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Design Description of FRC 2767 Stryke Force “Third Coast” Swerve Drive UnitsIntroduction and History (up to 2019)Stryke Force’s motivation to convert to Swerve Drive came from watching and being pushed around byanother west Michigan team, FRC Team 141 “Wobots.” (Imitation is the highest form of flattery.) Westarted in the 2011 off season with the commercially available “Revolution Swerve,” developed by 221Robotic Systems. We used it to learn about incorporating Swerve into a robot, and how to program anddrive it. We borrowed freely from the wealth of public information on swerve drive programming and(eventually) field-oriented control available on Chief Delphi. Special thanks are due to the user “Ether,”who has provided a wealth of very cogent information.We did not compete with the Revolution units due to high BOM cost and a strong desire to incorporatemore mechanical learning into our program. Stryke Force developed its first custom swerve drive unitas shown below for the 2012 game, Rebound Rumble.This was essentially a scaled-up version of the Revolution, with “wheel pant” skid plates so that it couldjump over the mid field barrier. It had 6” diameter wheels. Although we had success and were hookedon Swerve, there were several serious shortcomings with our first custom design: Robot center of gravity far too high—causing us to limit acceleration and forcing our driver to becareful to avoid tipping over. This obviated much of the intended advantage of swerve.Overall space claim on the robot and weight were too high.

Too much weight on the perimeter of the robot—limiting rotationalacceleration/maneuverability.Difficulty driving straight in autonomous and dealing with obstacles.Our design for Ultimate Ascent in 2013 attempted to address some of these shortfalls by reducing wheelsize to 4”, removing discrete bushings for the azimuth rotation (employing the housings themselves),and adding posi-traction (described more completely later in this document). We also added a servoactuated, dog clutch two-speed transmission. We also 3D printed timing belt pulleys to facilitate thedesign of the two-speed transmission.This swerve design was a large improvement. The ¼” throw posi-traction was helpful in improving thecontrol of the robot, but also made it sway during hard maneuvering and the recoil of the Frisbeeshooter was visible in the mechanism. There were some other, more significant, shortfalls: COG still too highSpace claim and weight higher than desired.Shifting time limited the utility of the transmission.High maintenanceo Custom wheels/treado Multiple chains, which are pinch hazards, potential failure points, and need to beadjusted periodically.o Multiple light duty belts, which were prone to wear & failure

ooInsufficient thickness in mounting plate on robot (not shown), leading tobending/alignment issues.Motor mounting issues leading to belt walk.Our next iteration, developed for 2014’s Aerial Assist game, began an emphasis on simplification. Themost significant and visible change was the elimination of the horizontal drive shaft and itssprockets/chain by direct driving the axle, and the corresponding move to “dually” wheel sets. Thewheels were 4” VEX Versawheels/Versahubs with some secondary machining ops to accommodate thebevel gear set. The top driveshaft bearing was changed from a ball bearing to a needle bearing. Overallheight and complexity were reduced relative to previous iterations. We found reliability was improved,and the maintenance load on the pit crew was dramatically reduced. We also started to noticeimproved handling. It was visibly unique, and our team started to identify with it. Playing off the “WestCoast Drive” name, we’ve taken to calling our dual wheel posi-traction swerve a “Third Coast Drive,” inreference to the Great Lakes region.Items targeted for improvement after competing with this first generation Third Coast Drive: WeightVertical drive shaft cantilevered off lower bearing too far—led to occasional bent drive shafts.This was in part due to some shafts with improper heat treatment.Conversion of azimuth chain to beltMethod of setting bevel gear mesh2015’s game, Recycle Rush, really didn’t emphasize the traditional capabilities of Swerve. However,since we had a lot of comfort and experience with it, we did it simply for the advantages ofmaneuverability in tight quarters. The main differences between this design and the former were thatwe changed how the axle was constructed and the bevel gear adjusted, and we converted the azimuthfrom chain to belt drive. We also eliminated the bolted connection between the side plates and the topplate. In this version, this last change meant that we bolted a retainer for the axle bearings in from thebottom (not easily seen below). At the time, we thought this was necessary for maintenace on thewheels. As in 2014, the design required secondary machining ops on the wheels to accommodate thebevel gears. The whole axle assembly also had quite a few spacers.

