Ball Magnets

[scawsome]

Hi, everyone!
I am from team 230 and our team is trying to decide if we should put effort into designing a ball magnet. We were pretty sucessful without one at WPI but I saw that ball magnets were pretty useful at other competitions. I wanted to ask other teams several questions.

1) How useful are ball magnets when shooting or pushing balls into the goal?
2) What kinds of ball magnets (roller,suction,etc.) worked well?
3) How have teams done with scoring and pushing balls into the goal without a ball magnet device?

Any opinions or suggestions will be very helpful.

[BJC]

I will put it this way.

I would rather have a good ball magnet then a good kicker.

That being said depending on where your kicker kicks you limit your options.

Also by far the best ball magnet is the pincher which is generally about as wide as the robot and uses a spinning top roller to pull the ball into a clamped position. When doing this you want to make sure that you use small diameter rollers and cover it with somthing that doesn't slid on the ball (your choice). Also for best results the top roller should be spinning twice the floor speed and able to stop when a ball is possessed. Maby use a beam sensor. However, if trying this don't use a fisherprice because it will stall.

For reference, 217 67 910 27

[Zach O]

I'll go ahead and answer in the same format you asked in.

1) Ball magnets are incredibly useful when trying to kick (or shoot, whichever you prefer to call it) into the goal. They allow you to accurately line up the ball towards the goal and move around with the ball with ease. As for pushing a ball into a goal, they are not so helpful. Once a ball is in the "ball magnet", you need to either kick of off or sometimes it will shake lose. If you plan on pushing a ball into the goal while trying to use a magnet, you might need something to get the ball out of you control first... if that makes sense.

2) There have been several types of "ball magnets" that have worked well for teams this year. Rollers have been the best, with one roller on top rolling inward to your robot and one stationary roller on the bottom, keeping it from going too far into your robot. Some teams also use vacuums, which have worked to a certain extent. Some teams have paired the two together.

3) Teams have had varying success when pushing balls without a ball magnet. It can be done, and some teams do it well. Others just can't seem to put them in.

[ttldomination]

Via a search, you'll find a thread that talks about the effectiveness of different ball controls. I believe the thread is called "All about Ball Control".

I've seen plenty of the robots that done extremely well while pushing the balls. Teams 1466 and 34 only pushed the ball at Peachtree, they were pushing in 6-7 balls a match.

Really depends on what your ultimate goal is. My team has spent the better part of 2 months working on different ball control mechanisms because our design depends on it.

[Jeffy]

During autonmous, many teams have been successful without a ball magnet. After that, the only time I have seen it work with any success was for a zone 1 plow bot that can plow the balls along the wall and into the goal.

I would highly reccomend that if you have a kicker, you have a ball magnet to go along with it.

The simplest way to make a ball magenet is to have a roller towards the top of the ball and a wall 3" inside of your robots frame.

From what I have learned working for a week on the prototype and going through a competetion with it is that the roller type ball magnet has very small tolerances. To compensate for this, our team uses two foam wheels that deform and make up for some of this, and you can change the wheels or file them down accordingly. Also, you can add a slick substance such as electrical tape to your wall to change that distance.

Ours uses a fischer price in a 25:1 gearbox and 2.75" foam wheels. I would suggest that you try and gear it a little lower, or a little higher. Our gear ratio works, but only because of a clutch that saves it from stalling on the ramps and such.

edit: listen to the guy below me, one of the most impressive ball possesion mechanisms at KC.

[Tarzan19]

We, from the start tried to implement a ball magnet, however we had complications from the start as well. During the competition, we started out with two motors turning a tube of pvc wrap with surgical tubing. about have way through this device broke, so we improvised. we stationized the motors, slid a pool noodle over the pvc, and it work better than we could have ever imagined.

[BIGWILLI2081]

Our team used a ball magnet, and it was extremely useful. It centered the ball in the middle of our robot and made it much easier to control the ball. Our robot could move backwards, move forwards, turn sideways, and sometimes move side to side (we had mechanum wheels) with the ball. As for pushing it into the goal, we just had a button on the control that we pushed to run the roller in reverse. This worked great because we could posses the ball and then spit it out as we started towards the goal. The most successful teams have had rollers as their ball magnets. They use a bar of some sort with a non-slick surface wrapped around it. Pushing the balls in without a ball magnet seemed difficult for many teams. As they would go up the little bump before the goal, the ball would go underneath them. So, if you are planning on pushing a ball in without a ball magnet, you will want to make sure there is a type of bar blocking the ball from going underneath.

