettes or syringes, and
mesotherapy devices often move loads
linearly instead of rotationally. For these
medical devices - as well as pick-andplace
machines and fluid regulation
valves for industrial applications - en -
gineers must develop their own system
to translate rotary motion into linear
motion.
As you focus on creating the core
product, it pays to delegate miniature
motor selection and transmission system
design to motion specialists who can
save valuable development time. This
article provides a detailed look at some
of the linear motion options available.
Rotation can be converted into linear
motion via a screw-and-nut system
assembled on the motor shaft. There are
two main types of screw-and-nut systems:
ball screw and lead screw.
A ball screw, shown in Figure 1, operates
on rolling contact between the nut
and a screw. The ball is recirculated
along a helical groove. The rolling components
keep friction low, while allowing
high efficiency greater than 90 percent
along with a high load capability.
A lead screw, shown in Figure 2, is
typically composed of a stainless-steel
screw and a plastic nut. Both components
are in direct contact, generating
more friction than the ball screw system.
This can be a good, economical
option if cost is a concern. The nut
material generally affects the life and
maximum load capability of the system.
However, with two preloaded nuts,
axial play can be eliminated. There are
typically two types of linear options to
consider:
* Option 1: The lead screw is directly
integrated into the motor.
* Option 2: The lead screw is mounted
on the motor shaft.
52
Fig. 1 - A ball screw system.
Fig. 2 - A lead screw system.
Option 1: Motor with Integrated Lead
Screw
Standard linear actuators, which are
often called digital linear actuators
(DLAs), are fully integrated linear
mechanisms that use a can stack stepper
motor. This is a generally cost-effective
choice. Inherent with stepper technology,
the motor itself is a positioning system,
so the control does not need position
feedback. The DLA can be driven in
full steps, half steps, or micro steps,
depending on the required resolution.
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In addition, the detent torque from the
motor allows the DLA to hold its position
when the power is removed.
For the linear transmission, a nut is
overmolded onto the rotor assembly, as
shown in Figure 3, with a special material
that optimizes the friction. This creates
an efficient system with a long lifetime.
Some products even have a special
ball bearing assembly in which the ball
bearings are preloaded with a wavy washer
to reduce axial play, also shown in
Figure 3. This improves linear positionMedical
Design Briefs 2021 Technology Leaders
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Medical Design Briefs: 2021 Technology Leaders

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