Fascinating, you can see the cavitation on the upper side starting earlier since there is leakage to the next cavity on the bottom, an asymmetry since it is the upper gear that is being driven.
thats actually a very good catch. Just to further explain, different gear faces are in action (sealing) on top and bottom gear because of which gear is being driven.
Really nice catch. I just watched it for the third time and hadn't previously noticed the tiny gap between the leading faces vs. the closed gap between the trailing faces (which would be reversed if the lower gear was the driven one), and the way that the fluid flow through this gap causes the development of the upper vortex to "lead" the lower vortex by a tiny fraction of a second. Absolutely fascinating, cheers!
I had pump gears in my hands with many many tiny grooves in it. These where from aircraft pumps that have pretty high rpms. They explained what cavitation is, but with this kind of video it would be much easier to understand.
@@remkojerphanion4686 these gears are hard chromed on the surface, and once this chrome is gone, it looks like a hole in the teeht, pinhole on the surface, and below this its getting larger. This can lead to a total fail once a chung breaks out of it. If you google "pump cavitation" you see crazy pictures.
@@remkojerphanion4686Cavitation destroys metal in the surface little by little, so while it's not exactly "wear" it is damage that will eventually become terminal
@@ChoChan776 The term "wear" implies parts damaging other parts. On a larger scale this is catastrophic damage, while in something small like an oil pump it has basically no impact on the life of the pump.
Love your stuff man, it inspired me to branch out on my own, and without better timing i'm building a 4,500lbs. CNC router to do my own work. Please keep us updated with controller configuration with the CNC controls.
This!…. Is a good example of accelerated wear on mechanical gears. Fluid viscosity also has a major role in this too. This is a very good video capture!
Someone once told me that the gears in this type of oil pump create half of the heat in the oil in an engine, and I can believe it. I wonder if some channels in the gears to allow flow would reduce cavitation more than they reduce oil flow.
@@izzmus All pumps work by creating a pressure differential between the inlet and the outlet, regardless of design. "Cross-flow" channels would reduce the pressure difference, and any modification which does that is going to reduce flow rate. I'm just an armchair enginerd, but I'm gonna guess that you would probably wind up in a situation where your pumps last twice as long due to reduced cavitation, but you'd need to put two of them in series to achieve the desired pressure/flow which is obviously going to leave you worse off in terms of overall efficiency of the system.
the design of the pump specifically pump curves over RPM is what makes this a non issue. this pump was ran outside of its parameters to cause cavitation
Interesting to note that the vortices (after the vaccuum collapses) is sucked into the next void - denser material tends to be drawn into vortices, suggesting that metal chips from erosion will be kept within the vortices somewhat instead of travelling downstream (I think you can see metal flakes around the vortices in this video). I suspect purging the first bit of fluid just before startup should reduce fowling due to this (or a magnet on the outer wall near the vortices to draw metal fragments out before they get a chance to flow downstream). this is great footage though, lots to learn!
fun fact, the gears look like they're pushing liquid from the right to the left of the screen, but the fluid actually follows the teeth around the outside of the gears. so the direction of fluid flow is actually left to right in this type of pump
@@Maxumized the lines/patterns you can see at the right side where the gears mesh together? That's not a shockwave. It's just the boundary between fast and slower moving liquid. On the left side, where the cavitation bubbles collapse, you can see quick flashes from dark to bright. Those are shockwaves. You can't really see the shockwave front propagate though, the video would have to be filmed at hundreds or thousands times higher framerate for you to be able to see that.
oookay... i did not understand the left-to-right-thing yet, but i did a fast research so i think you are talking about this mechanism, right? ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-TSnrjYH3ghE.html
Well this is the coolest video I'll probably watch all day. Who knew this would be happening within the pump. As someone who is now machining gears out of my home hobby shop, I never would have thought of this. Great video and thanks for sharing.
Получается, что износ внутренней стороны зуба, которая не прилагает усилие на ведомую шестерню, происходит не в результате загрязнения, а в результате длительной кавитации! Офигенно, никогда об этом не задумывался.
Еще, если присмотреться внимательно, то видно, что очаги кавитации присутствуют именно на технологических неровностях самой шестерни. Чем выше культура производства детали, тем меньше износ получается.
Cavitation causes more than annoying noise, it can actually wear the metal surprisingly fast. Ship propellers wear away due to cavitation, more so before they were engineered to minimize it.
That's what I'm thinking... and fluids are incompressible, so I'm guessing the sides of this are open to allow for the fluid displacement during the mesh? And the cavitation still happens because the low pressure is created before the fluid can fill back in fast enough? @TransAmDrifter, a gas would act as a cushion, but liquids would need a place to go as they don't compress
a possible improvement to minimize root damage will be a smoother transition in the valley so the pressure relief wave front will not curl around the root.
Now THAT is somming every mech engr needs to see. The talk and prevention always deals with centrifugal, but here is an effect that extends the 'textbook' to show how the theory is applicable IF you actually understand that theory. Imagine a test question or bonus points for such a condition!
It's worth noting that the fluid pumping occurs out of frame on the exterior part of the gear. The meshing area is not intended to move fluid but is instead powering the other shaft and preventing backflow from high(right side of frame) to low (left side).
Quite unusual to have a pump like this. The normal ones are Impeller, centrifugal or medical dosage pumps. The throughput of Liters/minute of one of these is very small I imagine, plus very noticable cavitation implosions in the video at the leading edge of each cogtip..😮😮
We need small troughs hollowed out in the pump housing, through which the pumped liquid can flow freely into the spaces between the teeth where low pressure is generated. This will not reduce the efficiency or tightness of the pump, and will prevent cavitation. Perhaps the energy losses caused by it will also be reduced.
