------------------------------------------------------------------------ > I don't think I've asked this question before. > > A customer has an expression piano, make escapes me at the moment, > basically soft and loud or normal and accent. > > I've had to work in the valves rebuilt by someone else that used > inferior leather "donuts" on the stems to adjust valve spacing. > > I'm testing the valve chest, with the pneumatics attached, by itself - > not yet connected to the tracker bar. > > So my question is, what would be the minimum vacuum to activate the > valve to shut the pneumatic ? > > Thanks, ------------------------------------------------------------------------ Hi, That's sort of a loaded question, so I'll qualify it first. What should the minimum vacuum level of a test pump be set to before it is connected to a stack to test the valves? The answer is 5 inches. Another way to phrase the question is: How many inches of vacuum should be required to seat all of the valves in the stack? Again, the answer is 5 inches. BTW, if all the valves seat at 5 inches, each individual note should operate easily, or collapse the bellow when the trigger tube is opened. In fact, I normally keep uncovering note nipples to see how many notes will activate before the combined losses of the valves changing state, the open bleeds to the atmosphere, and all other losses, are so great that no more notes will activate. Normally, I can get to ten notes before things stop working. The most I've ever been able to get is 27, but the stack had a 1-1/2" supply. Now, let's get fussy! Once the valves have all sealed at 5 inches, how low can you turn the vacuum level and expect any particular note to function? The answer is 3 inches. While I have rebuilt stacks that could function on 1-1/2 inch of vacuum, they had to be 100% air tight with as little as 3 inches of vacuum. And the only stack that has operated that well is the modern Universal stack. BTW, at 5 inches of vacuum, 41 notes could be operated simultaneously, but they had to be opened (or turned 'on') one at a time. In every test I've ever run, it took at least 5 inches of vacuum to create enough energy to activate a note on a piano. Naturally, the more notes you activate simultaneously, the more "air flow" you need. Notice I didn't say more 'vacuum'. But this is where things get a little technical. Let's say you have a 3/8" vacuum supply line connected to the stack. Only so much air can flow through that hose at 5 inches of applied vacuum. Increase the diameter of the hose to 1-1/2 inches, and it's a whole other ballgame. That's because you have four times more air flow just by increasing the diameter, even though the vacuum level remains the same. To do more accurate measurements (and figuring), you have to know the exact cubic area of the striker pneumatic, the size of the intake and exhaust ports, and the transient loss as the valve changes state (which is a function of the valve gap, the pouch size, the well size, and the bleed size). Now that I've gone through all of that, let me tell you how I check a stack. I connect up a length of hose, put it in my mouth, and suck. What I hope to feel as soon as I start sucking is 'the valves all seating' and then a tiny amount of air flow. If it takes less than a few seconds of sucking before my lungs fill with air... there's a problem. What I aim for is being able to keep sucking for at least 12 seconds before my lungs are completely filled with air. Basically, it's the same 12-second test that's used to test a lower section. And that all relates to the old adage that you should be able to pump up the lower section, stop pumping, and walk around the piano one time, and the music will still be playing when you get all the way around and sit down to start pumping again. That is a well rebuilt (or built) player mechanism. Lastly, the question arises: How can I tell how much vacuum it takes to seal all of the valves? If you have a variable vacuum source, that's pretty easy. Simply connect the stack to the vacuum source and slowly turn up the level until all the valves seal. How can you tell when the valves seal? Again, that's fairly easy. By placing your hand near the exhaust port on the vacuum pump, you can feel when the air flow drops significantly. (You can also hear a change in the way the pump sounds, but that's a bit harder to explain because it's a subjective word, i.e., struggling.) Then, disconnect the supply from the stack (leaving the pump set at the point where the valves all sealed) and measure the vacuum level. Also, as you might expect, valves that are horizontally inclined will require a higher initial vacuum level to seat than valves that are vertically inclined. However, once they have all seated, you should be able to reduce the vacuum level to at least 7 inches and expect every note to operate flawlessly (independently -or one at a time). Hope this helps. Musically, John A Tuttle Player-Care.com ======================================================= This email has been used to create another webpage at Player-Care.
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