 The Clarinet BBoard
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Author: Gordon (NZ)
Date: 2008-03-10 00:26
I put this link here in case it is of interest to anybody:
http://www.saxontheweb.net/vbulletin/showthread.php?t=79549
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Author: Gordon (NZ)
Date: 2008-03-10 13:01
You're most welcome, David.
It's the sort of thing I ponder about while servicing instruments.
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Author: skygardener
Date: 2008-03-10 14:19
From the above mentioned post- "For getting very low sluggishness, ideally we would use a spring with: [...] 3. A lot of initial relaxed-state curve in the spring, so that by the time the spring is placed in the cradle, it is near its elastic limit. (However it must not actually reach its elastic limit during operation.)"
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I have a question about the spring being near it's elastic limit. Since we don't really have the option of changing the size of springs on clarinets (except to a thicker size), what if you DON'T want all the potential power that the spring has? I may have asked this before in the "Springs" thread. What if you want to get a fast spring but you don't want the stiffness of a thick spring? I have thought about thinning the diameter of the spring and leaving the base at the thickness to fit the hole, but there is heat involved in that process and I suspect that the heat would ruin the desired properties of the metal.
I guess I want the key to have a lot of power when I lift but not when I push the key. ![[rotate]](http://test.woodwind.org/clarinet/BBoard/smileys/smiley25.gif) Maybe I should take some time off, go to a park and watch the pigs fly by.
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Author: BobD
Date: 2008-03-10 17:11
Very interesting read,Gordon, as well as the responses. I also ponder these things but without the extensive experience you have in instrument work. I do have a problem relating to elastic limit,however, since it is primarily a theoretical term and you probably won't find it listed in any mechanical property data sheets for metals and alloys. Yield Strength is the usual mechanical property listed and this is defined by the amount of "offset" used on the stress-strain curve. However, I guess one could use the term relative to the spring itself. When you "anchor" the spring in the post and then put a bend in it to adjust the force desired you have changed the properties of the material. The bent section now has a higher yield strength than the unbent section...and you have also changed the position of the fulcrum.
As you point out it is a "merry go round" of compromises.
Bob Draznik
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Author: Gordon (NZ)
Date: 2008-03-10 22:12
"What if you want to get a fast spring but you don't want the stiffness of a thick spring? I have thought about thinning the diameter of the spring and leaving the base at the thickness to fit the hole, but there is heat involved in that process and I suspect that the heat would ruin the desired properties of the metal."
When I encounter a ridiculously thick spring, with excessive 'sluggishness', and its demands are not high, such as F/C on clarinet, then I use a tungsten carbide bur (or diamond wheel) in my dental handpiece to put flats on the spring, perpendicular to the plane in which it operates. That is the most effective way to reduce its stiffness. This is without taking the spring off the instrument. No heat involved. (Heat does little to change the stiffness of a steel spring.)
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Author: BobD
Date: 2008-03-10 22:46
Great idea,Gordon. And you're right "stiffness" is a function of thickness(section modulus as I recall) and alloy composition. Not sure what heat sky is referring to....the flattening or the grinding. By the way what make/model dental handpiece do you use?
Bob Draznik
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Author: Gordon (NZ)
Date: 2008-03-10 22:53
"I do have a problem relating to elastic limit,however, since it is primarily a theoretical term and you probably won't find it listed in any mechanical property data sheets for metals and alloys. Yield Strength is the usual mechanical property listed and this is defined by the amount of "offset" used on the stress-strain curve."
Elastic limit is typically defined in mechanical engineering texts something like "The stress at which a permanent extension occurs. The metal is no longer elastic."
this is somewhat different from "yield stress". A typical definition would be "The stress at which the metal stretches without further increase in load" Think of bubble gum held vertically.
Without getting into intricate and complicated detail, the elastic limit is very clearly shown on the specification stress-strain graph for a material. It is where the line stops being straight. Yield strength is the first peak of the graph. Sometimes yield strength is quite close to elastic limit, so the two are not highlighted in specs as different. Graphs for different materials (and different states of hardness and tempering) can be very different.
http://www.anasys.co.uk/library/dma1_1.gif
http://www.indiana.edu/~quake105/images/fig8.jpg
http://aemes.mae.ufl.edu/~uhk/STRESS-STRAIN.jpg
The term "yield strength" is probably used rather loosely at times.
"However, I guess one could use the term relative to the spring itself. When you "anchor" the spring in the post and then put a bend in it to adjust the force desired you have changed the properties of the material. "
Not really. You have forced the spring past its elastic limit/yield stress, in order to acquire a new shape, without changing the properties. This enables to spring to have a "pre-tension" when hooked around the spring cradle. In effect, it is operating still on the flat section of the graph, but further up that flat section.
".. The bent section now has a higher yield strength"
No. The yield strength is inherent in the material itself, unless it is altered by some means such as heat treatment or work hardening. Very little work hardening occurs during the pretensioning you describe.
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Author: Gordon (NZ)
Date: 2008-03-10 23:02
"By the way what make/model dental handpiece do you use?"
My dental lab micromotor unit is now 20 years old and has served me well. It is a Japanese brand, Morita. Model: Roxy 2. No regrets at buying quality on this one!
A lot more torque is available at low revs on the newer ones.
I use this at least once - often many times - on 90% of repair jobs.
If it died, I would replace it with the best available within hours. My most useful equipment after my screw drivers, feeler, and key bending tools.
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Author: BobD
Date: 2008-03-10 23:15
Where the stress/strain "line" starts deviating from a straight line is hard to define and that's why it isn't considered a mechanical property of commercial materials. It's a theoretical concept. Bending the spring does in fact change it's yield strength "at that section" since permanent deformation involves work hardening. Yield Point (your yield stress) is a property of ductile materials and does not apply to materials used for springs. We could continue our exchange directly if you care to....I don't have your email address.
Bob Draznik
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Author: skygardener
Date: 2008-03-11 00:29
The heat I was talking about is built up during the metal removal. I had thought the best way to thin it and keep an even shape would be to use a bench motor and sand paper or a file. I don't know if this heat is enough to cause a change in the metal, but it is a thought.
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Author: Gordon (NZ)
Date: 2008-03-11 09:43
Bob, I think the work hardening after a single bend in a needle spring is minuscule.
We could easily get into these sorts of intricate details...
http://en.wikipedia.org/wiki/Yield_(engineering)
We could also consider "endurance limit".
But what is the point?
I used the term "elastic limit" partly because of the audience, who does understand elastic bands, but for whom "yield" cold mean all sorts of different things.
Depending on the intricacies of how you define the terms, for the spring steel (& SS) we use for needle springs, elastic limit and yield point are pretty close together.
Fundamentally, elastic limit is an important consideration for our needle springs, because:
1. If we exceed it during the expected normal operation of the spring, then the string gets weaker from then on. (In practice, any work hardening seems to be relatively irrelevant.)
2. We MUST exceed it when we bend the spring for pretensioning. And we must be able to significantly exceed it without reaching brittle failure.
For the purposes of this discussion, no more complexity is really necessary.
Post Edited (2008-03-11 09:46)
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