[Chris is me]

Hi, everyone!
I am from team 230 and our team is trying to decide if we should put effort into designing a ball magnet. We were pretty sucessful without one at WPI but I saw that ball magnets were pretty useful at other competitions. I wanted to ask other teams several questions.

1) How useful are ball magnets when shooting or pushing balls into the goal?
2) What kinds of ball magnets (roller,suction,etc.) worked well?
3) How have teams done with scoring and pushing balls into the goal without a ball magnet device?

Any opinions or suggestions will be very helpful.

It's better to do one thing well than two things average. The position no one wants to be in is that you can do neither well because you tried to do both.

In my mind, what made you guys win WPI is that you did one aspect of the game well. You kicked balls from every zone and scored consistently doing so. Your game routine made it very hard to turn the tide of the match after the first 10 seconds of teleop. You let your partners deal with the nuances of the game involving possession and set up a balanced alliance.

My team was an example of the latter. We wanted awesome kicking, possession, and hanging, and it took us until Saturday to even kick a possessed ball at all.

So with that warning... Do you guys have a practice robot? Are you able to build something around your kicker without modifying it? How much weight can you play with? If you have a practice robot, then you get to prototype changes before you make them - significantly less risk. If you have one, you can just try it and see if you like the results. If you need to modify your kicker and don't have a practice robot, that's a very significant risk. Is that one you're willing to take? Weight dictates changes, too.

If you need help with a vacuum, drop me a PM and I'll tell you what my team did wrong and how to do it right.

[ALTrammell818]

For a simple ball magnet that works well just apply a layer of duct brand carpet tape before every match to the front of your robot on a metal bar or rectangle of poly-carb. It is 100% legal as long as you use certain brands. Duct being one of them.

[jblay]

Ball magnets are super useful especially when you are trying to gain control of balls that are in the middle zone. However a ball magnet could limit how high or far you kick and I know that a huge part of your game plan is scoring from the far zone in autonomous.

[Alex Dinsmoor]

For a simple ball magnet that works well just apply a layer of duct brand carpet tape before every match to the front of your robot on a metal bar or rectangle of poly-carb. It is 100% legal as long as you use certain brands. Duct being one of them.

This was determined to be illegal at the Detroit event this weekend because it was leaving residue on the balls and causing them to stick in the ball returns.

I would not suggest using tape as a way to control the ball. Engineer something that would not affect the balls in the negative way that tape does.


What my team did during the last week of build when we realized that we needed a ball roller was attach a pvc tube to a 64:1 FP motor and covered the pvc with surgical tubing. This was mildly effective, but always let us have pretty good ball control without the risk of carrying.

[Chris is me]

I'd like to think the decision is a bit deeper than "yes, maintaining possession of the ball is a good thing!". Of course one would rather have possession than none. Whether or not you want it or can implement it is the real question!

Engineering isn't about whether or not it would be nice to have a feature, but the cost / benefit analysis of implementing said feature too.

[fuzzy1718]

When we were formulating our pick list for west mich, ball magnet robots were ranked higher than non-magnet ones. IF we saw two teams as equals, the one with the ball magnet got ranked higher always; so if you are debating about developing one, I highly suggest that you do. It could mean the differance between being in the playoffs and not.

[Alex Cormier]

My team will be re-doing our ball magnet in the upcoming weeks. What we are doing is a dual roller setup. On the bottom, we are putting a square shaft at angle with dycem (http://www.amazon.com/Dycem-Self-Adhesive-Strips-Original-Blue/dp/B000C7EUQG) on it. On the top we are using the Entrapption Star (http://andymark.biz/am-0619.html) rollers from AM. This will be awesome and already proven deadly from numerous teams.

[ALTrammell818]

They stated at Detroit that only certain brands were allowed. We used Duct Brand Carpet Tape during the Elims.

Basically you have to use tapes that are proven to "hold their shape" and not leave residue.

