The physics of a dropweight tensioner

[nousername]

Originally Posted by 1012007

lol i do let gravity do its job. I also have only strung 2 rackets for self.

but your comments implied you don't understand how to let gravity do it's job in a dropweight machine. i hope you do understand that by clamping the string while you are pushing the rod down or pulling it up you surely mess up the tension ... you get that, right?

(oh, by the way, i just read one of your other posts... and dropweight machines never need calibration, to answer your question. that's one of their advantages.)

[aussie]

Originally Posted by 1012007

lol i do let gravity do its job. I also have only strung 2 rackets for self.

Always gently lower the tension rod with your hand underneath the rod. The rod will rest either above horizontal, below horizontal or in the horizontal position. Once it rests, remove your hand from under the rod and then adjust the rod (if necessary) to rest again hopefully in the near horizontal position. There must never be any forcing of the rod to reach the near horizontal position.

[split-step]

Originally Posted by nousername

first, it's amazing how a touch of a 1st yr engineering course can impress so many!!! =) ...

I found this to be very condescending (esp with the smiley added). It may not have been your intention but that's just how it reads to me. IMO it added nothing to your post but rather put down the OP and the responders.

Nice write up Lethalfang.

[jcstennis]

this is awesome... what a high quality, well thought out explanation!

i wonder who invented a drop weight stringer (not the method or math behind it... that has been around forever for other uses)... you would have been a great pitch man for this type of invention!

[lethalfang]

Originally Posted by jcstennis

this is awesome... what a high quality, well thought out explanation!

i wonder who invented a drop weight stringer (not the method or math behind it... that has been around forever for other uses)... you would have been a great pitch man for this type of invention!

This is one of the earliest dropweight machine I know. They didn't use it to string tennis racquets, but........

[nousername]

Originally Posted by split-step

I found this to be very condescending (esp with the smiley added). It may not have been your intention but that's just how it reads to me. IMO it added nothing to your post but rather put down the OP and the responders.

Nice write up Lethalfang.

right ... sorry ... not thinking before speaking ...

[Bud]

Originally Posted by lethalfang

This is one of the earliest dropweight machine I know. They didn't use it to string tennis racquets, but........

Is that a catapult?

[lethalfang]

Originally Posted by Bud

Is that a catapult?

Yeah..... but I think it's just a model.

[lionel_101]

Just wondering,

If I set my tension to be at 50 lbs and when I pull the string and the bar is horizontal is it exactly 50 lbs that nice.

If I ratchet it up once more and now the bar is pointing about 5 degrees upward, did I just increase the tension on the string or is the tension still 50lbs?

I am pretty dumb and didn't understand any of the math stuff, but it seems if the bar is pointing up then the tension on the string is higher then what it is set at, otherwise the bar would keep falling. Isn't this the reason when it drops below horizontal you need to keep ractheting it up more and let it drop again until it is horizontal?

Can some tell me if I am right or wrong if the

Bar pointing up means to much tension,

Bar pointing down means to little tension

Bar pointing horizontal means it is just right.

Thanks

[christo]

I love my dropweight and especially with poly hybrids i can get the mains just lovely and horizontal with 1 go, sweeet! I paid $120 for mine and it is seriously bulletproof

[Bud]

Originally Posted by lionel_101

Just wondering,

If I set my tension to be at 50 lbs and when I pull the string and the bar is horizontal is it exactly 50 lbs that nice.

If I ratchet it up once more and now the bar is pointing about 5 degrees upward, did I just increase the tension on the string or is the tension still 50lbs? You decreased the tension (about 0.2 lbs - value according to LF's earlier post when reference tension was 60 lbs.)

I am pretty dumb and didn't understand any of the math stuff, but it seems if the bar is pointing up then the tension on the string is higher then what it is set at, otherwise the bar would keep falling. Isn't this the reason when it drops below horizontal you need to keep ratcheting it up more and let it drop again until it is horizontal? No, when it drops below horizontal, the tension is also less, by the same amount, as when it's above horizontal. If it's 5 degrees above or below horizontal, it will be 59.8 lbs. (assuming a reference tension of 60 lbs.)

Can some tell me if I am right or wrong if the

Bar pointing up means to much tension? Incorrect (see above).

Bar pointing down means to little tension? Correct (see above).

Bar pointing horizontal means it is just right? Correct (see above).

Thanks

As a simple empirical proof of the above, try the following:

1. Tie a weight (anything close to a pound or more) to the end of a 12"-15" long piece of string. **You can also hold a fairly heavy shoe by the end of one of the laces for the same effect **

2. Tie the string to the end of your finger.

3. Place your elbow on the edge of a table and rotate your forearm so that it points straight to the sky (weight should be hanging directly beside your forearm).

4. Rotate your forearm so it horizontal and hanging off the table (weight should be hanging 90 degrees from your forearm, now).

5. Observe the effort it takes to resist the weight as your forearm moves closer to horizontal.

6. Now, let your arm drop below horizontal and see if you can feel the weight decrease (less effort) the further your arm drops.

