I don't like to rain on anyone's parade, and I certainly encourage
others to (as I do) put their creative efforts to work. So I hope the
info in this message is used as intended and not viewed as some sort
of criticism. I've attempted to very succinctly lay out a process for
reviewing one's own design of any mechanical device and project. If
you have no interest in the contents and proceedures in here, no one
will be offended.
This proccess can certainly be done by an individual, especially if
they can be objective about their own work or ideas. However, this
proccess is best done as a small group brainstorming session. It's
interesting to note that most of the disasters occurring with car
recalls, building collapses, etc, are the result of overconfidence and
other ego related behavior affecting decisions rather than simply
overlooking a detail or other technical lack of ability. Last minute
changes and rushed (shortened) proceedures by upper management cause
over 95% of recalls in the car business.
We do learn from our mistakes, so failure (of any efforts) often lead
to the greatest successes. There will be equipment/mechanical failures
in any endeavor, and these should be taken as "lessons learned"...
this is part of the design and engineering proccess. Anyone building a
mechanical device is engaging in engineering and design whether as an
amateur or professional, and experience is as valuable as studying the
science of engineering.
Still we want to minimize risks and eliminate those things that are
known to be hazards, or minimize them in other ways, such as through
proccess and understanding. An open wheel turning is one of the most
dangerous things to anyone within the vicinity of the machine, yet a
simple guard will not eliminate but absolutely minimize the danger of
a spinning wheel. Electrical hazards cannot be eliminated if
electricity is involved, but a GFI outlet minimizes the risk to
acceptable levels.
Keep this in mind...
The art of designing mechanisms always seeks to minimize risk and find
an acceptable compromise between opposing forces or contradictory
parameters.
So there is always the risk of a (mechanical) failure. One only has to
watch the History Channel to see Greatest Engineering Disasters, to
know that even professionals screw up. I remember a building that
collapsed around 30 years ago. The building's "structure" was entirely
made of reinforced concrete. The architect had figured the loads for
the building seemingly proper and included the added weight of using
concrete as opposed to Steel Framing only. The problem was that he had
used the load bearing and weight formulas for cured (dry) concrete and
forgot to figure the loads of wet concrete, which is not only much
heavier, but weaker to boot. that building collpased like the WTC with
the concrete floors falling on the lower floors causing a chain
reaction with complete structural failure...it resulted in the death
of over 100 construction workers.
In the design that has drawn so much attention here, my overwhelming
concern was in part on the high probability of a minor failure
becoming a catastrophic chain reaction.
It is almost certain that at some point due to excessive loads, the
bearings in the conveyor rollers will fail (I am very familiar with
this equipment). In itself this is relatively minor.
After a bearing failure Shear loads can build extremely fast and cause
sudden unexpected failure of other parts. We conventionally think of
solid steel rollers as being rigid, however there will be components
that act to store energy like a spring and release that energy
suddenly when the component fails...and parts fly.
Once the bearing fails the driving wheel with all of it's kinetic
energy together with the energy of the vehicle dropping onto the floor
of the roller box, and the torque of the car's engine tranfers the
total sum of all that energy in one instant. This can tear the roller
our of the frame and fling it a few inches or several hundred feet
depending on all the variables. The result is truly unpredictable.
Additionally, with or without a roller failure the entire roller
mechanism can be suddenly ejected out behind the driving wheel. The
only force preventing this is friction between the "box" and the
ground. The rollers being configured to cradle the wheel is too
shallow to act as much of a stop. The lack of a solid base is perhaps
the greatest liability in this design. Without the roller box being
mounted securely gives the greatest potential for unexpected events
and very unpredictable chain reaction failures.
If the roller box is only partially ejected there is still a great
potential for a chain reaction failure. If the driving wheel only
kicks it out partially, it is quite likely to twist/pivot the roller
box opening another opportunity of moving parts becoming bound up. so
now there is the sum of all that mass moving (papercrete mix, spinning
driven wheel and driveline) with a fairly likely chance that it can
all be suddenly concentrated on any single component in the system as
things end up misaligned and get bound up...so at that point there is
another potential for catastrophic failure due to the unpredictibility
of many variables.
My purpose here is not to beat a dead horse but offer some technical
understanding to everyone here to assist them in building their own
equipment if they choose. While the science of engineering can be
carried out to cutting edge design (finding the extreme line between
high-performance and system failure), as DIYers our designs are
usually has much simpler goals so generally we can use common sense to
predict modes of failure and reasonably asses and predict the
probability and severity of equipment failures. I'll continue to use
the roller design for reference but it is not my intention to pick on
it anymore.
