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I finally spent some time to really understand how the IHMT/I-HMT transmission on the YT3 series(YT359/YT347) Yanmar tractors works and now I understand more clearly where the benefits come from and even why it makes hydrostatic noise at a stand-still. Hopefully I can shed some light on this for others as well. There is a good video on YouTube that shows the internal workings/cut-away of the transmission, with a factory rep explaining. I had to watch it a few times before it sunk in. Here, I'll borrow a picture from this video and another on the basic hydrostatic transmission workings to illustrate. See links for them at the end. Apparently this transmission is used in some military vehicles. Due to its efficiency in the working range, I'm predicting it could become the transmission of choice in the higher end hydrostatic tractor market. It's a pretty ingenious design.
I admit I had some reservations about going with it, really due to the newness, but I went with it due to the Yanmar name and quality and it was explained to me that the transmission can be much more easily pulled and serviced if required, through the bolted plate in the rear housing. Also, I wanted the speed control of a hydrostatic transmission, but didn't like the fact that they substantially heat up when pulling heavy loads. I've previously experienced this when pulling a harrow or plow for a couple of acres, albeit with a smaller unit. With the YT359, I have not noticed the transmission heat up really, but it is not an apples-to-apples comparison. I'll leave that to someone who has two comparable size units to try. Based on my understanding of the transmission though, the IHMT should remain cooler. No issues so far, but I'm only at 230 hours. Much of that has been reasonably heavy loader work.
Let's start with a bit about a standard hydrostatic transmission. Take a look at this diagram taken from Mekanizmalar's video on YouTube. This shows a single wheel load and an electric motor, but it illustrates the concept well. You might want to open this and the second picture in separate windows to refer to while reading.
This transmission uses an axial piston pump, to the left side with the red handle on its swash plate and an axial piston motor to the right with a fixed plate, powering the wheel to the far right. The connection between the two is purely hydraulic. Instead of an electric motor shown to the far left, the power source would be the compact tractor's diesel engine. This would power the axial piston pump, which sends pressurized fluid along the red line to the axial piston motor, powering the load.
With the swash plate tilted as shown and when the diesel engine turns the pump, each of the pistons rotates around the central shaft, compressing and elongating (due to the swash-plate angle), drawing in fluid from the return side (blue) and pumping it out on the high-pressure side (red). The more the swash-plate is tilted using the red ball handle, the faster the pump moves fluid and the faster the wheel turns. The swash-plate gives infinite speed adjustability. If the swash-plate is tilted perfectly vertically, no compression or elongation happens and no pumping occurs. Also, the tractor can't move, as the hydraulic motor is hydrostatically locked. If the swash plate is tilted in the opposite direction, the pump pumps backwards and drives the wheel in reverse.
The highest efficiency of a standard hydrostatic drive is when it is pumping very little fluid, as low hydraulic flow-rates produce low friction energy loss. This corresponds to the tractor moving, but at a virtual stand-still. The faster you go, the more the transmission pumps. Pumping fluid generates heat, wasted heat, which only makes things worse because you have to get rid of that heat. It also lessens the life of the fluid. Ideally, you want to pump as little as possible.
Now suppose we could get the zero-pumping state of the transmission to correspond with some non-zero speed? If we set this speed at the typical working speed of the tractor for each gear, then when working at this speed we have optimal efficiency (no pumping). This appears to be what Yanmar aimed for with the IHMT. Also, when working slightly away from this optimal speed, we still would minimize the amount of pumping, because only the amount of pumping needed to create the speed offset would be required.
With the workings of a normal HST transmission in mind, it will be easier to explain how the IHMT works. Here is a screenshot from the IHMT YouTube video; Daniel is the guy's name. This is the actual unit used in the YT359/YT347.
We can first make correspondences to similar pieces of the normal hydrostatic transmission. The hydraulic motor output of the transmission, connected to the wheels would be where Daniel's left hand is (our right, yellow arrow). There's a lighter blue flange hidden by his hand you can see elsewhere in the video that looks like it bolts to a gear. That gear would be connected to the wheels through the three-speed range gearbox. The white arrow shows the fixed plate of the motor side. This is a complement to the light blue pump side variable swash plate that is shown by the red arrow to the left. The silver round body he is pointing to is connected directly to the diesel motor output. It actually houses the pistons for both the pump side (light blue arrow) and the motor side (light green arrow). The pump and one side of the motor always spin together at the same rate as the diesel motor.
Now suppose you set the swash plate to vertical. No fluid would be pumped, so the motor and pump would be hydrostatically locked together, just like in the case with the standard hydrostatic transmission at zero speed. However, here the tractor is moving, as the output is spinning exactly at the motor's speed. With no pumping, the efficiency should be essentially the same as a gear tractor. Changing the swash plate angle from vertical in either direction will start the pump pumping to either add or subtract from the ideal "no-pumping" speed. I assume that Yanmar designed the system so that this no-pumping speed is in the middle of the speed range for that gear, but I'm not sure if there is a good reason to bias that toward the slow or fast side of the range. Yanmar, if you are reading, you might want to consider a dash light indicating hydrostatic lock or some kind of efficiency metric that could be aimed for while working. It would allow us to maximize fuel efficiency.
OK, so we've covered the case where we are moving......now what is happening with the transmission when the engine is running and you are stopped? Well, certainly the unit can be set with the engine disconnected using the clutch, but that's not the interesting case. Let's suppose you are just about to move the tractor. You would be stopped with the transmission in gear, the forward lever engaged, and the brake off. How does the transmission produce this zero speed at that moment? The system uses what is called "Powered Zero," similar to a CVT transmission. In "Powered Zero", the swash plate is set such that the motor side of the transmission operates in reverse to exactly counteract the rotational speed of the diesel motor, keeping the pump output (blue ring under Daniel's hand) & tractor stationary. Then, as you increase the accelerator pedal, this causes a change to the swash plate angle to reduce pumping, which then counteracts the motor rotation less, causing the tractor to move forward. As you continue to depress the accelerator to increase the speed of the tractor, you are further changing the swash plate angle which eventually brings you to the hydrostatically locked, maximum-efficiency speed. Further throttle increases beyond this maximum-efficiency cause the pump to start pumping in the forward direction, adding to the speed of the engine shaft, all the way to the top of that gear's range.
In "Powered Zero," the transmission is pumping fluid, so this is not an ideal place to be operating. But, no one really will have the tractor sitting for long periods of time in "Powered Zero," it is transitory and also, the pumping is not with much resistance when stationary. I suppose you could force a non-ideal case by running in a high gear with a really slow forward speed, pulling a heavy load. Most folk realize that you should be matching the gear range to the work you are doing. Net is, to maximize efficiency under load, pick the right gear and aim for the mid-range of the gear, thus minimizing fluid pumping under load.
"Powered Zero" also explains why there is hydrostatic noise when the tractor is running but not moving. The tractor is pumping fluid to stay stationary. This noise seems strange at first, because one gets used to things making more noise when working harder and something making noise when stationary seems out of place. Here it makes sense and interestingly, the hydrostatic noise will decrease as the tractor speed increases and pumping decreases toward the working range.
I should mention another thing regarding the transmission noise at a stand-still. There is an option that the dealer can program to change the behavior a bit to reduce this. The tractors seem to be shipped with the option that yields transmission noise when the tractor is sitting there idling out of gear with the parking brake on. It seems the default setting is consistent with being in "Powered Zero". I complained about the noise when test driving it. The dealer modified the program setting and after, it enters "Powered Zero" only when preparing the tractor to move by releasing the brake and setting the reverser handle to forward or reverse.
I hope this has been helpful. The links to the two videos are below.
Standard hydrostatic design - Hydrostatic Transmission - YouTube
IHMT explanation - Yanmar Showcases YT3 Tractor - YouTube
I admit I had some reservations about going with it, really due to the newness, but I went with it due to the Yanmar name and quality and it was explained to me that the transmission can be much more easily pulled and serviced if required, through the bolted plate in the rear housing. Also, I wanted the speed control of a hydrostatic transmission, but didn't like the fact that they substantially heat up when pulling heavy loads. I've previously experienced this when pulling a harrow or plow for a couple of acres, albeit with a smaller unit. With the YT359, I have not noticed the transmission heat up really, but it is not an apples-to-apples comparison. I'll leave that to someone who has two comparable size units to try. Based on my understanding of the transmission though, the IHMT should remain cooler. No issues so far, but I'm only at 230 hours. Much of that has been reasonably heavy loader work.
Let's start with a bit about a standard hydrostatic transmission. Take a look at this diagram taken from Mekanizmalar's video on YouTube. This shows a single wheel load and an electric motor, but it illustrates the concept well. You might want to open this and the second picture in separate windows to refer to while reading.
This transmission uses an axial piston pump, to the left side with the red handle on its swash plate and an axial piston motor to the right with a fixed plate, powering the wheel to the far right. The connection between the two is purely hydraulic. Instead of an electric motor shown to the far left, the power source would be the compact tractor's diesel engine. This would power the axial piston pump, which sends pressurized fluid along the red line to the axial piston motor, powering the load.
With the swash plate tilted as shown and when the diesel engine turns the pump, each of the pistons rotates around the central shaft, compressing and elongating (due to the swash-plate angle), drawing in fluid from the return side (blue) and pumping it out on the high-pressure side (red). The more the swash-plate is tilted using the red ball handle, the faster the pump moves fluid and the faster the wheel turns. The swash-plate gives infinite speed adjustability. If the swash-plate is tilted perfectly vertically, no compression or elongation happens and no pumping occurs. Also, the tractor can't move, as the hydraulic motor is hydrostatically locked. If the swash plate is tilted in the opposite direction, the pump pumps backwards and drives the wheel in reverse.
The highest efficiency of a standard hydrostatic drive is when it is pumping very little fluid, as low hydraulic flow-rates produce low friction energy loss. This corresponds to the tractor moving, but at a virtual stand-still. The faster you go, the more the transmission pumps. Pumping fluid generates heat, wasted heat, which only makes things worse because you have to get rid of that heat. It also lessens the life of the fluid. Ideally, you want to pump as little as possible.
Now suppose we could get the zero-pumping state of the transmission to correspond with some non-zero speed? If we set this speed at the typical working speed of the tractor for each gear, then when working at this speed we have optimal efficiency (no pumping). This appears to be what Yanmar aimed for with the IHMT. Also, when working slightly away from this optimal speed, we still would minimize the amount of pumping, because only the amount of pumping needed to create the speed offset would be required.
With the workings of a normal HST transmission in mind, it will be easier to explain how the IHMT works. Here is a screenshot from the IHMT YouTube video; Daniel is the guy's name. This is the actual unit used in the YT359/YT347.
We can first make correspondences to similar pieces of the normal hydrostatic transmission. The hydraulic motor output of the transmission, connected to the wheels would be where Daniel's left hand is (our right, yellow arrow). There's a lighter blue flange hidden by his hand you can see elsewhere in the video that looks like it bolts to a gear. That gear would be connected to the wheels through the three-speed range gearbox. The white arrow shows the fixed plate of the motor side. This is a complement to the light blue pump side variable swash plate that is shown by the red arrow to the left. The silver round body he is pointing to is connected directly to the diesel motor output. It actually houses the pistons for both the pump side (light blue arrow) and the motor side (light green arrow). The pump and one side of the motor always spin together at the same rate as the diesel motor.
Now suppose you set the swash plate to vertical. No fluid would be pumped, so the motor and pump would be hydrostatically locked together, just like in the case with the standard hydrostatic transmission at zero speed. However, here the tractor is moving, as the output is spinning exactly at the motor's speed. With no pumping, the efficiency should be essentially the same as a gear tractor. Changing the swash plate angle from vertical in either direction will start the pump pumping to either add or subtract from the ideal "no-pumping" speed. I assume that Yanmar designed the system so that this no-pumping speed is in the middle of the speed range for that gear, but I'm not sure if there is a good reason to bias that toward the slow or fast side of the range. Yanmar, if you are reading, you might want to consider a dash light indicating hydrostatic lock or some kind of efficiency metric that could be aimed for while working. It would allow us to maximize fuel efficiency.
OK, so we've covered the case where we are moving......now what is happening with the transmission when the engine is running and you are stopped? Well, certainly the unit can be set with the engine disconnected using the clutch, but that's not the interesting case. Let's suppose you are just about to move the tractor. You would be stopped with the transmission in gear, the forward lever engaged, and the brake off. How does the transmission produce this zero speed at that moment? The system uses what is called "Powered Zero," similar to a CVT transmission. In "Powered Zero", the swash plate is set such that the motor side of the transmission operates in reverse to exactly counteract the rotational speed of the diesel motor, keeping the pump output (blue ring under Daniel's hand) & tractor stationary. Then, as you increase the accelerator pedal, this causes a change to the swash plate angle to reduce pumping, which then counteracts the motor rotation less, causing the tractor to move forward. As you continue to depress the accelerator to increase the speed of the tractor, you are further changing the swash plate angle which eventually brings you to the hydrostatically locked, maximum-efficiency speed. Further throttle increases beyond this maximum-efficiency cause the pump to start pumping in the forward direction, adding to the speed of the engine shaft, all the way to the top of that gear's range.
In "Powered Zero," the transmission is pumping fluid, so this is not an ideal place to be operating. But, no one really will have the tractor sitting for long periods of time in "Powered Zero," it is transitory and also, the pumping is not with much resistance when stationary. I suppose you could force a non-ideal case by running in a high gear with a really slow forward speed, pulling a heavy load. Most folk realize that you should be matching the gear range to the work you are doing. Net is, to maximize efficiency under load, pick the right gear and aim for the mid-range of the gear, thus minimizing fluid pumping under load.
"Powered Zero" also explains why there is hydrostatic noise when the tractor is running but not moving. The tractor is pumping fluid to stay stationary. This noise seems strange at first, because one gets used to things making more noise when working harder and something making noise when stationary seems out of place. Here it makes sense and interestingly, the hydrostatic noise will decrease as the tractor speed increases and pumping decreases toward the working range.
I should mention another thing regarding the transmission noise at a stand-still. There is an option that the dealer can program to change the behavior a bit to reduce this. The tractors seem to be shipped with the option that yields transmission noise when the tractor is sitting there idling out of gear with the parking brake on. It seems the default setting is consistent with being in "Powered Zero". I complained about the noise when test driving it. The dealer modified the program setting and after, it enters "Powered Zero" only when preparing the tractor to move by releasing the brake and setting the reverser handle to forward or reverse.
I hope this has been helpful. The links to the two videos are below.
Standard hydrostatic design - Hydrostatic Transmission - YouTube
IHMT explanation - Yanmar Showcases YT3 Tractor - YouTube
