Wire line computer analysis
Wire line computer analysis
(OP)
Hello
Any one familiar with the process of using a taut wire to check if a series of bores are in line? Typically piano wire stretched with a 60 pound pull, centered to the outermost bores and then a measurement is made to the wire from the inner bores. By analysing the results one can determine if the bores are "in line"
There is some serious math involved with determining the sag of the wire which must be incorporated in the results. Some one at work has made an excel spread sheet but it is cumbersome and (in my opinion) not accurate.
So,, does anyone have or have a link to software that would do the analysis. Ideally something that would make a pictorial reptesentation of the relationship of the bores to a true line.
Thanks
Ralph
Any one familiar with the process of using a taut wire to check if a series of bores are in line? Typically piano wire stretched with a 60 pound pull, centered to the outermost bores and then a measurement is made to the wire from the inner bores. By analysing the results one can determine if the bores are "in line"
There is some serious math involved with determining the sag of the wire which must be incorporated in the results. Some one at work has made an excel spread sheet but it is cumbersome and (in my opinion) not accurate.
So,, does anyone have or have a link to software that would do the analysis. Ideally something that would make a pictorial reptesentation of the relationship of the bores to a true line.
Thanks
Ralph





RE: Wire line computer analysis
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Wire line computer analysis
One issue with your posited procedure is that ostensibly, you're trying to perform a least squares regression of the bore centers to a line. However, by centering the cable on the outermost two bores, the line is is arbitrarily fixed and the remaining bores are compared to THAT line as opposed to the nominal best-fit line through ALL the bores.
TTFN
RE: Wire line computer analysis
What level of uncertainty are you trying to achieve?
TTFN
RE: Wire line computer analysis
RE: Wire line computer analysis
I'm oft amazed at the precision these techniques provide.
Is there a reason surveying isn't used?
RE: Wire line computer analysis
If yes, then sag of the line is not a concern.
RE: Wire line computer analysis
The accuracy depends a lot on the enviroment but in good conditions one can easilly diffentiate up to .0002". The measurement is relative, to one side one may have 6.0205 and the other side 6.021. This would only mean that from the edge of the wire to the bore differs by .0005 or that the bore is .00025 off a true line.
Based on an analyis of the result one can determine the location of the bores are true to the ends. This then can determine if the housing can be moved to create a "better" alignment or if some of the bores need machining the correct the location.
So... the sag is important, even though not much ~.0015 over 20 feet it does affect the results one is trying to obtain.
Ralph
RE: Wire line computer analysis
• the sag is small;,
• the wire is inextensible axially,
• the wire has zero bending stiffness,
• the two endpoints are at the same level,
the catenary equation can be approximated by a parabolic equation.
It is then relatively easy to show that the vertical sag at midspan is given by
w*L^2/(8T)
where
• w is the weight per unit length of the wire,
• L is its horizontal length, and
• T is its tension.
HTH
RE: Wire line computer analysis
RE: Wire line computer analysis
As an example, if you use kilonewtons per metre for w, metres for L, and kilonewtons for T, the formula will give you your sag in metres.
(Putting it crudely, the kilonewtons sort of cancel each other out as you evaluate the formula.)
RE: Wire line computer analysis
dwedel
Hotrod Big Engines!
For site policies and guidelines
see FAQ731-376
RE: Wire line computer analysis
Assume this chain is slowly shortened, the "sag" will be reduced. Is it a catenary throughout? Or, at some point as the catenary approaches a straight line it is no longer a catenary.
Is "my" tensioned wire even properly considered a "catenary"? Are the formula to determine the shape of a catenary valid for my tensioned wire?
Denial (Structural) makes this comment....
the catenary equation can be approximated by a parabolic equation.
Does this mean my (tensioned wire)is closer to a parabolic than a catenary? or just an easier formula?
RE: Wire line computer analysis
I don't know when it is safe to assume that it is a parabola
The horizontal force, in terms of total cable length and sag is
Fh = w / (8*h) * (S^2 - 4*h^2)
The y coordinate (height) of any point in terms of the horizontal force
y= Fh / w * (cosh(w * x / Fh - 1 ) )
We assume that the origin is at the center of the span.
Total span = L
Sag in the cable = h
So, the coordinates of the endpoints are (+/- L/2,h).
