14 replies to this topic

[Birch]

As I understand, concept threads are not really allowed, but I have just so little desire to actually try it out this idea for myself, I thought I could just tell you my thinking and someone who knows more than me about electronics and stuff could try it out or something.

My basic idea is to move the location of the motors in a flywheel blaster. Doing this, one could in theory raise the revolution speed of the flywheels greatly, without using insane motors or equally insane batteries. By harnessing basic mechanical laws, this design would be able to compete with other plunger-based homemades in range and fps. Here is my basic design:

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Locating the motors in the behind the darts allows for their to be a mechanical advantage system of wheels and pulleys. This means I can spin the back motors at a slower or even similar rpm to what is required in a say a stryfe, and achieving much better performance. The motors would need to have really Of course this would add a lot more bulk to the blaster, but it would still be much shorter than a double rainbow, which this would be similar in size to.

The reason I have so little desire to try this out for myself is because I have no where near the amount of cash that would be needed to build this thing, nor do I have the knowledge or the patience to work with something like this. In addition, this thing would require either expertly crafted or 3D printed parts. I would love to see this idea put into practise by someone, and to see if my ideas would actually be practical.

Edited by Aeromech, 23 November 2015 - 01:22 AM.

[snakerbot]

RPM doesn't directly influence fps, per say. It's the speed of the edge of the flywheel. Since your flywheels are connected by a belt, the edge speed is the same on both. This has no advantage over simply feeding the dart in the larger wheels in the back, and has the added disadvantage of extra friction in the system.

[Lunas]

RPM doesn't directly influence fps, per say. It's the speed of the edge of the flywheel. Since your flywheels are connected by a belt, the edge speed is the same on both. This has no advantage over simply feeding the dart in the larger wheels in the back, and has the added disadvantage of extra friction in the system.

not to mention the added mass will reduce the run time significantly that said you would need to maintain tension on the belts or bands making this blaster even heavier this would be a good thing for if we needed a high torque motor in this type of setup but we don't torque equates to how much mass can be tossed out rpm gives you how fast it will be going relative to how much kinetic energy can be transferred to the dart as in how much the dart slips on the flywheels as it moves through...

The faster the flywheels are spinning the faster the dart will end up how much momentum the dart picks up is dependent on the grip and slippage. Torque is a matter of how much the wheels will slow down when they are hit with the load of the dart...

[Birch]

RPM doesn't directly influence fps, per say. It's the speed of the edge of the flywheel. Since your flywheels are connected by a belt, the edge speed is the same on both. This has no advantage over simply feeding the dart in the larger wheels in the back, and has the added disadvantage of extra friction in the system.

Wait, actually? i thought about it like a bike. When you want the back wheel to go faster you switch the gear on the pedal to a larger one. I may be totally wrong and I'm sorry, bu the maths could probably be done and there could still be some for having an indirect connection between motor and flywheel.

[snakerbot]

Wait, actually? i thought about it like a bike. When you want the back wheel to go faster you switch the gear on the pedal to a larger one.

Which increases the edge speed of that gear. Edge speed = gear (or flywheel) radius * rotational speed (in radians/sec). The same rotational speed with a larger radius means faster edge speed, but the pulley doesn't do you any good. Just make the flywheel bigger and feed the darts into that.

the maths could probably be done and there could still be some for having an indirect connection between motor and flywheel.

I can't see one. Sprockets and chains are useful in bikes because it allows for a torque and speed transformation from the pedaling speed of the rider. The difference is that on a bike the limiting factor is the rider's legs, but we can spec flywheel motors to about whatever we want.

Edited by snakerbot, 08 March 2015 - 10:16 PM.

[Lunas]

this is all gear ratio the smaller gear in the front would be more rotations for every one rotation of the torquey slower motors in the rear example a 3 to 1 gear ratio as in for every one rotation of the big wheel equals 3 rotations of the smaller wheel this also adds speed to the small wheel the small wheel will spin 3 times faster than the big wheel. idea is not bad but makes for a clunky heavy gun and honestly i would just stick with current designs they are simpler and easier to work with than figuring this out and then keeping your fingers out of the danger zone...

You could design it to be driven by 1 motor but there will be another gear involved to get them spinning the right way.

[CS- Tiff]

Your better off just making an afterburner.

