Understanding V5 Mechanical Launching Systems

The most common way of launching objects with a V5 robot is by spinning a wheel at high speeds and then feeding the object into the wheel. This document explains the relevant physics behind a spinning system, what happens when an object is launched, and how you can adjust a system to launch objects better.

Physics behind Spinning Objects

Rotational energy, the measurement of the energy contained in a spinning object, is defined by the equation:

ERotational = 1/2 Iw2

  • I stands for rotational inertia (also called the “Moment of Inertia” or “MOI”), which is a measurement of how hard it is to turn the object.
  • w is the speed at which the object is spinning.

This means that we can change two variables—either the rotational inertia of our system (I) or the speed at which it spins (w)—to change the rotational energy in our launcher system.

So why do we care about the rotational energy in our launcher? The Law of Conservation of Energy states that energy is neither created nor destroyed, only transferred. This means the launcher system will transfer some of its rotational energy to the object that we are launching, and it is that energy that makes the object launch through the air!

An object moving in a direction has an linear energy, defined by the equation:

ELinear1/2 mv2

  • m stands for the mass of the object
  • v is the velocity of the object

This means an object launched at a certain speed has a set amount of energy. This value is fixed for a certain speed, but the energy in our launcher is not. The energy in our launcher right after a launch will be less than right before because of the transfer of energy to the launched object. By changing the energy in our launcher system before the launch, we can change the proportion of energy transferred to the launched object, and in doing so affect both how well the launcher launches the object and how prepared it is to launch the next object.

What is a Flywheel?

As mentioned above, one of the ways we can change the rotational energy of our launcher is by changing the rotational inertia of the system. It is important to know two things: First, every object has a certain rotational inertia value about an axis of rotation, and second, rotational inertia of all parts of a system add together to make the rotational inertia of the system. An object that is used to increase rotational inertia of a system is known as a flywheel, and there is a new VEX V5 Flywheel Weight to do exactly this in the V5 ecosystem.

Impact of a Flywheel on System Performance

The biggest thing to understand is how different moments of inertia in a system affect its performance.

If we increase the moment of inertia, rotational energy will increase (as shown by the first equation above). With more energy in the system at a certain speed, it will take more time to get the energy in the system so spin up time will increase. With more MOI, the RPM drop after a launch will decrease and an object will generally be launched further. With a decrease in moment of inertia, we get all of the opposite effects: rotational energy and spin up time will decrease, RPM drop will increase, and both the energy transferred to the object and how far the object will go will decrease.

Higher MOI Lower MOI
Higher current draw on initial spin up Lower current draw on initial spin up
Less speed needed to launch object the desired distance Higher speed needed to launch object the desired distance
Less speed drop when object is launched (less time between launches) Higher speed drop when object is launched (more time between launches)

How to use the V5 Flywheel Weight

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The V5 Flywheel Weight can be mounted two different ways. First, a standard ½” pitch square mounting pattern allows the flywheel to be mounted to the 48T, 60T, 72T and 84T high strength gears. Second, a standard 1.875” hex mounting pattern allows the flywheel to be mounted to a versahub, which can be mounted to a high strength shaft with a versahub adapter. The image to the left shows the mounting holes on the V5 Flywheel Weight. The red holes match the standard square mounting pattern and the blue holes match the versahub hex pattern.

Assembly1.png

An example showing V5 Flywheel Weight Mounting Example #1.

Assembly2.png

An example showing V5 Flywheel Weight Mounting Example #2.

As with everything that is manufactured, all parts have a tolerance in their design due to small, unavoidable inaccuracies in the manufacturing process. The V5 Flywheel Weight is not an exception to this rule, and there is potential for a small amount of asymmetry in the flywheel that results in vibration. Vibration in your robot can loosen bolts, make your launcher inaccurate or even damage robot components. There are two ways to combat this. First, if more than one flywheel is being used, the flywheels can be rotated relative to each other such that they cancel out each others’ asymmetrical balance. Second, if there is only one flywheel being used, a bolt can be placed in an unused mounting hole to counteract the asymmetrical balance. In both cases, it is advisable to use a trial and error process to figure out what configuration is best.

