Ran across some very interesting reading regarding hydrostatic transmissions and their designs. A lot of it is in engineer speak but the interesting parts, namely the different types, and the speed to torque ratio should be understandable by anyone who gets the basic principle.
Graph B on Fig. 3. though a little hard to see (hold control and push the + key to zoom, ctrl+0 will return to normal) is very interesting since that's the type most commonly used in tractor transmissions. Efficiency is in the toilet til you get above crawling speeds (note this isn't wheel rpm since the pumps drive the differential input shaft in most designs).
One thing I found interesting is that the pump and motor are very sensitive about their shaft speeds when it comes to power transmission efficiency. If you're idling with the HST pedal floored or crawling along you're using a lot of power just to generate heat. It's not a problem since the system has a cooling fan but interesting nonetheless.
Some of the articles at the bottom are pretty good too.
Graph B on Fig. 3. though a little hard to see (hold control and push the + key to zoom, ctrl+0 will return to normal) is very interesting since that's the type most commonly used in tractor transmissions. Efficiency is in the toilet til you get above crawling speeds (note this isn't wheel rpm since the pumps drive the differential input shaft in most designs).
One thing I found interesting is that the pump and motor are very sensitive about their shaft speeds when it comes to power transmission efficiency. If you're idling with the HST pedal floored or crawling along you're using a lot of power just to generate heat. It's not a problem since the system has a cooling fan but interesting nonetheless.
Some of the articles at the bottom are pretty good too.