Wood durability and string tension - Woodworking Talk - Woodworkers Forum
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post #1 of 13 Old 08-02-2020, 03:20 AM Thread Starter
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Wood durability and string tension

Hello everyone. I'm about to dive into a project of making a stringed instrument. I am a total novice and will have to do loads of research along the way. That being said, there is one thing I feel I need to understand before I even start, and that is how to calculate if a certain piece of wood is strong enough to withstand a certain amount of tension coming from the strings.

The tension of the strings is easy enough to calculate. Let's imagine I have X strings, each with Y pounds of tension, totaling Z pounds. Near to both ends of the piece of wood, the strings are held in place on tuning pins that have been driven into the wood.

So, where do I start? How do I know whether a given piece of wood will crack or break or whether the tuning pins will burst out of the wood? Is it dependent on all 3 dimensions of the piece? And, of course, it is dependent on the type of wood, but which properties of the wood? Density? Anything else? Do the strings have to line up with the grain or can they be at an angle?

As you can tell, I'm quite clueless, so any help would be appreciated!
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post #2 of 13 Old 08-02-2020, 06:21 AM
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Welcome to the forum. Reads like you have quite an ambitious and interesting project. There are some stringed instrument makers on this forum. They may be able to help you.


I will be interested in hearing the answers.


George
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post #3 of 13 Old 08-02-2020, 10:12 AM
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Welcome to the forum, or welcome back since you joined in 2017! Add your first name to your signature line so we'll know what to call you and it will show in each post. Add your location to your profile, as well.

An acoustic guitar, for example, generally has a top made of Spruce with Spruce bracing to help it withstand the tension of the strings. There is usually a Maple bridge plate under the bridge and that keeps the ball ends of the strings from pulling through the softer Spruce top.

But it sounds like you're going to have a different configuration with pins at each end. What are you building? We do like photos!

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post #4 of 13 Old 08-02-2020, 10:40 AM
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When in doubt ... test to failure!

Make a one string test board using the materials that you would ultimately be using. Attach the string to the tuning pegs, using rosin to prevent them from untwisting. Attach a fish weighing scale in the center and see what happens if you "max out" the tension .... maybe nothing.
Then depending on the number of strings, calculate the total tension on the neck. You can do another test using a similar neck piece or one using downward pressure with support at the ends. Test to failure.


If you are satisfied it won't fail under actual use, go for it!

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)
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post #5 of 13 Old 08-02-2020, 03:07 PM Thread Starter
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Quote:
Originally Posted by difalkner View Post
Welcome to the forum, or welcome back since you joined in 2017! Add your first name to your signature line so we'll know what to call you and it will show in each post. Add your location to your profile, as well.

An acoustic guitar, for example, generally has a top made of Spruce with Spruce bracing to help it withstand the tension of the strings. There is usually a Maple bridge plate under the bridge and that keeps the ball ends of the strings from pulling through the softer Spruce top.

But it sounds like you're going to have a different configuration with pins at each end. What are you building? We do like photos!

David
Hi David. Will do!

I can't provide exact pictures because it's a new design. Basically, I want to make an electric version of an acoustic pedal clavichord, visible here (the thing on the ground). Being acoustic, that instrument has a soundboard, but I'm thinking more along the lines of an electric guitar, i.e. a solid block of wood with no soundboard, using magnetic pickups to amplify.

I plan on having a total of 54 strings in 27 courses. I'd like the lowest note to be quite low, so that means I'll need sufficient length. I don't know the tension on the strings, in fact I was hoping to work backwards, that is, once I understand how much tension a given piece of wood can withstand, I can then figure out what sort of string tensions I can safely use.

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Originally Posted by woodnthings View Post
Make a one string test board using the materials that you would ultimately be using. Attach the string to the tuning pegs, using rosin to prevent them from untwisting. Attach a fish weighing scale in the center and see what happens if you "max out" the tension .... maybe nothing.
Then depending on the number of strings, calculate the total tension on the neck. You can do another test using a similar neck piece or one using downward pressure with support at the ends. Test to failure.


If you are satisfied it won't fail under actual use, go for it!
Hello. This is kind of what I was thinking. But then I get a bit fuzzy on how things would scale from one string. Like, if I deduce that a 1" wide piece of a given wood at a given thickness breaks at 10 lbs of tension, does that mean a 10" wide piece breaks at 100 lbs? And what about the location of the pins? Is this scaling only accurate if the 10 tuning pins are spaced evenly apart, an inch apart?

Sasha
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post #6 of 13 Old 08-02-2020, 03:28 PM
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shivasage must be an engineer, he's using too many eng terms in the same post. rather than calculate stress and tension for x,y&z... i agree with woodnthings, make a test board.

i just made a door harp and had no idea what i was doing to begin with. first i made a test board for testing strings. i tried nylon string, fishing line and various wires. finally bought a set of 5 guitar strings for $4 from amazon. i used brass screws with a 1/16" hole drilled in the screw to loop the wire in. finally i made an actual test door harp with pine face, mdf body and thin plywood back. currently the test door harp is hanging on my back door. i'll eventually get around to make a couple of nicer door harps

edit: sasha posted while i was typing. i still think your overthinking this. using wood your not going to test for breaking, i'd thing any bowing or movement would affect your tune. 27 strings is a lot of strings. even if you made a 27 string test board, you'd have to test it for a year or two at various humidty to see what you can handle.

i think your alternative to multiple multi year testing is to over build it, plain and simple. you can probably base your design on a known design plus

Last edited by _Ogre; 08-02-2020 at 03:37 PM.
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post #7 of 13 Old 08-02-2020, 03:39 PM
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Quote:
Originally Posted by shivasage View Post
Like, if I deduce that a 1" wide piece of a given wood at a given thickness breaks at 10 lbs of tension, does that mean a 10" wide piece breaks at 100 lbs? And what about the location of the pins? Is this scaling only accurate if the 10 tuning pins are spaced evenly apart, an inch apart?
I think other factors will come into play here; specifically the grain in the boards used where the pins attach. If the grain is tight and running perpendicular to the string I think you'll be ok but if the grain is wide and running with the strings then you're liable to see splitting.