[IndySam]

My team will be re-doing our ball magnet in the upcoming weeks. What we are doing is a dual roller setup. On the bottom, we are putting a square shaft at angle with dycem (http://www.amazon.com/Dycem-Self-Adhesive-Strips-Original-Blue/dp/B000C7EUQG) on it. On the top we are using the "45 - Technokats" (http://andymark.biz/am-0619.html) rollers from AM. This will be awesome and already proven deadly from numerous teams.

If you are going to give a team credit for the name then you have to credit team 1018 The Pike Robo Devils and Mike Trapp. They developed the Entrapption star sold by AM and used by 45.

[kwotremb]

They stated at Detroit that only certain brands were allowed. We used Duct Brand Carpet Tape during the Elims.

Basically you have to use tapes that are proven to "hold their shape" and not leave residue.

Actually it was a Scotch brand carpet tape that was supplied by the inspectors and deemed legal. As said above, we used it the entire second day. I dont think it worked quite as well, but it didnt leave the residue.

If others use this dont do it if you just have a pusher bot. We received a penalty during the finals because if you push a ball into the goals the ball will not come off and you will receive a penalty for carrying a ball. You have been warned.

Here is an example of what it is, it should be available at a Home Depot:
http://www.drillspot.com/products/295314/3M_CT2010_15X42_Ind_Carpet_Tape?s=1

[MarkoRamius1086]

On the subject of tapes, my team found two types of material very effective. The first was pipe insulator, very cheap and effective. The second was grip tape, the kind used in stairwells. We ended up using the grip tape, only because it was very very rugged, for the pipe insulator tended to discintegrate after awhile due to the high rpm we had our spin bar moving at.

[ALTrammell818]

Actually it was a Scotch brand carpet tape that was supplied by the inspectors and deemed legal. As said above, we used it the entire second day. I dont think it worked quite as well, but it didnt leave the residue.

If others use this dont do it if you just have a pusher bot. We received a penalty during the finals because if you push a ball into the goals the ball will not come off and you will receive a penalty for carrying a ball. You have been warned.

Here is an example of what it is, it should be available at a Home Depot:
http://www.drillspot.com/products/295314/3M_CT2010_15X42_Ind_Carpet_Tape?s=1

As Kyle stated it is a terrible Idea to use it if you don't have a decent kicker!

[Chris is me]

As Kyle stated it is a terrible Idea to use it if you don't have a decent kicker!

230's kicker is certainly "decent", they won a regional with it... :rolleyes:

[Alex Cormier]

If you are going to give a team credit for the name then you have to credit team 1018 The Pike Robo Devils and Mike Trapp. They developed the Entrapption star sold by AM and used by 45.

ahh, very nice. I was not sure. I corrected my post.

[Tom Bottiglieri]

Corvette is to cars as Ball Magnet is to rollers.

Please use the term accordingly.

[BJC]

I think that as the season progresses team's ball magnets will advance and improve to the point where if you don't have one you won't be a contender, because someone will be able to do your job better than you.

That said let me share a quote from during build season.

Me-"So how well does the ball magnet work?"
Mentor Tim-"Well, it works really well going forward."
Me-"Soooo, then whats the point of having one? Couldn't we just push it?
Mentor Tim-silence
Me-silence
Mentor Tim-smirks

Anyways, the point is that I've seen a lot of ball magnets that hardly do anything. So, if your going to put the time into making one, make sure you know exactly how your going to make it and how its going to work. Don't just go at it blind. Good Luck:)

[Andrew Remmers]

I will put it this way.

I would rather have a good ball magnet then a good kicker.

That being said depending on where your kicker kicks you limit your options.

Also by far the best ball magnet is the pincher which is generally about as wide as the robot and uses a spinning top roller to pull the ball into a clamped position. When doing this you want to make sure that you use small diameter rollers and cover it with somthing that doesn't slid on the ball (your choice). Also for best results the top roller should be spinning twice the floor speed and able to stop when a ball is possessed. Maby use a beam sensor. However, if trying this don't use a fisherprice because it will stall.

For reference, 217 67 910 27

We use a FP on our ball magnet and we have no problems with it works great! I'm not sure what the ratio to the roller is but i know we use a FP motor on the roller and we have faced no problems with it besides the wires not being properly crimped.

just my .02

Andrew

[jblay]

I think that as the season progresses team's ball magnets will advance and improve to the point where if you don't have one you won't be a contender, because someone will be able to do your job better than you.


