Our goal is to improve our Vex Robot's power rating, and we can do that by improving speed without sacrificing pulling power.
To increase/help pulling power, we:
• changed the wheels to increase friction;
• added two smaller wheels in the front;
• changed the gear train;
To increase speed, we wanted to incorporate bevel gears, but we ended up:
• increasing wheel size;
• changing the gear ratio.
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Step 1: Materials and Building Our Modified Sled
You'll need hex wrenched to build the sled.
We used the same materials as the basic sled on LearnMate, but we used extra materials to widen it.
Step 2: Materials for Robot
- Various wrenches (hex wrenches, muscle wrench)
- (2) 2.75" wheels
- (2) 4" wheels
- (2) 36-tooth gears
- (2) 64-tooth gears
- (8) 12-tooth gears
- Various screws
- Bearing flat
- Kep's nuts
- Angle bars
- Square bars
- (2) motors
- (1) battery
- (1) controller
Step 3: Preparation for Building Final Robot
- Remove the top portion of the BaseBot that includes the battery and controller.
- Remove the angle bars on the left and right of robot.
- (Your robot should look like the 1st picture)
- Move the bearing flats farthest from the motor one space closer to the motor (on each side).
- You'll also need to add a bearing flat 2 spaces from the front (on each side).
Step 4: Building the Final Robot
- The changes that we wanted to make to the gear train needed space, so we added length to the the angle bars on the front and back.
- We also moved the gears around a little bit in order to change or Gear Ratio and (W)output while keeping in mind the Output Torque
- We built a sled that would attach to the robot to hold weights
- We increased the size of the wheel to increase speed.
Step 5: The Gear Train and Ratio
Our gear ratio is 15:1. This is because 60 to a 12 is (5), 5 times the 36 that is on the same bar is 180 divided by the twelve would equal 15 divided by 1 because the two twelves make a ratio of 1:1, would all equal 15:1.
Step 6: Input and Output RPM
Each motor has its own level of power that it exerts. This is the (W)input. For our robot the Left motor had an RPM of about 116.88 with an efficiency of 0.97. Our right motor had an RPM of about 114 and an efficiency of 1.02. So we add these two efficiency numbers together to show that our robot is about1.99 efficient. Our robots MAX efficiency is known to be 1.67N*M so we will use that for later calculations.
RPM is found by using this equation: ( (# of turns)/ time) * ((60)/1)
The (W)output depends on the gear ratio. In our robot's cad we have a 60 tooth gear attached to a 12 tooth gear on the same bar a a 36 tooth gear connected to two more bars of 12s. All in all this makes our gear ratio 15:1. what we would do now is divide 15 by one and multiply with the (W)input. Our robot's (W)output would be 29.85
[To make this process more understandable say we had a 60 tooth gear attached to a 36 tooth gear. 60 divided by 36 would equal 1.67 and then you would times it by your (W)input.]
Step 7: Output Torque
Torque= force times distance.
(W)output torque is calculated by dividing your (W)input with your gear ratio number that you multiplied earlier with the (W)input.
For our robot it looked like
As the speed and driver gear ratio increase the Torque will go down!!!!
Step 8: Actual and Theoretical Speed
Actual Speed is measured by dividing Centimeters over Time considering the size of your wheels. The bigger the wheels the faster your robot will go.
Our robot had big wheels that accumulated and actual speed of about 797.9 cm/s.
Theoretical speed is calculated with this equation:
(W)output * (2 Pi r) / 60
When we calculated our robots theoretical speed we got 898.5 cm/s
Step 9: Iterations
- We wanted to incorporate bevel gears (1st picture) into our final design, but the way we tried to proved unsuccessful.
- The next thing we tried was to have a simple 2.7:1 gear ratio, but we decided to change our gear train to include 12-tooth gears.
Step 10: Final Design and Conclusion
The final design ended up with the Bar connecting to the motor having a 60 tooth gear on it and connecting with that gear was a 12 tooth gear. On the same bar as the 12 tooth gear there was a 36 tooth gear that connected to a twelve that was connected to the neutral 12 that created space for the wheels to fit.
Our frame was a little bit on the bulky side and kind of flimsy due to the screws coming undone every time the robot shook and so if the weight was placed in the middle instead of the wheels that could have been a flaw.
Our robot had equally big wheels. The back wheels moved, but the front ones were free ranged and did not move on their own.