In the off season between 2015 and 2016, we created a Third Coast Drive T-Shirt Cannon Robot. For thedrive units in this design, we hoped to get rid of the hassles and time penalties of azimuth zeroing byshifting to absolute encoders. We developed an in-house method of mounting an optical encoder on aVex Planetary Gearbox output—only to find out that Vex simultaneously released a similar product witha more robust magnetic-based encoder. We happily adapted to using theirs. Other refinementsincluded using a composite saddle (wet layup over printed ABS core), exploring using a gear set ratherthan pulleys and belts for azimuth control, moving to Colson wheels to improve wear, elimination ofseveral axle spacers, smaller wheel size to reduce the overall gear reduction needed for a target speed,elimination of secondary machining on the wheels, and the axle bearing retainers. Overall, this module,which never saw FIRST competition, was very light and a nice upgrade but the fabrication of thecomposite saddles was time intensive and we didn’t have faith in them or the azimuth gear set given theposi-traction axial motion.In our initial estimation, 2016’s game, FIRST Stronghold, did not lend itself to Swerve Drive. The amountof physical abuse we expected the robots to take gave us pause, and the wide variety of orientation inobstacles made us worry about swerve’s appropriateness. For example, if the dual wheels were notaligned with an obstacle when they hit it, the impact with the leading wheel would create a torqueshocking the azimuth belt, potentially skipping teeth or breaking it. Thus, we went to an eight wheel

tank drive with the outer wheels raised slightly for maneuverability. Each set of four wheels weremounted in suspended “skis” intended to smooth out impacts going over obstacles. In hind sight, FRC16 “Bomb Squad’s” robot for that game proved that Swerve could still be used to very good effect.Hat’s off to their team’s insights on how to make that all work out.With the wide-open field of 2017’s game, FIRST Steamworks, we went back to swerve. The 2017generation of Third Coast Drive started with what we learned from the T-Shirt Cannon Robot and put alot of emphasis on simplicity, ease of manufacturing, and weight reduction. Key design elements wereshifting to a Nylon mounting “bonnet” vs. the traditional aluminum (weight savings), moving to 775Prodrive motors, a 2:1 bevel gear set, and reduction of posi-traction throw to facilitate a stable shootingplatform. This design is described in detail below:2017 Stryke Force Third Coast Drive Detailed DescriptionWheel/Axle Assembly: Use of durable 2.5” Vex Colson wheels with ½” hex bores and Andy Mark’s 2:1bevel gear set along with AndyMark CIMiles allowed us to move to lighter high speed motors whilekeeping things simple and the drive pulley ratios reasonable. Earlier Third Coast Drive designs usedlower speed CIM or mini-CIM motors. To use the Andy Mark gears on axles made from Vex ½”Thunderhex, we removed the flange on the large gear’s back side, opened the 3/8” hex to ½” hex(drilling and broaching) and drilled a hole pattern in its face matching the one in the Colson wheels. Alltorque was transmitted through the hex flats; the screws going through the hole pattern were simply tokeep the face of the gear flush against the wheel face, counteracting the moment created by the bevelgear mesh forces. Note that the opening of the 3/8” hex bore to the ½” hex bore had to be done verycarefully. We used a mill and a dial indicator to set up the job. The concentricity of the bore to the geartooth pattern is critical to properly setting backlash. Lack of concentricity will force unnecessarybacklash. Unnecessary backlash will lead to premature gear wear, noise and poor control. Similarly,care must be used in the broaching operation.

The wheels were axially held in place on the axle with three snap rings (not shown); one for the gearedwheel (opposite side from the gear to take the mesh thrust), and one on each side of the non-gearedwheel. The location of the snap ring grooves in the axle was determined by the location to hold thegeared wheel in the nominal mesh location relative to the mating gear on the vertical drive shaft. Thenon-geared wheel was located symmetrically about the vertical drive shaft relative to the geared wheel.This was done so that scrubbing torques are equalized and the robot can stay stationary whenazimuthing a swerve unit. One way to think about it is that essentially a virtual wheel is created on thevertical axis. The axle length was set carefully so that it could just be rocked in position in the saddlewhen the bearings were not in place. This elimated the need for a complicated saddle with bearingretainer plates.Saddle: The incremental design goal for the Saddle in 2017 was for it to be one piece and machinable bybuild team students using basic skills. The Saddle started life as a ¼” wall 5”x5” 6061-T6 Aluminumextrusion. The extrusion was cut to length on a horizontal bandsaw and then cut in half using a verticalbandsaw. The width was determined by what was necessary to provide complete support for theazimuth pulley diameter and the height was what is left after cleaning up the bandsaw cuts. The nextstep was to mill the saw-cut faces in order to square them up. After the faces of the horseshoe weresquared up, the axle shaft holes were carefully drilled and reamed to fit 1-1/8” OD Thunderhex flangedbearings. The top hole was then drilled out to intersect the axle shaft axis at right angles. This holemust intersect the axle shaft hole for proper gear mesh. The bolt pattern at the top was also done onthe mill at this time and the holes were subsequently tapped. In order to minimize weight and sweptdiameter, the edges were chamfered and the bottom corners cut off around the axle bearings. Notethat in CAD, these corners are radiused, but on the actual robot, they were cut off at 45 degrees—simplyeasier for fabrication. On our prototypes we didn’t even bother with these cuts at all. Note thatminimization of swept diameter is important because we set up the wheels so that the swept path isalmost tangent to the frame—wheelbase reduces as swept diameter increases, and wheelbase isimportant for stability/handling.

