Hydraulic power transmission — device for varying torque Which hydraulic unit is specifically used to transmit and vary (increase/decrease) torque to the driven shaft in fluid couplings and automotive drivetrains?

Introduction:
Many hydraulic devices sound similar but perform very different functions—pumping water uphill by water hammer, boosting pressure, storing energy, or transmitting torque. This question targets the component that manipulates torque between a prime mover and a driven shaft using fluid dynamics.

Given Data / Assumptions:

• Closed-loop hydraulic device using impeller, turbine, and stator.
• Operates with circulating working fluid (typically oil).
• Designed to multiply torque at low output speeds.

Concept / Approach:
A hydraulic torque converter is a hydrodynamic device with three main elements: pump (impeller) connected to the engine, turbine connected to the load, and a stator that redirects flow to provide reaction and torque multiplication at stall or low-speed conditions. As output speed approaches input speed, the torque ratio drops toward unity, functioning like a fluid coupling but with torque multiplication capability when needed.

Step-by-Step Solution:

Verification / Alternative check:
Automotive automatic transmissions historically use torque converters for launch and low-speed torque boost before lock-up clutches engage.

Why Other Options Are Wrong:

• Hydraulic ram: lifts water using water hammer; no torque transmission.
• Hydraulic intensifier: increases static pressure, not rotational torque transmission.
• Hydraulic accumulator: stores pressure energy.
• Hydraulic press: converts hydraulic pressure into linear force for forming.

Common Pitfalls:
Confusing torque converters with simple fluid couplings (which do not multiply torque) or with mechanical gear torque multipliers.

Final Answer: