How Does a Hydrostatic Transmission Work?

after that last video where we looked inside of a small transmission out of a lawnmower i got to thinking this is really fun but i wish i had a little bit more complicated transmission one with more gears inside one where i can actually see what's going on so my dad actually did have another lawnmower got a whole lot of lawn mowers laying around uh and so we decided to take that one apart but as we started uh dragging it into the shop we realized that this the the speed shift on it instead of being an individual gear selector it was a hydrostatic transmission well actually that's even better because i know about hydrostatic transmissions i've driven lots of lawn mowers with these kind of transmissions and probably you have too but i have no idea how they work never seen inside one and i've never really even researched the principle of what that even means so we decided to tear this thing apart and actually learn what's going on inside of it it was fascinating so we'll take you through that adventure and look at this thing step by step and look inside of it the only downside to it is that it needs to have all of the fluid inside of it in order to actually demonstrate how it works so we tear it open we drain all the fluid out of it so we won't be able to see the thing working but you'll get hopefully a really good idea of how the hydrostats hydrostatic transmission does what it does so check this one out so this is the view of the transmission itself here's the pulley that would be coming from the engine just a normal v-belt pulley so this is what we call the power input shaft underneath here is actually where the hydrostatic transmission part of it is and then back in here is the part that converts it back into the power for the drive the wheels now if i turn it here this part of it is the part that's actually attached to the lever on the side so as you're sitting on the seat you pull the handle and you can shift it into reverse by pulling it back or you can shift it into different speeds forward and the further you push the higher the speed is so it's called hydrostatic because instead of having a gas pedal that we have to press on or individual gears we simply move the stick to a position and let it sit there and then actually this little piece with the spring provides friction so that the handle doesn't vibrate to a faster or slower speed as we're driving so that's what this thing looks like on the outside next we'll take it apart and take a look at the gears inside of it and the whole mechanism that actually causes that thing to change speeds inside so here's the view of the side of the transmission this is the part that where the wheel would attach and so let's go ahead and take the top off of this thing we've removed all the bolts around the side and then remove the disc brake assembly here which i'll show you in a little bit and then this part right down here which we'll see inside was actually a piece that would um that would allow you to pull a lever on the back of the lawnmower so that you could push the lawnmower without having to fight back against all the hydrostatic of the engine inside and against the engine itself so we'll see how that little pin which was just attached to a lever would interact with the thing so let's take this top off now we remember that power came in through the pulley up here so it would turn and we can see it goes down into this assembly down in here this is actually the really interesting part of the whole thing here is a set of gears where power would come out of this shaft which is attached to to this good thing i'm not putting this thing back together again when i'm done this gear would fit inside this shaft down here and so the power input came from here the power output comes through this spur gear through this combination of gears here until it finally reached this set of gears and that was attached to the front and back to the to the two side axles now this part right here if we remember that plate this is the plate that was attached that hydrostatic arm so we could use this to change those speeds so we know that from this view we have power input the power output but then the power output just goes through the combination of gears until the axles and then here this key keyed means that it only has a certain position that it can fit is attached to that arm that allows us to change the speeds so as far as the hydrostatic part this is the part that i'm most interested in understanding i can see the gears they're pretty visual in fact this one kind of pushed out just a little bit but it's pretty easy to visualize that it's this part that i was really interested in so let's keep digging this thing apart until we get to this and see what's actually going on inside there all right my friends this right here is the actual hydrostatic transmission part of it so like we saw this is coming from the engine so this is the part where that pulley is actually being rotated at a more or less constant speed now your engine in that in that lawnmower might kind of speed up and slow down but for the most part this is staying at a constant speed this is that output shaft that goes to the wheel so we want this to be able to run at a variable speed so that's the whole idea of this kind of transmission is being able to get a constant to a variable speed and in this case this is the assembly that does that so right inside here is that splined shaft that goes down and if we spin that we can see that this piece right here this gray piece is spinning now unfortunately the hydrostatic transmission relies on the movement of fluid kind of basically the hydraulic system i don't have fluid in here this is completely drained so sitting on the table like this the hydrostatic is never going to work but we'll see if we can describe what it is and i'll put a link to some other videos that you can use to to maybe kind of visualize the process in motion a little bit better so if we take this shaft out so we take the shaft out and now if we reattach that plate to simulate this thing this hydrostatic part of it being shifted this is fixed in the back of that transmission housing so as you turn it this will rotate it just pivots a little bit it's really not that much this this angled cut right here this little slot is all the more that this can rotate both forward and reverse so there's not a whole lot of play in this but i can feel that there's some spring tension going on back there so let's see where those springs are attached to take this off and in here is this nice roller set because this thing this piece right here is where that spline shaft was attached to so this was rotating there has to be some sort of a ball bearing in here because this does not rotate around it only pivots a little bit this way so this is just spinning around like crazy underneath that so that's why we have these bearings now each one of these things is a little cup with a spring inside of it now there's five of these these little cups with springs they kind of act in this case a lot like the pistons inside of a car as the car has fuel and air mixture put into a cylinder and then when the spark plug ignites it causes a whole bunch of pressure inside that cylinder which pushes that piston down so we're creating pressure by increasing in that case the expansion of gas is creating that extra pressure and that pressure converts into force pushing the piston down well that's actually exactly what's happening here except we're doing it with a fluid instead of an air fuel mixture that's exploding we're just doing it with hydraulic fluid instead now if i take this very carefully because all those things will fall out if i'm not careful with it there's this very carefully cut slotted face in here now on each side there's a hole and that hole is going down to the bottom where it pulls fluid in from the surrounding and another one that pushes it out and actually goes down to here because this is on the top for the input power and there's another one just like it on the side for the output power because remember if i turn this and give you a view from the side remember this one as it turns is turning our output so this is the one that actually has to be doing the driving of our wheels this is the one that's being driven from the engine turn here relates to turn here or at least it should relate now each one of these is going to be is going to have to be pushed down so if we think about it like a car if we push down we're pushing out the fluid that's inside of it now the spring is going to try and push it back up so if it pushes back up it would create a suction that would pull in fluid but then if that fluid spins around to this side and gets pushed back out it'll be pushed out the other slot and sent off another direction so it's pulled in by the spring sucking the fluid up into the cup when it expands and then it's pushed back out when the cup is pushed down but how does it do that push down on one side and pull up with a spring on the other well that's the job of this thing rotating so if this is tilted we see that it tilts and it pushes down on the springs on this side but allows the springs on this side to relax so on this side it's filling up with fluid but as it spins and gets around to this side it's pushing out now as we change the angle more and less extreme we'll allow more fluid to flow in a second i'll go into the white board and show you how that how we can the this air the amount of push up and down relates to the fluid flow but if we can get more fluid to flow with every rotation more fluid means that it routes back through here over to this one and causes one to get pushed and another one to get to allow it to push the fluid back into the system so if we can accomplish that same thing by giving this a bit of an angle then we can use this as a basically a hydraulic generator or the pump to drive the fluid it's pushing it back around to here which actually acts as a hydraulic motor it sounds kind of weird but there is such a thing a hydraulic motor now if we change the um which of these is being pushed if it's this side being pushed down and this side being relaxed then the fluid is going to be pulled up this way and pushed out this way so we'll have a certain direction of fluid flow but if we rotate it the other way and allow this to be the push and this to be the relax we're going to push the fluid out this one and draw it up in the other way so the fluid is now being reversed directions which should mean that it would reverse the direction so when your hydrostatic lever when you push forward that relates to the angle of tilt one way but if we pull the stick back and go into reverse it's the angle of tilt the other way which is reversing the suction of this little pump and reversing the fluid flow direction so let me take this apart here and this apart here and we'll see that from the top the cuts and the holes in here which draw the fluid in are very similar to the ones here except one of these lines is connected here and the other two are open to outside through a filter so if we can draw the fluid in the line in between one way or the other way we can get that direction of spin to change so let's turn around and let's look inside of that transmission into that housing and see if we can see how the output shaft axle is being tilted it has that angle so that we still got that change in direction of the fluid so let's see inside of that and see if we can see that similarity so right back up in here in that depression back in there is where the output pump or the hydraulic motor in this case that's the piece that it's seated up against now it's kind of hard to see from the camera view but that is actually sitting at a permanent angle so on the input side we change the angle to change the flow rate on this one it's a fixed angle but we're going to change the speed to change the flow rate a really great operation now this you can see that this would fit in normal operation this is all back together this fits back in here and that plate in the back does not move it still has that bearing on it the bearing that looks like this because it still has to rotate this this piece is rotating that is fixed so there must be some sort of a bearing to relieve some of the friction there and that's what this does so this set of bearings and plate up against here right now it's vertical so it's uh pretty hard to lay something up against it but it's sitting at that permanent angle so let's go check it take a look at some of the math and see why that can change from a constant speed to a rotating speed just by taking advantage of the fluid dynamics so here's how that fluid rate the fluid flow rate is going to affect the input to the output side now the main thing about fluid flow is that it's kind of like speed a flow rate of fluid think about it kind of like speed but speed changing the speed is not the only thing that can change the amount of fluid flow now gallons per second is just one way of saying fluid flow rate there's quite a few of them but we'll just imagine it's gallons per second because most people know what a five gallon bucket is so they can easily relate that to a volume of fluid an amount of fluid now on the input side as we rotated that angle we were by changing the angle we changed how far up and down those little cups were able to move which meant that we're changing the amount the physical volume of fluid with each one of our rotation so the rotation stay the same speed but since we could grab more fluid with each rotation that meant that we had a higher fluid flow rate so if you write this as a fraction if the top of the fraction gets bigger the whole amount of fluid flow also gets bigger now over on the input the output side we had a constant angle so since that didn't change the amount of fluid flow rate had to be maintained you had a constant flow through the whole thing i couldn't back up anywhere so we had to maintain that somehow now since the gallons were fixed because that angle of that plate inside was fixed the only way to compensate from for that was to make the time for every one of its revolutions the amount of time it took for a cup to fill fill empty fill empty fill empty the amount of time went down so if the time that it takes goes down in fact that's speeding it up so we change the angle to change the increase the fluid amount and that increased the fluid flow rate but we also could change decrease the time per revolution and that would make the speed go up so on our input from the engine speed was the same but we changed the angle on the output side the angle stayed the same but the speed was able to change so that's how we could take advantage of fluid dynamics in order to make that hydrostatic transmission convert angle into speed pretty cool i sure hope you enjoy learning about this stuff just as much as i love learning about it it's mechanical electrical fluid all three different kinds of ways that we can get power to go from place to place all three of those fascinate me the hydraulics and mechanics mixed together in this one even without any electronics at all is just amazing so if you're really into this kind of stuff check out some of my other videos or even better why don't you go find some stuff that you're not using and just tear it apart for me it's been many many years that i've never known how this stuff works until i finally go in and just tear it apart and explore it have a great day go build something awesome take care

Channel: Peterson Brothers Automation

Views: 255,062

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Id: h3K_THDapTM

Length: 15min 21sec (921 seconds)

Published: Sat Sep 19 2020

Reddit Comments

[u/nicht_ernsthaft]

Neat.