Re: [Harp-L] widening out/opening up the backs of harps

[Joseph Leone <3n037@xxxxxxxxxxx>]

If you take a 2 foot (50.8cm) piece of metal stock that is 1/8th" (3mm) thick by 1/2" (12.7mm) wide and place it on the flat  between 2 common chimney bricks that weigh 6 lbs 14 ozs. (3,112 gms) and are set apart
about 20" (about 45.72 cm) apart, you will notice that the bar sags. If you place the same piece on edge, there will be no noticeable sag. So Michelle my Belle is correct. The lip on the covers act as buttresses. And Inasmuch as you carried this harp for 45 years (about 1/100 millionth of the earth's life), I can see where crinkeling the cover plate could have happened. I suspect it was in your back pocket. And thus received an
eclipse. lol

smo-joe


On Mar 9, 2016, at 7:22 PM, Rick Dempster wrote:

> I have carried a harp in my trouser/jeans hip pocket, daily, for forty-five
> years, Michelle.
> Just relaying my experience.
> But to continue from your theoretical point of view, once that right-angle
> flange has a kink in it,
> it's all rapidly downhill. Guess I should do a 'control' experiment, if we
> keep up the
> discussion. I'll let you know when I build up the energy!
> Cheers,
> RD
> 
> 
> On 10 March 2016 at 02:11, Michelle LeFree <mlefree@xxxxxxxxxxxxxxxxxxxxx>
> wrote:
> 
>> Rick Dempster wrote:
>> 
>> I agree, no difference worth noticing; but what, for example, Joe Spiers
>>> does makes the plates stronger,
>>> so my 'pocket' harp does much better with this particular mod.
>>> RD
>>> 
>> 
>> I have to disagree here, Rick. stock SP20 cover plates are significantly
>> stronger than "opened" plates for two reasons.
>> 
>> First, from a vector mechanics (physics) viewpoint, the vertical component
>> of the right angle flange at the back of  the harp contains a lot more
>> metal to resist vertical downward force (or upward on the bottom plate)
>> than opened plates do. This is the same concept that engineers apply in
>> their use of structural I beams.
>> 
>> That vertical part of stock cover plates also contain struts that help
>> stabilize the cover plates. When you open the back you flatten those struts
>> out.
>> 
>> The comb's plastic struts support the center of the cover plates but the
>> other two mechanisms for resisting downward forces are gone when you
>> flatten those vertical flanges.
>> 
>> But, it does look cool. 8^)
>> 
>> Michelle
>> 
>> 
>>