Medical Design Briefs - July 2021 - 17

also requires more power due to the
lower efficiency.
Gearbox selection: The output torque
and the input speed will help determine
the gearbox. The Portescap catalog indicates
that the R13 gearbox can be a
potential option for working with both
screws. This gearbox has a maximum
output torque of 0.25 N*m - greater
than the application requested torque
previously calculated. The maximum
input speed of this gearbox is 7,500 rpm.
Ratio selection: Thanks to the maximum
recommended input speed of the
gearbox, users can define which maximum
ratio to choose. To define the maximum
ratio, divide the maximum input
speed by the output working speed.
Compare the result with the ratio available
(see Table 4). Note: In general,
choose the closest and smallest ratio
available in order to have a working
input speed lower than the recommended
maximum input speed.
Motor selection: To choose the motor,
calculate the application input torque of
the gearbox. Since this is a continuous
application, the motor must have a maximum
continuous torque higher than
the application gearbox input torque
(see Table 5). For both screws, the 12G88
motor, which has a maximum continuous
torque of 3.5 mNm, can be used.
Custom Motor Assembly Selection
The electrical power, efficiency, and
total dimension can now be calculated for
each custom motor design (see Table 6).
The 12G88 215E has the following specs:
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Technically, both strategies can work,
but certain application requirements can
favor either option 1 or option 2. For
example, if battery life carries more importance
than the package, option 2 appears
to be the better choice. Indeed, the total
efficiency is 30 percent compared to
option 1 at only 18 percent efficiency.
Option 2 presents a size concern because
its gearbox has one more stage than option
1, making it 3 g heavier and 3.1 mm longer.
Partner to Define Both Device and Motor
Requirements
For linear applications, motor suppliers
can support the development team by
Medical Design Briefs, July 2021
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Table 5. Torque calculation at the motor level.
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Table 6. Power calculations for each custom motor design.
offering standard linear motors or by creating
a linear, custom motorized design.
In addition to defining the device's technical
requirements, both the motor supplier
and application designer should
also determine the requirements at the
motor level. With a full understanding of
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the project's needs, design teams can find
the best compromise between the tool's
technical and commercial requirements.
This article was written by Clémence
Muron, Application Engineer for Portescap,
West Chester, PA. For more information, visit
http://info.hotims.com/79415-371.
17
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Medical Design Briefs - July 2021

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