Keywords

1 Introduction

The lightweight of hydraulic components and systems is one of the important approaches to achieve weight reduction in construction machinery and vehicle engineering. This not only helps to reduce the weight of the equipment itself, but also improves the overall mobility and load-bearing capacity of the machinery. Furthermore, it aligns with the requirements for energy conservation and emission reduction. In all kinds of hydraulically driven mobile equipment: 1% weight reduction of the pump truck can achieve 0.6–1% [1] reduction in fuel consumption; each 1% reduction in aircraft weight improves performance by 3–5%, and can reduce fuel consumption, improve load, and reduce operating costs[2].

At present, in the construction machinery and vehicle engineering equipment, hydraulic cylinders for one of the most lightweight potential hydraulic components, some developed countries have been through Carbon fiber, compact design [3, 4] and other new materials and technology means to achieve lightweight hydraulic cylinders (including integrated in the hydraulic cylinder on the oil block, etc.). However, according to some current hydraulic cylinder related standards, such as JB/T10205-2010 Hydraulic cylinder [5] designed and manufactured hydraulic cylinder “thick and bulky” problem still cannot be ignored. However, the issue of “thick and bulky” hydraulic cylinder design and manufacturing still cannot be disregarded in compliance with some of the most recent hydraulic cylinder standards, such as JB/T 10205-2010 Hydraulic Cylinder. One of the reasons for this problem is the “nominal pressure” as the basic parameters of the hydraulic cylinder design and manufacture of hydraulic cylinders.

Hydraulic cylinder is one of the hydraulic components with the most potential for lightweight, and it is a project involved in the national key research and development plan. Avoid using “nominal pressure” as the basis for hydraulic cylinder design, testing, and using. Instead, use “rated pressure” as the fundamental hydraulic cylinder design and manufacturing criteria in order to achieve the lightweight hydraulic cylinders that is a necessary condition for advancing Chinese hydraulic cylinder technology.

2 Why Hydraulic Cylinders Are “Big and Bulky”?

An example of hydraulic cylinders design is given to illustrate the point. For instance, there is a valve-controlled hydraulic cylinder system, the choice of the rated pressure of the hydraulic valve (or rated supply pressure) of 21 MPa, the user also needs to give the basic parameters of the hydraulic cylinder in order to carry out the purchase of hydraulic cylinders. Because the 21 MPa pressure value is not the GB/T 2346-2003 [6] “Fluid Transmission System and Components Nominal Pressure Series” in the specified pressure value, can only be selected in the GB/T 2346-2003 specified 25 MPa pressure value as the nominal pressure of this hydraulic cylinder.

Hydraulic cylinder design and manufacture based on the basic hydraulic cylinder parameter of 25 MPa nominal pressure. According to JB/T 10205-2010 “6.2.7 Pressure resistance: Hydraulic cylinder block should be able to withstand the nominal pressure of 1.5 times the pressure, there shall be no external leakage and damage to parts and other phenomena.” and JB/T 11718-2013 Hydraulic Cylinders-Technical conditions for cylinder tubes [7] given in the calculation formula, cylinder material strength requirements of the minimum wall thickness δ01 is:

$$\delta _{{01}} \ge \frac{{p_{{\max }} D}}{{2\left[ \sigma \right]}} = \frac{{1.5p_{{\text{n}}} D}}{{2\left[ \sigma \right]}} = 18.75\frac{D}{{\left[ \sigma \right]}}{\mkern 1mu} {\mkern 1mu} {\mkern 1mu} {\text{(mm)}}$$
(1)

where

pmax—cylinder pressure, MPa;

pn—nominal pressure, MPa, pn ＝ 25 MPa;

D—cylinder diameter, mm;

[σ]—permissible stress of cylinder material, MPa.

As stipulated in JB/T3818-2014 Hydraulic Press-Technical requirement [8] “The self-made hydraulic cylinder should be subjected to pressure test, the pressure holding time is not less than 10 min, and there should be no leakage, permanent deformation and damage. The compressive test pressure shall comply with the following requirements: (a) When the rated pressure is less than 20 MPa, the test pressure shall be 1.5 times the rated pressure. (b) When the rated pressure is greater than or equal to 20 MPa, the pressure test pressure should be 1.25 times.” According to the design and calculation of the above requirements, cylinder material strength requirements of the minimum wall thickness δ02 is:

$$\delta_{02} \ge \frac{{p_{\max } D}}{2\left[ \sigma \right]} = \frac{{1.25p_{{\text{e}}} D}}{2\left[ \sigma \right]} = 13.125\frac{D}{\left[ \sigma \right]}\,\,\,({\text{mm}})$$
(2)

where

pe—rated pressure, MPa, pe ＝ 21 MPa;

Other parameters have the same meaning as above.