In analyzing our own designs common sense and finding analogs (simlar
examples) in other parts of our lives are important. If someone has
seen a batting practice machine throwing baseballs or used a table saw
can understand about the roller box being ejected out behind the
driving wheel.
First make a rough sketch of the entire system as intended.
1. Car sits with the driving wheel on roller box.
2. roller box transfers load to driven wheel.
3. driven wheel connected to axle, differential and output shaft.
4. output shaft attached to mixer blade.
In operation what are the forces involved:
1. weight of rear wheel being suspended on rollers (stored energy).
2. gas engine supplying torque.
3. kinetic energy of spinning drivetrain to include driveshaft,
vehicle differential, driving wheel.
4. Frictional forces between driving and driven wheels and rollers.
5. kinetic energy of spinning mass of driven driveline.
6. kinetic energy of mixer contents.
note: while this energy may dissapate very quickly, in motion, all of
the papercrete mass moving/spinning carries tremendous kinetic energy!
The purpose here is not to quantify the forces, but simply to know
where they are. Additionally it is very important to list those
parameters of operation that are out of the normal intended
operation...like the vehicle remaining stationary while it
is "driving", or the tire which normally sits on flat ground (with
relatively large contact area) now contacting two small diameter
rollers (much less contact area than road).
Add arrows to your sketch to show where these forces are being
applied... show direction of movement and large forces (such as the
weight of the mixer and the car being raised above ground level.
Next make a list of expectations of the design..brainstorm here!
1. Vehicle remains stationary but drivetrain is running.
2. Rollerbox tranfers energy from driving to driven wheel
while...aha! ...roller box is stationary.
3. driven wheel attached to axle and differntial tranfers power to
mixing blade while
4. mixing blade chops and mixes slurry.
note: include obvious expectations in the design such as elevated
mixer to remain stationary on platform (only mixer blade and contents
move). Be as thorough in this as possible.
Lastly, beginning at the input side (car or truck) assess potential
failuresand possible consequences (in the system)...
Engine quits, causing vehicle to jump off rollers.
Engine quits and jumps rollers causing a collision between car and
mixer platform.
Engine quits and mixer mechanism slows to a stop.
Engine quits and mixer inertia kicks roller box away from driven wheel.
etc, etc etc...
Work through the whole design one system and critical part at a time.
determine probability and severity of potential failure mode.
Assign risk reduction strategy to limit variables and consequences...
Attach tie down chains at all four corners of vehicle to prevent
jumping off rollers or into mixing platform.
This last step is the main difference between professionals and DIYers
where DIYers can close the gap between them and the professionals, by
how thorough this proccess is accomplished.
Also once a corrective change is implemented another round of what-ifs
must be applied to at least the changed part of the system, but it is
wise to go through the entire system several times.
The degree to which one goes through this proccess is determined by
several risk factors...such as:
Cost (of wasted materials and failed part replacement, lost labor,
etc).
Severity of potential failure mode (a bruised ego and late for dinner,
vs. the unintended severe injury/death of a bystander or self).
confidence of each corrective action to be effective...Putting a well
secured guard over the open driven wheel gives high confidence for
effectiveness of the fix, even though it is possible that someone
could loosely attach the guard where it would be effective.
Other risk factors (such as timing) can and should be factored in so
the end result is (as many as possible) that all factors are taken
into account of which actions and corrective actions can be, cannot
be, should and should not be taken.
Finally, a decision is made whether to go forward with the build...or
not.
Certainly we all understand the risk of unexpected costs. Many people
here I assume are as interested in saving money as they are to save
Mother Earth due to limited finances. Apply the same criteria to
injury (and other) risks. If you cannot reasonably have confidence in
a safety fix, then depending on the severity of the potential injury
(sliver in finger vs. being crushed) some builds might have to be put
on hold or the design changed drastically. Perhaps the build needs to
be completed in a certain amount of time, so timing can determine a
green light (or red) on going forward with a project.
This in a nutshell is what engineering and design field does as their
meat and potatoes. It is not a simple proccess but it is not difficult
as much as tedious, time consuming and ego bruising! lol...
every creation of our projects in essence go through this process,
albeit in much less detail. everytime a hobbyist, designer, builder,
PC'er envision and build anything... their creation did indeed go
through at least a very simple form of this proccess.
With all of the talent and creativity here, I figured that there might
be those that want this info as there is no reason that people here
can't achieve results even better than the "professional builders".
The scale of papercrete building and mixing is great enough to warrant
caution and professional like attention to detail, but this is in the
ability of nearly if not all people, so the sky's the limit to
creativity and dreaming.
-Todd
> slurryguy <slurryguy@...> wrote:
> Recent posts by Shaman_Circle, daleandbren, bornofthehorses, and
many
> others about
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