SPYDERLK
Super Star Member
- Joined
- Feb 28, 2006
- Messages
- 10,160
- Location
- VA
- Tractor
- JD2010, Kubota3450,2550, Mahindra 7520 w FEL w Skid Steer QC w/Tilt Tatch, & BH, BX1500
With either given motor the lowering of torque with that motor as flow decreases is because flow has to supply both motor motion and leakage through the motor. If there were no leakage both of the motors would deliver their characteristic Torque -- ~1200 vs 1900ft-lb@ the 2500PSI relief setting on most tractors -- regardless of flow. The big motor would run slower of course because its larger displacement requires proportionately more flow for each revolution. In the ideal/no leakage situation the flow strictly causes the revolution, and the resistance to the revolution (load) causes the pressure. Thus the Torque you can apply is limited to tractor relief pressure acting on the motor displacement. If you were pumping grease yould see each motor run at constant speed with increasing load. Pressure would rise accordingly and the motor would suddenly stop when you reached your tractor relief pressure. The motor T would still be almost the same, but no motion. Just a bunch of grease flowing through a relief - the only place it can go since it cant leak past the motor element.
In our real world there is leakage. At any given fluid pressure across a motor (or pump) it is proportional to the fluid viscosity and the motor size. Even though leak paths are as small as feasible some flow is always wasted and this waste increases as the fluid thins out as the system warms up. The big motor is going to leak more. When fully heated there may be enough leakage past the motor elements to tax the self leaking and delivery capabilities of a fully warmed small tractor hyd pump. The pressure from the pump cannot rise to its full relief pressure while supplying both leak and displacement flow. Thus this "slip" effectively limits Torque below what you would achieve with a bigger delivery pump. --- But that potential for T is there. Just do your high T work right away while the fluid is cool and thick.
In our real world there is leakage. At any given fluid pressure across a motor (or pump) it is proportional to the fluid viscosity and the motor size. Even though leak paths are as small as feasible some flow is always wasted and this waste increases as the fluid thins out as the system warms up. The big motor is going to leak more. When fully heated there may be enough leakage past the motor elements to tax the self leaking and delivery capabilities of a fully warmed small tractor hyd pump. The pressure from the pump cannot rise to its full relief pressure while supplying both leak and displacement flow. Thus this "slip" effectively limits Torque below what you would achieve with a bigger delivery pump. --- But that potential for T is there. Just do your high T work right away while the fluid is cool and thick.