Well they also work going backwards depending on if you gear the thing properly. The trouble really occurs when you are turning. We find that if we turn too quickly the ball slips out.

If any team has found a solution to this I'd love to know how.

[Chris is me]

I think that as the season progresses team's ball magnets will advance and improve to the point where if you don't have one you won't be a contender

2006:
"Can't reload? You're done." (522)

2007:
"Can't place tubes? Count yourself out!" (Several division finalist ramps)

2008:
"If you can't hurdle, you can't win." (148)

2009:
"If you can't floor load, you won't make the finals." (118, Archimedes)

Just a healthy reminder to never say never when it comes to robot functions.

Despite my posts on the matter and my inability to hold my tongue, I do believe possession is the most important part of the game. At least, for my robot. I just can't help but feel for certain robots and teams, ESPECIALLY 230, this is not a cut and dry issue where everyone should just come and say "Yeah, yeah, you NEED this, or you suck and won't make elims".

I'll shut up now, I just felt compelled to add that.

[BJC]

2006:
Just a healthy reminder to never say never when it comes to robot functions.

Despite my posts on the matter and my inability to hold my tongue, I do believe possession is the most important part of the game. At least, for my robot. I just can't help but feel for certain robots and teams, ESPECIALLY 230, this is not a cut and dry issue where everyone should just come and say "Yeah, yeah, you NEED this, or you suck and won't make elims".


I'm sure that 230 is good, but I'm also sure that if they made a good ball magnet they would be better.

After all being able to shoot from the far zone is a great attribute, why not do it all match all leave the opposition with nothing to score with... oh, you don't have a ball magnet so it would take forever to line up... BUT WAIT, NOW YOU DO HAVE A BALL MAGNET, I want YOU on my alliance.

:D

[BJC]

We use a FP on our ball magnet and we have no problems with it works great! I'm not sure what the ratio to the roller is but i know we use a FP motor on the roller and we have faced no problems with it besides the wires not being properly crimped.

just my .02

Andrew

I was refering to when you are using a pincher where the rollor (fisherprice motor) has to stop suddenly. Fisherprices can't handle this because they will stall. I'm not sure if you are using this ball possession method, but if you are you must have modified the motor in some way.

[NOV8R]

As the American Express ad says "Don't leave home without one" Here's a video of our ball magnet and shooter working together. Btw, the ram light on top tells the driver when he's in possession of a ball. http://www.rambotics.com/video5.html

[Chris is me]

I was refering to when you are using a pincher where the rollor (fisherprice motor) has to stop suddenly. Fisherprices can't handle this because they will stall. I'm not sure if you are using this ball possession method, but if you are you must have modified the motor in some way.

Friction-based power transfer (polycord, timing belt flipped upside down, etc) that works like a clutch when sufficient resistance to spinning is met can do this without modifying a motor.

[BJC]

As the American Express ad says "Don't leave home without one" Here's a video of our ball magnet and shooter working together. Btw, the ram light on top tells the driver when he's in possession of a ball. http://www.rambotics.com/video5.html

Very nice, we have a light for that purpose on our robot too. On a side note did you model after that robotic soccer league ball magnet. They look very similar.:rolleyes:

[Tom Bottiglieri]

2006:
"Can't reload? You're done." (522)

2007:
"Can't place tubes? Count yourself out!" (Several division finalist ramps)

2008:
"If you can't hurdle, you can't win." (148)

2009:
"If you can't floor load, you won't make the finals." (118, Archimedes)

Just a healthy reminder to never say never when it comes to robot functions.

These are edge cases, not the norm. I would rather improve my robot's functions than take a chance on being an outlier.

[jblay]

2006:
"Can't reload? You're done." (522)

2007:
"Can't place tubes? Count yourself out!" (Several division finalist ramps)

2008:
"If you can't hurdle, you can't win." (148)

2009:
"If you can't floor load, you won't make the finals." (118, Archimedes)



522 and 148 were both the final pick in their divisions and in 2007 the only team that could not control tubes that went far was 190 and that was an alliance that was just strangely awesome in it's success so I don't think that really proves your point however I do agree that a team can be extremely successful as a scorer without ball control as long as they have great driving and great strategy as 230 has already proved by winning wpi very convincingly.

Also in this game you can be a strong defender or a striker with a robot that just moves well. 2265 proved to me at NYC that any team can come out and play absolute shut down defense if they have a proper drivetrain and good driving. 34 is nothing but a mecanum driving bot and they were one of the best scorers in the early regionals.