Get with Don Teach, a friend for the last 40 years or so and a truly knowledgeable resource on pianos and other stringed instruments. See if he can help you, he has a world-wide reputation for solving player piano and related instrument issues.

David

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post #8 of 13 Old 08-03-2020, 10:03 AM
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Wood calculations are tricky. For large structures "Wood as an Engineered Material" is the handbook of standard calculations and property values for woods. BUT this is numbers game. It's poor design to have the integrity of a wood structure depend on a single member or fastener, so the values are averages which may vary widely but have overall validity for a structure with many members. I've seen some very odd exponents in some of the formulae. Behavior of wood often does not scale up or down linearly.

There's some wise advice in the previous posts. There's nothing like accumulated wisdom for what has worked and what hasn't in similar, even if not identical, applications like yours.
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post #9 of 13 Old 08-03-2020, 11:56 AM
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Stringed instruments often have truss rods

The truss rod does most of the work when the strings are trying to break the neck of guitars. They are adjustable and fit into a channel in the neck. The same approach could be used on a wider solid body, just use several rods. Think about the tension and compression forces and how and where they are acting on the body of the instrument ......



Interesting video;

The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)

Last edited by woodnthings; 08-03-2020 at 02:19 PM.
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post #10 of 13 Old 08-03-2020, 01:15 PM
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I think that the first thing I would do is make a full or half scale drawing of the instrument. Then I would place force vectors on that drawing.


Your strings will be in tension, but the wood will have two forces acting on it(then). The major force will be compression. There will also be a fending force because the strings will be above the wood. Think of any other directions of force (vectors) that may be involved. Eg. the pins will be trying to lean in their holes.


Last, as Woodsnthings suggested, build a model or two. Or more if necessary.


George
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post #11 of 13 Old 08-05-2020, 09:49 PM Thread Starter
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Quote:
Originally Posted by _Ogre View Post
shivasage must be an engineer, he's using too many eng terms in the same post. rather than calculate stress and tension for x,y&z... i agree with woodnthings, make a test board.

i just made a door harp and had no idea what i was doing to begin with. first i made a test board for testing strings. i tried nylon string, fishing line and various wires. finally bought a set of 5 guitar strings for $4 from amazon. i used brass screws with a 1/16" hole drilled in the screw to loop the wire in. finally i made an actual test door harp with pine face, mdf body and thin plywood back. currently the test door harp is hanging on my back door. i'll eventually get around to make a couple of nicer door harps

edit: sasha posted while i was typing. i still think your overthinking this. using wood your not going to test for breaking, i'd thing any bowing or movement would affect your tune. 27 strings is a lot of strings. even if you made a 27 string test board, you'd have to test it for a year or two at various humidty to see what you can handle.

i think your alternative to multiple multi year testing is to over build it, plain and simple. you can probably base your design on a known design plus
Not an engineer, just a lowly musician. Thanks for the advice. Yes, I'm leaning towards overbuilding. I was thinking that the test for breakage would at least give me some numbers to work with, so I know what overbuilding would even entail.
Quote:
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I think other factors will come into play here; specifically the grain in the boards used where the pins attach. If the grain is tight and running perpendicular to the string I think you'll be ok but if the grain is wide and running with the strings then you're liable to see splitting.

Get with Don Teach, a friend for the last 40 years or so and a truly knowledgeable resource on pianos and other stringed instruments. See if he can help you, he has a world-wide reputation for solving player piano and related instrument issues.

David
That's curious. I was under the impression that, because wood more easily moves across the grain, the strings should run parallel with the grain to avoid exacerbating any movement.

Quote:
Originally Posted by woodnthings View Post
The truss rod does most of the work when the strings are trying to break the neck of guitars. They are adjustable and fit into a channel in the neck. The same approach could be used on a wider solid body, just use several rods. Think about the tension and compression forces and how and where they are acting on the body of the instrument ......



Interesting video;
How To Build An Electronic Autoharp - YouTube
Good idea on the truss rod. It seems like a middle ground between sticking with a wooden frame and going all out piano-style with a cast iron frame. Well, not so much 'middle', more like 5%, but you get the idea.
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Sasha
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post #12 of 13 Old 08-18-2020, 06:38 PM
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I know nothing about building instruments but will add a though I learned building a Murphy bed where there are spring stresses placed on various parts requiring the use of plywood instead of solid wood. Engineered wood is stronger and generally more predictable than solid wood. Basically (3) 1/4" thick pieces of say hard maple epoxied together is stronger that (1) solid piece of 3/4" maple.

A 3/4" sheet of finish grade plywood from the local lumber company has 5-7 layers. A 3/4 x 3/4 strip can be broken fairly easily across the grain. The same thickness of Baltic birch has around 14 layers and that same 3/4" strip is pretty difficult to break.

Just food for thought.
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post #13 of 13 Old 08-19-2020, 10:22 AM
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You are trying to rethink something that has been figured out centuries ago.

The construction of a stringed instrument takes this into account.

The grain orientation of the top & the construction of the neck are designed to counteract string tension.

Guitars have an adjustable tension rod imbedded in the neck.

Robert
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