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PostPosted: 26 Mar 2020, 07:52 
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Due to unfortuante events world wide, I now find myself with some time to focus on testing table tennis products again. I've already decided on the robot I'll use to feed the balls (vedroitt's TenniRob) and have been in discussion with vedroitt for some time now so I know it's the best robot to meet my needs.

I've also invested in some other DIY bits of equipment and am trying to source a solid material with a coefficient of restitution (COR) as near to 1 as possible. I know Wikipedia says stainless steel can be as close as 0.93 and that is probably why the ITTF use:

Quote:
A standard steel plate is made of Steel S235 with a surface roughness of Ra <= 1.0 µm and Rmax <= 7.5 µm. The size is no smaller than 20mm thick and 200 x 200 mm large.

(source ITTF Technical Leaflet T3 - The ball page 3 of 9)

to test balls, but I'm after something I can attach rubbers to which has a COR of 1 and therefore removes the "bat" out of the equation. The bat can be factored back in during other types of test.

Do you know of a solid substance with a COR of 1?

Thank you


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PostPosted: 26 Mar 2020, 10:43 
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According to the Wikipedia page on COR:
https://en.wikipedia.org/wiki/Coefficient_of_restitution
here are the materials nearest to 1:
acrylic 1.06
PET 0.95

So either get a nice thick piece of acrylic, or flatten out a PET bottle :)

You may want to stick (fnarr, fnarr) with what the ITTF uses for its ball bounce test, as that would be in some way shape or form, comparable.

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PostPosted: 26 Mar 2020, 13:08 
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I think the old Konrad Tiefenbacher articles used a block of marble to test.

Imo there is no need for perfect 1. The ball itself will be the bottleneck at some point. A ball bounced in a 10mm hinoki blade is pretty much the same as that on a marble block.


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PostPosted: 26 Mar 2020, 21:10 
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Retriever wrote:
You may want to stick (fnarr, fnarr) with what the ITTF uses for its ball bounce test, as that would be in some way shape or form, comparable.


Yep, considered that but getting a steel square 20cm by 20cm by 20cm today is not going to be practical.

Acrylic is the closest I've found so far too and you could cut a bat shape out of a sheet of acrylic making an acrylic bat. Not ITTF legal but it would give a better idea of the propeties of the rubber.


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PostPosted: 26 Mar 2020, 21:13 
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lasta wrote:
I think the old Konrad Tiefenbacher articles used a block of marble to test.

Imo there is no need for perfect 1. The ball itself will be the bottleneck at some point. A ball bounced in a 10mm hinoki blade is pretty much the same as that on a marble block.


If I had a substance with a COR of 1 and then tested the ball bouncing on that substance I'd know the COR value of the ball. Any deviation from that value of the ball could then be attributed to the rubber when the ball is tested again, this time with the rubber attached to the substance with a COR value of 1. I'm trying to make the testing as simple and specific as possible.

At a later date I could test the rubber on custom blades I've had made which are identical except one has a balsa core the other a "wood" core.


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PostPosted: 27 Mar 2020, 06:32 
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Debater wrote:
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Yep, considered that but getting a steel square 20cm by 20cm by 20cm today is not going to be practical.

Yes, but getting a steel cube 20cm by 20cm by 20 cm would be quite a bit more practical than what you have considered.

Don't thank me, pedantry, unlike table tennis prowess, can never be overrated.

On a more serious note, the COR of things like acrylic and even the previously mentioned steel are very much estimates, so however impractical the steel cube is, it is accepted by the ITTF to do things with.

What are the impracticalities involved with the steel?
Knowing how to get it? You have to use your block.
Cost? You'll just have to steel yourself for the price as it won't be a steal.
Secure storage? Everyone will want to steal it.
Humidity free storage? So it doesn't <NZ accent> rust </NZ accent> in piece(s)?

Can you tell that no TT is having some affect?

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PostPosted: 27 Mar 2020, 08:06 
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Classical Golf is the only "ball & impactor" game to take the COR of the playing materials in serious manner.


https://www.youtube.com/watch?v=dOBmBfb-bz0

COR Stands for "coefficient-of-restitution"
http://www.golfclub-technology.com/coef ... ution.html
TAKE NOTE: It is impossible for the collision of your driver head and your golf ball to produce a perfectly elastic collision (COR of 1.000) in which all energy is transferred, for two reasons:
1.The clubface and the ball are made from completely different materials.
2. The clubhead and the ball are of two totally different weights.


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PostPosted: 27 Mar 2020, 23:33 
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I do prefer some plywood materials for making the head of table tennis racket.

