People suffering from degenerative hip or knee joints find sitting and rising from a seated position very difficult. These people can rely on large stationary chairs at home, but must ask others for assistance when rising from any other chair. An orthopedic surgeon identified to the MSFC Technology Utilization Office the need for development of a portable device that could perform a similar function to the stationary lift chairs. The MSFC Structural Development Branch answered the Technology Utilization Office's request for design of a portable powered seat lift. The device is a seat cushion that opens under power, lifting the user to near-standing positions. The largest challenge was developing a mechanism to provide a stable lift over the large range of motion needed, and fold flat enough to be comfortable to sit on. CAD 3-D modeling was used to generate complete drawings for the prototype, and a full-scale working model of the Seat lift was made based on the drawings. The working model is of low strength, but proves the function of the mechanism and the concept

Weddendorf, Bruce

English

NASA Technical Reports Server

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DESIGN OF A PORTABLE _)0WE_,ED SEAT LIFT
: Bruce WeddendorfNASA Marshall Space Flight Center
MSFC, Alabama 35812
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ABSTRACT
People suffering from degenerative hip or knee joints find sitting and rising from a seated position very difficulL
These people can rely on large stationary chairs at home, but must ask others for assistance when rising from any
other chair. An orthopedic surgeon identified to the MSFC Technology Utilizatiou Office the need for development
of a portable device that could perform a similar function to the stationary lift chairs. The MSFC Structural
Development Branch answered the Technology UtiliTation Office's request for design of a portable powered seat lift.
The device is a seat cushion that opens under power, lifting the user to near-standing positions. The largest
challenge was developing a mechanism to provide a stable lift over the large range of motion needed, and fold flat
enough to be comfortable to sit on. CAD 3-D modeling was used to generate complete drawings for the prototype,
and a full-scale working model of the Seat lift was made based on the drawings. The working model is of low
strength, but proves the function of the mechanism and the concepL
INTRODUCTION
This paper describes how the portable powered seat lift prototype was designed. It includes how requirements
were derived and how they were met through the design of the prototype. Also included are the lessons learned from
building and testing the working model and possible improvements to the design to make it lighter and less costly to
manufacture.
BACKGROUND
People suffering from degenerative hip or knee joints find sitting and rising from a seated position very difficult.
These people can rely on large stationary chairs at home, but must ask others for assistance when rising from any
other chair. An orthopedic surgeon identified to the MSFC Technology Utilization Office the need for development
of a portable device that could perform a similar function to the stationary lift chairs. The MSFC Structural
Development Branch answered the Technology Utilization Office's request for design of a portable powered seat lift.
DESIGN REQUIREMENTS
Engineers in the MSFC Structural Development Branch began the design process by developing functional
requirements for a portable seat lift. These requirements were generated with the help of the orthopedic surgeon
requesting the technology. The first requirement established was strength: The portable powered seat lift must
support a load of 300 pounds with a factor of safety of 2 on yield. This requirement was established to prevent
collapse of the seat lift during use and applies throughout the lifting range. The seat lift mechanism also must lift a
300-pound weight from the fully closed position. Maximum lift time was decided to be 10 seconds. A maximum
weight of ten pounds for the seat lift was established. This weight was not met with the prototype design, as
meeting the requirement of low cost and ease of fabrication were considered more important. In addition to
guidelines laid out by the orthopedic surgeon, this effort included research by MSFC engineers. The research
included videotaping several people standing and sitting in front of a grid and entering the hip positions into the
CAD system. This information was used for generating the range of motion requirement for the seat lift.
CONFIGURATION SELECTION
Several types of overall lifting schemes were considered, including crutch type lifts, a walker with a built in lift,
and others. When the criteria of ease of use by most patients and portability were applied, the powered opening seat
cushion configuration was selected. The seat lift configuration chosen has an unobtrusive, briefcase-like appearance
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when being transported, and operation should be familiar to anyone who has used a powered lift ann chair. This
configuration requires a slim overall height, but this height is still significant and may pose problems in some cases
when the user sits at a low table. This choice of configunuion also requires the user always to sit on the lift when
they need its assistance, making comfort of the seat over long periods a concern. Use of the device in a chair on
casters may pose problems, as there may be a tendency for the chair to roll back away form the user when lifting.
PROTOTYPE DESIGN PHILOSOPHY
Design of this prototype was based upon meeting the requirements for lift angle and height, strength, and overall
size as described above. In addition to these requirements, the design was based upon production of one or two units
using commonly available machine tools. No molds, dies, or special tools were required to build the prototype
design. Cost was considered to be a major design driver, and simplicity of fabrication took precedence over weight.
Further influencing the design was the intended use of the prototype as a test article to prove if the portable powered