The weight per unit length = w
Total length of wire/cable = S
Length along the cable from the origin = s
Fh is the horizontal force component everywhere, and is equal to half the tension at the center.
grabbed from http://
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: Wire line computer analysis
RE: Wire line computer analysis
Ralph2 asks whether his wire can validly be considered a chain for this exercise. The key assumption a wire might violate is the one about negligible bending stiffness. I do not know of any simple rule of thumb that provides an answer here. Not only does wire have a bending stiffness, but it also usually has a non-straight unstressed shape as a result of being wound into a coil for transportation.
You could always do a quick calculation as follows. Calculate the plastic moment capacity of the wire's cross-section, and divide that by the proposed tension. That will give you a length, and that length will be the maximum possible deviation that the actual wire could have from its theoretical path.
Please note that Greg's last statement seems to have fallen victim to a cut&paste&edit error. It should read:
"Fh is the horizontal force component everywhere, and is equal to the tension at the centre." (The word half needed to be removed.)
PS. For the etymologically inclined, the word catenary comes directly from the Latin word for chain.
RE: Wire line computer analysis
I did get to compare the laser to a machinist's boring bar, so I am confident of the .001" results I was getting.
I researched the possibilities of wire-lining and went with the laser because of the "art" that seemed to be part of the wire-line process. The fact that the laser alignment tool also helps us align couplings made the tool an easier sell.
dwedel
Hotrod Big Engines!
For site policies and guidelines
see FAQ731-376
RE: Wire line computer analysis
How complicated / easy is using the Pruftechnik BoreAlign system. Unless I am confused with something else, one rotates a target inside the bearing bore. Kind of a little 4 wheeled truck with a mast that intersects the center of the bore. It seems that this could lead to errors if the bore was out of round..
Have you had your Pruftechnik BoreAlign long.. are you happy with it? (we have the turbalign and use it only for alignments)
RE: Wire line computer analysis
I believe the more accurate tool is called CentrAlign. Ludeca is the US importer of the tool.
I have three digital pictures of the 82KVG and a PDF of the basic report that I printed out that I could send you via email.
You are correct about most of the operation. The target is rotated in the bores and the laser is stationary outside the unit. I used the oil pump flange which spaced the laser about 12” from the nearest bearing bore. The wheeled cart you spoke of, is actually the magnetic base for the rotating point on the mast you talked about. The target is mounted on a spring loaded holder, which pushes a rod out against the bearing bore. The rod will touch down on several points. You basically have an inside mic, that is constrained to rotate about a point somewhere near the center of the bearing bore. The computer then computes a bore center line. You can see all the data points as you take them as well as the averaging the computer does. Thus the tool operator can see an out of round bore.
For accuracy, I tested taking the target out of the bore in between sets of measurements to see how well the sets of measurements compared. I also reconstructed the mast when working on the 82KVG, and the measurements before and after seemed to agree well. The vibration was a problem. The 82KVG was 14’ long with 8 bores. At the #7 and #8 bores, I had to use the maximum averaging of the computer to get results with lower standard deviations. I felt that a standard deviation that was .0005” when I wanted my resolution to be .001” was acceptable. The older sensor that I rented, may have contributed to the vibration problem. You must be careful with thermal growth of the tool as I fought with high radiant heat sources and cold drafts. On the 82KVG I compared the results to a machinist boring bar, and it seemed to be accurate. In the vertical plane, one bore was .0014” high, and another bore was .0019” low. The horizontal plane had one bore +.0016” and another -.0014”. These measurements were from a line of best fits and of course the ten-thousandths accuracy must be taken with a grain of salt considering the standard deviation of my measurements. The crankshaft web deflections also agreed with the measurements we made.
Overall the tool works well for us. I will try to train one of our mechanics this summer on its operation, which I think will go well. The higher order math is done by the computer and data doesn’t need to be transferred from a pad to computer. Bucking the laser in, seemed really simple to me. Unfortunately I cannot compare directly to the wire-line method as I have never done it. However, as the wire line method was described to me, the laser makes some of the measurement and interpretation simpler.
If you are thinking about this option, renting the tool worked well for me. Perhaps the salesman would be willing to give you a demonstration also. Hopefully this doesn’t sound too much like a sales pitch, because it is just my experience.
dwedel
Hotrod Big Engines!
For site policies and guidelines
see FAQ731-376
RE: Wire line computer analysis
Ralph
RE: Wire line computer analysis
dwedel
Hotrod Big Engines!
For site policies and guidelines
see FAQ731-376