And this is how the pump wear away, there just need for be a few particles as hard or harder than the gears for the cavitation to smash those particles on the gear and start digging in it.
This is NOT all gear pumps. Because of the low teeth number count, the tooth foot has been enlarged so the opposite tooth head can fit, but in case of a gear pump it should be enlarged more to allow oil flow as soon as the next toth is in contact. You can see when the cavitation starts the next tooth is already in contact. Teeth correction reduce pump global flow a bit, but reduce oil squeeze (friction so power need, cavitation so ) by a lot.
This actually makes more sense why we need oil on our gears and between moving parts. I knew this happened, but didn't understand it the way it was shown. Thanks for enlightening me today.
We mainly need the oil to reduce friction and to carry away the heat generated by any remaining friction (there will always be some), along with the additional functions of corrosion protection (rate of metal corrosion is greatly reduced if the metal is protected from contact with atmospheric oxygen) and detergent action (cleaning away any deposits/buildups caused by a wide range of contaminants). Cavitation is an undesirable by-product of having oil in between the gears, the shockwaves that are generated when each cavitation vortex collapses actually eat away at the gears little by little. And as @dalem04 says, it also degrades the oil and makes it less stable/effective over time. This is why regular oil changes are the best thing you can do for anything that uses a gear-driven pump for any purpose (internal combustion engines, hydraulic pumps, etc.)
this is why we need oil? did you really just say that? this is BECAUSE of the oil. and it's actually doing damage to the gear. it's just many many times better then friction without a libricant. but this problem of cavitation is 100% not existant if the gears are not submerged.
Oil Pumps are usually Straight Teeth - the flow doesn't go between them, and straight teeth have a wider spacing between each tooth on a single gear, to carry that oil around the outside of them, between them and the housing. Helical gears would push sideways in to the housing, meaning they would need flat roller bearings, where the straight cut gets no sideways thrust, so the side of the gear can float on a thin layer of oil between the gear and the housing sides.
Man… I wonder how dope the first mechanics of Alexandria who thought of gears felt. Must have been like climbing 28 mountains at once. What a fascinating process that must have been.
I guess during normal operation you don't want that to happen as it will as I know quickly erode the gears and the pump casing and hydraulic pumps are in general very reliable what I know from all hydraulic machines I use at my work that just run and run. Does this happen during normal operation of gear/hydraulic pumps? I think of how devastating cavitation is to centrifugal pumps.
Notice how the spiraling of the collapsing vacuum alternates each and every time it's rotational direction. Goes back and forth spinning in opposite directions each time as it follows the new vacuum cavitation in expanse collapsing the old cavitation and it's newly becoming vacuum.
its more like an implosion. its not air bubbles. its technically space bubbles. its what happens when there is not enough fluid to fill the entire void and so it strings out. looks like air space in there, but its total vacuum. space space lol
I get it! Because the top gear is the one which is pushing the bottom one, the contact point is always on the right side so the water can move through the tiny split on the left
За этим можно очень долго наблюдать) Кавитация "пролезла" даже в зубчатую передачу) Обратите внимание - масло, хоть и почти несжимаемая жидкость, но вролне способная расширяться! С образованием пузырьков растворённых газов - каверн.
This why water pumps in engines fail, the cavitation destroys the gears that pump the coolant and eventually they fail. The mess they are in when they are removed you’d think they would use a harder metal but nope they are designed to fail eventually.
yes - I think many gear pumps do this. They have a cutout in the side wall so fluid can get into the teeth just after they've sealed but before they cavitate
Yeah usually something like having the sealing face taper to a point or a ramp after the teeth have come into mesh. Basically it just creates an open cavity on the low pressure side to relieve the pressure differential as they come back out of mesh from my recollection.
So, this is a design flaw, right ? It should be designed in such a way that cavity volume remains constant. Without cavitation, this pump would lock, and the energy to create it is mostly lost.
Actually, as I understand, compression doesn't play a role here. The vacuum bubble is created when the liquid is not able to fill the expanding volume fast enough. Then the liquid comes in at a very high speed (the bubble collapses) and material erosion takes place.
@raulkaap you're right that compression isn't neccasary in cavitation, however in this case there is compression, note the volume before and after the gear teeth mesh, this is why they can output rather high pressures (in the thousands of psi). The op was referring to the operation of the gear pump, not the cavitation event.
If you look really really closely (bottom left; ignoring everything else), you can see compression and even a pressure squirt from the slit-like space between the gear teeth on the left when the bottom gear's tooth reaches its greatest height. No surface is perfect. There is a vacuum that is created and the oil on the left quickly rushes in to fill that void and makes that swirl.
This is beautiful. I wonder how to solve this issue? It seems that the easiest solution would be to just move the gears 1 or 2 thousandths apart. I guess you're losing pump efficiency but at least you're not destroying your gears
The problem here arises from the fact that completely sealed "cells" of low pressure are created between the teeth - if the profile of the tooth was changed to allow for an opening to equate the pressures that should prevent the cavitation from happening or at least diminish its strength.
At the same time, the gear rotor pump is unbeatable in simplicity. Lots of better pumps exist, but they're all more complicated and prone to failure than this little guy
The nucleus of the cavitation will sometimes just keep being dragged into the gear action. If there's abrasives in the fluid it could lead to unusual patterns of gear abrasion in rare cases, might be super confusing to have happen to you.
Beautifull! But why does this effect begins earlier in the gap between 2 upper teeth and later in the gap between 2 lower teeth? 🤔 I though it would be the same , bit I clearly see a difference.
What frame rate? Visually, it's a great bit of footage. Do you think many folk watching are aware that the oil flow through the pump goes around the outside of the gears, between them and the housing?
it took me a bit to understand your comment, so flow goes from left to right, and as you say, between the gears and the housing and not between each of the gears / where the camera is focused, is this right?