Getting back to the point of this thread, I would personally make the roller run by a motor, because an idling roller really defeats the purpose since you guys don't have trouble pushing the ball. I would slap it on the bot and see how it works during practice matches. If anything you could always go back to the way things were. There isn't that much risk in this situation and the reward could be great. You guys are one of the best scoring machines out there but if you have a roller that doesn't take away from your other success you could become that much more difficult to defend and ultimately defeat.

[Josh Fox]

Corvette is to cars as Ball Magnet is to rollers.

Please use the term accordingly.

What is the actual distinction between a "ball magnet" or rollers?

I was under the impression ball magnet was a blanket term for any ball-control device.

[NOV8R]

Hi BJC....Yes we found that robocup ball dribbler video early on in the design process. Our first prototype worked well enough to keep our interest and our refined ball magnet is even better than even the one in the video. Here's another video http://www.rambotics.com/video7.html that shows our driver practicing with our practice robot and improved dribbler.. We thought ball control was more important than a kicker to practice with so we didn't even put a kicker on our practice bot.

[Tom Ore]

Well they also work going backwards depending on if you gear the thing properly. The trouble really occurs when you are turning. We find that if we turn too quickly the ball slips out.

One thing we do to help this is to adjust the speeds of the Mecanum wheels so the center of turn of the robot is near the center of the ball.

We use a FP on our ball magnet and we have no problems with it works great! I'm not sure what the ratio to the roller is but i know we use a FP motor on the roller

We also use and FP and I believe we use about a 4:1 speed increase to a 1" diameter roller.

When doing this you want to make sure that you use small diameter rollers and cover it with somthing that doesn't slid on the ball (your choice)

We covered our roller with a small diameter bicycle inner tube. To get the inner tube stretched over the roller, we put the aluminum roller inside a steel pipe and stretched just a bit of the inner tube over the pipe. Pressurizing the pipe inflates the tube and the roller drops right in - then trim the inner tube to length.

A couple of other notes. To get variable shot selection we reverse the direction of the roller just before the kick to push the ball out in front a bit. By varying the lead time, we can get a good shot from all three zones even though the kicking force is the same all the time.

Also, when driving forward with the ball magnet pulling the ball into the robot, our robot would start to "power hop". We reverse the roller with speed propotionate the forward speed to avoid this.

[Tom Bottiglieri]

What is the actual distinction between a "ball magnet" or rollers?

I was under the impression ball magnet was a blanket term for any ball-control device.
Magnets hold on to things. They don't drop the things they are holding on to when they move backwards or turn around.

148 has a ball magnet. 217 has a ball magnet. 1114 has a ball magnet. These types of things are often imitated, but never duplicated.

[jblay]

Magnets hold on to things. They don't drop the things they are holding on to when they move backwards or turn around.

148 has a ball magnet. 217 has a ball magnet. 1114 has a ball magnet. These types of things are often imitated, but never duplicated.

What would you consider something like what 1726 has where the ball spins in front of the bot instead of inside it. They still hold onto the ball when they back up its just different. It would be ironic if that had another name since 1726 coined the term ball magnet I believe.

[Chris is me]

Magnets hold on to things. They don't drop the things they are holding on to when they move backwards or turn around.

148 has a ball magnet. 217 has a ball magnet. 1114 has a ball magnet. These types of things are often imitated, but never duplicated.

The term "Ball Magnet" originated from 1726, who has a good, but imperfect single roller. I think it would be unfair to take this away from them because they're not as good as Team IFI.

My team's been using the name, as every possession mechanism we've made is an iteration of that original design. I guess we'll stick to "the vacuum" now, though.

[s_forbes]

Magnets hold on to things. They don't drop the things they are holding on to when they move backwards or turn around.


Single rollers can be ball magnets, too. They're the ferrite equivalent of the rare earth ball magnets used by 148/1114/1868/etc. :)

[PAR_WIG1350]

Magnets hold on to things. They don't drop the things they are holding on to when they move backwards or turn around.

148 has a ball magnet. 217 has a ball magnet. 1114 has a ball magnet. These types of things are often imitated, but never duplicated.

