Describe a hydraulic transmission, please, to this non-engineer?

Question

Describe a hydraulic transmission, please, to this non-engineer?

Answer 1

A hydraulic valve assembly, for use in a hydrostatic transmission, for controlling fluid transfer between a first, a second and a third line, wherein two of the lines define first and second pressure lines, within a closed-loop circuit. The valve assembly comprises: a valve body having ports in communication with the three lines; a spool bore; a valve spool, adapted for sealing reciprocation within the spool bore, having a first and second end portion, a connecting portion, and a first and second bypass o*****e within the valve spool; and dampers for centering the valve spool. This spool is movable from a neutral position occurring when the fluid pressure forces in the first and second pressure lines are substantially similar, to a first or a second position occurring when the fluid pressure force in the first pressure line is greater or less than that in the second pressure line, respectively. The bypass orifices are enabled in the neutral position, but are substantially disabled in the first and second positions. A hydraulic system utilizing the valve assembly and a method for increasing the width of the dead band of the hydrostatic transmission in a neutral mode of operation are also set forth.

Answer 2

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http://auto.howstuffworks.com/automatic-...

Answer 3

Since this question was posed in the rail category, here is a rail category answer.

The only locomotives I know of with hydraulic transmissions were those purchased and tested by Southern Pacific.  These were of a design by the German manufacturer Krause Maffei.

The prime mover was superior to any US designed diesel motor producers, but it was the hydraulic transmissions that did them in.  More to the point, it was the drive shafts.

They were very comfortable and well behaved locomotives, but, with the heavy tonnage and grades they were required to overcome, the drive shafts were not equal to the challenge, and broke on a regular basis, therefore failing to be useful.  As far as the hydro-mechanics were involved, the technology was the same as described above, only very heavy duty.

The last of these engines on the SP wound up as a "camera car", where it was outfitted with motion picture cameras for right of way photography from the engineer's view point for use in the company's simulator in Cerritos, California.  SP's was the first and hundreds, well into the thousands, actually, attended the program for promotion, which was basically a six month college course packed into a three week period.

I have an operating manual for this particular beast if you need more technical data and can be contacted via this site.

Answer 4

Take two fans (the motorized kind that spin, not the  hand held kind) and point them at each other. Leave one off. Turn the other one on. You will see that the one left off begins to spin, due to the air forced toward it by the one that is on. This is how power is transferred in a hydraulic transmission, only instead of air used as the working fluid, inside the transmission is a light oil.

A mechanical transmission uses a clutch to transmit power, instead of a hydraulic drive.