seat lift concept is technically and medically feasible. Because the powered seat lift prototype is intended for use
with patients in a controlled and supervised testing environment, no effort was made to generate or meet safety
requirements beyond simple structural strength.
MECHANISM DESIGN
Design of the mechanism proved challenging because of the large range of modon, high forces, and thin
packaging envelope. Many different overall mechanism types and layouts were considered before the final type was
developed (see figure 3). This mechanism features front and rear facing arms, attached to shafts mounted in bearings
on the stationary base, with the front arm crossing over the shaft for the rear arm. The front arm is pivoted behind
the rear ann pivot, which allows both arms to be as long as possible. The two shafts on which the arms are pivoted
are geared together with a ratio chosen to give the right amount of lift at the front and the rear of the seat. The front
of the seat is pinned to the front arm, while the rear of the seat is supported by rollers pinned to the rear arm. The
system of two arms and the seat has one degree of freedom and can therefore be controlled at any one location. The
control location chosen for the mechanism is the rear arm shaft. Crank arms f'vced to the rear arm shaft are pinned to
connecting finks that are pinned to a slider on a track fixed to the base frame. The links and crank arms convert the
linear motion of the slider into rotation of the shafL The position of the slider on the track is determined by an acme
screw which runs through it. The screw is supported on thrust bearings and features a worm gear made onto its
forward end. The worm gear is driven by a worm direcdy connected to the motor shaft. This system has a large gear
reduction which allows the use of a small motor. The large gear reduction cannot be back driven, so it will remain
stationary unless the motor turns.
VERIFICATION OF THE GEOMETRY
The geometry of the mechanism was tailored to match the lift curve generated by a 5 foot 10 inch subject from
the data. To check the validity of the design, two stationary wood models of the lift at different heights were made
and tested subjectively by people of varying size. The results were as expected, so no changes to the geometry were
made. The tests demonstrated that the increasing tilt of the seat as the lift progresses allows shorter users to be
supported closer to the front of the lift, and taller users closer to the rear. The wood models also allowed us to learn
that handles were necessary on the rear corners of the seat lift. and that under one of these would be a good location
for the control switch. The width of the seat lift was also determined by use of the wood models. Originally, the
planned seat lift width was as close to the width of a chair as possible, with minimum clearance for the chair arms.
This was found to be a poor assumption, as the user's hands must fit between the seat rift and the chair arms as they
hold the handles at the rear of the seat. The width of the design was reduced to allow this clearance. See figure 1 for
the overall dimensions and figure 2 to see the motion of the lift.
DESIGN METHODS AND DETAILS
In order to package the mechanism within the conf'mes of the seat lift envelope without any interferences, a 3-D
model of the entire assembly was made using Intergraph EMS computer aided design (CAD) software. Strength
critical parts were first sketched and hand analyzed for strength, then sized and input into the CAD. Other parts
391
which were not as highly loaded were sized in the CAD by clearance constraints. The solid modeling allowed
clearances between all parts to be verified throughout the range of motion of the mechanism. As the design
progressed, several design changes were incorporated as new problems surfaced. For example, a potential pinch
hazard between the seat top and the base was eliminated by making the sides of the seat from soft closed cell foam
(see figure 3). Pinching from the internal mechanism is also a consideration. The prototype design does not include
protection for this type of pinching, as it is intended to be used as a development and test article only, and pinching
could only occur with a deliberate and deep insertion of the hand into the opened seat lift. A production model may
require a guard for certain parts of the mechanism.
DESIGN DRAWINGS AND DOCUMENTATION
Engineering drawings of all parts and assemblies of the lm3totype design were made. The drawings were made
from projected views of the parts and assemblies of the CAD solid model. There are over 90 sheets of drawings
documenting the prototype design, which meet the MSFC drawing standards.
WORKING MODEL
A working model of the prototype design has been built which is very close in appearance and operation to the
prototype, except it is of low strength. It was built at the MSFC model shop using the prototype drawings as a
guide. The overall dimensions of the model are made to the drawings, but the materials and tolerances are changed.
The gears of the model are standard low-strength aluminum and brass, instead of the high-strength steel parts of the
prototype. The model does use the same bearings, a similar aluminum frame, and identical electric parts as the
prototype would. Similarity of the model to the prototype allows testing of the mechanism and electric system
function for less cost than construction of the prototype itself. The model is a valuable demonstration tool for the
portable powered seat lift as well as helping in the development of the prototype.
Construction and testing of the model taught several imporlant lessons. Building the model verified the
assembly of the mechanism and electric hardware was possible without interference. Testing the model uncovered a
problem with the original location of the limit switches which interrupt the circuits of the control switch when the
lift reaches fully closed or fully open. The closed position switch was damaged by the rear arm whose position it
was suppose to sense. This was because the mechanism had enough momentum to move slightly even after the
motor power was cut. This problem was solved by using a different type of limit switch and relocating both limit
