Medical Design Briefs - July 2021 - 13

this area can be
Ball Nut
deflector
deflectorbridge
type
Screw
Shaft
Fig. 4 - Deflector-type ball screw.
Ball Nut
deflector
used as a pilot surface
for instruments
or components attached
to the nut
(see Figure 4).
How to Prevent
Backlash. To eliminate
backlash or axial
play
between
the
screw shaft and the
Screw Shaft
oversized ball
Fig. 5 - Ball screw preload using oversized ball bearing.
Ball Nut
lead
lead
nut, NSK uses slightly
oversized balls to create
a light preload
(see Figures 5
and 6). The
elastic deformation
of the
balls creates
an internal force
between the nut and the
screw shaft to eliminate clearance. The
result is precise linear movement of the
object attached to the ball screw nut with
zero backlash between shaft rotation
and nut linear movement. This preload
allows for better positioning control for
medical applications with multi-directional
axial loads.
4-point contact
Screw Shaft
Fig. 6 - Ball screw with four points of contact.
bridge-type deflector
recirculation
piece to guide
the ball bearings
between
adjacent
ball thread
grooves or
raceways
providing
the recirculation
necessary
for a
ball screw.
Since the
deflectors
sit
below the
surface of
the
nut
Motorized ball screw
actuator (MBSA).
Medical Design Briefs, July 2021
Intro
Cov
body outside
diameter
,
High positioning accuracy is the result
of a precision ground ball screw with a
preloaded nut that eliminates backlash.
Lead accuracy is measured by calculating
the difference between the actual
linear distance traveled by the ball screw
nut in one shaft revolution versus the
theoretical distance traveled (equal to
the lead). The International Organization
for Standardization (ISO) has set
lead accuracy standards for precision
ball screws. To conceptually visualize the
accuracy, the average human hair is
0.080 mm in diameter. For the MBSA
Series, NSK uses a C3 accuracy ball screw
which has a lead accuracy of 0.006 mm
per shaft revolution. The stepping
motor paired with the ball screw has 200
counts per revolution, resulting in a
motor resolution of 0.005 mm for a 1
mm lead ball screw. When measuring
lead accuracy over 300 mm (or 1 ft) of
travel, the actual nut position deviation
from the theoretical nut position is a
maximum of 0.008 mm for a C3 accuracy
ball screw. Comparatively, a lead screw
can have a deviation as large as 0.250
mm in 300 mm of travel. Therefore, the
MBSA Series is the optimal solution for
www.medicaldesignbriefs.com
ToC
+
-
A
µ
Motorized ball screw actuator
(MBSA) used in a blood analyzer.
highly precise medical devices and laboratory
equipment.
Integrating Ball Screws and Motors. In
the development of the MBSA Series,
NSK replaced the electric motor output
shaft with a journal that is part of the ball
screw shaft. The motor directly rotates the
ball screw shaft, which translates the nut
for linear motion. This design eliminates
the need for a separate motor-to-ball
screw coupling. The MBSA Series allows
for a compact space-saving design that
reduces system inertia and eliminates
alignment error that can occur when the
motor and ball screw are separately
mounted. This new product series is small
enough to fit in the palm of your hand.
The MBSA Series is also designed to
accommodate a configurable encoder
mounted to the back of the stepping
motor. The stepping motor can communicate
with a wide variety of controllers/drivers.
Users can customize it by selecting
the desired controller/driver.
As our world continues to evolve,
medical devices and laboratory equipment
used in diagnostics, syringe
pumps, microfluidics, imaging, cancer
treatment, and other applications
requiring automation will benefit from
these types of integrated solutions.
This article was written by Jon Lorello,
Linear and Mechatronics Specialist and New
Business Development Manager for NSK
Americas, Ann Arbor, MI. For more information,
visit http://info.hotims.com/79415370,
or call 888-446-5675.
13
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http://info.hotims.com/79415-370 http://www.medicaldesignbriefs.com
Medical Design Briefs - July 2021

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