Pneumatics that are not spring-loaded are far less likely to be adversely effected by the bias of the pneumatic cloth. There are those who say that the cloth on striker pneumatics should be stretched slightly when it is glued to the pneumatic. Their claim is that the pneumatic will have a natural springiness, and will therefore open back up faster than one where the cloth is simply glued on without stretching. I have a natural tendency to stretch the cloth slightly when gluing it to the long sides of the bellow. This helps insure that the cloth is laying properly and that the corners, at the front, are nice and tight (sealed). Since the cloth is more easily 'stretched' when it's biased as I explained in my article, it's pretty easy to 'over-stretch' thin cloth. This has a negative effect of not allowing the bellow to full open when it's in the relaxed state, and would be a case for advocating that the cloth should be biased in a manner opposite to the way I explained.
That said, I never cut the long side of thin pneumatic cloth. The cloth is designed to rip in a straight line along the length of the cloth. When ripped in this fashion, the area of the cloth that gets glued to the front of the bellow is the non-stretchy part. Therefore, all of the bellows I make are, what I call, correctly biased so they won't stretch across the span.
I might add that I fairly recently revisited the reference that depicted cutting thin cloth cross-wise instead of ripping it lengthwise. On page 25 of the book, "Rebuilding the Player Piano", by Larry Givens, he explains his process for cutting the cloth, and also has a picture which shows the layout pattern. However, he also goes on to say that the cloth can be ripped lengthwise, and that doing so saves time. So the question arises, why do it any other way? The answer is simple. MONEY! Point-in-fact, it is more economical to cut the strips cross-wise.
Since the average striker pneumatic has a perimeter of 13 inches (which includes the overlap) and the (usable) width of the cloth (which is sold by the running yard) is 53 inches, you can cut four 13+ in. strips if the pattern is arranged in a cross-wise manner. (Bear with me here.) Taking for granted that the average span of a striker pneumatic is 1-1/4", you could get 88 strips of cloth from a length of cloth that is only 27-1/2 in. long. To get the same number of strips if the pattern is arranged in a lengthwise manner, you would need a minimum of 39 in. of cloth because you can only get 42 strips (1-1/4" wide) from a piece of cloth that is 53 in. wide. Furthermore, you end up with quite a bit of leftover cloth.
Generally speaking, suppliers sell cloth by the running foot even though they say it's sold by the running yard. The operative word here is 'running'. If you order four feet of cloth, you're ordering 1-1/3 running yards. So, a supplier will sell you 2 feet, 3 feet, or four feet of cloth without a problem. However, they won't sell you a 28 in. or a 40 in piece. Therefore, if the strips are cut cross-wise, you only need three feet cloth. But you need four feet if you rip the strips lengthwise.
In the information I provide at the web site about how much cloth is needed, I recommend getting one yard of thin cloth. Using the cross-wise pattern explained earlier, you can easily get 88 strips out of a yard of cloth. However, if the perimeter is 14 inches, you have to modify the cutting pattern. See Graphic:
For larger bellows with a perimeter of 15" and a 2" span, you'll need four feet of cloth. To see a graphic of the layout of how to cut the cloth, see the graphic below.
On the other hand, if you wanted to rip the cloth lengthwise, you would have to get at least four feet of cloth if the perimeter of the bellow is anything more than 12 inches (including the overlap).
The bottomline is that those who are willing to do a little extra work and carefully measure and lay out everything as efficiently as possible can save 25% or more on their cloth purchase. Considering that my labor rate is $95.00/hr., if I save 30 minutes ripping the cloth instead of drawing out an elaborate pattern and hand cutting every piece of cloth, I save the customer $47.50 in labor but spend an additional $15.00 of their money on materials. As you can easily see, it's all about reference points, not the bias of the cloth.... in this case.
Also take note that ALL bellows cloth has a bias (See YouTube video -click here) and Wikipedia webpage -click here. While it's not absolutely necessary when covering striker pneumatics, most professionals agree that bellows should be constructed such that the cloth doesn't stretch across that span of the bellow.
Hope this helps answers your question.
John A Tuttle
At 02:08 PM 5/11/2006, you wrote:
comments: Hello John,
I just had a quick question for you. I read your article on pneumatic cloth and you said that pneumatics with springs should not have the 'stretchy' direction of the cloth across it's span. Is this correct with small striker pneumatics as well? I had received conflicting information on this.
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