If the same material is chosen for the above two design calculations and the cylinder diameter is the same, comparing Eq. (1) with Eq. (2):

$$\frac{{\delta _{{01}} }}{{\delta _{{02}} }} = \frac{{18.75\frac{D}{{\left[ \sigma \right]}}}}{{13.125\frac{D}{{\left[ \sigma \right]}}}} = 1.429$$
(3)

From Eq. (3), it can be seen that δ01 calculated in accordance with the design of JB/T 10205-2010 is 1.429 times larger than δ02 calculated in accordance with the design of JB/T 3818-2014. It could be concluded that the hydraulic cylinder designed according to JB/T10205-2010 has the problem of “thick and heavy”.

3 Discuss the Term “Nominal Pressure”

  1. (1)

    Differences and similarities in the definition of the term “nominal pressure” in various standards

In the current standards, the term “nominal pressure” and its definition can be found mainly in:

  1. (a)

    In GB/T2346-2003, the term “nominal pressure” is defined as: “A pressure value assigned to a component, line or system for the purpose of facilitating the representation and identification of the pressure series to which the component, line or system belongs.”

  2. (b)

    In GB/T 17446-2012 Fluid power systems and components-Vocabulary [9], the term “nominal pressure” is defined as: “a component, a piping or a system for the purpose of convenient designation and indicating its belonging to a series”

Although there are differences between the above two definitions, such as the definition of GB/T 17446-2012 does not specify the “pressure series” or “nominal pressure series”, but there are some common points:

  1. (a)

    The purpose is the same, to facilitate identification and/or representation.

  2. (b)

    The object is the same, for components, piping/pipes, systems.

  3. (c)

    The source of the pressure values is the same, to be specified/assigned.

  4. (d)

    The results of the designation/assignment are consistent, to require an artificial selection of a pressure value in the series.

It should be evident from the definitions of the terms that the “nominal pressure” is solely for labeling and/or indication purposes, and hydraulic cylinders cannot be designed or operated based on this value.

  1. (2)

    Evolution of the “nominal pressure” in the hydraulic cylinder standard

JB/T 10205-2010 Hydraulic Cylinder was released on 02-11-2010, implemented since 07-01-2010, and replaced JB/T 10205-2000 Hydraulic fluid power cylinders-Technical specifications.

In JB/T 10205-2010, it states, “3 Terms and definitions: The terms and definitions established in GB/T 17446 and the following terms and definitions apply to this standard.” “5.3 Basic parameters: The basic parameters of hydraulic cylinders shall include cylinder bore, piston rod diameter, nominal pressure, stroke, and mounting dimensions.” “6.2.7 Pressure resistance: The cylinder of the hydraulic cylinder shall be able to withstand a pressure of 1.5 times the nominal pressure without external leakage and damage to parts, etc.”

After checking, in JB/T 10205-2000 Hydraulic fluid power cylinders-Technical specifications (has been replaced) there are no provisions on hydraulic cylinder parameters or basic parameters of hydraulic cylinders. Therefore, it can be determined that the “nominal pressure” as the basic parameter of the hydraulic cylinder should be from JB/T 10205-2010 Hydraulic fluid power cylinders-Technical specifications.

In JB/T 10205-2000 defined “3.1 nominal pressure: The nominal value of the working pressure of the hydraulic cylinder. That is, under the specified conditions of continuous operation, the design life of the working pressure can be guaranteed.” And stipulates: “4.1.1 Nominal pressure series shall comply with the provisions of GB/T 2346.” “4.2.6 Pressure resistance: The block of a hydraulic cylinder shall be able to withstand a pressure of 1.5 times its maximum working pressure without external leakage or damage to parts, etc.”

After the above comparison, the definition of “nominal pressure” in JB/T 10205-2000 is obviously different from that in GB/T 2346-2003 and GB/T 17446-2012. However, it is basically the same as the definition of “nominal pressure” in GB/T17446-1998 Fluid power systems and components–Vocabulary (which has been replaced).

Therefore, JB/T 10205-2010 changed the definition of terms “nominal pressure” as the basic parameters of the hydraulic cylinder. This is the cause of the design and manufacture of hydraulic cylinders in accordance with JB/T 10205-2010 prevailing “thick and bulky” the root cause of the problem.

It should be noted that the definition of the term “nominal pressure” in JB/T 10205-2000 is similar to the definition of the term “rated pressure” in GB/T 17446-2012. However, it is unclear why JB/T 10205-2000 selected “nominal pressure” and “maximum working pressure” instead of “rated pressure” at that time. While, in GB/T 17446-2012, the “maximum working pressure” is categorized under “Pressure terminology related to fluid drive systems”.

In GB/T17446-2012, the term “3.2.464 nominal pressure” is defined as “see ISO2944 and Fig. 1. “However, Fig. 1 does not include “nominal pressure,” and the relation between “nominal pressure” and other stress terms cannot be established.

Fig. 1
An engineering diagram of the S 2 hydraulic cylinder. It includes the top and side views of the cylinder. It has 3 concentric circles, which are divided into 4 quadrants. 2 sets of concentric rings are inserted in each quadrant.

