West Coast Drivetrain and Gearbox

West Coast Drivetrains are the simplest yet one of the most robust and common drivetrains present in FRC. They're commonly referred to as WCD's and utilize a tank drive, which means the wheels are parallel to each other on both sides of the robot. Differences in the speed of either side allow the drivetrain to turn. Versions without omni-wheels at the edges use a "drop-center" in order to ease turning (the center wheel is slightly lower than the others). Follow the guidelines and videos below to create a WCD!

Links to use:

JVN Calculator: https://docs.google.com/spreadsheets/d/1jKvegQ52v2o2tBl1QVohmnE2E6etftGUg3plpFB1f3A/edit?usp=sharing

Chain Calculator: http://www.botlanta.org/converters/dale-calc/sprocket.html

West Coast Drive Design

Use JVN Calculator to determine gearbox

  1. Free speed of ~14 fps is good, faster/slower based on game
  2. Games with distinct sprints/defense would prefer shifting gb, although the brushless motors have enough power to not warrant a shifting gearbox in most conditions
  3. Traction limited means that the gearbox has more power than the wheels can transfer to the ground
  4. The stall Current draw will depend upon multiple factors. If the drivetrain is traction limited, the stall current will not be reached for more than a couple of seconds. If the drivetrain is not traction limited, it is important to make sure that the stall current will either not be sustained for a large period of time or meets the 40 Amp limit
  5. Less reductions = less friction = more power
  6. Keep in mind sizes of gears, and how gearbox can look with various gear combinations

Chain should be calculated using botlanta chain calculator

  1. Even number of links to remove the need for half links
  2. Chain pitches #25 - 0.25 inches, #35 - 0.375 inches
  3. Use center to center distance, not horizontal (refers to dimensions in your CAD sketch)
  4. Add an extra 0.012 - 0.018 inches to the calculated center-to-center distance to account for tensioning issues with chain (can be reduced for smaller chain loops, 0.018” works perfectly for 9” CC distance)
  5. Chain CAD is mainly aesthetic, but a simple loop would still allow to check for interference

Drivetrain width/length

  1. Based on game requirements
  2. Try to never use maximum dimensions allowed
  3. Long/narrow robot = better turning, square robot = better stability
  4. Consider possible designs of other manipulators as well as maneuverability thru the field
  5. Center wheel drops between 1/16 - 1/8 inches depending on the length of the drivetrain and number of wheels (2019 8WD 1/8 inch drop - very stable, 2020 6WD 1/8 inch drop - very rocky)

2x1 Tubing Choice and Pattern

  1. Various combinations are possible, 0.125" tubing with regular bearings or 0.0625" tubing with versa bearing blocks (the numbers are the thickness of the tubing)
  2. Bearing blocks + 0.0625" tube is lighter, 0.125" tubing is stronger
  3. Regular 2x1 pattern is 5/32" holes, 0.5" from corner of tube, 0.5" spacing

Bumpers

  1. Various ways to mount bumpers, one effective way is to use 1x1 tube on top of wheels (2019) and regular bumper brackets
  2. The 1x1 tube can be mounted with 2x1 supports on either end of the drivetrain or supports in between wheels depending on other manipulators

Bellypan

  1. Pocketing bellypan with the X pattern can take a lot of machining time
  2. An alternative is to use 3/32 or 1/16 sheet metal without any pocketing
  3. These sizes can become really weak if pocketed
  4. Non-pocketed bellypan poses problems during assembly and pits since loose parts cannot fall down the robot and be recovered easily

Gussets

  1. Full width gussets make alignment and assembly easier
  2. Should have diagonal supports
  3. Don’t forget about reliefs for the CNC if the gusset interfaces with any sharp corners

Gearbox mounting

  1. Try to use 2x1 hole pattern or another VexPro Cots Pattern (patterns found on vexpro gearboxes) to make replacement easy if necessary, also requires less manufacturing/replacement time
  2. A common VexPro Cots pattern is 1.875” radius circular set of six holes around the bearing, with top and bottom ones removed
  3. DO NOT use extremely long screws (> 3 inches) to mount the gearbox and hold the gearbox together, gearbox should be able to come out as a separate assembly without any hassle

Encoders

  1. Falcon 500 and NEO motors have an encoder built in
  2. If encoder is needed, make sure it has a 1:1 ratio with the wheel otherwise encoder resolution is lost (applies to all manipulators)
  3. Encoders can be placed directly on the shaft or have a connecting gear

Gearbox Design (Assuming ratios are known from previous steps)

Gear Mesh

  1. Use VexPro chart on gears to find pitch diameter for all the gears (alternatively, you can take the number of teeth and divide it by the Diametral Pitch of the gear, ex. 36/20 = 1.8" pitch diameter)
  2. Create circles of pitch diameter and tangent them in order to get the center of each gear/shaft/motor. Add 0.003 to one of the pitch diameters in each gear interaction in order to prevent excess friction
  3. Overall shape and main holes in one sketch, pocketing in a separate sketch
  4. Try to use similar shape for both gearbox plates (derive the first plate to create similar shape without memorizing dimensions or copy/paste the sketch)
  5. Create all required parts, assemble, check for any interference, make any changes, then pocket
  6. VexPro Hex gears have bosses on either side of the gear, so gears can be flush with each other and bearings without a need for spacers
  7. All standoffs are 0.500 OD, irrespective of screw size
  8. For mounting motors, try to use 2 opposing holes in order to have maximum strength
  9. DO NOT using long screws for mounting everything, make the gearbox a standalone assembly that can be mounted to the rest of the drivetrain with relative ease
  10. Try to place screws so that they can be easily accessed, generally gearbox screws should be accessible from the outside of the robot so that changes can be made without removing manipulators on the inside
  11. CIM Pinion gears can be assembled with 0.0625 distance from the face of the motor, no need to CAD the key or the retaining ring
  12. Standoffs should not interfere with the path of the chain since that will mess up the chain calculations
  13. Make sure that all shafts are retained, generally done using tapped holes on either end along with a screw and a washer (for drivetrain gearboxes)