How can one conveniently measure wire rope tension? Does anyone have a reference for a deflection/force specification for various wire rope sizes? The Loos gauges seem the only game in town, though they are expensive, and to measure both my 1/4" lowers and 5/16" uppers and stays would require that I buy TWO of their gauges! What are the alternatives to Loos? Thanks, jv
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Excellent question!
My problem also, two sizes of wire requiring two gauges. I have thought that a small torque wrench from Sears Hardware could be adapted. You would need a good reference rig to set the marks but then would be good forever. Or until the next boat anyway.
My problem also, two sizes of wire requiring two gauges. I have thought that a small torque wrench from Sears Hardware could be adapted. You would need a good reference rig to set the marks but then would be good forever. Or until the next boat anyway.
Tension
A comment on the Loos gauge. I just bought the larger of their Pro model. I does cover the spread from 1/4 to 5/16. The labeling indicated this was a new version that added the 1/4-in. size capability. Yes it is expensive, but I found that my careful previous attempts at adjustments were about 50% to 75% too low. I saw a device called the "Rig Stick" on a Brion Toss video. It was basically a specialized yardstick to measure stretch. Has to be way cheaper than Loos, but I think they went out of business.
A comment on the Loos gauge. I just bought the larger of their Pro model. I does cover the spread from 1/4 to 5/16. The labeling indicated this was a new version that added the 1/4-in. size capability. Yes it is expensive, but I found that my careful previous attempts at adjustments were about 50% to 75% too low. I saw a device called the "Rig Stick" on a Brion Toss video. It was basically a specialized yardstick to measure stretch. Has to be way cheaper than Loos, but I think they went out of business.
Simple Checker
Hi John, I made my own arrangements to set my rig tension by pulling the wire horizontally using a spring balance and measuring the deflection on a stick held against the wire, with the other end resting against the mast as a reference. I mark the wire with a small piece of tape, the mast with pencil and the stick where it touches the wire with pencil plus another mark 3/4" towards the mast. The calculations are not difficult and I normally work 50" up from the lower end of the turnbuckle, deflect 3/4"" and a pull of 39 lbs equates to a tension of 2000 lbs. This takes into account that the length of main shroud on my 376 is 276"from deck to spreader. The calculation is not difficult and can be done for any other length or tension. t=T*d(1/50-1/L-50) where t=pull on spring balance, d= deflection, T= tension in wire,L=length of span between clevis pin and spreader. All measurements in inches and pounds (despite the Metrication America act of TWO decades ago) You can measure the height to the spreader by pulling a tape up on a halyard. QED Regards, Don
Hi John, I made my own arrangements to set my rig tension by pulling the wire horizontally using a spring balance and measuring the deflection on a stick held against the wire, with the other end resting against the mast as a reference. I mark the wire with a small piece of tape, the mast with pencil and the stick where it touches the wire with pencil plus another mark 3/4" towards the mast. The calculations are not difficult and I normally work 50" up from the lower end of the turnbuckle, deflect 3/4"" and a pull of 39 lbs equates to a tension of 2000 lbs. This takes into account that the length of main shroud on my 376 is 276"from deck to spreader. The calculation is not difficult and can be done for any other length or tension. t=T*d(1/50-1/L-50) where t=pull on spring balance, d= deflection, T= tension in wire,L=length of span between clevis pin and spreader. All measurements in inches and pounds (despite the Metrication America act of TWO decades ago) You can measure the height to the spreader by pulling a tape up on a halyard. QED Regards, Don
Nice!
That's great Don! That's one of the things I am looking for. I'm not sure I understand it all. Will this apply to all wire sizes? What's teh "50" for? Is it for the 50" up from the pin? Also, is it: t=T*d((1/50)-(1/(L-50))) Thanks, jv
That's great Don! That's one of the things I am looking for. I'm not sure I understand it all. Will this apply to all wire sizes? What's teh "50" for? Is it for the 50" up from the pin? Also, is it: t=T*d((1/50)-(1/(L-50))) Thanks, jv
I believe neither equation is correct...
...if you come up a distance x from one end of the stay, the equation is: t = T*d*((1/x) + (1/(L-x))) which is different from both Don and John. You can show this by resolving the tension T on either side along the direction of the pull, and making a small angle approximation that sin(a) ~ tan(a). You may really want to solve for T T = t/(d*((1/x) + (1/(L-x)))) Note that you can use any measure of length, as long as d,x,L are all the same (in, ft, m, etc), and T will have the same units as whatever you use to measure t (lb ). Also the quantity ((1/x) + (1/(L-x))) is a constant as soon as you choose x, simplifying the calculation. In the example Don gave (L=276", x=50", d=0.75", t=39lb), I calculate T=2,129 lb. Derek
...if you come up a distance x from one end of the stay, the equation is: t = T*d*((1/x) + (1/(L-x))) which is different from both Don and John. You can show this by resolving the tension T on either side along the direction of the pull, and making a small angle approximation that sin(a) ~ tan(a). You may really want to solve for T T = t/(d*((1/x) + (1/(L-x)))) Note that you can use any measure of length, as long as d,x,L are all the same (in, ft, m, etc), and T will have the same units as whatever you use to measure t (lb ). Also the quantity ((1/x) + (1/(L-x))) is a constant as soon as you choose x, simplifying the calculation. In the example Don gave (L=276", x=50", d=0.75", t=39lb), I calculate T=2,129 lb. Derek
How about resolvinng to a torque measurement?
