# Is Anyone Successfully Shooting with a Belt?

Theory says driving a frisbee pinched between a straight belt and a straight wall should work exactly the same as pinched between a wheel and a curved wall. As long as the shear forces, speeds, and contact times are the same, it should work the same.

We all have seen dozens of curved shooters, and straight wheeled shooters (one and two wheel varieties). But I have not seen anyone claim success with belt shooters, specifically straight belt shooters. Has anyone out there done it?

If not, any theories as to why not?

Theory may say that it will work, but physics might not agree.
Having a straight belt might cause drag and inconsistency.

Any thoughts on why physics might not agree or why a straight belt might cause drag and inconsistency more than a wheel, absent any friction between the belt and fixed items?

Team 75 succssfully used a belted shooter last year. Its performance is highly comperable to the one wheel+curved wall design, although some aspects proved to be serious problems. Things like belt inefficieny/stretching/grip/wear and overal mechanism complexity made the shooter somewhat troublesome. However, there is no “physics” reason (that I can come up with at least) to say that a shooter with belts is any less functional than a shooter with a curved wall and one wheel. The principle is the acceleration period, which depends on the required change in momentum of the projectile. The larger the change in momentum is, the longer the wall or belt is required to be to reach the target speed.

I should also add that a curved wall and wheel is no differet in terms of friction from a belt and a wall; using belts is simply “unrolling” the wheel into a flat surface. I’ll try to explain as best I can by saying that 2pir would be the belt+wall length required to achieve the same performance as a wheel with that same r plus a curved wall. Unless you take into account other factors such as ceptripetal force and angular acceleration, then things change slightly.

It’s good to know the principle works with the foam balls from last year, but I was hoping to find someone who made it work with frisbees. If nobody has done it yet, there could be unknown issues that make it more difficult than it looks.

My team was going to prototype a belt shooter, but then we ran some calculations.
The Frisbee’s linear is theoretically half of the belt or wheels tangential velocity. So, the larger the wheel, the larger the tangential velocity. The belt has the same linear speed the tangential speed of the pulley that it rides on. Most belt pulleys are small when compared to the wheels that most teams are using–around 2 inches in diameter verses 6-8 inches in diameter-- so in order for the belt shooter to move the Frisbee as fast as a larger wheeled shooter, it has to move faster. Moving faster increases the chance for slippage between the belt and the Frisbee.
If you had belt pulleys of a larger diameter, and a belt that has a high grip on the Frisbee, I seen no reason that a belt shooter would not perform the same, if not better, that a wheeled shooter.

I see what you are saying - with a 2" pulley, the motor has to go faster to get the same surface speed as an 8" wheel. But whether we have a belt going 100 inches/sec or a wheel with a surface speed of 100 inches/sec, if the friction is the same and the normal force is the same, then the slippage is the same.

It seems like is should work, but has anyone demonstrated that it can practically be done?

We tried using belt to shoot frisbees, they didn’t work. The solid metal pulleys do not compress, neither do the frisbees. This causes problems achieving enough friction to throw the frisbee.

See http://www.fightingcalculators.org/blog/?p=91 and the rest of our blog. In concept, the belt shooter was far more attractive to us than a wheel shooter, so we focused on making it work over other designs that took fewer iterations, and are now very happy with what we’ve come up with, and moving forward with the competition design (with no illusion of it being final).

As a engineer who has spent over 20 years designing belt conveyors I think there are some significant losses that need to be considered with using a straight line belt shooter vs a curved track wheel shooter.
First you need to consider the losses from the slider bed behind the belt. Using “standard” rough top PVC belt with a polyester fabric carcass the friction coef. is about .28 using a metal backing, .24 using uhmw, and mabe you can get it down to .20 using Teflon.
You also need to consider the losses of bending and unbending the belt as it goes around the drive and idler rollers. We have seen this factor play a significant roll in overall losses as the speed of the belt increases such as is needed for a shooter.
Finally you have losses from moving the bearings on two wheels and the load on the bearings from with belt tension.
Using a curved shooter with a single wheel the only losses are in the bearings of the wheel and the deflection of moving the rubber of the tire.

