Any system, elevator or not, should be designed based on its load and performance requirements. The gearing and number of motors is based off of how much load the elevator experiences under use, how quickly you want to reach your target distance, and how much current you are drawing. System design is a balancing act of requirements, and understanding how each variable affects the others is important when designing a system.
At a base level:
-Higher gearing decreases speed and increases the torque (how much load can be moved, which decreases the current draw per motor)
-Increasing motors adds power (how easily load can be moved), decreases the current draw per motor, and increases the number of motors (obvious, but worth considering).
So where to start? I would start by considering how my system will handle one of the most common high-load scenarios, which for most, is stall. Stall is basically when you hold your system (an elevator, in this case) still at a single point. There are two primary ways of stalling a system: a) a separate mechanical locking brake, or b) powering the motors just enough to hold the system in place without moving it. Each comes with their own advantages and disadvantages.
A mechanical brake allows you to avoid stalling the motors (this is good because stalled motors draw current continuously, heat up, and potentially die/burst into smoke), but comes at the price of an additional mechanical brake system. If you choose this route, you don’t need to worry too much about your motor overheating in stall because you’re not really stalling your motor for any significant amount of time, especially since you’re only pushing current through your motor during the short durations that you are moving the system.
Stalling the motors to brake the system simplifies your mechanical system, but provides another constraint for you to design around: holding voltage. Holding voltage is the voltage (between 0 and 12 volts) that you must run the motor at to provide enough power to counteract the load of your system (the weight, in this case), to keep it in place. With resistance in the system being mostly the same, if we look at the classic V=IR, the higher our holding voltage is, the more current the motor draws. The more current the motor draws, the hotter it gets, and the hotter it gets, the more likely the motor will burn out. This means that when designing a system that stalls motors to brake, it is important to know what holding voltage is “safe”.
Luckily for us, VEX has done some independent testing to provide us with these values. If we visit motors.vex.com, we can see a list of motors that have been tested (as well as the Falcon 500 and the NEO, both of which are missing tests). If you click on any of the non-brushless motors, you will see a series of graphs describing the outcomes of various tests on the motor (here is the page for the 775pro: https://motors.vex.com/vexpro-motors/775pro). What we care about is the Locked Rotor Stall Test. On the X-axis is time in seconds, the Y-axis is output torque from the motor, and each line is the motor at different voltages. Some of the lines drop off immediately, some stay for a little and then drop off, and others move far along the X-axis. These represent how long the motors can survive at stall at the given voltage. As you can notice, the trend is such that the motors last longer the lower voltages they stall at, which supports our V=IR math we brought up earlier. If you notice, only three lines - 6V, 4V, and 2V - last beyond the length of a match. Of those three, we really only have one - the lowest, 2V, that stays mostly consistent throughout a match’s worth of constant running (unlikely scenario, but remember the motor keeps getting hotter throughout the length of a match, and in elims there can be some back to back matches pretty quickly). As a result, some people (myself included) tend to try and keep our system holding voltage at 2 volts or less when using 775pros.
The next question, then, is how we determine holding voltage. There’s a couple ways to do it, but the way I’ve learned how is the following:
Holding Voltage = (Applied Load (lbs) * Pulley Radius (in) * 12 volts) / (Total Gear Ratio * Motor Stall Torque (in*lbs) * # of motors)
Some things to note:
-In the JVN Calc, the linear system tab uses pulley diameter. To turn into radius, multiply the diameter cell by 0.5.
-The math requires that the motor stall torque must use the same force units as the applied load and the same length units as the pulley radius. Since the stall torque in JVN Calc is provided in NewtonMeters, I multiply the Motor Stall Torque by 8.850745792 to convert the NewtonMeters to Inch*Pounds. I suggest making an additional cell in JVN Calc that shows you the holding voltage of your system.
Once you’ve figured out how you are handling stall (I usually use motor stalling, but it is up to you), you can play around with the variables of motor quantity, motor type, and gear ratio to figure out how to best accomplish your system performance goals (go x inches in t amount of time) within your system requirements (Pull no more than n amps of current per motor at stall with a holding voltage under n volts)