What would it be if you applied two closely spaced pins, as on a Loos gauge, and torqued them a certain amount - say, 10 degrees or so? I imagine you could calculate tension from the pin spacing, number of degrees of rotation, and torque. Anyone? Thanks, jv
What would it be if you applied two closely spaced pins, as on a Loos gauge, and torqued them a certain amount - say, 10 degrees or so? I imagine you could calculate tension from the pin spacing, number of degrees of rotation, and torque. Anyone? Thanks, jv
Torque based tension measurement system
My first pass says that it is very simple: if the pins are separated by a distance d, and the angle turned is theta, then the required torque To is To= T*d*sin(theta) where T is the cable tension. Since theta is small, we can probably use a small angle approximation that sin(x) ~ x (x in radians) and use To = T*d*theta*0.0175 where theta is in degrees, and the constant 0.0175 converts degrees to radians. Note that this is just a first pass, We'd have to look further to see if the wire diameter and pin diameter affected the results significantly. Is there any way of checking this formula against a Loos gauge - I have never seen one? Derek
My first pass says that it is very simple: if the pins are separated by a distance d, and the angle turned is theta, then the required torque To is To= T*d*sin(theta) where T is the cable tension. Since theta is small, we can probably use a small angle approximation that sin(x) ~ x (x in radians) and use To = T*d*theta*0.0175 where theta is in degrees, and the constant 0.0175 converts degrees to radians. Note that this is just a first pass, We'd have to look further to see if the wire diameter and pin diameter affected the results significantly. Is there any way of checking this formula against a Loos gauge - I have never seen one? Derek
Wow...
You guys go all out to find a way to "conveniently" measure rig tension! Unfortunately I think I have more money than math skills, so I guess I'll have to stick to the Loos
You guys go all out to find a way to "conveniently" measure rig tension! Unfortunately I think I have more money than math skills, so I guess I'll have to stick to the Loos
Construction
OK, now that we have the math, how do we construct the device? Seems like you'd need a very stiff lever arm of some kind (length = d). On one end you'd put a bolt or nut (for use with the torque wrench) and a way to attach to the wire, and on the other you need a pin that would cause the deflection through theta. You'd also need a reliable way to measure theta. The device could be based on either a constant torque and use the variance in theta to determine tension, or you could use a set theta (5 deg?) and change the torque wrench settings. Seems like using a constant torque setting would be easier. Hmmm... I guess you could keep the torque AND the angle constant and using a varying distance between the bolt and the deflection pin. Lots of interesting possibilities here. Lots of room for missing a critical variable too. You have to factor in the amount that the lever arm will bend... you have to make sure your torque wrench is very accurate... Does the wire thickness affect things??? Now you know why people simply buy a Loos gauge (or two).
OK, now that we have the math, how do we construct the device? Seems like you'd need a very stiff lever arm of some kind (length = d). On one end you'd put a bolt or nut (for use with the torque wrench) and a way to attach to the wire, and on the other you need a pin that would cause the deflection through theta. You'd also need a reliable way to measure theta. The device could be based on either a constant torque and use the variance in theta to determine tension, or you could use a set theta (5 deg?) and change the torque wrench settings. Seems like using a constant torque setting would be easier. Hmmm... I guess you could keep the torque AND the angle constant and using a varying distance between the bolt and the deflection pin. Lots of interesting possibilities here. Lots of room for missing a critical variable too. You have to factor in the amount that the lever arm will bend... you have to make sure your torque wrench is very accurate... Does the wire thickness affect things??? Now you know why people simply buy a Loos gauge (or two).
I have a very simple idea...
...involving a piece of metal and a spring balance. Unfortunately, I have other things to do (sometimes!), but I'll make a drawing and post it as soon as I can find a few minutes. Derek
...involving a piece of metal and a spring balance. Unfortunately, I have other things to do (sometimes!), but I'll make a drawing and post it as soon as I can find a few minutes. Derek
My Humble Apologies
Sorry guys - There were TWO typos in my previous mail. Yes the formula should have added the contributions from the top and bottom spans. John, you clearly intend putting this into a spreadsheet - as I did. My intention was to make the contribution from the lower span dominant so the accuracy of measurement of the length of the upper span was less important. And yes I did measure from the lower clevis pin in the bottlescrew (turnbuckle) Also the TOTAL span length from lower clevis to spreader end is 216"- not 276" as I typed. I converted everything from metric and didn't spot the mistake. Indeed to formula should have read:- t=T*d{(1/50)+[1/(L-50)]} ye gods I hope it is right now!!! And my problem was typing formulae using only the primitive WP on this page. If I may comment on the concept of using two pins like the Loos gauge, the problem is that it is difficult to estimate the force needed to put two kinks into the wire as this requires the strands to slide on one another. Also the wire diameter comes into the formula in quite a big way and it must not flatten on the bend - which is one reason each size of Loos only works for a limited range of wire diameters - and not at all for rod rigging. Anyway, my 376 manual says just to wind the turnbuckles up as tight as one can without using an extension on the spanner. I did this and the only obvious effect, apart from an acceptable luff, is that the main hatch sticks a bit when the sun is on it. The guys who race Sigma 33's over here know when their rig tension is enough because they can't open the toilet door! Such is the world of yacht racing. Kind regards to all.