S2 hydraulic cylinder shape diagram

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In ISO/DIS 5598:2020(E) [10], the definition of “3.2.478 nominal pressure” includes a reference to “see also ISO 2944 and Figure 20”. However, “nominal pressure” is not depicted in “Figure 20”, making it difficult to establish the connection between “nominal pressure” and other pressure terms.

It remains unclear whether this is due to an omission or other reasons. Moreover, according to the definition of the term “nominal pressure” in GB/T 17446-2012, it should also be the term for pressure associated with components and piping, but it is not indicated under the definition of the term “3.2.464 Nominal pressure” or in Fig. 2.

Fig. 2
A double bar graph plots barrel weight and cylindrical barrel weight reduction versus cylindrical diameter slash limit travel. The cylinder barrel weight at nominal pressure of 25 megapascals has the highest value at 310 for 320 slash 1250. A reduction line follows a decreasing trend from 0 to 35%.

Hydraulic barrel weight comparison

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Taking the pressure terms displayed in Figs. 1 and 2 of GB/T 17446-2012 as an example, Table 1 specifies the pressure terms and definitions necessary for the design, manufacturing (testing), sale, and utilization of hydraulic cylinders.

Table 1 Pressure-related terms and definitions in GB/T 17446-2012
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Refer to the terms and definitions related to pressure stipulated in GB/T 150.1-2011 Pressure vessels—Part 1: General requirements [11], as shown in Table 2.

Table 2 Pressure-related terms and definitions in GB/T 151.1-2011
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An attempt can be made to give some of the pressure terms closely related to the design and manufacture of hydraulic cylinders have the following relationships:

  1. (a)

    (a component or a piping) burst pressure (3.2.86) ＞ (a component, a piping or a system) nominal pressure (3.2.464) ＞ (a component or a system)maximum pressure (3.2.428) or (a component or a piping) proof pressure (3.2.575) ＞ (a system) maximum working pressure (3.2.429) or (a component or a piping) rated pressure (3.2.597).

  2. (b)

    The “nominal pressure” in the fluid transmission systems and components may be defined: the maximum pressure or proof pressure plus the permissible upper limit of its average display value, that the limit deviation rounded upwards to the pressure value specified in the nominal pressure series.

Based on the above findings, the S2 hydraulic cylinders with sliding bearings on the rear ears in standard JB/T6134-2006 Hydraulic cylinders for metallurgical equipment [12] are designed, and the shape diagram is given in Fig. 1. According to the nominal pressure of 25 MPa and the area ratio φ1.4 given by the standard, the weight of the cylinder barrel at the limit stroke section is calculated, and the data can be obtained as shown in Table 3.

Table 3 Cylinder barrel weight at nominal pressure of 25 MPa
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Without changing other conditions, the rated pressure of 21 MPa as a design parameter, based on the above formula to calculate the limit stroke of the hydraulic cylinder barrel weight as shown in Table 4.

Table 4 Cylinder barrel weight at rated pressure of 21 MPa
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Comparison of the data in Tables 3 and 4 leads to Fig. 2, in which the barrel weight and cylinder wall thickness are drastically lowered while the other conditions remain the same. It shows that the use of rated pressure design and manufacture may significantly reduce the weight of hydraulic cylinders, to help solve the problem of hydraulic cylinders “thick and heavy”.

With the “rated pressure” as the basic parameter of the hydraulic cylinder, the relationship between the “rated pressure” and the “proof pressure “is determined according to the standards observed. Meanwhile, according to the relationship given in item (b) above to calculate the specified “nominal pressure”, which may become a new hydraulic cylinder design, test specifications.

4 Conclusion

Through the above analysis and demonstration, the following conclusions could be drawn:

  1. (1)

    According to the current standard, “nominal pressure” as the basic parameter of the hydraulic cylinder design and manufacture, makes the strength margin of the hydraulic cylinder is too large, “thick and bulky”. It seriously affects the lightweight design and manufacture of hydraulic cylinders.

  2. (2)

    Neglecting the permissible upper limit deviation of the average display value of the proof pressure, the establishment of “K × rated pressure = proof pressure ≤ nominal pressure” (Note: K is a multiple of the standard) abbreviated pressure terminology relationship equation, which may be important for fluid transmission systems and components.

  3. (3)

    In standards related to hydraulic cylinders, it is reasonable to set the “rated pressure” as the basic parameter of the hydraulic cylinder, which can effectively promote the lightweight design and manufacture of hydraulic cylinder in China.

The current “thick and bulky” status quo of hydraulic cylinders, although it is advantageous for manufacturers to sell hydraulic cylinders by weighing and pricing. From a long-term perspective, it will be unfavorable for hydraulic industry and even the national level and the effective use of limited resources.

At the standard level to establish the “rated pressure” as the basic parameter of the hydraulic cylinder, although there are still different opinions, but in order to realize the lightweight design and manufacture of hydraulic cylinders is also imperative.