Azimuth Pulleys: The azimuth pulleys were 3D printed by our sponsor in polycarbonate using acommercial FDM type printer (Fortus400MC). A flange for the swerve unit side was also printed (visiblein assembly views, but not shown below). We have found that the teeth profiles need to be tweaked abit (opened 0.002”) to get the timing belt to settle into them fully. This is important because if theteeth don’t settle in fully, the belt teeth will jump under load and you’ll lose wheel alignment.Note that to be able to take advantage of the absolute encoder on the output of the azimuth gearbox,these two pulleys must be the same number of teeth. We used 44T in 2017. Also note that we now useHTD, 5mm pitch, 9mm wide belts. To minimize “backlash” due to belt stretch during direction reversalsand possible slipping of teeth, these belts need to be fairly tight. Earlier versions used XL type beltswhich weren’t quite as smooth or robust to the loads. The pulleys were a little wider than the belts inorder to accommodate the posi-traction motion. One thing we struggled with in this design wasdurabiltiy of the hex driven azimuth pulley. We prevented outright failure by JB Welding an SAEaluminum washer in around the boss surrounding the hex. This washer took the hoop stress andprevented the cracks we were seeing at the hex vertices. The hex fit could still loosen a bit due to wearover the course of a tournament, leading to some backlash in the system. We inspected for this closelyand changed them out as soon as we saw one with some relative motion. The hex shaft-mountedazimuth pulley was retained on its shaft with a washer and button head screw. An off the shelfaluminum ½” hex pulley could be used, but we were looking to save weight and eliminate the backlashassociated with the typical clearance fit. Note that in order to keep the overall packaging as tight/lowas possible, we cut down the stock Versaplanetary hex shafts to custom length and re-drilled/tapped theends. Off the shelf shafts could be used if the azimuth actuator axis is moved further away from theSwerve Drive. The larger clearance pulley could also likely be an off the shelf pulley with a large bore.The Swerve Pivot Hub would just need slight re-design to accommodate it.

Swerve Pivot Hub and Mounting Hub:These were the two “complicated” parts in the system which were done on a sponsor’s CNC lathe withsecondary ops on a mill.The Pivot Hub (left) was aluminum and supported an off the shelf 6” long Ø3/8” case hardened steelvertical drive shaft (McMaster-Carr). This support was accomplished with a 7/8” OD flanged bearing(AndyMark) at the bottom, and a 7/16”OD needle bearing at the top (McMaster-Carr), both of whichwere pressed in. The separation of the two bearings nicely supported the vertical drive shaft. One keyto success was to get the lower bearing close to the bevel gear. If this distance was too long, the 3/8”diameter drive shaft could bend under the combined loading of the bevel gear thrust and wheel sideloading. The Pivot Hub was bolted into the top of the saddle using flat head screws. It sandwiched theprinted azimuth pulley and a spacer such that those printed plastic parts were vey well supported. Theheads of the flat head screws were slightly recessed below the face they went into. This was becausethat face served as a thrust bearing for the underside of the Mounting Hub. Essentially, ¼ of the robotweight less the posi-traction force (described below) acted on this thrust bearing. The hole pattern wasoriginally determined by our use of off the shelf sprockets for our chain driven azimuth. In this iteration,it was vestigial, and since it drove us to make the milled “scallops” to clear the heads it was redesignedfor 2018 as will be seen below. The moment was transferred through the assembly to the robot frameby two separated cylindrical faces. The first was the scalloped face and the second was the areaimmediately below the snap ring groove. The two faces and the thrust face were all carefully deburredand lubricated (we used “Super lube”). The Mounting Hub interfaced with these faces as describednext.The Mounting Hub (right) was made from cast nylon (McMaster Carr), which made a nice bushingmaterial and was still strong. Delrin would also likely work well. The robot frame sat on the flange withthe bolt pattern. The holes making up the bolt pattern in the flange were tapped and this was how theSwerve Drive unit attached to the robot.

