Klippermate - reference vs actual tension

[a0f6459]

This thread is mainly to post my findings. I know most will say it doesn't matter and that the reference is just a number and that it is the end results, regardless of how much the tension drops that matter. I agree, but for those that care to be as accurate and consistent as possible, this may help.

I had been struggling with always having my tension come off 6 lbs lower than my reference pull tension immediately after stringing. This is as measured by RaquetTune. (Yes, I have seen the post that says reference tension does not = actual tension.)

Steps I took to try to get my actual stringbed tension to come close to my reference pull tension.

1) Check my clamps to make sure they were not slipping
2) Clean my clamps and gripper
3) Pull tension on each string for approx 10 seconds
4) Use the Jay Cee method
5) Ensure the weight is on the right direction


No matter what I did I would still come in 6 to 7 lbs lower than my reference. Finally I decided it was time to calibrate the Klippermate. Yes, I know that gravity never changes, but I wanted to see if the sticker on the tension bar matched up with what I was actually pulling.

I had seen some threads doing a test using a luggage scale. A couple of people found that the Klippermate was pulling about 4 lbs light. I wanted to see if this was the case for my stringer.

I tested the luggage scale using different dumbbell weights to make sure it was accurate. I then pulled tension at 50, 55, and 60 lbs on a scrap piece of string that I attached to the scale. The scale was attached to one of the mounts, screwed down like a racket would be.

RESULTS:

My Klippermate was pulling about 4 lbs light. At higher tensions it was pulling slightly lighter.

This means when I pulled 50 lbs, the actual tension was ~46.5
When I pulled 55, the actual tension was ~51
When I pulled 60, the actual tension was ~ 55.5

So for me, now I can confidently know that I must set my reference tension about 4 lbs higher in order to get an accurate stringbed tension.

Why is this important to me? I plan on testing copolys and I came across this article that makes alot of sense.

http://www. guts and glory tennis.com/blog/2011/03/the-definitive-guide-to-stringing-polys-and-co-polys/

 

[spacediver]

good stuff - always good to know the "ground truth".

I am, however, surprised that discrepancy isn't consistent across pull tensions on a drop weight.

 

[a0f6459]

Check out post #38 by SW Stringer.

I find my test to be consistent with what he found.

http://tt.tennis-warehouse.com/showthread.php?t=192374&highlight=stringbed+klippermate&page=2

 

[max]

You should share your findings with the company; they'd likely like to hear this. Have you tested other stringing machines?

 

[a0f6459]

I just bought the Klippermate a few months ago. This is my first stringer.

In reading other posts by SW Stringer, he seems to believe that the Klippermate was designed to mimic a lockout stringer, an Ekelton or something.

Whether this is true or not, I do not know.

All I do know is when I put the Klippermate to the test, these were the results.

Like you have said many times, it's consistency that is key right? Now I know that the tension is off approx. 4 lbs and knowing this will help me be more confident in my string jobs.

For example, I always thought I was 6 -7 lbs off and that I was doing something wrong. However, now that I know, being only actually 2 -3 lbs off from reference, I can live with because I know other factors like friction, tie-off, etc accounts for that.

 

[DE19702]

You bring up a good point. Apparently, the scale on the Klippermate is not correct. It is only stuck on with glue, so perhaps there is some leeway when the factory sticks it on. Also, as you can see by the scale, the weight references are evenly spaced so that the distance between 30-35 pounds is the same as the distance between 60-65 pounds. I would have thought that the the scale length would get smaller as the weight went up. In other words, as length of the arm got larger it would pull more in proportion to the length than when it is smaller. In any event, you can get a very consistent string job no matter what tension you are pulling with the Klippermate.

