governing characteristic to the design of the base of the arm assembly was the
limitation of width. As stated
previously, the arm assembly was not to increase the width of the chair by more
than six inches. Since the stationary
plate was the part that extends the farthest from the wheelchair, the width of
the stationary plate was the limiting factor in its design.
brainstorming and discussion led to the decision that both the twist and the
bend motors of the arm should be mounted in the base of the arm to save weight
in the lower arm. With this in mind,
initially the stationary plate and corresponding components were designed to
accommodate both motors in the base.
The initial design of the stationary
plate was a 0.50-inch thick piece of aluminum that measured 5.00 x 7.00 inches
(see Figure 4.3). The large 3.50-inch
diameter hole in the plate was for the placement of the twist bearing, and the
smaller 1.00-inch diameter cutout supports the bearing for the twist motor
shaft. An 18-volt Black and Decker
Firestorm cordless drill motor was selected for both the twist and bend motions
of the arm. The twist motor was mounted
directly to the bottom of the stationary plate in a vertical orientation (see
Figure 4.4). The drive gear for the
twist motion was linked to the motor by a 0.4375-inch shaft supported by the
aforementioned bearing. The drive gear
was a 2.00 diameter simple spur gear that had a face width of 0.3125
inches. Rotational motion was
transferred to the large twist gear via the drive gear.
large twist gear was a major component in the operation of the arm. The gear was a 5.00-inch diameter spur gear
with a 0.3125-inch face width. The
diameter was limited to 5.00 inches because of the width of the stationary
plate. A larger diameter gear would hang
over the edges of the plate and cause operational and safety issues. The importance of the twist gear was that the
rest of the arm was mounted to it. The
shoulder mounting brackets would sit on top of the twist gear and be bolted
from the bottom of the gear.
to the bottom of the twist gear was the shoulder motor assembly (see Figure
4.5). This assembly consisted of a spacer, motor mounting plate and motor. The 0.75-inch aluminum spacer was to sit
inside the twist bearing and support all of the moment experienced by the
joint. A 0.25-inch plate, which served
as a mounting plate for the motor, was located at the bottom of the of the
spacer plate. The motor was bolted to this plate in a vertical position. This design allowed the shoulder bend motor
to twist with the rest of the arm.
bend the shoulder joint, holes were to be machined through the centers of the
spacer and mounting plate so that a 0.4375-inch shaft could be run from the
shoulder motor up to the shoulder joint.
A 1.00-inch OD bearing was placed in the center of the twist gear to
support the shaft but also allow it to rotate independently of the twist
gear. A 0.75-inch beveled gear was used
to drive the bend of the shoulder. This
beveled pinion drove a 2.00-inch bevel gear, which was fitted to a 0.50-inch
diameter shoulder shaft, which was supported by 1.00-inch diameter bearings
mounted inside the shoulder mounting brackets.
The shoulder shaft was also connected to the lower arm tubing, which was
placed between the shoulder mounting brackets, and was held in position by
snap-rings on each end.
previously mentioned, both the twist and bend motions were to be driven by the
same type of motor. The common use of
motors allowed for a lower overall cost, fewer part numbers, and easier
repairs. The Firestorm drill motor
applies enough torque that after a small amount of gearing, the torque applied
at the shoulder for the bend motion would easily meet the nearly 500 in-lbs
required to move the arm under maximum loading conditions. The torque needed to twist the arm is more
than enough to over come the friction of the bearings, which is easily done by
the motor even before gear reduction.
The Firestorm also operates at a fairly low rpm so that gearing can slow
the rotation of each of the two motions to a manageable speed. One problem with the Firestorm drill motors
is that even though the gearing can slow down the operation of each of the two
joints, they will still be much faster than the targeted 5 rpm speed (~0.5m/s
gearing ratios selected were governed by space limitations of the base and
shoulder joint. The twist gearing was
2.5:1 and the bend gearing was 8:3.
These gear ratios were chosen to produce the largest torques and the
slowest rotational speed without changing the configuration and design of the
rest of the components. For example, the
bevel gear in the shoulder could not be any larger without interfering with the
lower arm tubing, and the shoulder pinion could not be any smaller because of
the size of the motor shaft it is attached to.
investigation into electric motors led to the acquisition of less expensive,
more powerful, and slower motors that better suited our arm than the previously
selected Firestorm motors. This change
in motor selection required an immediate and extensive redesign of the base of
the arm. The new twist motor is a window
lift motor (see Figure 4.6), and the new bend motor is a high torque, low rpm
motor. Due to the size and orientation
of the new bend motor, it could no longer be mounted in the base and was moved
to the lower arm. The twist motor is
still in the base but numerous changes have been made.
new stationary plate has been extended to be 5.00 x 8.00 inches and no longer
has a uniform thickness (see Figures 4.7 and 4.8). To mount the twist motor under the stationary
plate length-wise, the plate is 0.85 inches thick at one end and 0.625 inches
at the other. This mounting
configuration is also why the stationary plate had to be extended out to 8.00
inches. The change is so that the motor
will fit tightly against the bottom of the plate and a minimum amount of weight
would be added to the base. A 3.5-inch
diameter circle is cut 0.5625 inches deep for the placement of the twist
bearing assembly. Also, a 1.125-inch
diameter hole is cut to allow clearance for the new twist drive pulley. A 4.00 x 4.00-inch square area concentric
with the bearing cutout is cut 0.125 inches deep so a bearing retaining plate
can be mounted flush to the top of the stationary plate.
the motor across the bottom of the plate and having two different thicknesses
required a change in the rear mounting bracket also (see Figure 4.9). The placement of the rear bracket is still
the same and so are the majority of the dimensions, except now the rear bracket
has to go around the motor. Therefore, a
cutout was made to the old design (see Figure 4.9) to make sure the mounting
bracket would fit around the motor.
