The Essence of Hydraulic Transmission

Transmission, short for power transmission, refers to transferring mechanical power (motion and force) from the power provider to the power demander. If the power does not match (such as speed too fast, insufficient force), a transmission mechanism is needed for conversion.

Transferring motion and force means:

• Overcoming load force to make objects perform the required motion;
• Moving objects to make the acting force reach the required magnitude.

For example:

• Hydraulic lift: requires precise arrival at height, load force within allowable range is sufficient;
• Waste paper baler: requires applying predetermined force, compression thickness is secondary;
• Ceramic tile press: requires two compressions — first apply force to achieve density, then press to predetermined thickness (motion and force cannot simultaneously satisfy arbitrary requirements).

Hydraulic technology, short for “hydraulic transmission technology,” uses liquid pressure to transfer motion and force (hydrostatic); transfer using liquid kinetic energy is called hydrodynamic transmission.

Among other transmission technologies (mechanical, hydrodynamic, pneumatic), hydraulic is often given priority due to “powerful flexibility, small actuator mass and volume.”

In this text:

• Load: objects (equipment, machinery, workpieces) requiring hydraulic drive;
• Load force: force to be overcome in driving the load (including gravity, friction, deformation resistance, inertial force);
  
  • Positive load: load force opposite to motion direction, hydraulic excels at overcoming (no upper limit);
  • Negative load: load force same as motion direction (will move without hydraulic), hydraulic can regulate its speed.

Hydraulic overcomes load force and achieves motion through “adding oil” and “releasing oil.”

A general hydraulic system consists of hydraulic pump, hydraulic valve, hydraulic actuator, and auxiliary components:

• Hydraulic valve: indispensable component of hydraulic system;
• Hydraulic actuator:
  
  • Hydraulic cylinder: achieves translational motion (linear motion), limited stroke, can have no internal leakage;
  • Hydraulic motor: achieves rotational motion (rotary motion), unlimited stroke, has internal leakage;
  • (Note: “motor” in this book specifically refers to “hydraulic motor,” according to GB/T 17446—2012 standard).