Well I might as well give it a stab, and it's a basic stab at that /forums/images/graemlins/grin.gif
Basically, an HST is a large volume hydraulic pump with a variable high pressure output that is used to spin a mechanical gear reduction transmission. Both the hydraulic and mechanical assemblys can be housed in the same unit (which is more common) or they can be in sparate housings but they have to be connected to one another and they are completely different assemblies that are totally dependant upon one another.
The engine drives the hydraulic pump and the faster the engine spins the the pump the higher the hydraulic output of the HST's pump.
Generally the output of the pump is controlled by a swash plate. There are different devices but it is the most common. Picture a butterfly valve in a tube. When it's closed it completely blocks the passageway and theres no output pressure. The more it is opened, the more of the passageway is opened up which inturn allows more volume of what ever is being transmitted through the pipe to pass through. In our case where talking about hydraulic fluid.
The swash plate of an HST works in a similar fashion but it works in a way to where the pressurized fluid can be redirected to allow for forward or reverse rotation of the driven gear transmission.
Imagine the steps/vanes in large water wheel and how the water falling from a trough above it collects in these steps/vanes which pushes down on the wheel which spins it.
The geared transmission of an HST has a similar step or vane device that is connected to the gears and when the pressureized fluid hits it, it will inturn spin the gears which inturn spins the axles etc.
The more the volume & pressure of fluid that hits these vanes, the faster it spins the gear tranny.
There is no gear reduction in the hydraulic assembly of an HST. It's job is to provide a high pressure fluid output that can be varied in both pressure and direction. Since theres no gear reduction, when the tractor encounters a hill or other obstacle and you press down harder to hopefully increase power output, what is actually happening is the preesure will start dropping due to the engines speed dropping and no matter how open the swash plate gets, the loss of pressure will slow the tractor down. The load on the engine and HST could eventually stall the engine out. Letting up on the HST in this case can actually increase the pressure enough (constricting the channel increases the pressure) to where the engine won't stall. That is why many say to let up on the HST pedal when the tractor starts to bog down.
To enable tractors with HST's to climb up inclines with heavy loads with little strain, the manufacturers devised multiple gear ranges in the mechanical gear reduction assembly of the HST. Some like my
BX23 only have a hi and lo range while larger tractors have 3 or more ranges. By selecting the proper gear range before one encounters an incline etc, the tractor will have more than enough available power to carry it's load above and beyond....... Some models alow the ranges to be changed while under load, most don't though....
The hydraulic section working in concert with the mechanical section is the happy medium of this whole assembly. /forums/images/graemlins/grin.gif
Since an engine on a tractor needs to maintain as constant an rpm as possible when operating certain implements, a way had to be devised to vary the speed of the tractor without varying the speed of the engine (like in an automobile). Mechanical gear reduction is the way this has traditionally been done, but the desire arose for a more constantly variable speed control, hense the HST. The tractor can go as fast or as slow as needed without having to adjust the engines rpm or change gears and also without effecting the implement it's driving.
Well my breaks over gotta get back at it,
Clear as mud? /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif
Volfandt