Marble material impactor.
https://patents.google.com/patent/US3199872A/en

Beach pebble used as an impactor
https://patents.google.com/patent/US5749793A/en


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PostPosted: 28 Mar 2020, 01:02 
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You know, this doesn't really make sense to me. The article says that if e > 1 then the collision results in the release of energy (they give the example of nitrocellulose balls colliding and exploding..). But you find a LOT of materials with e > 1 (most elastomers, in fact). Take polycarbonate, for instance - e = 1.46. What does this mean? If you bounce a steel ball (e ~ 1) on a polycarbonate surface, does this mean that every bounce will be 46% higher than the previous bounce, and eventually it'll be bouncing itself into the stratosphere? Weird, no? If you bounce a polycarbonate ball on a polycarbonate surface, would it bounce (1.46^2 = 2.13) times (113%) higher each time?

Iskandar


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PostPosted: 28 Mar 2020, 05:21 
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Retriever wrote:
Debater wrote:
Quote:
Yep, considered that but getting a steel square 20cm by 20cm by 20cm today is not going to be practical.

Yes, but getting a steel cube 20cm by 20cm by 20 cm would be quite a bit more practical than what you have considered.

Don't thank me, pedantry, unlike table tennis prowess, can never be overrated.

On a more serious note, the COR of things like acrylic and even the previously mentioned steel are very much estimates, so however impractical the steel cube is, it is accepted by the ITTF to do things with.

What are the impracticalities involved with the steel?
Knowing how to get it? You have to use your block.
Cost? You'll just have to steel yourself for the price as it won't be a steal.
Secure storage? Everyone will want to steal it.
Humidity free storage? So it doesn't <NZ accent> rust </NZ accent> in piece(s)?

Very few suppliers in the UK with steel of the right type in that dimension and transport is a nightmare. Acrylic much easier to get.


Can you tell that no TT is having some affect?


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PostPosted: 28 Mar 2020, 05:22 
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igorponger wrote:
Classical Golf is the only "ball & impactor" game to take the COR of the playing materials in serious manner.


https://www.youtube.com/watch?v=dOBmBfb-bz0

COR Stands for "coefficient-of-restitution"
http://www.golfclub-technology.com/coef ... ution.html
TAKE NOTE: It is impossible for the collision of your driver head and your golf ball to produce a perfectly elastic collision (COR of 1.000) in which all energy is transferred, for two reasons:
1.The clubface and the ball are made from completely different materials.
2. The clubhead and the ball are of two totally different weights.


There's a third. Golf rules dictate the allowable COR value of golf head design to try and limit distance - currently 0.83. I've contacted golf trajectory makers and none say their products would work for table tennis. There is also the fact that a golf ball is stationary at impact. A table tennis ball isn't.


Last edited by Debater on 28 Mar 2020, 05:26, edited 1 time in total.

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PostPosted: 28 Mar 2020, 05:24 
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iskandar taib wrote:
You know, this doesn't really make sense to me. The article says that if e > 1 then the collision results in the release of energy (they give the example of nitrocellulose balls colliding and exploding..). But you find a LOT of materials with e > 1 (most elastomers, in fact). Take polycarbonate, for instance - e = 1.46. What does this mean? If you bounce a steel ball (e ~ 1) on a polycarbonate surface, does this mean that every bounce will be 46% higher than the previous bounce, and eventually it'll be bouncing itself into the stratosphere? Weird, no? If you bounce a polycarbonate ball on a polycarbonate surface, would it bounce (1.46^2 = 2.13) times (113%) higher each time?

Iskandar


Don't forget to factor in, amongst other things, gravity.


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PostPosted: 28 Mar 2020, 12:50 
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Gravity is already taken into account. By definition e = velocity before bounce/velocity after bounce, and therefore e = SQRT(height before bounce/height after bounce). So if e >1 the height after bounce will be higher than the height before bounce. Gravity's already factored in. Sounds like perpetual motion to me.

Iskandar


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PostPosted: 28 Mar 2020, 22:21 
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iskandar taib wrote:
You know, this doesn't really make sense to me. The article says that if e > 1 then the collision results in the release of energy (they give the example of nitrocellulose balls colliding and exploding..). But you find a LOT of materials with e > 1 (most elastomers, in fact). Take polycarbonate, for instance - e = 1.46. What does this mean? If you bounce a steel ball (e ~ 1) on a polycarbonate surface, does this mean that every bounce will be 46% higher than the previous bounce, and eventually it'll be bouncing itself into the stratosphere? Weird, no? If you bounce a polycarbonate ball on a polycarbonate surface, would it bounce (1.46^2 = 2.13) times (113%) higher each time?

Iskandar

If some future day tt-maker comes out with a magic material COR >1 ITTF will nip it in the bud, I know for sure they will.


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PostPosted: 29 Mar 2020, 02:58 
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I'm sure there wouldn't be any need to.. it'd be totally uncontrollable.. :lol:

(Igor comes up with the weirdest stuff... :lol: )

Iskandar


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