Hydraulic transmission: It uses hydraulic oil as working medium, converts mechanical energy of prime mover into pressure energy of hydraulic oil through power element (oil pump), then through control element, and with help of actuator (oil cylinder or oil motor), pressure energy is converted into mechanical energy to drive load to achieve linear or rotary motion, force and speed of actuator are adjusted by remote control of control element and adjustment of flow.
When outside world disturbs above system, output of actuator generally deviates from original set value, resulting in a certain error. Hydraulic control: Like hydraulic transmission, system also includes power elements, control elements and actuators, and power is also transmitted through oil.

 

Difference between the two is that hydraulic control has a feedback device. Function of feedback device is to feed back output of actuator (mechanical quantities such as displacement, speed, force, etc.) and compare it with input (which can be variable or constant). Deviation after comparison is used to control system so that output of actuator changes with change of input or remains constant.
It is a hydraulic transmission system that constitutes a closed loop, also called a hydraulic follow-up system or a hydraulic servo system.
Hydraulic transmission system uses on-off or logical control elements. For its control purpose, it is to maintain stability of set value or simply change direction, also called fixed value and sequence control elements.
Servo control element used in hydraulic control system has a feedback structure and is controlled by an electrical device. It has high control accuracy and response speed, controlled pressure and flow often change continuously. Output power can be enlarged.
Proportional control is a type of control between above two. Proportional control valve used is a new type of electric-hydraulic control element developed on the basis of on-off control elements and servo control elements. It combines some characteristics of above two types of elements and is used in situations where on-off control by manual adjustment cannot meet requirements, but does not require same strict pollution control requirements as servo valve for hydraulic systems.

Among four major types of transmission methods (mechanical, electrical, hydraulic and pneumatic), no power transmission is perfect, and hydraulic transmission has following extremely obvious advantages:
(1) From a structural point of view, its output power per unit weight and output power per unit size are the best among four types of transmission methods, and it has a large torque-to-inertia ratio. When transmitting same power, hydraulic transmission device has a small size, light weight, small inertia, compact structure and flexible layout.
(2) From perspective of working performance, speed, torque and power can be adjusted steplessly, action response is fast, direction can be changed and speed can be changed quickly, speed adjustment range is wide, and speed adjustment range can reach 100:1 to 2000:1; action is fast, control and adjustment are relatively simple, operation is relatively convenient and labor-saving, it is easy to cooperate with electrical control and connect with CPU (computer), which is easy to realize automation.
(3) From perspective of use and maintenance, components have good self-lubrication, easy to realize overload protection and pressure maintenance, safe and reliable; components are easy to realize serialization, standardization and generalization.
(4) All equipment using hydraulic technology has good safety and reliability.
(5) Economic: Hydraulic technology has strong plasticity and variability, which can increase flexibility of flexible production, make it easy to change and adjust production process. Manufacturing cost of hydraulic components is relatively low and adaptability is relatively strong.
(6) Hydraulics can be easily combined with new technologies such as microcomputer control to form "machine-electricity-hydraulic-optical" integration, which has become a trend of world development and is easy to realize digitalization.

 

Everything has two sides, and hydraulic transmission is no exception:
(1) Hydraulic transmission inevitably has leakage due to relative motion surface. At the same time, oil is not absolutely incompressible. In addition, due to elastic deformation of oil pipe, hydraulic transmission cannot obtain a strict transmission ratio, so it cannot be used in inline transmission chain of machine tools such as threaded gears.
(2) There are losses along flow of oil, local losses and leakage losses, and transmission efficiency is low, which is not suitable for long-distance transmission.
(3) Under high and low temperature conditions, it is difficult to use hydraulic transmission.
(4) In order to prevent oil leakage and to meet certain performance requirements, manufacturing precision of hydraulic components is high, which brings certain difficulties to use and maintenance.
(5) It is difficult to check fault, especially in units where hydraulic technology is not popular. This contradiction often hinders further promotion and application of hydraulic technology. Hydraulic equipment maintenance requires experience, and it takes a long time to train hydraulic technicians.