Also, the step on the bottom of the stationary plate causes the two
places where the rear mounting bracket is bolted to the plate to be at different locations. This meant that one side had to be shortened
to ensure that the fit was correct.
A new rubber insert was also selected for
the mounting brackets. The new insert is
thinner and can be attached by adhesive.
This change makes the machining of the brackets less expensive since the
tapped holes are eliminated from the cutout where the rubber inserts are
placed. The thinner inserts changed the
cutout dimensions slightly but not extensively.
The front mounting bracket was not affected by the new motor or change
in the stationary plate. The only
changes to the initial design of the front mounting bracket was the elimination
of the tapped holes for the rubber insert and the dimensional changes for the
cutout where the insert is placed.
new twist motion utilizes a belt and pulley system instead of gears (see Figure
4.10). This change was made because the
new motor moved the center of the motor shaft further from the center of the
bearing hole. A benefit of this change
is the pulley also acts as a safety device.
The pulley will slip and prevent the motor from burning up if the arm is
constrained from twisting for any reason.
This limitation on the twist torque may also prevent people from getting
hurt by inadvertently twisting the arm into a person standing close to the
drive pulley is a very intricate piece that is machined from solid aluminum
(see Figure 4.11). The lower side of the
pulley is designed to slide over the existing 0.562-inch square drive of the
twist motor and is held in place by a setscrew.
The upper section is a 0.75-inch diameter cylinder with a 0.125-inch
radius groove cut around it. The groove
creates a 4.50-inch diameter pulley that accepts a 0.25-inch diameter belt.
twist pulley (see Figure 4.12) is similar to the twist gear originally designed
in that it still supports the shoulder mounting brackets and therefore the rest
of the arm. The pulley is a 4.75-inch
diameter circular plate of aluminum that is 0.4275 inches thick. A 0.125-inch radius groove is cut around it
to create a 4.50-inch diameter pulley that accepts a 0.25-inch diameter
belt. Since the bend motor is now in the
lower arm, the hole for the bearing and motor shaft have been eliminated. The twist pulley is mounted to the center
twist bearing piece, which is part of a 3-piece bearing design and will be
twist belt is a black neoprene belt with a round cross-section. The diameter of the cross-section of the belt
is 0.25 inches and thus matches the grooves in the two pulleys. The belt has a length of 399 mm.
redesign accommodates the twist motor and also improves upon the first design
in a number of ways. The use of the
pulleys not only acts as a safety device, but they are also cheaper than the
gear design used initially. These
pulleys are easily machined from stock aluminum and do not have to be specially
ordered. The new design also has a more
desirable gearing ratio for the twist motion.
The pulleys produce a speed ratio of 9:1. This ratio produces more torque to overcome
the friction in the bearings and also slows the rotational speed considerably.
initial bearing design was 0.50-inch thick bearing. The outer diameter was 3.50 inches, and the
inner diameter was 3.00 inches. This is
a very simple and effective design. The
choice of material was that of oil impregnated brass because of its wear and
frictional characteristics. This was an
improvement upon the previous year’s design because it was cheaper and reduced
the material required.
redesign of the base also brought about a redesign of the twist bearing. A three-piece bearing assembly was designed
to take the place of the single bearing (see Figure 4.13 and 4.14). The main piece of the assembly is the center
bearing. This piece is bolted to the
twist pulley from the top and is the only moving component in the bearing
assembly. The lower part of the bearing
has a diameter of 3.375 inches. This is slightly smaller than the hole in the
stationary plate where it is placed so that it can rotate freely without
contacting the stationary plate. The
upper section of the center bearing piece has a 2.75-inch diameter. This part extends up through the center of
the upper bearing piece and the bearing plate.
This is the end that is bolted to the twist pulley.
other two parts of the bearing assembly are very similar and only have one
difference. Both are 0.125 inch thick and
have a diameter of 3.50 inches. The
difference is that the upper bearing piece has a 2.75 inch diameter hole cut
through it. Both pieces are pressed into
the stationary plate and do not rotate.
The lower piece goes under the center piece, and the upper piece fits
around the upper section of the center bearing piece.
three pieces of the twist bearing assembly are made of oil impregnated
nylon. Nylon was chosen over brass due
to its availability and lower cost. The bearing assembly is designed such that
all points of friction are nylon against nylon.
The nylon-to-nylon decision was made to try and minimize wear and noise. These contact areas are the entire bottom of
the center piece against the lower piece, the ledge of the center piece against
the upper piece, and the OD of the upper section of the center piece against
the ID of the upper piece.
bearing assembly is held in place by a bearing plate. This 4.00 inch square plate bolts to the
stationary plate in the previously described cutout and holds the bearing
assembly in the stationary plate.
final base assembly is shown in Figure 4.15 with the stationary plate, twist
pulley, and twist bearing plate all transparent so that the rest of the
components can be seen. Finally, an
exploded view is shown in Figure 4.16.
This shows each component of the redesigned base assembly and how they
Table of Contents
Section 5: Lower Arm