For whatever reason, I’ve always been more
than a little fascinated by things involving water.
Particularly the devices we use to carry it
around, distribute it, keep it away,
or even just make it look pretty.
Be it fountains, aqueducts, water towers,
roofs, or even a plain ‘ol pipe,
there’s a lot to whet your inquisitive appetite.
Such as these things!
Ah, the humble lawn sprinkler.
The irrigating sound of summer.
Impact sprinklers like these have been around
for a very long time and I’ve always been
a little curious about them.
See, the way they move back and forth is…
intriguing.
An oscillating sprinkler like this guy makes
total sense.
It’s just got a little water wheel in there
that acts like a motor, robbing some of the
kinetic energy of the flowing water to power
a little gear train
which makes the holey stick thing go back and forth.
These dudes, though, well I’m sure you’ve
heard how in one direction they go all…
[tick tick tick tick tick tick tick tick]
but in the other direction
it’s more of a
[tstststststststststststststs]
Why does it speed up?
And how does that make it go in the other
direction?
Well, I wanted to answer these questions for
myself and also you
because it turns out these sprinklers are downright fascinating, and for a bunch of reasons.
Naturally, I have one right here.
These sprinklers do that thing where their
principle of operation is incredibly simple,
but the design which makes it possible is
visually complex.
Let’s start with where the water comes from.
Hook a garden hose up to this bit and water
will flow up into the sprinkler head
(and, if you wanted to, out that way to another hose for another one down the line).
Now obviously the head is designed to spin
and it’s pretty easy to turn.
But notice that it has a lot of vertical play.
I can actually lift up on it quite a bit, and if I hold it in this position it becomes a bit harder to turn.
When the sprinkler is being used, the water
pressure forces the sprinkler head up against
its stop, the effect of which is that it takes
a fair bit of force to turn it.
You’ll see why that’s important shortly.
Next let’s look at where the water comes
out -
it’s here, from this little brass nozzle.
But notice how there’s a thing in the way.
This little armature
♫ sudden low-fi fast jazzy dance hall music ♫
(or something like that)
Can you believe I didn't put a ton of jokes in this bit?
This little armature serves to redirect the water’s path.
It will first hit this angled piece which
throws it slightly to the left.
But immediately after that it will hit this sort
of scoop shape, throwing it back to the right
and roughly on its original path.
The armature isn’t fixed, in fact it can
move but a spring acts to return it to its
resting position.
When the water comes flying at the armature
as the restriction in flow created by the
nozzle causes its velocity to increase dramatically,
the armature gets flung with a fair bit of force to the left.
That force gets absorbed all nice and soft
like by the spring, but of course the spring
returns the favor and eventually flings the
armature right back where it started.
And by the time it gets there, it’s moving
pretty fast.
The energy the armature picked up as it was
thrown to the left by the water never went away,
instead it was just dampened and absorbed by
the spring.
The spring then works to push the armature
back to its resting position, and once it
gets there the stored energy gets released
all at once in a momentous occasion.
The sudden impact as it slams against the
head imparts a fair bit of torque.
Bringing a moving object to a sudden stop
isn’t exactly easy.
That’s our friend Force equal Mass times Acceleration.
When the armature is initially flung to the
left by the jet of water, the total force
(and thus, torque) generated isn’t that
substantial.
It gets moving, or is accelerated, fairly
slowly, and the opposing force of the spring
(another source of acceleration) is also pretty
weak and so happens over a fairly long period of time.
But once it returns to its starting position, it’s moving at a relatively high speed and hits a solid barrier.
The near instantaneous acceleration that results
from this impact, multiplied by the mass of
the armature itself, imparts a substantial
amount of rotational force
(or torque) on the sprinkler head.
That knocks it slightly to the right, or clockwise.
And this is where the resistance brought about
by the lifting force of the water comes into play.
With the sprinkler head forced against its stop, now we have to overcome friction to get it to move.
While there is a counterclockwise force imparted
as the armature is flung to the left
(and as the spring resists that movement),
the small acceleration component diminishes that force to the point that it can’t overcome static friction.
The upshot of this is that the sprinkler head
can only rotate when there is a substantial
peak force caused by an impact, which means
it will only move clockwise.
But of course, it can also move the other
way.
Just as your Auntie Clockwise!
[awkward silence]
That was a dumb joke, but in case you didn’t
know these don’t have to change directions;
you can make them just go in circles like
most online arguments.
If this little thing is flipped up, well then
it will just go ‘round and ‘round and
‘round and 'round.
But flip it down and when it hits these little
things
(which are adjustable so you can control its range of movement) it will change directions at each end.
