My L3901 seems WAY under-powered to me

[CH4Ohio]

It kind of works like that.

Think like this then.

On a geared transmission, you have usually 4 main gears and 3 ranges. Forget the range part of the transmission now. You're going on the hill in 4th and it starts to bog down, you downshift to 3rd, or 2nd if need and so on.

On an hydrostat transmission, you're going on the same hill, pedal smashed all the way down and it starts to bog down. You lift the pedal slightly till it stops bogging down. Pretty much like downshifting on the geared transmission.

In the scenario you're describing, I believe that the engine stops bogging down because when you back off the HST pedal, you're allowing more fluid to flow through the bypass circuit instead of forcing it to drive the wheels. In effect, you're increasing the "slip" between the engine and the wheels. The tractor will of course slow down -- it still might not be able to pull the hill -- that depends.

The more you back off that pedal, the slower you'll go, and the less the engine will bog, until you're finally not moving at all.

My point is, when you downshift to a lower gear on a standard transmission, you get more torque because of the mechanical advantage. When you decrease pressure on the HST pedal, thus decreasing the amount of fluid driving the wheels, you DO NOT get more torque. You might save the engine from bogging down by increasing the slippage, but you don't have more power to the wheels.

Make sense?

 

[RickB]

"When you decrease pressure on the HST pedal, thus decreasing the amount of fluid driving the wheels, you DO NOT get more torque."

Actually you do because you are decreasing the transmission drive ratio.

 

[ptsg]

In the scenario you're describing, I believe that the engine stops bogging down because when you back off the HST pedal, you're allowing more fluid to flow through the bypass circuit instead of forcing it to drive the wheels. In effect, you're increasing the "slip" between the engine and the wheels. The tractor will of course slow down -- it still might not be able to pull the hill -- that depends.

The more you back off that pedal, the slower you'll go, and the less the engine will bog, until you're finally not moving at all.

My point is, when you downshift to a lower gear on a standard transmission, you get more torque because of the mechanical advantage. When you decrease pressure on the HST pedal, thus decreasing the amount of fluid driving the wheels, you DO NOT get more torque. You might save the engine from bogging down by increasing the slippage, but you don't have more power to the wheels.

Make sense?

It makes sense to me.

Also why the geared tractor can simply drop down a gear or two on the road and on the move versus having to fully stop an HST tractor do change ranges in the middle of the road.

 

[CH4Ohio]

It makes sense to me.

Also why the geared tractor can simply drop down a gear or two on the road versus having to stop an HST tractor do change ranges in the middle of the road.

That's one thing that I really like about my L5740HST. It has a separate HI/LO selector on the steering column in addition to the standard RANGE selector. So for each HST range, you can just tap that lever up or down into HI/LO effectively giving you 6 different ranges. When I'm pulling a baler and loaded wagon up a hill and start to bog down, I can stay in HIGH range but drop in the LO setting on the fly.

I know I should look to see what this is actually called, but you guys probably know already. Anyway, I find that I use this feature a LOT when working in all the ranges. VERY convenient.

Should add -- I don't bale hay in HIGH range -- No way. Just when I'm driving back home from a neighbors and transporting equipment that I encounter a pretty steep hill. She will move free-wheeling loads down a level road in HIGH range. But when I encounter a slope (work), have to drop into a lower range. Actually usually use my geared tractor for baling.

 

[CH4Ohio]

"When you decrease pressure on the HST pedal, thus decreasing the amount of fluid driving the wheels, you DO NOT get more torque."

Actually you do because you are decreasing the transmission drive ratio.

With all due respect, I don't think so.

Drive ratio refers to gears usually. This doesn't work like an automatic transmission or CVT in your car that actually has gear ratios.

It's a hydraulic fluid pump that pumps constantly at a rate determined by RPM and it drives / turns a hydraulic motor. When you back off the pedal, you're letting more fluid pass through the bypass circuit and causing less fluid to be applied to the wheels. You can't do more work by applying less pressure. But you can take some load off the engine by applying less pressure because you're allowing for more "slip" between the engine and the drivetrain.

 

[RickB]

Drive ratio is shaft rpm in vs shaft rpm out. What is in the middle makes no difference. Until you understand how the pump displacement changes while the motor displacement remains constant your confusion will persist.
Your assumption that the hydrostatic pump operates at a fixed displacement is incorrect.

 

[ptsg]

With all due respect, I don't think so.

Drive ratio refers to gears usually. This doesn't work like an automatic transmission or CVT in your car that actually has gear ratios.

It's a hydraulic fluid pump that pumps constantly at a rate determined by RPM and it drives / turns a hydraulic motor. When you back off the pedal, you're letting more fluid pass through the bypass circuit and causing less fluid to be applied to the wheels. You can't do more work by applying less pressure. But you can take some load off the engine by applying less pressure because you're allowing for more "slip" between the engine and the drivetrain.

With a variable displacement pump, the fluid doesn't go through a bypass. It's simply not pumped at all with the swash plate at 90 degree angle with the input shaft. Once you start angling that swash plate, it will start pump accordingly.

 

[CH4Ohio]

Drive ratio is shaft rpm in vs shaft rpm out. What is in the middle makes no difference. Until you understand how the pump displacement changes while the motor displacement remains constant your confusion will persist.
Your assumption that the hydrostatic pump operates at a fixed displacement is incorrect.

I see what you mean! Looked at a diagram online. I thought it was controlling fluid flow using a bypass circuit. Wrong. Thanks for the education on that point.

The swashplate mechanism is ingenious. However, I still maintain that decreasing pedal pressure, and thus fluid flow, reduces power to the wheels. That is definitely the case with both of my HST tractors.

I have one tractor that has Stall-Guard and it will compensate by revving the engine up when needed. I usually don't operate with it turned on. But at a constant RPM, I definitely have less power at the wheels with less pedal pressure.

 

[SDT]

"When you decrease pressure on the HST pedal, thus decreasing the amount of fluid driving the wheels, you DO NOT get more torque."

Actually you do because you are decreasing the transmission drive ratio.

Bingo.

SDT

 

[SDT]

It kind of works like that.

Think like this then.

On a geared transmission, you have usually 4 main gears and 3 ranges. Forget the range part of the transmission now. You're going on the hill in 4th and it starts to bog down, you downshift to 3rd, or 2nd if need and so on.

On an hydrostat transmission, you're going on the same hill, pedal smashed all the way down and it starts to bog down. You lift the pedal slightly till it stops bogging down. Pretty much like downshifting on the geared transmission.

Agreed.

It actually does "work that way."

Lifting the HST pedal does, indeed, increase effective gear ratio (reduce speed).

SDT