I would like to get some opinions on using a low flow hydraulic MOTOR to drive a high flow pump ? Would the high flow pump be able to create a useable pressure to drive a piece of equipment that requires the greater flow ? Or will the system loss to friction , restriction and mechanical drag make it impossible to realize any useful gains ?
Great question with many potential applications.
The short answer to your question is YES. A hydraulic motor can absolutely be coupled to a pump in a useful way. You would effectively have made a hydraulic transformer. You are exchanging low flow @ high pressure for high flow @ low pressure, with some transmission loss.
What kind of efficiency to expect? Let's say for example that you couple a bent-axis piston motor @ 5000 psi with a vane pump @ 400 psi. Expect approximately 93% efficiency from the bent axis unit and 80% from the vane unit. So a net efficiency of 0.93 * 0.80 = 0.74 for the transformer. Therefore if you input 10 gpm flow to the bent-axis side, expect approx 92 gpm flow through the vane pump.
Note that there is another efficiency loss from the high-pressure pump which was supplying your bent-axis unit. Let's estimate around 18% of energy lost between that pump and whatever control valves might be involved. A more efficient design would use an electric motor to replace the bent-axis unit. However in many applications, it is not practical to install an electric motor for this purpose. Further, the high-pressure supply pump may continue to spin regardless of how much flow is actually required. Installing the additional electric motor would not provide much additional efficiency benefit in that case.
Regardless of the efficiency discussion, the entire topic has many many applications for "2-speed" devices requiring high flow @ low pressure, up to a certain distance. Then briefly requiring low flow @ high pressure, to complete the actuation. One classic example is the compacting cylinder used in recycling machines. As this cylinder extends, pressure slowly builds due to the elastic nature of the recycled material (eg. cardboard, plastic). Pressure increases rapidly towards the end of cylinder extension, as air pockets in the material are reduced. Pressure will peak at the end of the stroke. Finally, the cylinder will retract under negligible load.
Lastly here is an article I wrote going into much more detail on this topic for various different industrial and mobile hydraulic applications. https://www.fluidpowerworld.com/white-paper-design-concepts-for-power-limiting-open-circuit-architectures/