Lets go over some basics of engine, Ordinary hydrostat (there are hydro's that are NOT ordinary) range shifter, and final drive gears.
Think of the engine as more like a stationary engine that runs at what ever rpm you set it at and a governor assembly that tries to keep it at that rpm as the load increases. This is unlike an automotive engine that you are constantly changing the rpm and torque output.
The ordinary HST is a variable displacement pump, that is fastened directly to the engine output shaft of the engine, sometimes with a dry clutch in some models, and No clutch in many models. Attached to the variable displacement pump via oil passages or in a few machines by oil hoses is a fixed displacement hydraulic motor. Yes there is a few with variable displacement, but they are NOT the ordinary HST's I am talking about here.
The variable displacement hydraulic pump is controlled either directly by the "go pedals/pedal" or thru a hydraulic servo in some models. But many in this size class have just direct connection to the inside of the pumps "swashplate" The swash plate is on a pivot that the pedal controls the angle of the pivot action of the swashplate. (those of you with helicopter experience are now saying "swashplate", I know what that is!". riding on that swash plate are usually 9 "slippers" that are fastened to pistons. The angle of the swash plate determines the flow rate of the fluid out of the motor thru the passages and on to the hydraulic motor. As the pistons and slippers rotate around the "suck" fluid from the return and push it out the forward ports as they come around to them. Unless the swash plate angle is zero. Then no fluid flows or if the swash plate move the other direction the fluid moves the other direction to the ports and the tractor goes in reverse. So Zero angle = neutral, no movement, forward angle = flow to forward ports and hydraulic motor moves forward and backward angle the flow goes the other way and tractor goes the other direction. The tractors (like mine) with with hydraulic assist do the same thing only it give you some hydraulic assist to move the swash plate so you foot doesn't have to do all the work of applying pressure to it all the time. and of course "cruise control" figures in here also if your tractor has that feature. Some do some don't.
So let's go over what we have so far. Engine, is the prime mover, usually run at a fixed RPM, and maybe a clutch or NOT, and a variable displacement pump under control of the operator by the pedal system controlling the swash plate angle, followed by a powerful hydraulic motor able to go either direction depending on the ports the oil flows into from the pump. There is NO mechanical connection between the engine and the pump. Just a hydraulic one. Now. on to the Range transmission.
The range transmission is either a two or 3 gear ranges ordinary old non synchronized transmission. Nothing fancy here. Just 1890's technology here. The power comes out of the range transmission on to the final reduction drives and power is tapped off here for the front differential propeller shaft for 4WD with a shifter of course to engage this, either direct mechanical or electric over hydraulic shifter.
So here is what happens when you "attack" a hill in let's say HI range on your range shifter and your tractor has a big load in its loader bucket. The power needed to get up the hill is great, the puny engines RPM is starting to fall because of this load. Your first impulse of what you have done all of your life is "stomp on that accelerator pedal to make more power . Only it just makes things worse. your tractor bogs down even more. Here is why. When you increase the angle of the swash plate, you are making MORE oil flow from the pump headed to the motor, which puts even MORE load on the prime mover, the engine. You are calling for more speed, but speed is not what you need, you need more torque. If you cannot get any more torque from the engine by the fixed throttle and the governor has already tried to maintain the RPM level and can do no more then the RPM's are going to fall, the engine is going to fall out of its "torque curve" sweet spot, and you are not going to go up that hill any faster, or maybe not at all. What could happen is that the pressure builds in the pump to the point even that the pumps relief valve opens or starts to. We are talking about pressures around 5000 lbs per square inch here or more.
BUT if we let off of the swash plate, we reduce the flow rate and we slow down, we lower the pressure but we also give the engine a little breather and remove some of its load and possibly let the RPM's stabilize or even go up a bit and get closer to the torque/RPM curve that the engine wants to operate in. NOW to be honest, you should not have been a darn fool and attacked that hill in HI range anyway with it taller gearing. You should have looked at that hill at the bottom and said "I am heavily loaded, and I better downshift the range selector to M or even LO if it is really really steep.". But you got to make that determination "before" you attack. You cannot "shift on the fly, and to swap those gears on a hill is a dangerous proposition. You better be riding those brakes, and you better be in 4WD as you come to a stop, you will have to mesh that old coffee grinder transmission gears while trying to keep the tractor from rolling backwards downhill. Use caution.
Yes, for those of you with some advanced hydrostats, are chomping at the bit about your 2 speed variable displacement hydraulic motors. Keep chomping because this "book" is about ordinary hydro's,Not advanced hydro's with really cool features. You have to pay actual good money for those "bells and whistles" that many think they don't need.
So you have to have some "smarts" before you are in trouble climbing that hill. But stomping the forward pedal is NOT the answer. Notice I said you had a puny engine. Yes it is very puny. Mine has around 35 horsepower and my tractor fully ballasted with loader attached is over 6000 lbs. without a load in the bucket. So think about that a bit. How many cars weight 6000 lbs. Not too many nowadays. How many cars have 35 hp or so engines? Not too many. Tractors are all about gear reduction to increase torque, and they don't need to go real fast, they have lots of torque to pull, and low speeds, but there is a limit if you gear it too high. Think ahead. We tend to "load up" out tractors with liquid filled tires or wheel weights, and lots of rear ballast not to mention the payload we put in the bucket or grapple, but the engines just aren't that big for the most part.
I hope this "book" has helped a little bit, if not, well, I was just watching TV over the laptop anyway.
Thanks for the detailed summary. I think I get it. I am curious about something. As I was demoing the tractor, I was going easy on the gas (forward hydro pedal) and he told me not to do that. Can you think of any reason I shouldn't go easy on the hydro pedal?