How?
Well, look at what it does.
The action of moving the little directiony
changey bit causes this small metal piece to move.
And when it’s in this position, it limits
the movement of the armature
by catching it with this little tab.
Now you might think this wouldn’t do a whole lot,
but it makes one significant change
to the action of the armature.
It changes when the impact occurs.
Now, as before, the armature picks up speed
from the jet of water throwing it to the left.
But rather than have a nice and subtle spring
slow it down and reverse its course,
it slams right into the metal piece right away.
That impact imparts pretty much the same amount
of torque as before, but in the opposite direction.
The energy the armature gains from the water
pushing it to the left goes right into the
sprinkler head, rather than into the spring
as an intermediary.
That high peak torque can overcome the static
friction and nudge it to the left, or counter-clockwise.
When it’s going in that direction, the impacts
occur much more frequently because the spring is
for the most part out of the picture, and
the distance the armature can travel is quite small.
Plus, thanks to that limited movement, the water
jet now becomes not only a source of energy
for pushing the armature to the left, but also a dampener when it returns.
After making the impact, it will of course
return to its resting position
(partly because of the spring and partly just from bouncing off the tab thing),
but it doesn’t have enough speed to overcome the jet of water and so a second impact doesn’t occur.
Instead, the armature just gets pushed away again
relatively gently,
without enough acceleration to produce a torque which can overcome the static friction.
Since the only impact occurs on its outward
swing, the only movement generated is in the
counterclockwise direction.
And really, that’s it!
As far as I can tell these got their name solely
from the fact their driving force
comes from an impact.
And they’re not the only device to use this
principle.
Ever heard of an impact wrench?
These things, rather than just spin a socket
with a plain ol’ motor, instead spin a large
metal thing inside them called a hammer.
And not, like, this kind.
It’s just called a hammer.
That hammer, after it’s up to speed, periodically
strikes an anvil which creates a large amount
of torque as the energy from the spinning
hammer is imparted into the stationary anvil.
The socket is attached to that anvil, and so
in effect gets struck in a twisted way repeatedly
by the hammer.
It’s the same principle as using a high
amount of acceleration
to create a large amount of instantaneous torque.
Because…
that’s what it is.
The distinctive sound of an impact wrench,
either air driven or as is the case here cordless
is the result of the hammer repeatedly striking
the anvil,
and this process is what creates the repetitive bursts of very high torque.
[rapid, loud clicking/banging sound]
In these sprinklers, though, it’s a lot more simpler
and also just easier to see because
you can actually see it.
The impact of the armature also helps the
sprinkler be a better sprinkler.
See, the nozzle shoots water pretty far at
a slightly upward angle giving it great coverage
far from itself, but pretty lousy coverage right
in front of it.
As the armature slaps the stream of water
like a curious racoon, it periodically disrupts
the stream and gets some water close to home.
When the direction changes, it gets even closer.
And also, fun fact, many of these sprinklers
are adjustable beyond simply their angle of sprinklage.
You might have noticed this little set screw.
Tighten that and this pin will enter the jet stream and break it up, reducing the effective range of the sprinkler.
You can also achieve similar deflection with
this little hat thing.
A nice touch.
These really are pretty versatile sprinklers,
unless of course what you need to sprinkle isn’t round.
Like a cake.
So the next time you hear the irrigating sound
of an impact sprinkler,
just remember that it’s really quite clever.
And make no mistake, this is a technology.
While I often focus on things like audiovisual
equipment and other eletronicals,
something as simple as a water distribution device deserves due diligence.
Orton Englehart used human ingenuity to invent
this thing back in 1933,
and while it may seem like a simple and down-to-earth device, it is still full of lessons we can learn from it.
Thanks for watching.
♫ impactfully smooth jazz ♫
But the design which makes it possible is
visual…
When water comes flying at it as the restriction
in flow created by the nozzle causes its velocity
to increase dramatically
[said mockingly]
...the armature isn’t fixed, in fact it can move but a spring…
da di duppa dough
And once it gets there, the…
that wasn’t right.
It gets moving, or is accelerated, fairly
sssssssssssssssssss
lowly.
Bringing a moving object to a sudden stop
isn’t exactly easy.
That’s our…
what was that?
There was a noise.
The distinctive sound of an impact wrench
either.. air… ugh
You can control its range of movement.
That…
I missed some words.
You better believe I know what armatures are, now.
Sometimes they're the spinny bit of a motor, other times they're the wire framework of a figurine or model.
And other times they're offensive or defensive appendages on animals and junk. Like armor.
But yeah, anyway, armatures.
