Re: [Harp-L] Steel Reed Thickness

["Vern Smith" <jevern@xxxxxxx>]

There are two limits of interest:

The elastic limit is the maximum stress from which a part will spring back 
and recover its previous shape.  We must exceed this limit to change the 
gap.

The fatigue limit (less than the elastic limit) is the maximum stress below 
which the metal will not fatigue from repeated cycling.  Steel has a fatigue 
limit but non-ferrous (not containing iron) alloys such as brass and bronze 
do not.  In non-ferrous metals, any cyclic stress produces some fatigue. 
However, the number of cycles to fatigue failure is a non-linear function of 
stress level.  For example, reducing the stress level by one-half will much 
more than double the number of cycles to fatigue failure.

The stress cycling of a reed is the worst kind becauses it stresses the 
surfaces of the reed in both tension and compression. Fatigue begins as 
microscopic cracking along the borders of the metal crystals.  These 
eventually join to produce larger cracks.  Eventually, a large crack causes 
part failure.

If I were to undertake to measure reed thickness in the harp, I would attach 
pointed probes to the jaws of a micrometer.  Reeds do not have a single 
thickness.  They are tapered with greater thickness near the rivet and less 
near the tip.  One reed can vary from a thickness of as much as .010" near 
the rivet to as little as .002" at the tip.

Vern
Visit my harmonica website www.Hands-Free-Chromatic.7p.com


----- Original Message ----- 
From: "fjm" <mktspot@xxxxxxxxxxxxxxxx>
To: "h-l" <harp-l@xxxxxxxxxx>
Sent: Friday, April 27, 2007 7:59 AM
Subject: Re: [Harp-L] Steel Reed Thickness


> The one reed set I looked at under magnification sure looked like the 
> milling marks were across the width of the reed not down the length of it. 
> If I understand Vern's explanation of ss versus cuprous alloys then the ss 
> reeds will last forever by virtue of them being able to cycle infinitely 
> as long as the stress limit is never exceeded.  One would hope that Seydel 
> engineered the reeds to operate within that constraint. How exactly would 
> you manoeuvre  a micrometer into a reed slot to gauge the thickness 
> anyhow?  The one I have is wider than the slots.  fjm
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