Four bar linkage: a basic lift design
A four bar linkage is a great lift design because it is based on a parallelogram. Why is that important? This example that you will build is probably too small for a competition robot but you will learn some important design principles:
1. In a gear train, all of the drive shafts must go through the SAME set of parallel frame pieces. Otherwise the gears will pull apart and slip.
2. Keps nuts do not allow connected parts to move. Lock nuts tighten on the screw itself so they DO allow connected parts to move. (See Vex Tools.) This design uses four lock nuts. Where are they used?
3. Bearing flats allow square axles to spin. Lock bars have square holes and turn with the axle. (See Understanding Bearings.) You will use both bearing flats and lock bars in this design.
4. Lifts require a lot of force. For your competition robot (not this example) you should use a heavy duty 2-Wire 369 motor and extra strength gears. See Vex Gears and Vex Motors for more information.
Build this basic four bar linkage. Note that the pictures are three different views of the same lift.
Using Easy C for Cortex, create a new competition project. Click the Operator Control tab, drag in an Aracade-2 motor control. Download your program and control your lift with the joystick. In which port should you plug in the motor?
Write an EasyC program that raises and lowers the lift autonomously during the autonomous period then lets you control it with a joystick after 20 seconds. BE CAREFUL. Be sure you manually push the lift into the down position before running your program. Start with a 2 second wait and increase it if necessary. If your arm goes too far, turn off the controller right away before you damage the motor.
Write a program that lets you raise your lift using button 5 until a limit switch is activated.Then the "up" button should stop working but you should be able to lower the lift using the "down" button. First attach a limit switch to your lift as shown below:
Your program will use a "Joystick Digital to Motor & Limit" command. You will need to figure out the settings and where to plug the limit switch into the controller (it is NOT a motor!). In this example only the up direction is limited. You would need to use a second limit switch to limit the down directions