Bearing or Bushing: Which One Do You Need?

With the introduction of the High Strength Shaft Ball Bearing, VEX users now have access to two different ways of supporting rotational systems in their robots. The part known as the “bearing flat” is actually known in industry as a bushing because it has no moving parts. Both bearings and bushings work by reducing the friction between the rotating shaft and the fixed support. Bushings—the “bearing flat” or “High Strength Shaft Bearing” in VEX (referenced in this document as bushings)—do so by providing a smooth, round surface that the shaft can contact. Bearings, on the other hand, contain many small balls that roll when the shaft spins. Despite reducing friction, neither bearings or bushings eliminate it completely. Because of their different constructions and a couple of other factors, bearings and bushings have different strengths, weaknesses, and use cases.

 

Strengths Weaknesses
Bearing
  • Reduced Friction relative to bushing
  • Able to take more load
  • More robust
  • Can do things a bushing can't do
  • Performs well at high speeds
  • More expensive
  • Heavier
  • Harder to mount
Bushing
  • Easy to use
  • Less expensive
  • Lighter
  • Good for most applications
  • Weaker
  • Not good at high speeds

If we are looking at a spinning mechanism in the context of its energy, as we have done previously in this guide, bearings or bushings constantly “leak” energy away from the system in the form of heat through friction. The rate at which they do so, however, is different. Bushings lose energy from the system faster than ball bearings, and the impact is significant.

mceclip0.png

We ran a series of tests with a launcher, first using bushings and then using bearings. In both versions, the launcher had 2 bearings/bushings geared at 600 rpm and 2 bearings/bushings geared at 3600 RPM, using two V5 Smart Motors with blue cartridges. The difference between the bearings and bushings was significant. This is the graph of the motor velocity during a normal spin up.

The bearings achieved a significantly higher stable top speed and accelerated faster than the bushings did. In the context of energy, this means that the system with bearings was able to keep more energy in the system and launch its object farther and faster than the system with the bushings. The difference in efficiency was roughly 8%, with a difference in 300 RPM at the output of the gearbox.

mceclip1.png

With the same setup, we measured the current draw of one of the motors during a normal spin up of the launcher. Just like the last test, we did one test with bushings and another with bearings, with an otherwise identical setup. The difference in current draw was significant, with the bushing-based launcher drawing more than double the current of the bearing-based launcher. This is the graph of the current draw over time.

mceclip3.png

Finally, to demonstrate the impact of flywheels discussed earlier in this article, we ran a test tracking the RPM of one of the motors while launching 3 disks. One test had no flywheels while the other had two. This is the graph:

There are a couple important things we can see in this graph:

  • RPM drop—the difference between the target RPM (600) and the slowest RPM right after a shot—was significantly reduced in the test with 2 flywheels. The tests with 0 flywheels had a ~150 RPM drop while the test with 2 flywheels had a ~75 RPM drop.
  • Recovery Time—the time it takes the launcher to get back to the target RPM (600)—was reduced significantly in the test with 2 flywheels. This makes sense as a lower proportion of the total energy is transferred to the launched disk, as discussed earlier in the article.
  • Overall launch time was reduced by ~40% per shot and overall for the test with 2 flywheels.

Conclusions

  • It is easiest to think about launchers in terms of their rotational energy and launches as a transfer of that rotational energy to the launched object.
  • Flywheels allow you to increase the rotational energy in your launcher, allowing you to launch objects farther. The new V5 Flywheel Weight opens up the options for flywheels in VRC and the V5 ecosystem.
  • V5 Flywheel Weights may need to be attached in a way that is relative to each other to reduce asymmetrical balance produced during the manufacturing process.
  • Bearings and bushings “leak” energy from your launcher through heat from friction. Using the new high strength bearings over traditional bearing flats (bushings) can allow you to achieve a higher top speed in your launcher and reduce the sustained current draw of your launcher motors. This increases the energy in your launcher system while keeping your motors cooler.

For more information, help, and tips, check out the many resources at VEX Professional Development Plus

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