The good ole string line

Date 03-17-2009 03:09 Edited 03-19-2009 05:30

Hello All, for many of you this is old hat and you probably have some of your own variations to add or detract from it, yet I believe for many of the newer crowd this might be helpful.
When I have needed to line up troughs or conveyor sections for short and longer runs it helps to keep everything relatively level and straight......., those belts don't like going around corners so well.
I try to start out with centerlines on each end of each section. I also use some tabs tacked to one of the sections so that it can be hung from the other one to allow for levelling and straightening by way of adjustable jackstands, overhead cranes, or any other available means. Using a block of equal thickness and orienting the string line directly over the centerline on each end allows the two center marks in the middle of the two sections to be lined up and moved right or left to line them up. When all four marks are lined up with the string the sections are going to be relatively straight. Having similar thickness blocks also allows for the height of the string to be compared at the blocks on each end and at the center joint to verify that the two sections are flat and level in relation to one another. I have included a series of pictures that will hopefully explain some of what I have spoken of here. I would welcome any and all comments from others for accomplishing this task as I am set to learn here as well. Best regards, Allan

Edit: for those of you who looked closely enough you may have noticed that the center joint of this fit-up is an 1/8" low, just noticed that as I was going through the pictures.

By natecf

Date 03-19-2009 03:27

Hello Allen,
   We too use the string method for most of our line up work. Its quick, easy, and cheap. unfortunately, on longer runs, like over 20' , all the string tension in the world can't overcome gravity and sag, as your ruler shows. So to overcome this, depending on the structure we are building, we use the string for straightness left and right, but up and down along the length is verified by a fairly affordable laser level or a good pair of EYEcrometers.
   The laser level, is used basically the same way the string is used. most levels have the beam at a set height from the base. So with the level at one end , adjust the beam so it is the same height at the other end of the unit you are trying to straighten. once you have the two ends the same, then check the measurements down the line.
   A couple of important notes to keep in mind, - the laser beam gets wider the further away from the source that you are. so when measuring the beam height, always use the beam's centerline as your reference.
   point 2, - we typically set the level on a stand or horse at the end of the structure and not actually on the structure itself. otherwise, any movement of the stucture could change the laser setting at the far end. this is one thing that the string is not affected by .
Hope some of this info will be helpfull

By aevald

Date 03-19-2009 05:10 Edited 03-19-2009 05:27

Hello Nathan, completely agree with some of the limitations of string lines/piano wires and as you also included, you do have to consider the "spread" of the beam on most "inexpensive" lasers and can generally use the center of the dot to gauge the actual center.
I try to stress to students that whether you use a string line, piano wire, laser, optical level or transit, you are relying on a straight line of one form or another to base straightness, level, slope, or plumb on. The method used will generally be selected by the limitations of the situation as well as the availability of the technology. We often talk of weather having a limiting effect on the use of string lines/piano wires, and gravity, as you mentioned. Side to side motion generally affected by wind, up and down by gravity, piano wire being the better choice with regard to the gravity issue and the wind.
I also discuss some of the benefits of laser and string/piano wire over optical choices. For starters, optical choices require at least 2 operators and sometimes 3, 1 to view the targets and 1 or 2 to locate the targets to line up the optical sight line. Lasers are definitely the easiest to set-up and work with from a manpower standpoint, generally requiring the least additional help. String /piano wire options can be employed fairly readily by a single operator but require a greater degree of set-up and time. I do have a bunch more information and examples to illustrate the principles that I included in this post, when I have the tiime to organize and properly present them I will include them with this post or start a new one. I appreciate you taking the time to include your post on this thread. Thanks and best regards, Allan

By natecf

Date 03-20-2009 02:14

no problem Allen, it's always interseting to see other techniques and practices that are out there, I'm always on the hunt for a better, faster way to work. after so many years working at the same shop with the same guys, its easy to become stagnant with regards to new ideas. thats one of the things I enjoy about the forums, there is such a wealth of knowledge and insight, that your bound to learn something anytime you log on.
I look forward to your future posts on this or other subjects.
Nathan

By DaveBoyer (*****)

Date 03-20-2009 03:26

There is a chart in the old American Machinists' Handbook that gives the ammount of sag at given intervals along the length for an .058" diameter piano wire tensioned to 60#. This is how they established a straight line in pre-laser days. I guess the chart is in Machinery's Handbook too, at least in the older editions.

By aevald

Date 03-20-2009 06:59

Hello Dave, thank you for including that information, I hadn't been aware of that before now. I'll have to take a look in some of my copies and see if it is still there. Thanks again and regards, Allan

By rlitman (***)

Date 09-14-2010 22:22

Ideally, it's a catenary. With a known cable tension, length and cable weight, one can calculate the sag (this varies with gravity as well, but the amount gravity varies so little that it is immeasurable in any shop conditions). The cable will hang in a hyperbolic-cosine shape.
Here's a simple calculator for this:
http://www.spaceagecontrol.com/calccabl.htm

With that calculator, you can figure the sag in music wire, string or spaghetti. Whatever.
Of course, the formula does not compensate for the springiness of the hanging material, so short runs of music wire would be flatter, because they store some energy in their elasticity (unlike chain).

The relationship between sag and tension is inverse, so it requires infinite tension to achieve zero sag.

By 803056

Date 09-16-2010 01:24

The ingenuity of true tradesmen never ceases to amaze me.

Best regards - Al

By welderbrent (*****)

Date 09-16-2010 14:21

A subject as 'simple' as a string line and I am learning many new things.

Thanks Allan and others. Great Ideas.

Have a Great Day, Brent