Pressures p (the physical concept is pressure) is the force per unit area, which is related to the size of the load, i.e.
p=F/A （2-1）
In the formula p–pressure (commonly known as pressure) (MPa);
F–Load force (commonly known as external force) (N);
A–cross-section area subjected to the load force (mm²).
In the absence of mechanical and fluid friction, fluid flow resistance (including both resistance and local resistance), and other factors, this formula (2-1) explains the physical meaning: the external load force \( F \) applied to the hydraulic actuator generates the hydraulic system fluid pressure. The hydraulic actuator then generates this fluid pressure on the fluid.
This indicates that the pressure of the hydraulic system originates from the external load. Therefore, according to equation (2-1), the maximum pressure that the system can withstand is determined by selecting the area A. External loads can be classified into two types. One type is a load force (F) that produces linear reciprocating motion. The other type is a torque (M) that generates rotary motion. When the external load is torque, the pressure \( p \) in the motor with a back pressure \( p_0 = 0 \) can be derived from formula (2-2). At this time, it can be considered that the pressure \( p \) is determined by the external load provided by the torque \( T \)., that is:
p=T/V （2-2）
In the formula p–Pressure (MPa);
T–load external torque (N – m);
V–hydraulic motor displacement (mL/r).
When the hydraulic external load force or torque acts in the same direction as the hydraulic actuator’s movement, we call this external load a “negative load.” Alternatively, we can also refer to it as an “over-running load.” Examples include the weight of a crane falling or the self-weight of an object moving in the same direction. In such conditions, the actuator will show a vacuum state in the oil inlet chamber over a certain period of time. Consequently, this can lead to the oil return chamber stalling, becoming uncontrollable, and potentially dangerous. Furthermore, even if the system design accounts for negative load conditions, the system will still experience energy loss during operation. As a result, this energy loss can lead to serious system heating and other issues.
For fluid drive system pressures, the following standard classes are available according to national standards:
Table 2-1 Fluid transmission system and components nominal pressures series (GB/T2346-2003)
(Unit: MPa)
| 1 | [1.25] | 1.6 | [2] | 2.5 | [3.15] | 4 | [5] | 6.3 | [8] | 10 | ||
| 12.5 | 16 | 20 | 25 | 31.5 | [35] | 40 | [45] | 50 | 63 | 80 | 100 | |
| 125 | 160 | 200 | 250 |
Note: The values in parentheses are non-preferred values.
The nominal pressure series in Table 2-1 is given after grading the various quantities with the priority coefficient \(q_r\), as the theoretical common ratio, specified in national standard GB321-1980. The theoretical common ratio \(q_r\), defined as \(\sqrt[r]{10} \) (r = 5, 10, 20, 40). Table 2-2 shows the pressure classification for hydraulic systems and components.
Table 2-2 Pressure Classification of Hydraulic Systems and Components
| Pressure category | underpressure | medium pressure | medium and high pressure | high pressure | ultrahigh pressure |
| Pressure value/MPa | 0~6.3 | 8.0~21 | 25~31.5 | 35~63 | >70 |
| Typical applications | machine tool | farm machinery | bulldozer | presses | Disaster relief equipment |
The above pressure classification concepts are based on the development of hydraulic technology. Currently, hydraulic pump manufacturers are developing from a nominal pressure of 31.5MPa to a class of 35MPa. Advanced manufacturers, especially large ones, have developed their products to the 42 to 48 MPa level. These products have appeared in their product catalogues. The highest pressure level of the products has reached 63MPa (produced by Bosch Rexroth), see Table 2-3.
Table 2-3 Pressures Application Values for Hydraulic Components
| Application Industries | machine tool | shipbuilding | press machine | construction machinery | Recent development | Anticipated development |
| Pressure value/MPa | 21 | 25 | 35~63 | 31.5~35 | 42~48 | 56~63 |
When applying the concept of fluid pressure to a product, we often specify the maximum pressure that the product (e.g., a hydraulic pump) can withstand. This pressure should guarantee that the product lasts through its design life. This applies whether the product is in continuous operation or briefly reaches a certain limit. This gives the following indications of pressure values:
(1) Maximum component operating pressure \(p_{max}\) The maximum permissible operating pressures of a hydraulic component during brief operation of the system.
(2) Component minimum working pressure \(p_{min}\) Refers to the system steady state operating conditions of the hydraulic components (pumps) allow the lowest working pressure.
(3) Element Peak Pressure \(p_{p}\) The highest pressure that the system allows a hydraulic element to reach in an instant.
(4) Nominal component pressure \(p_{n}\) Refers to the maximum allowable working pressure of the hydraulic components under steady state conditions of continuous operation of the system.
Various manufacturers use the pressure value indicators mentioned above for their product samples. In many cases, the maximum working pressure and nominal pressure are essentially the same. However, the difference mainly arises from the manufacturer’s emphasis, which helps to facilitate user selection.
It is important to emphasize here that the formation of pressure originates from the load. The pressure formation has nothing to do with the flow rate of the system or its formation. Pressure and flow are related only in the case of fluid flow, which is the Bernoulli equation.
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