7. Lift your arm back to vertical then try it again.

** the heavier the weight... the more you'll feel the effect.

[Bud]

Originally Posted by lionel_101

Just wondering,

If I set my tension to be at 50 lbs and when I pull the string and the bar is horizontal is it exactly 50 lbs that nice.

If I ratchet it up once more and now the bar is pointing about 5 degrees upward, did I just increase the tension on the string or is the tension still 50lbs?

I am pretty dumb and didn't understand any of the math stuff, but it seems if the bar is pointing up then the tension on the string is higher then what it is set at, otherwise the bar would keep falling. Isn't this the reason when it drops below horizontal you need to keep ractheting it up more and let it drop again until it is horizontal?

Can some tell me if I am right or wrong if the

Bar pointing up means to much tension,

Bar pointing down means to little tension

Bar pointing horizontal means it is just right.

Thanks

For 50 lbs:

Cos 5 = cos (-5) = 0.996, i.e. if you set the mark at 50 lb, but the bar is either 5 degrees above or below horizontal, the actual tension is 49.80 lb.

Cos 10 = cos (-10) = 0.985, i.e. if you want 50 lb., you get 49.25 lb.

** See OP's original post for diagram.

[tennisfreak15347]

I have one question here. you said that if I put an extra force of 1lbs on the end of the tension arm to force it to horizontal, I'd be overtensioning by 15lbs. What if I raise the tension arm and let it swiftly drop down, then it becomes horizontal? would that be along the same effects?

[lionel_101]

Thanks for the info Bud.

Being simple minded as I am, it doesn't make much since to me, but I looked at it some more and it looks like to me like ....

1. When the bar is lower then horizontal, the string can't be at 50 lbs, but must be less depending on how low the bar is. This makes sense.

2. When the bar is level and completely horizontal then it is at the set tension of 50 lbs, no more and no less. This makes sense.

3. When the bar is ratcheted up one more notch from horizontal, it has to be also at 50 lbs, no more and no less. Sounds confussing now.

a. It can't be less then 50 lbs because it was at horizontal at one time and thus at 50 lbs.

b. It can't be more then 50 lbs because the drop weight isn't heavy enough to drop down the bar anymore because the tension on the string is at 50 lbs.

To test it out, I pulled some string through the first mains and mark the string at the frame.

a. When I put the string in the tensioner and started to tension it up, the mark move away from the fram as expected.

b. When the bar was level and horizontal (at 50 lbs), I marked it again at the frame .

c. When I ratcheted the bar up agian one more notch, the mark did not move. When I move it up again another notch, the mark didn't move. When I move the bar passed 50 degrees or so, the bar moved back towards the racquet and the mark went inside of the frame and lost tension.

All I can conclude is that....

If the bar is lower then horizontal, then keep ratcheting it up until it is level or slightly above level and then you will be at the tension you set it at, in this case 50 lbs.

What do you guys think?

[Hokiez]

Measuring how far the mark moved isn't an accurate method as that would only measure the elasticity of the string, not the tension. Over wide ranges, there will be a relationship between tension and elongation works, but we're talking from 0 pounds up to whatever tension it takes to physically break the string, say 500 pounds. Within the range your tensioning, the string won't stretch enough for you to measure by eye. The tension however will be higher and you'll be clamping and holding that tension. So if you reach horizontal at 50 pounds, ratchet it up and allow it to drop where the arm is now above horizontal, the tension on the string will be lower. The tension is only 100% correct at exactly horizontal. Above or below that = lower tension.

[Bud]

Originally Posted by tennisfreak15347

I have one question here. you said that if I put an extra force of 1lbs on the end of the tension arm to force it to horizontal, I'd be overtensioning by 15lbs. What if I raise the tension arm and let it swiftly drop down, then it becomes horizontal? would that be along the same effects?

Then you run the risk of breaking the string... since that would momentarily place much more than 15 lbs. of tension on the string.

The best thing to do if you see it's not going to stretch any more and naturally fall to horizontal (even very slowly) is lift the weight and remove some of the tension... then let the weight re-tension the string on its own.

[Bud]

Originally Posted by lionel_101

Thanks for the info Bud.

Being simple minded as I am, it doesn't make much since to me, but I looked at it some more and it looks like to me like ....

1. When the bar is lower then horizontal, the string can't be at 50 lbs, but must be less depending on how low the bar is. This makes sense.

2. When the bar is level and completely horizontal then it is at the set tension of 50 lbs, no more and no less. This makes sense.

3. When the bar is ratcheted up one more notch from horizontal, it has to be also at 50 lbs, no more and no less. Sounds confussing now.

a. It can't be less then 50 lbs because it was at horizontal at one time and thus at 50 lbs.

b. It can't be more then 50 lbs because the drop weight isn't heavy enough to drop down the bar anymore because the tension on the string is at 50 lbs.

