First, the size of the friction force is not stable. Ordinary friction is along with the speed become larger and larger, but in the region of the static and low speed running motor internal frictional resistance, not only did not increase when working speed, decrease instead, becomes a drag. The hydraulic motor and load, on the other hand, is after mooring compressed hydraulic pressure driven by rising.
Second, leakage amount is not stable. Leakage of hydraulic motor is not the same, every moment with the rotor movement phase Angle change as a cyclical fluctuations. Because of time into the motor at low speed flow is small, leak possession is more big, the proportion of the leakage of instability will significantly interfere with the motor work flow numerical, so yes speed instability.