I believe that magnets do drop things, some more than others, but magnets do drop things. A ball magnet is really any mechanism designed to posses balls. I understand not counting adhesives as ball magnets, but vacuums, rollers, and pincher bars all fall into the category of 'ball magnets'.

[MrForbes]

"ball magnet" is just a silly name I came up with for this video

http://www.youtube.com/watch?v=KRqj0Mgbq10

good to see it continues to stir up so much controversy

And kudos to those teams who made one that works well....

[ttldomination]

Hi BJC....Yes we found that robocup ball dribbler video early on in the design process. Our first prototype worked well enough to keep our interest and our refined ball magnet is even better than even the one in the video. Here's another video http://www.rambotics.com/video7.html that shows our driver practicing with our practice robot and improved dribbler.. We thought ball control was more important than a kicker to practice with so we didn't even put a kicker on our practice bot.

Nice design. It looks like your driver is "tender" with the robot when he is driving it. I would really recommend he push the robot to its limits and see where they are. At competition, it's really hard to be that tender and still expect to score 4+ goals a match.

I too was under the impression that a ball magnet was a general word for something that possessed a ball, and wasn't aware that there is a design aspect to the word.

[Vikesrock]

Magnets hold on to things. They don't drop the things they are holding on to when they move backwards or turn around.


Unless said magnets are part of a magnetized screwdriver and you are trying to place a screw in a very awkward place.

[BJC]

Unless said magnets are part of a magnetized screwdriver and you are trying to place a screw in a very awkward place.

It's kind of funny how off topic this thread has gotten.

Anyway, I would suggest if you don't want to do anything else line the front of the robot with memory foam (the stuff in the goal). That in itself will help alot:ahh:. Although, if you are going for more of a BALL MAGNET (I think i used it right) then it will take testing and probably some practice rounds.

Just remember, if you go with a single rollor get it as high on the ball as possible. If you are going for a pincher use a smaller diameter roller and be careful the bottom roller clears the bump (many teams who use this designed for it since the beginning). Also remember that a vacuum takes the most weight. Finally, all of these take time to impliment so you need to be sure your prepared to possibly use all your practice time to get it to work; they can be tempramental.

Although, like stated earlier you don't really have anything to lose, so I would go for it.

Good Luck:)

[lenny8]

i don't know if anyone's seen these http://www.andymark.biz/am-0619.html

its what my team uses and other teams are starting to use and no complaints

[MarkoRamius1086]

Mind telling me exactly how that works? It appears as if the arms would just push the ball away...

[lenny8]

Mind telling me exactly how that works? It appears as if the arms would just push the ball away...

here's a picture of our teams bot just for visual aid. http://www.chiefdelphi.com/media/photos/35174

they way we use it is use those rollers to grab the ball then once its grabs the ball it keeps the ball spinning on a igus shaft. works great.

[Andrew Remmers]

I was refering to when you are using a pincher where the rollor (fisherprice motor) has to stop suddenly. Fisherprices can't handle this because they will stall. I'm not sure if you are using this ball possession method, but if you are you must have modified the motor in some way.

i guess i misunderstood what you said we just have a roller no pinching systems :/ sorry for the misunderstanding

[Alex Cormier]

here's a picture of our teams bot just for visual aid. http://www.chiefdelphi.com/media/photos/35174

they way we use it is use those rollers to grab the ball then once its grabs the ball it keeps the ball spinning on a igus shaft. works great.

Do you have any other pictures of this setup?
Do you press fit them on the igus shft, or do you glue them on?

I bought some of those recently and explained in my earlier post I wish to copy the design.

[MarkoRamius1086]

Oh... so they are flexible enough that they have a low enough friction coefficient that the ball follows the bar instead of bouncing away?

[IndySam]

Do you have any other pictures of this setup?
Do you press fit them on the igus shft, or do you glue them on?

I bought some of those recently and explained in my earlier post I wish to copy the design.

They are mounted on a square shaft. The igus is the idler roller.

[lenny8]

Do you have any other pictures of this setup?
Do you press fit them on the igus shft, or do you glue them on?

I bought some of those recently and explained in my earlier post I wish to copy the design.

actually the " entrappion" stars themselves are on a a tight fit on a 1/2 in square shaft.

if u look at the photo closely we used a 3/4 in igus shaft that is used to not only to keep the ball from going too far in but also to keep the ball spinning on the shaft to have better ball control.