The Tractor Pull project for Vex Robotics challenges the student to find ways to increase the robot's work without increasing force. Students are free to change their robot in any way except having over 2 motors in total. All iterations must be recorded in the notebook.We came up with a good-design to carry some weight while losing not too much speed.
Step 1: Acquire the Materials Needed
You will need:
(1) Vex EDR controller
(2) Vex EDR 2-wire motors
(2) 60 tooth gears
(6) 36 tooth gears
(4) Knobby tires
(2) Bar 1x25
(6) Flat Bearings
(A lot +30) Keps nuts
(A lot +20) Shaft Collar
(8) Drive Shafts (at least 3" long)
(1) 9-hole long bar
(1) 5-hole angle bar
(2) 16-hole long bar
(13) 0.182" ( 4.6 mm ) Plastic spacers
(A lot +30) 8-32 BHCS x 1/2" ( 12.7 mm )
(A lot +30) 8-32 BHCS x 1/4" ( 6.4 mm )
(4) 6-32 BHCS x 1/2"
Ductape OR a 5 hole angle bar w/ a battery holder.
(1 or more) Open end wrench
(1 or more) 3/32" Hex Key
(1 or more) 5/64" Hex Key
(1) Saw (recommended)
Step 2: Assemble the Chasis
Connect a bumper with two 10-hole angle bars with 8/32 x 0.5" screws. Now connect another bumper to the other end of the two 10-hole angle bars like shown in the picture. Screw the front end of the VEX EDR controller the 4th hole from the front of your Chasis. Screw the rest of the holes in place. It should fit perfectly.
NOTICE: Controller is upright in adjustment step, it is optional to make this change.
Step 3: Construct the Left Gear Train
Build like shown in the video.
The gear ratio is 1.6 to 1
We chose this gear ratio since it offered more speed in exchange for a bit of torque. The input RPM is about 228 output RPM is 380. Our input torque is 1.67 N*m and output torque is 1 N*m.
Step 4: Construct the Right Gear Train
Assemble the right gear train similar to the left but mirror the location of all gears, motor, and etc.
Step 5: Place Long-bar and Wheels
Place the 16 hole long bar on left and right side on each side's 3 axles. This is to cover and protect the gear train. Make sure all axles have cuff's on both sides of the long bar or a wheel followed by a cuff and a cuff to either side like shown.
The theoretical speed will be 248.7 cm/s but the actual speed is 125 cm/s.
Step 6: Connect Motors to Controller
Take the motors 2 pin port to the appropriate motor ports on the VEX EDR Controller. It doesn't matter which one you put it in or which way just make sure that you keep account of the motor port #.
Step 7: Connect the Battery
Make sure you plug in a fully charged battery into the vex robot. Plug it in like shown.
Step 8: Create a Custom Battery Holder
You can do it anyway you like or even just leave the battery on top of the vex robot but we recommend duct tape or using a battery holder with a 5 hole angle bar.
Step 9: Create the EasyC Program for the VEX EDR Robot
Open easyC V4 for Cortex. Select New Standalone project. Choose Autonomous project. Place 4 Motor module blocks in main programming window. Place wait block after 2 motor blocks for how long you want to go. Set motor number to the motor port on the controller. Make the first two go to 127 PWM and the last two to 0 PWM.
Step 10: Load Program Code to Robot
Tether the robot via USB tether cord. Do not unplug until done. Turn on the robot and watch it run.
Step 11: Make Adjustments
We added 2 more chasis bumpers to hold our vex controller upright since the battery wasn't connected when leveled to the robot.
You may need to adjust your robot code as well. if it goes backwards reverse RPM to -127. If one goes the other direction, Change the pwm to the same one that is going forward.
Our initial design:5/22
Place a compound gear train with driving gear of 60T gear connected by axle bar to a 12T gear connected to a 60T gear connected by axle bar to a 12 T gear that is the driven gear. This gives us a 1 to 25 gear ratio increasing speed by 25 times.
5/22 Also added weight trailer and removed regular wheels so we can carry the weight and bigger wheels should give us a longer distance in a shorter time.
5/27 Had problem fitting our gears and found out that our torque is decreased 25 times so we dissembled the robot and looked into other designs
6/1 We changed our design 1 to 1.6 and changed our chassis to current
6/10 added 2 chassis bumpers to hold VEX EDR controller upright and motors in place
Final Design: The one showed in this instructable.