Drive ShaftNeedle Bearing location“Bushings”Thrust “Bearing”SurfacesDrive ShaftFlanged BallBearing locationPositraction Description:For a robot to efficiently drive straight, the following must be satisfied:1. All wheels have same surface velocity. Usually:a. Same diameterb. Same rotational speed2. Same traction. A result of:a. Evenly distributed power.b. Evenly distributed traction.3. All wheels pointed in the same direction (aligned)Pos-itraction helps with item 2b. Three points define a plane. More points are odd men out—inengineering speak, the plane is “overconstrained.” In practical terms, when four rigid swerve units areput on the ground, manufacturing tolerance stackup or post manufacture movement (such as from adamaging collision, or drop) cause one of the points to come off the ground. The robot will then be lessstable than it would otherwise be, possibly rocking (depending on frame stiffness), or will at least haveless traction on the higher wheel. Even aligned, if the traction isn’t similar between wheels, the robotwill not drive straight without some other correction. The posi-traction spring is sized to push theSwerve Pivot Hub and Mounting Hubs apart with a force roughly 20% of the fully weighted robot. Thus,the robot normally rides with its “suspension” bottomed out on the thrust “bearing.” However, whenone wheel starts to become unweighted or even comes off the carpet, the spring will push back downand keep the wheel in contact with the ground. We know from practical experience that the positraction works and will make up for significant frame bending. It also helps control when accelleratinghard, driving onto a shallow ramp or over minor obstacles. Unfortunately, the amount of travel must belimited to fairly small amounts, or the robot is not a stable shooting platform. We limited travel to

approximately 3/32” for the 2017 robot because of the recoil from the Shooter. The inside surface ofthe Mounting Hub Nylon was protected from the end of the steel spring by a washer.Unit Assembly:The unit, less the Mounting Hub, Washers, and Spring were assembled (as shown below) off the robot.MeshAdjustmentScrewShimming of the bevel gear set was either done now or after mounting on the robot and is describednext.Once assembled into the saddles, the horizontal component of the mesh was adjusted by jacking theaxle back and forth several thousandths from the nominal location. This jacking was accomplished usingthe ¼-20 button head screw threaded into the Thunderhex bore (tapped) on the non-geared wheel side.Mesh was adjusted vertically with shims (not shown) between the back of the vertical shaft bevel gearand the support bearing inner race. Mesh is proper when the gears line up as shown (maximizingfacewidth engagement), have minimal backlash, and move freely through full rotation. Once set, theopposing ¼-20 button head screw was tightened to lock the shaft in position. Both axle screws weredoped with Blue Loctite to prevent loosening. This process was checked again after operation underload. Once re-adjusted after “burn in” we did not have to revisit the mesh during a season.Final Assembly:The Mounting Hubs were bolted into the robot frame. The posi-traction spring was set on the Pivot Hubwith a washer on top and then pushed into the Mounting Hubs from the bottom. The spring wascompressed until the snap ring groove at the top of the Pivot Hub came through the top of theMounting Hub. A second washer was placed on top of the Mounting Hub and a 7/8” snap ring was putin place to hold the whole thing together. The top washer protected the nylon of the Mounting Hubfrom the steel snap ring as it rotated with the Pivot Hub. Posi-traction motion could be reduced by

adding washers between the Mounting Hub and snap ring if necessary. Posi-traction force could beadjusted by adding washers inside the Monting Hub or grinding down the spring as needed.Drive PulleyLocationOther Notes: In 2017, we opened the bore of the vertical drive shaft bevel gear (it comes 8mm) and welded itto the 3/8” drive shaft. We have also succesfully cross-drilled and pinned them, and used keyedconnections in the past.It is good practice to try to get the Drive Pulley down close to the needle bearing in order tominimize that cantilever and the resultant moment loads on the shaft. However, we have nothad an issue with the upper portion of the shaft bending with the gear ratios we’re using.It’s important to give careful thought to how motors and gearboxes are mounted. Belt loadingscan be significant and if the shafts are not parallel to the swerve drive unit axis, belts will walkand slip off pulleys. Also, note that our azimuth pulleys are significantly larger than the beltwidths in order to provide for posi-traction travel.