We prototyped a belt shooter. We used 6 in. wheels for the rollers and direct drove it from a CIM. We used the orange urethane belt (just like the polycord everyone used last year, except flat) for the belt.
As soon as we powered it up we noticed a huge problem. When you get the CIM up to full speed (the kind of speed you would need to launch a frisbee a reasonable distance) the centrifugal force caused the belt to “float” off of the rollers.
We did some more work on the prototype and put in a ridiculously tight tensioner to keep the belt on the rollers. Then we ran the launcher at about 60% power. We were able to get 20-25 feet of distance launching at ~30 degrees.
This distance could be enough for some applications, but not for others. We’re planning to prototype a two-wheeled shooter next.
We figured that if we got some belts that wouldn’t stretch then we could use those, but our options seem to be pretty limited as to the belts we can get.
My conclusion? Yes, belts can be made to work, but you could waste a lot of time trying to get it to work consistently. Time that might be better spent on designing and building a good climbing mechanism.

This struck me as an interesting question too. I would love to see it work, but I would advise against it.

Here’s why: last year (and this isn’t the first time I’ve told this story), we went with a two belted shooter. It was terrible. In addition to the angle being wrong, there was tons of slippage between the balls and the shooter, that caused lots of inconsistency. So, if you do decide to go with a belted shooted, make sure to do prototypes and dial in the compression before you put it on the robot.

It took us a few tries to get the friction in our system low enough to really do anything, but it has been worth it. Belt tension and support is critical. Pro tip: put the supporting material inline with the pulleys so that the belt only rubs on it when the frisbee goes through.

Aren,
Glad to hear it is feasible. If you don’t mind telling us, how far are you able to shoot?

In this thread I gave some of the details of our setup and what we’re getting out of it.

For space & weight concerns, we are considering a one or two belt shooter.

The main concern we have is for how to maintain consistent & adequate belt contact pressure/friction against the disk edge, for a decent amount of travel distance, while still ensuring that the disk stays centered on the belts, without developing up/down “shear” forces shifting alignment of disks with the the belts’ centerline(s), and raising friction against upper/lower guide-glide surfaces.

We prefer to have dual belts, in order to allow using lower belt speeds and to hopefully give less resulting belt slippage, from the velocity boost that having a 2nd MOVING opposing belt surface allows. We also plan to have the two belts running at a suitable differential velocity to impart adequate disk spin.

We will initially try this by using the very thin layered (<.001) and low stretch (kevlar-polyester?) urethane surfaced style of belts, and with a generous belt width of ~1.5"

We are thinking that pivot arms with an adjustable pressure spring loaded belt tensioning mechanisms, and idler rollers with slightly convex dished roller ODs, located at at least at the shooter’s outlet end, would work best to help belts maintain consistent grip. The dished rollers could also help shape the belt slightly to better contact & contain the disks on center.

A pair of these idler arms for each belt may likely be needed so that as the frisbee flexes the inlet pair of idler arms outward. the outlet pair of arms would close inward the corresponding amount to take up the resulting belt slack.

Having similar opposing idler arms for each belt at both ends should also help keep the belts in better and more consistent contact with the moving disks over a longer belt arc segment worth of contact, as disks pass through this idler arm “pinch zone”.

These pivot points mounting of these belt tesnion rollers should likely be placed on a rocker plate to better handle the dynamics of belt flexing motions as the disks squeeze their way through the roller pinch zone.

There would still also be a fixed idlers and a powered pulley located at each end of the belt oval, just beyond the swing arm tension idlers, but spaced wide enough so they would barely pinch the disk on entry & exit.

The opposing idler arms at the entry end would separate as the disk wedged into the V caused by the bends in the two belt paths. Then, as the belts separated at the entry end, being spread wider by the disk, at the shooters exit end, that pair of opposing rollers would be closing tighter until the disk reached the midpoint.

With the disk at midpoint, the entry idler arm rollers would begin closing back together, while the exit pair of idler arm rollers would start spreading to allow the disk to pass between them.

It would be kinf of like an intestine with fast moving walls to squeeze, flex and accelerate the disk through the shooter.

Any thoughts?

-Dick Ledford

Yea, I built a linear belt shooter, but the problem we have so far is that the belts we use are to thin and tend to shift up and down, due to this we experienced many a failures. Amidst my failures though I did find a fix for the issue of the frisbee slipping and not getting a proper grip, just stuck a strip of pool noddle on the wall. Still would like to test this with a wider, grippier belt.

extra photo of ghetto rigged shooter:https://lh4.googleusercontent.com/-GVoKLgc-9wA/UPTVAxxPv-I/AAAAAAAAA1I/66-9rdPou9o/s902/F723D38B-A46B-4BE4-AE76-5DD28F2C707C.JPG

NOTE TYPO IN ABOVE WRITE-UP – Belt urethane thickness should be <.100" for smaller pulleys and better convex shaping to curved edge of disk.

Basic dual belt concept with pivot bar idlers fixed => i.e without any spring loaded swing arms added between idlers & pivot bars.

-Dick Ledford