Sorry guys - There were TWO typos in my previous mail. Yes the formula should have added the contributions from the top and bottom spans. John, you clearly intend putting this into a spreadsheet - as I did. My intention was to make the contribution from the lower span dominant so the accuracy of measurement of the length of the upper span was less important. And yes I did measure from the lower clevis pin in the bottlescrew (turnbuckle) Also the TOTAL span length from lower clevis to spreader end is 216"- not 276" as I typed. I converted everything from metric and didn't spot the mistake. Indeed to formula should have read:- t=T*d{(1/50)+[1/(L-50)]} ye gods I hope it is right now!!! And my problem was typing formulae using only the primitive WP on this page. If I may comment on the concept of using two pins like the Loos gauge, the problem is that it is difficult to estimate the force needed to put two kinks into the wire as this requires the strands to slide on one another. Also the wire diameter comes into the formula in quite a big way and it must not flatten on the bend - which is one reason each size of Loos only works for a limited range of wire diameters - and not at all for rod rigging. Anyway, my 376 manual says just to wind the turnbuckles up as tight as one can without using an extension on the spanner. I did this and the only obvious effect, apart from an acceptable luff, is that the main hatch sticks a bit when the sun is on it. The guys who race Sigma 33's over here know when their rig tension is enough because they can't open the toilet door! Such is the world of yacht racing. Kind regards to all.
I agree with you Don
My very simplified analysis of the Loos type device ignored several factors, including the torque required to simply bend a thick wire into a shallow "s" shape. I looked at the problem this afternoon and decided that while it would be easy to build the device, it would really have to be calibrated experimentally for each wire diameter if you want to accurately measure tension. Easier to spend a few bucks and have it already calibrated by the manufacturer. Derek
My very simplified analysis of the Loos type device ignored several factors, including the torque required to simply bend a thick wire into a shallow "s" shape. I looked at the problem this afternoon and decided that while it would be easy to build the device, it would really have to be calibrated experimentally for each wire diameter if you want to accurately measure tension. Easier to spend a few bucks and have it already calibrated by the manufacturer. Derek
Jon, I agree with you. By now we'd be done
with the adjustment using the "pro" Loos gauge for much less than the cost of having a new model of "something" built. Besides, when having something custom built they charge extra for owner directed changes during construction. Gentlemen, I hope this is taken in the same spirit as is written in. I loved the interaction of everyone and the math involved. That,... totally left me off the end of the table as after seeing the equasions involved I realized I wasn't smart enough to crawl under the table. I think Jon had the right idea after all. The "Pro" model Loos gauge will fit the wire sizes you have. Also as Jon mentioned, after I measured my rigging with a Loos gauge I found that 1/4 of the required value was a very high measurement of what I had. Ray
with the adjustment using the "pro" Loos gauge for much less than the cost of having a new model of "something" built. Besides, when having something custom built they charge extra for owner directed changes during construction. Gentlemen, I hope this is taken in the same spirit as is written in. I loved the interaction of everyone and the math involved. That,... totally left me off the end of the table as after seeing the equasions involved I realized I wasn't smart enough to crawl under the table. I think Jon had the right idea after all. The "Pro" model Loos gauge will fit the wire sizes you have. Also as Jon mentioned, after I measured my rigging with a Loos gauge I found that 1/4 of the required value was a very high measurement of what I had. Ray
Let's continue though...
Of course it is easier and more practical to just buy the Loos gauge - but it is certainly interesting and informative to discuss the alternatives, and the principles involved. And, I was hoping that someone might come up with a commercial alternative to Loos, so we could compare function and value. I once say a gauge that had two rollers spaced about 8 inches apart, and a spring plunger in the middle connected to a pointer on a scale. It wasn't a Loos, I don't remember the maker. But, it was clearly different in principle from the Loos gauge. Has anyone seen this? jv
Of course it is easier and more practical to just buy the Loos gauge - but it is certainly interesting and informative to discuss the alternatives, and the principles involved. And, I was hoping that someone might come up with a commercial alternative to Loos, so we could compare function and value. I once say a gauge that had two rollers spaced about 8 inches apart, and a spring plunger in the middle connected to a pointer on a scale. It wasn't a Loos, I don't remember the maker. But, it was clearly different in principle from the Loos gauge. Has anyone seen this? jv
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