In a hydraulic motor, what aspect of the admitted fluid decides the torque output?

The torque output of a hydraulic motor is primarily determined by the pressure of the admitted fluid. When fluid enters the hydraulic motor at a certain pressure, it creates a force that acts on the motor's pistons or vanes, leading to rotational motion. The greater the pressure of the fluid, the higher the force exerted on these components, which in turn increases the torque produced by the motor.

While viscosity, temperature, and density of the fluid can influence various aspects of hydraulic system efficiency and performance, such as flow rate and resistance to motion, they do not directly dictate the torque output. Viscosity can affect how easily the fluid flows and can influence the efficiency of power transmission, but it is the pressure that fundamentally allows for the generation of torque in the motor itself. Thus, pressure is the key parameter that determines the amount of torque produced by a hydraulic motor when fluid enters it.