Medical Design Briefs: 2021 Technology Leaders - 50

TECHNOLOGY LEADERS Motors/Motion Control
Mechatronic Integration for
Precision Medical Device Linear Motion
D
uring the 21st century, our society
has been at the forefront of
discovering new solutions
through advanced technology
for medical device and clinical laboratory
applications. These innovative solutions
have utilized linear motion to
develop state-of-the-art medical imaging,
diagnostic, and surgical equipment.
Linear motion is a common need for all
types of advanced equipment and
machines; but precise, smooth, reliable,
and repeatable linear motion is fundamentally
important for several applications
within the medical industry. Pre -
cision ground ball screws have become
the preferred choice for precise linear
motion because they deliver smooth and
accurate movement ensuring reliable
and repeatable results.
Overview
The demand to reduce the size of linear
screw actuators, but maintain precision,
is evident in several medical diagnostic
and imaging applications. A diagnostic
device, such as a desktop blood
analyzer, can process multiple samples at
a faster speed and higher accuracy in
comparison to its predecessor that was
the size of an automobile. Healthcare
practitioners rely on small ball screw actuators
on syringe pumps to accurately dispense
precise levels of medication. In
imaging applications, such as CT scanners,
small actuators position aperture
plates to control the x-ray beam. In each
of these applications, a lead screw or ball
screw is combined with a small electric
motor to translate the motor's rotary
motion into linear motion (see Figure 1).
Lead Screws vs. Precision Ground Ball
Screws
In order to achieve linear motion for
medical applications, the traditional
solution combines a lead screw with an
electric stepping motor. Lead screws use
a V-shaped helical thread on the shaft
with a matching thread inside the nut,
similar to a nut and bolt you can get at a
hardware store. Lead screws (and ACME
50
Efficiency of Normal Operation: Converting
Rotary Motion to Linear Motion
μ: friction coefficient
100
μ=0.003
10
20
30
40
50
60
70
80
90
2
0123456789 10
Lead angle (degree)
Ball Screw Acme Screw
Fig. 1 - Ball screw vs. lead screw efficiency.
screws) rely on sliding contact between
the surfaces of the nut thread and the
shaft thread to produce linear motion.
The benefit of a lead screw is the small
nut relative to the axial load it can support
due to the large flat contact surface
areas of the mating threads. However,
the flat surfaces sliding across each
other generate heat caused by friction. A
large motor is required to overcome friction.
As a result, this solution does not
provide smooth consistent motion and
will require more maintenance due to
considerable wear. Backlash is inherent
with the typical lead screw design caused
by clearance between the nut thread and
the shaft thread. The backlash affects
the linear positioning accuracy of the
screw. (see Figure 2). NSK understands
the importance of minimizing or eliminating
backlash and now offers an alternative
solution.
NSK sought to integrate its precision
ground ball screw into an electric stepping
motor resulting in its motorized ball
screw actuator (MBSA) series. Similar to
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a lead screw, a ball screw also has a helical
thread on the shaft and inside the nut;
but a ball screw uses precision ball bearings
between the nut and the shaft.
Therefore, the ball screw thread profile is
different than that of a lead screw. The
ball screw has a U-shaped groove, or
Gothic arch, to house the precision ball
bearings (see Figure 3). The nut thread
profile acts as the outer raceway, and the
groove along ball screw shaft acts as the
inner raceway for ball bearing travel.
The ball bearings provide a rolling
contact point between the nut and the
shaft that lowers the coefficient of friction.
The result is a highly efficient
(from 90 to 95 percent) mechanism that
requires less torque to convert rotational
motion into linear motion, making ball
screws a better fit for the challenges
inherent to medical applications. By
using a precision ground ball screw, the
application results in lower operating
temperatures, smoother motion, re -
duced motor size, less energy consumed,
less wear, and longer life in comparison
to a lead screw solution.
In order to adhere to small size constraints
for the nut body, NSK uses a
Medical Design Briefs 2021 Technology Leaders
Fig. 3 - Precision ground ball screw gothic arch raceway.
μ=0.005
μ=0.001
DESIGN FEATURES
›
support bearings
›
›
›
›
μ=0.2
μ=0.1
Ball Nut
Gothic Arch
Screw
Shaft
Balls
Screw
Fig. 2 - Lead screw nut and shaft showing axial and
radial clearance.
thread contact
backlash - thread clearance
direction
of load
Nut
Efficiency h1
(%)
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