To test it out, I pulled some string through the first mains and mark the string at the frame.

a. When I put the string in the tensioner and started to tension it up, the mark move away from the fram as expected.

b. When the bar was level and horizontal (at 50 lbs), I marked it again at the frame .

c. When I ratcheted the bar up agian one more notch, the mark did not move. When I move it up again another notch, the mark didn't move. When I move the bar passed 50 degrees or so, the bar moved back towards the racquet and the mark went inside of the frame and lost tension.

All I can conclude is that....

If the bar is lower then horizontal, then keep ratcheting it up until it is level or slightly above level and then you will be at the tension you set it at, in this case 50 lbs.

What do you guys think?

Have you ever taken physics and/or trig? If you have it becomes very clear. Also, try the string thing I mentioned in my post... you'll FEEL the difference.

[Bud]

Originally Posted by lionel_101

Thanks for the info Bud.

Being simple minded as I am, it doesn't make much since to me, but I looked at it some more and it looks like to me like ....

1. When the bar is lower then horizontal, the string can't be at 50 lbs, but must be less depending on how low the bar is. This makes sense.

2. When the bar is level and completely horizontal then it is at the set tension of 50 lbs, no more and no less. This makes sense.

3. When the bar is ratcheted up one more notch from horizontal, it has to be also at 50 lbs, no more and no less. Sounds confussing now.

a. It can't be less then 50 lbs because it was at horizontal at one time and thus at 50 lbs.

b. It can't be more then 50 lbs because the drop weight isn't heavy enough to drop down the bar anymore because the tension on the string is at 50 lbs.

To test it out, I pulled some string through the first mains and mark the string at the frame.

a. When I put the string in the tensioner and started to tension it up, the mark move away from the fram as expected.

b. When the bar was level and horizontal (at 50 lbs), I marked it again at the frame .

c. When I ratcheted the bar up agian one more notch, the mark did not move. When I move it up again another notch, the mark didn't move. When I move the bar passed 50 degrees or so, the bar moved back towards the racquet and the mark went inside of the frame and lost tension.

All I can conclude is that....

If the bar is lower then horizontal, then keep ratcheting it up until it is level or slightly above level and then you will be at the tension you set it at, in this case 50 lbs.

What do you guys think?

Thanks for the info Bud.

Being simple minded as I am, it doesn't make much since to me, but I looked at it some more and it looks like to me like ....

1. When the bar is lower then horizontal, the string can't be at 50 lbs, but must be less depending on how low the bar is. This makes sense.

2. When the bar is level and completely horizontal then it is at the set tension of 50 lbs, no more and no less. This makes sense.

3. When the bar is ratcheted up one more notch from horizontal, it has to be also at 50 lbs, no more and no less. Sounds confussing now. No, it's LESS than 50 lbs. The tension will be the highest when the bar is horizontal. Anything more or less will be LESS than 50 lbs.

a. It can't be less then 50 lbs because it was at horizontal at one time and thus at 50 lbs. This is where the Trigonometry and Physics enter... it IS less than 50 lbs. when the bar is not horizontal (higher or lower).

b. It can't be more then 50 lbs because the drop weight isn't heavy enough to drop down the bar anymore because the tension on the string is at 50 lbs. You are correct... 50 lbs. is the max if the weight is set at 50 lbs. and there is no additional force pushing the weight down. Don't forget that string stretches.

To test it out, I pulled some string through the first mains and mark the string at the frame.

a. When I put the string in the tensioner and started to tension it up, the mark move away from the fram as expected.

b. When the bar was level and horizontal (at 50 lbs), I marked it again at the frame .

c. When I ratcheted the bar up agian one more notch, the mark did not move. When I move it up again another notch, the mark didn't move. When I move the bar passed 50 degrees or so, the bar moved back towards the racquet and the mark went inside of the frame and lost tension. Exactly... tension was pulled and the string stretched until you hit 50 lbs... then when you dropped below horizontal... the tension again dropped below 50 lbs. so the string retreated and the mark moved backward.

All I can conclude is that....

If the bar is lower then horizontal, then keep ratcheting it up until it is level or slightly above level and then you will be at the tension you set it at, in this case 50 lbs. Almost! ONLY at horizontal will the tension be exactly 50 lbs. Sometimes, it takes a number of 'ratchets' to remove the 'stretch' from the string. Once the stretch is all removed... at that same 50 lbs. the bar should stay horizontal and not move a millimeter. That is when you know... for that type of string with those unique stretch characteristics it is at exactly 50 lbs. of tension.

What do you guys think? See above

[lionel_101]

Thanks again everyone for your detailed inputs. I think I understand now, just hard for my brain to fully understand.

Best thing for me to do is to get it as close to horizontal as possible and then just set the clamp.

[Bud]

Originally Posted by lionel_101

Thanks again everyone for your detailed inputs. I think I understand now, just hard for my brain to fully understand.

Best thing for me to do is to get it as close to horizontal as possible and then just set the clamp.

That's the best way!