Mechanical Racquet string bed tension measuring devices? Any DIY?

[STRINGA]

So, what kind of dial gauge is needed? Is this something that can be found in a Car Supply Store or Home Depot?

This page shows a lot of them, you can buy them with any shop that sells machines for machining metal.

http://www.justoffbase.co.uk/Tool-Shop/Dial-Indicators

[rrg]

These are some very good thoughts.
I really like the pulley idea because it's right on the table and easy to add.

So in that idea, here is my twist, if I place a round washer inside a half tennis ball. This gives a more realistic impact size. Than attach it to the drop weight, per your drawing, until the bar is level I could see the tension on the racquet bed.

I like it.


T-ball, brilliant!

Remember the goal is to see some consistency across string jobs and the test mechanism should be less than say $100-200.

[Sir.N]

Hi all!

Something like this,perhaps...



The clock (dial-indicator) could possibly be replaced by a simple graded pin.

~S~

[STRINGA]

This is what we call concurrent engineering, combine ideas.
I will take some pictures of the tool we use, the basic idea is the same.

[STRINGA]

This is the tool that we use;

[Sir.N]

Hi Stringa!
Yes! Nice and uncomplicated mechanics - that's the way to go. As You said, the principle is the same, only the string between the piston and the weight differs.
Another aspect of the matter is that your method allows for easy centering of the device - the one I outlined lacks this freedom. But given that we are looking for a way to build upon the tensioning system already at hand in a stringer, this limitation must be overcome. Any ideas, folks?
For instance, making the cross-bar out of two pieces, connected with a threaded junction somewhere between hook and measuring clock - a few turns and the piston would be at the desired spot.
Yet another thing to consider: there are several types of stringing machines out there, with different ways to attach racquets (two-point, six-point, etc). Therefore, the simplest solution is to place the device across the racquet, parallel to the cross strings.

Stringa, since you are the one experienced in engineering matters, here are a few questions:
(1) given that the device would be situated on top of a racquet and thus slightly higher, would pulling such a device put more stress on the racquet or is that negligible?
(2) do you have any suggestions for dimensions and choice of materials? What would the cost be.
(3) look at the measuring bit from another angle. If we use a simple marked piston that tells us when the stringbed is deflected, say, 0.5 cm, and adjust our stringer until we find such a tension (around 18-20 kg?). This way we would not need the Dial gauge, but
how big an error in SBS value do you estimate we'll introduce from reading out the applied tension from the stringer?

Sir.N

[STRINGA]

Hi guys,

There are quite some issues to consider about creating a mechanical stiffness tester. As you will understand we learned a lot from developing the Stringlab and the Rucanor computer.
I have a mechanical tester on my list for some time because it has many advantages over the “vibration-type”..

A mechanical system could also be used for badminton.

My other development lesson is, that a lot of things seem easy to create but after some time you find out that there is a problem “around the corner” that you did not realize.

One of this issues could be in the suggested system is the tensioner as a “force generator”.
Is it more complicated to create a system that connects the stiffness tester to a tensioner than to add a simple weight-arm to the system?

A very basic question is: Do you want to measure only the stringing tension or do you want the influence of the string stiffness also?
The Stringlab only measures the stringing tension because the stringbed is hardly deflected. This means that it is possible to obtain a relation between the sbs and the stringing tension, like with the Tension Advisor.

If you apply 18 to 20 kg you really deflect the string bed which means that the string has to stretch so the stiffness of the string has influence on the test result.
The Rucanor stiffness tester worked like this.

So quite some basic questions to answer before we get to a design.

Keep in touch.
Stringa

[tball]

I would test both stringbed deflection and strings stretching at the same time, as it happens in a real shot. I saw some slow motion videos of a tennis ball impacting a racquet, and and the deflection was very deep. The ball sank ~3 cm on some shots.

This may no longer be the pure String Bed Stiffness, but I think it is more realistic.

We may need to apply a significant force.

By my calculations:

Ball's weight m: 57 g = 0.057 kg
Ball diameter: 6.5 cm = 0.065 m

Let's simulate a hard shot -- a 100 mph serve.


velocity 0 = 0 mph
velocity 1 = 100 mph == 44.70 m/s

Contact time: 10 ms = 0.01 seconds

Acceleration a = 44.70 / 0.01 == 4470 m/s2
Force = m * a = 0.057 * 4470 = 255 N

Translating to pound force:

255 N = 26 kg = 57 lbs

This is a very significant force acting perpendicular to the racquet face!

My feeling is that the bounce at this point will be determined predominantly by how far the strings themselves stretch (as opposed to how the entire stringbed deflects and springs back).

So, measuring a 5mm deflection may not be realistic.

But if we are simply looking for some baseline number for comparing racquet A to racquet B, it may be good enough.

-- Nick

[tball]

Based on the great ideas and pictures from above...

The simplest of them all:



Two sticks, a toilet plunger and a weight.
Simply measure once without the weight.
Then measure again, with the weight.

The difference in height == stringbed deflection.

[FredT]

Hi Tball,

You are certainly very creative.
But we do not agree completely about the philosophy behind the stiffness test.
The important issue in this matter is that you look for relations. It seems nice that you know the relation between what the player feels of the combination of string elongation and stringing tension. But the disadvantage is that you do not know the influence of each.

The advantage of measuring the SBS with a very small deflection is that it only relates to the stringing tension, so that you get a relation.
My idea is to add the elongation character of the string separately. As you can read in other discussions I am a big supporter of string classification.

http://www.stringforum.net/board/showthread.php?t=3655