[snakerbot]

this is all gear ratio the smaller gear in the front would be more rotations for every one rotation of the torquey slower motors in the rear example a 3 to 1 gear ratio as in for every one rotation of the big wheel equals 3 rotations of the smaller wheel this also adds speed to the small wheel the small wheel will spin 3 times faster than the big wheel.

But the speed of the edge of the gear would be exactly the same on the small and large gears. Unless you can't find larger flywheels and simply must use the stock ones, there is no advantage to this system over using large flywheels. And if you desperately need to use stock flywheels and want to make them spin faster, you're better off buying faster motors than dealing with belts and pulleys and having to make sure they're tensioned properly, and aren't going to explode from the speeds.

Your better off just making an afterburner.

This. I don't know of any really hardcore tests done, but I believe we've long since hit the peak flywheel velocity from single flywheel pairs.

[Lunas]

But the speed of the edge of the gear would be exactly the same on the small and large gears. Unless you can't find larger flywheels and simply must use the stock ones, there is no advantage to this system over using large flywheels. And if you desperately need to use stock flywheels and want to make them spin faster, you're better off buying faster motors than dealing with belts and pulleys and having to make sure they're tensioned properly, and aren't going to explode from the speeds.


This. I don't know of any really hardcore tests done, but I believe we've long since hit the peak flywheel velocity from single flywheel pairs.

which is why i said to stick with current designs no sense in trying to reinvent the wheel. current designs are very simple this would add un-needed complexity current designs 2 identical motors spin directly 2 wheels the opposite way of each other given the same power source they spin at the same velocity.

[jktrolldaddy]

Just wanted to say something, I'm in an engineering class in school right now, and we've actually tested this with VEX kits, and the darts fly MUCH farther when we set up the flywheels and motors the way Nerf does it... because you want a DISTANCE multiplier NOT A FORCE multiplier. So in turn, what you want is a motor with high torque, but also a high RPM.

As for the bike thing, your feet moving the pedals are FORCE multipliers and from front sprocket to back sprocket it's a DISTANCE multiplier then it spins a rod on it's inside which is a FORCE multiplier then that spins the wheel which is the last DISTANCE multiplier. So in turn, it goes Force to Distance to Force to Distance.

[truglodite]

Sorry I'm resurrecting this, but I have $.02 to add...

 

I don't think this argument between belt vs flywheel is as cut and dry as it may seem after reading this thread. I think some arguments are not adhering to the laws of science, and some are just not focusing their scientific eye on the right spot. So I'll start off with our working equation (how a nerf dart accelerates using flywheels/belts... any friction energy transfer method really):

 

Work on the dart ~ Ffriction * distance ~ u*N*A*d

 

u = coefficient of friction (I think 99.9% dynamic, however I talk about static below)

N = normal force (ie how hard you squeeze the dart)

A = surface contact area between the conveyor (flywheel edge, belt, whatever) and projectile

d = distance over which the frictional force is applied (can be very large for a belt)

 

... a very simplified version, but it is enough to make my point about flywheel velocity and size. Flywheel size and RPM aren't even part of this equation, or at least they have very little to do with the dynamic friction available to accelerate our dart. A bigger flywheel may increase A a little bit, you can squeeze the dart more for a bigger N, and use rubber coated ribs to maximize u... but I think a belt will win this battle more often than not. With belts, the friction occurs over a much longer distance, and is applied over a significantly larger area. The distance force is applied in a belt is easy to design, and relatively unlimited compared to flywheels (you can only make the flywheel so big...much easier to play with the distance of the front/rear pulleys on a belt). Of course with a belt, the same leverage over N can be had by squeezing the dart, and coatings can modify u... so those variables don't really enter this argument.

 

With so much extra leverage over d and A, belts already seem very interesting to me. Then when I think about controlling the belt in a way that allows the static friction to come to play, available force could go way up! That might be doable with a clutch/flywheel accelerated belt, hehe! Yeah you could also use a secondary flywheel and clutch to similarly accelerate a flywheel drive, but with the flywheel drives tiny d (and huge inertia... we won't get into that here), slipping is pretty much the best way to go. The big d of a belt means there may be enough time to maintain a static coefficient (inertia? shoot, those are some super slick lightweight parts in modern RC road race cars that would certainly spool up easier than a big ole flywheel). This would be so cool to see IMHO!

