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Since there were/are so many different player system manufacturers, and considering that none of them ever listed the amounts or types of pneumatic cloth they used, you have to figure it out for yourself. However, this is not a difficult task. We'll start with the heaviest cloth, then the medium weight cloth and then move on from there to the thinnest cloth.
Listed below are the various devices found in a normal 88-note player piano. The various highlighted terms are linked to definitions. To return to this page after reading the definition/s, simply use the BACK button on your web browser.
Generally speaking, all bellows are made the same way. One continuous piece of cloth is wrapped all the way around the perimeter of the bellow, and the ends of the cloth overlap at the hinged end of the bellow. The amount of overlap varies depending on the size of the bellow. But, as a general rule, the overlap is one-third of the width of the bellow board. All bellows open a certain maximum number of inches (see Special Note below). The amount that the bellow opens is called the 'span'. So, with regards to the bellows cloth, every bellow has two important measurements; the span, and the perimeter.
Typically, the heavy weight cloth is only used for the Exhausters and the Reservoirs. The average exhauster measures 15" X 12" and, therefore, has a perimeter of 54". The average span of an exhauster bellow is 6" or less. The average reservoir is usually slightly smaller than the exhauster if there are two reservoirs. If there is only one reservoir, it is always larger than the exhauster. Usually, there is a minimum of a three-inch overlap for both the exhauster and reservoir bellows. So don't forget to figure that into the total. For larger bellows, consider covering them with two pieces of cloth as per this method.. click here
Typically, the medium weight cloth is used for the air-motor (or wind motor), air-motor governor (or governor), the roll tracking mechanism (or tracker), the auto-sustain, and other peripheral devices like the Bass and Treble Soft pneumatics and any other 'control' devices. Generally speaking, this cloth is for medium size bellows with a perimeter of 15"-25".
The perimeter and span measurements for the above named devices varies to such an extent from manufacturer to manufacturer that it is impossible to quote any meaningful averages. The only way to insure that you purchase enough cloth to do the job is to measure each device separately. .
Finally we come to the thinnest cloth (or striker pneumatic cloth). This cloth is usually only used for the small striker pneumatics (or note bellows) that activate each of the notes of the piano action. However, some manufacturers also used this cloth for air-motors and other medium-to-small bellows. When in doubt, use a micrometer and measure the thickness of the cloth. All striker cloth is less than 0.010" thick. The average is 0.009".
Determining the amount of striker cloth you'll need is not very complicated. First, there are normally 88 striker bellows. Second, almost all striker bellows have a maximum span of 1-1/4". (Bellows with larger spans are usually only found in nickelodeons, orchestrions and band organs.) Third, almost all striker bellows have a perimeter (including the overlap) of less than 14". Doing a little quick math, we have 88 X 14" X 1-1/4" or 1,540 sq.in. of cloth.
Striker cloth is sold by the running foot, and the bolt of cloth is never less than 48" wide. Therefore, dividing the total number of square inches needed by 48" will give you the number of running feet needed to make all the bellows. Doing some quick math, 1,540 divided by 48 equals 32.083 inches, or approximately 2-3/4 feet. And since the cloth is sold by the running foot, you will need an average of three feet, or one yard of cloth.
SPECIAL NOTE: Please be advised that the above explanation presumes that the cloth with be cut in cross-wise strips and that you can fit four strips across the width of the cloth. However, remember that the cloth is only 52" wide and that 14" x 4 = 56". Therefore, you can only get three strips from a 48" wide piece of cloth. That being the case, you can still get at least 88 strips of cloth out of one yard if you use the left over piece and cut the strips lengthwise. (See Graphic) .
Special Note: People often ask me how to determine the correct span of a bellow if the cloth is missing or ripped. While it is somewhat difficult to figure out the minimum required span, it is easy to ascertain the maximum allowable span*. (See Note below.) The rationale for this formula is that if the span is any wider than prescribed below, the opposing sides of the cloth will touch each other when the bellow collapses. If that happens, the cloth will wear out faster because of the friction created when the two pieces of cloth rub against each other.
*Note: The only exception to the above 'rule of thumb' involves bellows that are folded as seen below. However, it should also be noted that bellows folded in such a fashion are always limited by some other factor so that the opposing edges never touch each other.
As a general guideline, the minimum required span for any bellow (that's used to control the operation of a mechanical function, i.e., tracker, sustain, bass/treble soft, auto-rewind, piano action, etc.), is determined primarily by the operating distance that the device must 'travel' from the 'On' position to the 'Off' position plus 25%. In other words, if the component being moved by the bellow must 'travel' a distance of 1" (one inch), the minimum 'travel' of the bellow must be 1-1/4". Do not confuse "travel" with "span". As explained previously, the 'span' of the bellow includes the thickness of the two pieces of wood. The 'travel' is the space "between" the boards. So, as seen below, SPAN = TRAVEL + A + B
When dealing with Air Motors, the important thing to keep in mind is that you want to operate the bellow "within the area of least resistance". And while that is a basic truth for all bellows, it's critical when it comes to air motors because if any of the bellows in the motor has any resistance to movement within the area of operation, the motor will NOT run smoothly. For this reason, the span of an air motor bellow is typically around 35% more than the actual 'area of operation', or the distance the bellow travels from open to closed while it's working. So, a bellow that requires a 'Traveling distance' of 1-3/16" will require a total travel of 1-3/4" to work efficiently. Generally speaking, the best rule of thumb, in this case, is to err on the side of too large of a span instead of too little. As well as showing how the travel is measured, the pictures below show what an air motor bellow should look like in operation when the cam is at its two extremes.
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This page was last revised on May 20, 2017.
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