We designed in features to adjust belt tension for both drive and azimuth belts. We mountedthe motors/gearboxes/encoders in either sections of tube, or printed structures as shown aboveand then bolted those down to the yellow Swerve Drive Rails using slots. The Swerve Drive Railswere welded or bolted into the robot frame. Note that the 2017 design accommodated either775Pros with Cimiles or CIMs. The higher unit in the picture above shows both in this screenshot. Note that we have also mounted swerve units individually rather than in pairs. This wasdone using sheet metal with edges braked for stiffness. We like the rails because they help keepthe units planar, which should reduce the need for posi-traction travel.We set our azimuth motor pointing down (the belt is under the C-Channel in the above screenshot) and our drive motor pointing up (belt over the C-Channel above). The components nestedwithin the belt paths to minimize overall footprint. One or the other of these could be rotatedand the drive motor brought in towards the Swerve Drive if that form factor is advantageous.In addition to the azimuth pulleys discussed above, we 3D print our drive pulleys to save weightand cost. We make our hubs out of ¾” aluminum hex to reduce the stress on the plastic. Weturn down the ends of the hex to ½” round, slit them and then clamp onto the shaft through theslit round section using two piece heavy duty aluminum clamping collars. When tighteneddown, they don’t slip.Closed loop tuning will likely be necessary to get the whole package working nicely. Without it,larger motors, and/or gear ratios may be required. Tuning is discussed briefly below, but adetailed description is another subject .Stryke Force teaches a course on tuning in the offseason. A recording of the most recent one is available on Youtube. There are links on ourwebsite. A lot of information we go over is provided in CTRE’s Talon SRX user manual/materials.o In 2017, we used a BaneBot RS550 for the azimuth motor since it had plenty of powerand was very light weight. At various times we have geared it from 64:1 to 100:1 usingVex planetary gearboxes with ½” hex output shaft and encoder stage. In the past wealso successfully used BaneBots’s planetary gearboxes. The azimuth pulleys were 1:1,and we used 2.5” Colson wheels set apart approximately 2-5/8.” Under positioncontrol, with the Talson SRX PID loop properly tuned, that range of ratios easily turnedthose wheels on carpet and did so very quickly. The tuning was hot enough that theazimuth control loop was marginally stable with the wheels in the air, but good oncarpet.

o In 2017, we used a Vex 775Pro drive motor with a CIMile and CIMcoder. The drivepulley ratio was adjusted to balance acceleration and top speed based on wheel size,the game and driver preference. In 2017 the drive pulley ratio was approximately 2.4:1.Since the Cimile has a ratio of 29:12 ( 2.42:1) and the AndyMark bevel gear set is 2:1,the total ratio used was 11.6:1 (2.42x2.4x2). Larger wheels would need more gearratio for similar performance. Note that if you use 775Pros, the motors need to becurrent limited or you will burn them up. The current limit necessary for robustness willdepend on how you gear and drive your robot. We also use several driver techniques tohelp with this issue. We used a CIMcoder to enable closed loop speed and positioncontrol.2018 Stryke Force Third Coast Drive Detailed DescriptionThe goals for 2018 were to improve our ablity to manufacture parts using in-house (non-CNC) resourcesand further reduce weight, and swept volume.Wheel/Axle Assembly: This was essentially unchanged from 2017 other than a minor chamfer on theends of the Thunderhex to make putting the axle in the saddles easier. One note: It makes sense tocheck the straightness of the hex stock before manufacturing the axles. In 2017 we saw some bent axleshafts which caused difficulties in setting the mesh, similar to a non-concentric opening of the bore inthe bevel gear.Vertical Drive Shaft: The 2018 vertical drive shaft was changed from 3/8” to 8mm. The primary reasonwas to avoid the necessity of re-boring the AndyMark bevel gear. However, once changed, a beneficialcascade resulted. We were able to use smaller bearings which drove smaller housings. These sizereductions, along with closer attention to detail in all of the other components allowed us to realize aweight savings approximately 20%, or 1 pound per swerve corner—an overall robot weight savings of 4pounds! We tested key elements of the changes in the offseason and were convinced we didn’t looseany significant durability and this was borne out during the season. One other note: We used hollow8mm shafts in 2018 (SDP/SI “pipe shafts”). This was done not so much for weight savings, but to enable

