Paper Tubes: Spiral vs Convolute

[Patrice Nadeau]

From reading the pyrotechnic litterature, I always thought that convolute
tubes are much stronger than spiral wound tubes. Therefore, they are
supposed to be the best for high pressure applications like rockets and
gerbs.

Recently, I established a good link with a maker of both spiral wound and
convolute paper tubes of all sizes. Speaking with one of their
knowledgeable person, they told me that spiral wound tubes are more
resistant (tube diameter, glue and wall thickness being the same) to
compression than convolute tubes. When I think that compression is just the
inverse force compared to pressurisation, then I would deduct that spiral
wound tube should also be more resistant to internal pressure compared to
convolute tubes. My same expert told me that the weakness of spiral wound
is for the bending strength: if you try to bend it, it will break open or
crush along the joints, which I agree from personal experience. Convolute
tubes are much more resistant to bending, which explains their main use as
rolls for fabrics since large rolls of fabrics are often transported on the
shoulder of people, where resistance against bending and break is of key
importance.

Did anyone ever heard similar stories/explanations for the relative strength
of convolute vs spiral wound tubes?

I guess I could ask for the technical specifications of the compression
strengths of their tubes, but I did not bother them yet with this issue.

[Lloyd E. Sponenburgh]

Post by Patrice Nadeau
From reading the pyrotechnic litterature, I always thought that convolute
tubes are much stronger than spiral wound tubes. Therefore, they are
supposed to be the best for high pressure applications like rockets and
gerbs.

.....snip

Post by Patrice Nadeau
Did anyone ever heard similar stories/explanations for the relative
strength of convolute vs spiral wound tubes?
I guess I could ask for the technical specifications of the compression
strengths of their tubes, but I did not bother them yet with this issue.

For a given wall thickness, the only reason I can find that a spiral tube
might have more strength than a convolute tube would be that the paper
grains are crossed in adjacent layers in the spiral version. But there are
other more mundane reasons to use convolute tubes for pyrotechnics.

First is that by their nature, convolute tubes are more expensive to make
than spiral -- convolute tubes must be made in fixed lengths, then trimmed
to size. Spiral tubes may be made by continuous processes. Since they're
more expensive, there is a tendency to allow the use of better paper - like
virgin kraft - for convolute tubes. Spiral tubes are most often used in
cost-sensitive applications, so they often contain the worst-quality paper
that will suit. Convolute tubes of virgin kraft are much stronger than
spiral tubes of recycled chipboard in the same wall thickness.

Second, spiral tubes' laminations almost never align precisely. Most often,
there is a distinct gap between the turns of a spiral. With thick paper,
this leaves a deep (often wide) helical groove down the inside of the tube.
This groove is an impediment to your making successful gerbs and rockets.

The groove can form a fire path down the fuel grain, allowing the charge to
burn progressively when it shouldn't. It's often the source of a CATO.

LLoyd

[h***@yahoo.com]

Patrice Nadeau posted:

" When I think that compression is just the
inverse force compared to pressurisation, then I would deduct that
spiral
wound tube should also be more resistant to internal pressure compared
to convolute tubes."

Sorry Patrice, but you're meaking an invalid assumption. To illustrate
the error using a trivial example, consider the internal pressure that
a soda can or a aerosol container can withstand, as compared to the
small amount of externally applied force that it takes to, when empty,
easily crush in your hand.

Technically it has to do with the fact that the grain many materials
are much stronger in "tension" than in "compression" (this is the 'you
can pull on a rope, but not push on it').

With the paper tubes used in fireworks become pressurized, the internal
pressure creates tension in the tube walls, with very little shear
force. Consequently you want the paper grain of the rocket, gerb, wheel
pusher, or paper mortar to all be running in the same direction. A
convolute tube provides this sort of strength, while a spriral wound
tube of the identical wall thickness much less so.

At first glance this is not self-evident but when you think about it a
little the concept becomes clear. (I had my face shoved in this sort
of thing when on one of my earliest jobs I was involved in the
engineering design of filament wound/expoxy rocket engine cases for
Polaris A3 and Sprint solid fuel rocket engines.)

Kindest regards, Harry C.