IV

 
BASE

 

 

4.1 MOUNTING BRACKETS

            Due to the limitation in mounting positions and the structural stability of the frame of the wheelchair, it was decided that the arm would once again be mounted to the right side of the frame of the wheelchair.  The frame is a rectangular cross-sectional beam that runs from the front to the back of the wheelchair (see Figure 3.1).  This provides for a strong mounting position between the front and rear tires of the wheelchair.  The only limitations that needed to be considered with this mounting location were clearance of the front and rear tires, location of the cross beam of the frame, and height to the wheelchair arm rest.

            The brackets (see Figure 4.1) are custom machined plates of aluminum that fit over the frame.  Rubber inserts are attached to the inside of the bracket cutouts and are the only parts of the entire arm assembly that contact the wheelchair.  The selection of the size of the mounting bracket cutouts is such that when the rubber inserts are added, the brackets fit snuggly around the frame.  The cutouts in the bracket are also deeper than the height of the tubing so a bolt can be placed through the bottom of the bracket.  This bolt can be tightened to squeeze the mounting brackets around the frame of the chair and secure the entire arm to the wheelchair.

            Two of these mounting brackets are used to mount the arm to the frame.  Both are bolted to the stationary plate (discussed in the next section) of the arm and clamped to the frame of the wheelchair (see Figure 4.2).  There is a small difference in the two mounting brackets.  Since the frame of the wheelchair is not parallel to the ground, the front mounting bracket was designed to be slightly taller than the rear bracket.  This difference ensures that the stationary plate of the arm is parallel to the ground.

            This type of clamp design offers several improvements over last year’s design.  The first improvement is that one person can mount the entire arm.  The tight fit of the mounting brackets around the frame allows someone to slide the arm onto the wheelchair, and the arm will stay in position while the person inserts and tightens the squeeze bolts in each of the mounting brackets.  Another improvement is the addition of the rubber inserts between the brackets and the frame.  The rubber not only helps clamp the arm to the frame, but also protects the frame from scratching during installation and removal.  The rubber also eliminates the metal-to-metal noise while the wheelchair is in motion.

           

4.2 BASEPLATE DESIGN

            The 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.

            Initial 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.

            The 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.

            Attached 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.

            To 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.

            As 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 linkage speed).

            The 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.

            Further 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.

            The 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.

            Orienting 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.

            The 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 wheelchair.

            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.

            The 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 discussed later.

            The 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.

            This 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.

            The 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.

            The 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.

            The 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. 

            All 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. 

            The 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.

            The 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 are assembled.

 

 


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Table of Contents

 

Section 3

 

Section 5: Lower Arm