switches to operate by the position of the slider block, which has a much more controlled motion. The most
important lesson of the model, however is that the motor originally selected is inadequate. This motor was selected
based upon its advertised power output, light weight, small size and low cost alone. No curves of torque vs speed
were available, as the motor came from a hobby shop. The motor makes very little torque at zero speed, where the
powered seat lift requires maximum torque, and therefore cannot lift the necessary load. A different motor must be
selected for the prototype which meets the torque requirement.
POWER REQUIREMENTS
The requirement of lifting a 300-pound load to the full extent of the lift in 10 seconds was used to calculate the
required power of the portable powered seat lift. Power required to perform this task is 61 watts or about 1/12 horse
power. Friction in the linkage can add significantly to the actual motor power required. No testing was available to
quantify this friction, so an attempt was made to calculate it. Minimum motor power was determined to be about
150 watts, but the accuracy of the calculations is still untested at this writing. The gear ratio can be varied between
the motor and the acme screw, depending upon the speed at which the motor will run and its torque output. The
prototype design has a reduction of 20:1 from the motor shaft to the acme screw. The 20:1 reduction allows the
motor to lift the seat in 720 shaft revolutions, with a speed of 4320 revolutions per minute achieving the desired
10-second lift. The torque requirement on the motor is 0.135 Nm (1.20 inch-pounds) minimum at startup to move
the seat using the 20:1 ratio, and neglecting friction. With the calculated friction, the motor should produce no less
than 0.33 Nm (2.93 inch- pounds). These numbers can be adjusted for different speed motors and their required gear
ratios, given that they produce the sufficient power.
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Large torsion springs were added to the prototype design to assist the motor, to balance the upward lift torque
requirement and the lowering requirement. These springs are not required for the system to function if a motor is
selected using the above criteria for motor power. Springs may prove beneficial in reducing wear in the mechanism
by off loading about 70 pounds of the user's weighL The springs are then energized by the motor in the closing
cycle of the scat lift making a more even split of work for the motor during opening and closing of the lift.
RECOMMENDED PROTOTYPE DESIGN CHANGES
The portable powered seat lift prototype design presented here is a good starting point for any effort to produce a
production portable seat lift. The prototype design drawings are complete, but changes to the design should be made
before even a prototype unit is built from them. These changes are mostly the result of learning from the model.
The most important change is the selection of a different motor with proven torque capability and the modification of
the motor mount and gear ratio to install it. The limit switches should be installed as they are now in the working
model. It may be better to eliminate the springs provided the motor chosen has enough torque to start the lift when
loaded. An area of concern is the friction in the plain thrust bearings retaining the acme screw. This friction has not
been measured at this time, but a bail thrust bearing of sufficient strength should be considered as a replacement in
this area.
DESIGN IMPROVEMENTS FOR PRODUCTION
Many changes in the design should be made to make it more suitable for mass production. These changes
should be made in conjunction with other changes made based on knowledge gained from construction and testing of
the prototype. Changes made for production only should focus on the areas of weight and cost reduction frrst. To
help meet these goals for the production design, molded reinforced plastic top and bottom halves for the seat lift
should be considered. These parts are complex assemblies on the prototype and can be made easily in one piece after
investing in the molds required to make them. The top may require the tracks on which the rear arm rollers rest and
the bracket to which the front arm pins to be die cast aluminum parts co-molded into the plastic shell. A similar
solution could work on the base plastic shell and the metal frame which supports the mechanism. The aluminum
frame supporting the shaft bearings and motor could be cast in one piece instead of machined from billet and welded
into an assembly as in the prototype. Each arm, with its attached shaft and gear could be made in one net shape
piece using powdered metallurgy. The large shafts could be made hollow, and the shaft gears do not have to go all
the way around, as only a partial rotation of each is made. Smaller shaft bearings and bearing supports may also be
possible. A study should be made to determine if springs in conjunction with a smaller motor would have a weight
and cost advantage to a larger motor without assist springs. Taking into account all the areas for possible weight
savings, the target weight of 10 pounds can be met, and perhaps significantly undercut.
DESIGN LICENSING
The portable powered seat lift prototype design is the property of NASA and a patent application will be filed to
protect the key design features. Prospective manufacturers are encouraged to contact the MSFC Chief Patent
Council, CC01, MSFC Alabama 35812 for licensing information.
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/_ SEAT UFT CLOSED2.3F
FRONT ISOMETRIC
SIDE vIEW.
TOP PARTIALLY CUT AWAY
SEAT LIFT FULLY OPEN
2/
f3o"
FRONT ISOMETRIC S/DE V/EWo
TOP PART/ALLY CUT AWAY
FIGURE I
394
FIGURE 2
PORTABLE POWERED SEA1" UFT
SHOWN IN FIVE POSITIONS INCLUDING
CLOSED (I) AND FULLY OPEN f5).
ALL VIEWS FROM SIDE. WITH TOP
PARTIALLY CUT" AWAY.
395
REAR CORNER HANDLES
CONTROL SWITCH
WORM GEAR
CONNECTING UNKS
ARM SHAFT & GEAR
MOTOR
BEARING SUPPORT FRAME
SPRING
(ON REAR ARM SHAFT))
UMIT SWITCH
BASE FRAME
FIGURE 3
ISOMETRIC vIEW. SEAT UFT FULL)" OPEN
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Design of a portable powered seat lift

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