equipment as needed. While pillars can
lift the same loads as actuators, their
guiding functionality, inherent in the
profile tubes, enables them to accommodate offset and moment loads without
the need to design an additional support
and guiding frame.
Some pillars are multi-stage, enabling
vertical movement in two or more stages. While a single-stage pillar has one
screw that pushes two tubes apart to lift
a load, a double-stage pillar has a double
acting mechanism (with two screws) that
pushes three separate stages apart (one
stage pushes against the second and the
second pushes against the third).
Telescopic pillars are able to achieve
a high stroke to retracted height ratio,
which is particularly relevant for surgical
tables (Figure 2) and procedure chairs.
This ratio allows a table or chair to be
positioned very low (easier access for patients) and travel very high (ergonomic
benefits for medical staff). Entry height
capabilities have become a matter of
regulation—evidenced by the Americans
with Disabilities Act (ADA) requiring
equipment with an entry height of 17-19
inches.
Scissor mechanisms can also be used
for patient lifting functions, especially in
procedure chairs. One advantage is a low
retracted height with high stroke, but
the downside is poor stiffness characteristics, and a much more complex chair
design with a high number of parts,
extra cost, and added assembly time. For
surgical tables requiring a small footprint
of the central frame, a scissor mechanism
with a corresponding load and stiffness
is simply too large to be a viable option.
In contrast, pillars can be supplied as
“plug-and-play” systems without the
issues associated with complex assembly,
performance, and size limitations.
Competing with electromechanical
actuator and pillar technologies, hydrau-
lic systems are often used to perform
positioning functions. While hydraulic
systems, in general, are compact and
provide high power density relative to
their size, several disadvantages make
them a less favorable option for new de-
velopments. Poor positioning capability,
slow speeds, high service requirements,
and the ever-present risk of oil leakage
have largely relegated hydraulic systems
to the second choice behind electrome-
chanics.
Checklist for Optimized System
Design
Optimized patient positioning systems
will rely upon a designer’s insights into
the performance and safety needs of an
application, and how potential issues
can be resolved at the outset. Here's a
checklist of basic objectives to help guide
the decision-making process:
Ascertain load requirements. Elec-
tromechanical actuators and pillars for
chairs or tables must be able to support
imposed patient loads, and load capaci-
ties will vary depending on the required
function. For example, when designing
a procedure chair to
handle up to 600 lb. pa-
tients, the load capacity
of the actuators can vary
between 2000 N for leg
adjustments, and up to
6000-8000 N for back
adjustment, and Tren-
delenburg (longitudinal
tilting of the body axis).
An important factor
to consider when
designing the lifting
function with a telescopic pillar is the
distance of the center
of gravity in relation to
the center of the pillar,
since the offset load
introduced by this can
introduce a significant
additional load. A pillar
load capacity of 4000 N
is generally sufficient to
handle a 600 lb. patient,
while 6000 N would be
more suitable for patient
loads up to 1000 lbs.
Safety first. The me-
chanical and electrical
safety of the equipment
is a key requirement that should not be
taken lightly. Depending on the chair or
table, a first failure safety factor of 2. 5 to
four times maximum load (up to which
there can be no collapse to harm the
patient) has to be implemented. Actu-
ators and pillars certified according to
the medical safety norm IEC 60601-1
should be chosen to reduce the risk to
the patient and to reduce final system
certification time.
Minimize system “play.” Overall system stiffness and low-play design can be
a key requirement when designing tables
and chairs for critical procedures, such
as eye surgery. Play can be introduced
from the actuator (overall construction,
screw design, brakes, and/or gears),
as well as from attachment points and
hinges. Additionally, equipment stiffness
can be affected, resulting in uncontrolled
shaking or wobbling. All of these adverse
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