The pressure p (the physics concept is pressure) is the force per unit area and is related to the size of the load, i.e.
p=F/A
In the formula:
- p — Pressure (commonly referred to as stress) (MPa)
- F — Load force (commonly referred to as external force) (N)
- A — Cross-sectional area bearing the load (mm²)
In the absence of mechanical and fluid friction, fluid flow resistance, and other factors such as resistance and local resistance, the physical meaning of formula (2-1) becomes clearer. Specifically, hydraulic system fluid pressure results from the external load force F applied to the hydraulic actuator. Consequently, this pressure is generated through the hydraulic actuator. In other words, the hydraulic actuator produces this pressure.
This indicates that the hydraulic system’s pressure originates from the external load. Therefore, we determine the maximum load the system can withstand by selecting the area A according to equation (2-1). External loads can be classified into two types: one is the load force (F) that outputs linear reciprocating motion, and the other is the torque (M) that outputs rotary motion. When the external load is the torque, the pressure in the motor back pressure P₀ = 0 conditions can be derived from the formula (2-2), at this time it can be considered that the force pp by the external load provided by the torque T decision.
p=T/V
In the formula:
- p — Pressure (MPa)
- T — External load torque (N·m)
- V — Hydraulic motor displacement (mL/r)
The direction of action of the hydraulic external load force or torque and the hydraulic actuator’s movement direction are the same. We call this external load a ‘negative load’ or ‘beyond the load (over-running)’ (such as the crane’s weight falling, the object’s self-weight, and the movement in the same direction). In this condition, the actuator, for a certain period, will show a vacuum state in the oil inlet chamber, and the oil return chamber is prone to stall, making it uncontrollable and dangerous. Even if the design accounts for negative load conditions, the system will still experience energy loss during operation. This loss can result in serious system heating and other issues.
For fluid drive system forces, the following standard classes are available according to national standards:
| Table 2-1 Nominal Pressure Series of Fluid Transmission Systems and Components (GB/T 2346-2003) | ||||||||||||
| (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 are given by taking the priority coefficient qᵣ specified in the national standard GB 321-1980 as the theoretical common ratio and grading the various quantities. The theoretical nominal ratio qᵣ is defined as ʳ√10 (r = 5, 10, 20, 40). Table 2-2 shows the classification of forces for hydraulic systems and components.
| Table 2-2 Hydraulic System and Component Pressure Classification | |||||
| Pressure category | low pressure | 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 | farm machinery | bulldozers | presses | Disaster relief equipment |
The above classification concepts are based on the development of hydraulic technology. Currently, hydraulic pump manufacturers are moving from a nominal pressure of 31.5 MPa to 35 MPa. The advanced manufacturers, especially the large ones, have developed their products to 42-48 MPa. These products are already present in their catalogues. The highest level has reached 63 MPa, see Table 2-3.
| Table 2-3 Pressure Application Values for Hydraulic Components | ||||||
| Application Industries | machine | shipbuilding | presses | construction machinery | Recent developments | Anticipated developments |
| Pressure value/Mpa | 21 | 25 | 35~63 | 21.5~35 | 42~48 | 56~63 |
When applying the concept of fluid pressure to a product, it often specifies the maximum force.
Here are the revised sentences with varied beginnings:
1. This force defines the highest level at which the product can operate effectively.
2. This force represents the design limit of the product.
3. This force guarantees the product’s lifetime.
This applies whether the product is a hydraulic pump or another type. It also includes whether the product guarantees continuous operation. Additionally, it considers when the product briefly reaches a certain limit value. This gives the following indications of force values:
- Maximum component working Pₘₐₓ refers to the highest allowable operating force of the hydraulic component during brief system operation.
- Component minimum working Pₘᵢₙrefers to the lowest allowable operating force of the hydraulic component (pump) under steady state system operation.
- Component peak pₚ refers to the instantaneous maximum force the system allows for hydraulic components.
- Component nominal pₙ refers to the maximum operating force hydraulic components can withstand under steady state system conditions during continuous operation.
Each manufacturer’s product samples often use the above force value indicators. However, the maximum working force and the nominal force mainly differ based on the manufacturer’s emphasis. Consequently, this differentiation helps facilitate user selection.
It is important to emphasize here that the formation of force originates from the load. The formation of force has nothing to do with the flow rate of the system. The Bernoulli equation links pressure and flow only in the case of fluid flow.
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