 

Also, at first glance I think an experimental belt drive may be doable without access to a mill & lathe. I know in the RC car hobby there are a lot of examples of belts running stable at very high speeds, and of course gearbox/motor setups that could be adapted somewhat easily in a typical Nerf hacker's garage. I haven't ran the numbers nor do I have equipment to experiment with, but I'm guessing if the dart emerges anwhere near an RC car's belt velocity, it would be very impressive. I think (due to radial vibrations), a similarly performing belt system could be much quieter than flywheel. Battery life would of course be reduced vs flywheel, but I think something like 20min trigger on time should be easy to do (should be enough, right?... extra lipos in the bag?). Yeah fast RC cars only last 10min, but they're also putting out a c**p ton more work. Spitting foam those setups might last hours. Anyhow, I just don't see battery life as a major design consideration here, but I'm not a veteran nerf warrior. I'm sure there are good arguments to have long battery life in the field. Please share them with us.

 

Anyhow, that's probably more TLDR than $.02, but I wanted to give my support to those interested in developing a belt nerf drive. It has favorable design trades that IMHO are worth looking in to. I'm fairly confident my arguments are well supported by physics, however I'm no nerf engineering genius (just an old Wolverine BSAE who loves to tinker with everything). Please share your arguments on the belt vs flywheel subject... maybe we'll end up with something to show at the end of it all. Maybe it'll fail, but I think it's got enough merit to deserve a go.

 

Kevin

Edited by truglodite, 02 May 2017 - 12:38 PM.

[NerfGeek416]

From a physics perspective, belts absolutely make great sense. But from a practical construction perspective? I suspect it's going to be very hard to get a belt up to 250FPS without breaking it. 

 

There are widely available setups which hit 150FPS on a single stage. In order to be worth the expense and complexity of a belt drive, I'd want it shooting over 220 at least with short darts. Otherwise, I'll stick to multistage flywheels systems.

 

I'd love to proven wrong, and it sounds like you have experience with high speed belt systems. so go for it.

[snakerbot]

I'm going to have to break this up here...

I don't think this argument between belt vs flywheel is as cut and dry as it may seem after reading this thread. I think some arguments are not adhering to the laws of science, and some are just not focusing their scientific eye on the right spot. So I'll start off with our working equation (how a nerf dart accelerates using flywheels/belts... any friction energy transfer method really):

You're the first person in this thread to actually mention feeding darts directly into the belt drive. I'll talk more about this later in this post but I think you're the one who missed something.

Work on the dart ~ Ffriction * distance

Right, for a constant F.

Ffriction * distance ~ u*N*A*d

Not right. Ffriction is in force units, distance is in distance units, so you get lbf*ft in imperial. u is unitless, N is force, A is distance^2, and d is distance, so you get lbf*ft^3.

... a very simplified version, but it is enough to make my point about flywheel velocity and size. Flywheel size and RPM aren't even part of this equation, or at least they have very little to do with the dynamic friction available to accelerate our dart. A bigger flywheel may increase A a little bit, you can squeeze the dart more for a bigger N, and use rubber coated ribs to maximize u... but I think a belt will win this battle more often than not. With belts, the friction occurs over a much longer distance, and is applied over a significantly larger area. The distance force is applied in a belt is easy to design, and relatively unlimited compared to flywheels (you can only make the flywheel so big...much easier to play with the distance of the front/rear pulleys on a belt). Of course with a belt, the same leverage over N can be had by squeezing the dart, and coatings can modify u... so those variables don't really enter this argument.

If your equations show that flywheel velocity doesn't affect dart velocity, then your equations are a gross oversimplification. My hunch is that the friction force changes dramatically with flywheel speed as a result of different amounts of slip. This also changes form the time the dart first touches the flywheels to the time the dart leaves them.

<everything else>

A lot of your points do make good sense, but I feel like you're missing some things. Giving the dart more time in contact with the belts/flywheels allows for more energy transfer to them, so yes, you get faster darts. Perfectly valid, although there are a lot of other variables to take into account. If the flywheels are not slipping when the dart leaves them, then the ability for the flys to accelerate the darts is not limited by friction, but by the motors' ability to maintain speed during the dart crush, which I don't see a belt drive helping with.

Sorry if it sounds like I'm being too cynical, but I do have a tendency to resist change, and I'm just trying to play the devil's advocate. That said, I've been wrong before. If this works, I'd love to see it.

[Eightdotno]

Why don't you use gears insteae, that way it's shorter, and you can put more power into it.