a potential shifting swerve design (ultimately not needed/used). Also new this year, after it was weldedto the shaft, we turned down the hub on the bevel gear to save some weight.Saddle: The incremental design goals for the Saddle in 2018 were weight reduction, improved loadpaths and development of a couple of fixtures to ease manufacturing. The weight reduction wasaccomplished by starting with a smaller 4” x 4” extrusion, reducing width and extending the sidewalltapers. The main fixture developed was a block to ensure the saddle side walls are supported whiledrilling/reaming the axle bearing openings. This improved our ability to make sure the vertical driveshaft axis and axle axis are perpendicular and in the proper locations. The vertical drive shaft hole wassized to press fit the lower shaft support bearing. This put shaft thrust and much of the radial loaddirectly into the saddle instead of the Swerve Pivot Hub. We also opened the bolt holes to clearanceholes and moved the tapped holes to the Swerve Pivot Hub. This was done primarily to improve themanufacturability of the Swerve Pivot Hub as will be seen below.Azimuth Pulleys: The azimuth pulleys were reduced from 44T to 38T to enable the width reduction inthe saddles. This potentially could have led to tooth slippage issues, but we upgraded our Azimuth drivemounts so that they are more robust (shorter load path) and convinced ourselves with testing that wewere still OK. As in 2017, the pulley was printed without the flange to maximize the tooth profileaccuracy. At this point, we believe this is not necessary. The pulley with the hex hole was printed inNylon with short carbon fiber (Onyx) on a MarkForged printer with continuous strand carbon fiberaround the hex hole to take the hoop stress, avoiding the epoxied SAE washer from 2017. The hexinterface was a press fit and we thereby eliminated the wear issues we saw in 2017. We printed three¼-20 holes in this pulley and later tapped them so that we could tie into them if it was ever necessary topull the pulley off the shaft. We printed a “spider” pulley puller to work with them. The puller had atapped central hole that we could use as a means to jack the pulley up off the shaft by turning a boltagainst the ½” hex shaft end. It worked very well, but we never had to use them at a competition.

SpiderpulleypullerSwerve Pivot Hub and Mounting Hub:The two “complicated” parts in the system were converted for in-house fabrication.The Pivot Hub (left) was converted to a three piece hybrid aluminum and printed plastic assembly. Theprinted plastic was MarkForged Onyx and had continuous strand Kevlar reinforcement in a few areas. A5/8” OD 2024 aluminum tube (McMaster-Carr) was pressed into it. This assembly supported the verticaldrive shaft (SDP/SI). Shaft support was accomplished with a 19mm OD flanged bearing (AndyMark) atthe bottom pressed into the saddle, and a 12mmOD needle bearing at the top (McMaster-Carr) whichwas pressed into the Aluminum tube. One end of the tube was reamed for the 12mm needle bearingand grooved for the snap ring. The Pivot Hub was bolted through the saddle and Azimuth Pulley intotapped holes in the Pivot Hub. This change avoided the conterboring/countersinking operation (andscalloping of the lower bushing) on the 2017 design. We also replaced the lower bushing area with analuminum tube/sleeve to reduce the likelihood of wear/galling due to plastics of the same type runningon each other. This short tube was trepanned at one end to keep the posi-traction spring centered. Athin steel shim washer prevented the spring from digging into the aluminum. All interfacing surfaceswere lubricated with Super lube. The step down in diameter at the bottom of the printed part was usedto interface with the bearing bore in the Saddle so as to drive concentricity of the assembly. Note thatthe printed part of this assembly could easily be machined from either Aluminum or cast Nylon.The Mounting Hub (right) was Onyx reinforced with continuous strand Kevlar in select areas. TheMounting hub bushing surfaces (IDs) were printed for slight interference. These surfaces weresubsequently cleaned up with a boring bar to ensure a smooth, print artifact-free surface finish so as toavoid interference with the positraction axial travel. This part could easily be turned from cast nylon.

For 2018 we limited positraction travel to to approximately 1/8”. The printed Nylon was protected fromthe end of the steel spring by another thin steel shim washer.Unit Assembly:The unit, less the Mounting Hub, Washers, and Spring was assembled

Vex Planetary Gearbox output—only to find out that Vex simultaneously released a similar product with a more robust magnetic-based encoder. We happily adapted to using theirs. Other refinements included using a composite saddle (wet layup over printed ABS core), exploring using a gear set ratherFile Size: 1MBPage Count: 25