 

[max]

You bring up a good point. Apparently, the scale on the Klippermate is not correct. It is only stuck on with glue, so perhaps there is some leeway when the factory sticks it on. Also, as you can see by the scale, the weight references are evenly spaced so that the distance between 30-35 pounds is the same as the distance between 60-65 pounds. I would have thought that the the scale length would get smaller as the weight went up. In other words, as length of the arm got larger it would pull more in proportion to the length than when it is smaller. In any event, you can get a very consistent string job no matter what tension you are pulling with the Klippermate.

You're right. That reminds me of the Lincoln quote when he talks about the arc of a knife blade.

 

[spacediver]

Also, as you can see by the scale, the weight references are evenly spaced so that the distance between 30-35 pounds is the same as the distance between 60-65 pounds. I would have thought that the the scale length would get smaller as the weight went up. In other words, as length of the arm got larger it would pull more in proportion to the length than when it is smaller.

why would you make that assumption? There's a thread on the physics of drop weight stringing machines here:

http://tt.tennis-warehouse.com/showthread.php?t=170414

I haven't read it yet, but I would assume that it's a simple lever, where the applied torque is simply force x distance. In other words, the torque (and ultimately the applied tension) has a linear relationship with distance from the fulcrum (string jaws).

 

[a0f6459]

Reading through the first post and explanation, I don't see any case being made about the distance being linear.

At least how I read it, he is saying that the distance is directly proportional to the reference tension. The post is trying to prove accuracy which it does.

So as it relates to this simple test:

This means when I pulled 50 lbs, the actual tension was ~46.5
When I pulled 55, the actual tension was ~51
When I pulled 60, the actual tension was ~ 55.5

This will always be the case and I can be confident that I am pulling the tension I want because the distance is directly proportional to the reference tension. As long as I always pull at that distance/reference tension, I can expect to get the same tension each and every time.

The test was never really to test the Klippermate itself, but the validity of the sticker and the distances marked on the rod. I don't see the results as contradicting the thread you have linked.

From the thread:

When the string is being tensioned, the rotational tensioner does not move. That's because the torque exerted by the dropweight is balanced by the torque exerted on it by the string.
Again, the equation for torque does not change: torque = force x distance.
On the left side, the distance in question is the distance between the center of the rotational axis and where the string applies its force. In other words, the radius of the tension head is the distance (r) of interest. The force in this case is the "reference tension" imparted on the string.
When the tension head is at rest, it means the torque on the left side equals the torque on the right side, therefore we can set up an equation:
torque left side = torque right side, therefore
f x r = M x g x R

Play with that equation a little bit, you get:
f = (M g / r) R

Mass (M), gravitational acceleration (g), and the tensioner's radius (r) are all constant, i.e. they never change. Therefore, the reference tension is directly proportional to the distance (R). That's why a dropweight tensioner is simple and requires no calibration, and is impeccably accurate.

 

[DE19702]

I stand corrected. According to the article, "[T]he reference tension is directly proportional to the distance.

 

[rjw]

can the sticker be relocated? hopefully that would solve everyone's problem

 

[a0f6459]

I stand corrected. According to the article, "[T]he reference tension is directly proportional to the distance.

Actually, I think you are correct in your original statement. In this case I don't believe proportional is talking about the each distance of the line on the rod being proportional, but the distance from the tension head itself as being proportional.

All it is saying is that for a given distance, the tension will always be the same.

 

[max]

can the sticker be relocated? hopefully that would solve everyone's problem

OP: if you can, calculate the right distance from the end of the drop weight rod to the end of the sticker and let us know.

 

[a0f6459]

OP: if you can, calculate the right distance from the end of the drop weight rod to the end of the sticker and let us know.

You know what is funny max? I went back to that thread I referenced earlier to look at the numbers that they put and it was you 3 years ago that asked the same question! hahah I guess you forgot.

Check out people's answers starting from your question in post #31.

http://tt.tennis-warehouse.com/showthread.php?t=192374&highlight=stringbed+klippermate&page=2

 

[spacediver]

Actually, I think you are correct in your original statement. In this case I don't believe proportional is talking about the each distance of the line on the rod being proportional, but the distance from the tension head itself as being proportional.

