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.