All it is saying is that for a given distance, the tension will always be the same..

I believe one implies the other. If distance from fulcrum to load point is proportional to tension, then the distance between each successive unit on the scale should be the same.

Think about it this way: suppose 10 cm on the scale = 10 pounds of tension. If distance from tension head to fulcrum is proportional to applied tension, then 20 cm must equal 20 pounds of tension, 30 cm = 30 pounds, and so on.

So each 10 cm = 10 pounds of tension. And each cm = 1 pound of tension. It doesn't matter where this cm is in reference to - any cm further away from any point will increase tension by 1 pound.

 

[a0f6459]

I believe one implies the other. If distance from fulcrum to load point is proportional to tension, then the distance between each successive unit on the scale should be the same.

Think about it this way: suppose 10 cm on the scale = 10 pounds of tension. If distance from tension head to fulcrum is proportional to applied tension, then 20 cm must equal 20 pounds of tension, 30 cm = 30 pounds, and so on.

So each 10 cm = 10 pounds of tension. And each cm = 1 pound of tension. It doesn't matter where this cm is in reference to - any cm further away from any point will increase tension by 1 pound.

This seems to make sense, but for me, only so far as it applies to the real world.

Give the test a shot. For whatever reason, the difference in reference tension and actual tension is clearly more the higher in weight you go. I'm just reporting my findings.

 

[beernutz]

Have you confirmed your RacquetTune findings with a digital scale?

 

[a0f6459]

Have you confirmed your RacquetTune findings with a digital scale?

Not sure what you mean, how do I do that?

 

[beernutz]

Not sure what you mean, how do I do that?

I apologize if I've misinterpreted what has been written, but it appears you believe the Klippermate is not actually pulling the tension that is reflected on the drop bar. I was just curious if you had used a digital scale with string on each end to pull tension and see how close or far off the scale reading was to the drop bar setting.

 

[a0f6459]

I apologize if I've misinterpreted what has been written, but it appears you believe the Klippermate is not actually pulling the tension that is reflected on the drop bar. I was just curious if you had used a digital scale with string on each end to pull tension and see how close or far off the scale reading was to the drop bar setting.

Yep, that is exactly what I did with a luggage scale. (wasn't digital)

I found that it was off by about 4 lbs from what the sticker said.

 

[Irvin]

Yep, that is exactly what I did with a luggage scale. (wasn't digital)

I found that it was off by about 4 lbs from what the sticker said.

I have a tension gauge and it is off by more than 4 lbs.

Irvin

 

[beernutz]

Yep, that is exactly what I did with a luggage scale. (wasn't digital)

I found that it was off by about 4 lbs from what the sticker said.

I agree with Irwin who I believe is implying you need to calibrate your scale with a known weight to make sure the scale is accurate. I tested my digital fish scale by using 50 lbs of weight (which I also weighted with another scale to be sure it was actually 50 lbs).

FWIW, I tested my old Klippermate and my Neos a while ago and wrote about it here: http://tt.tennis-warehouse.com/showpost.php?p=3662122&postcount=1
As I mentioned in that post, I ultimately did test the weight to be sure it was the weight I thought it was, and it was. So my Klippermate measured 2.7 lbs under 50 for an error rate of 5.4%.

 

[a0f6459]

I agree with Irwin who I believe is implying you need to calibrate your scale with a known weight to make sure the scale is accurate. I tested my digital fish scale by using 50 lbs of weight (which I also weighted with another scale to be sure it was actually 50 lbs).

FWIW, I tested my old Klippermate and my Neos a while ago and wrote about it here: http://tt.tennis-warehouse.com/showpost.php?p=3662122&postcount=1
As I mentioned in that post, I ultimately did test the weight to be sure it was the weight I thought it was, and it was. So my Klippermate measured 2.7 lbs under 50 for an error rate of 5.4%.

I have a feeling you didn't read the entire first post. I did test the luggage scale at first as well.