After the hydraulic cylinder undergoes a pressure test, occasional issues may arise where the fastening bolts of tie rod hydraulic cylinders or bolt-connected hydraulic cylinders become loose or the torque decreases. This is a critical issue that directly affects the reliability and safety of the hydraulic cylinder.

The bolts loosened after the pressure test – the root cause is that the pressure test simulates the worst working conditions in actual operation, amplifying the weaknesses in the design or process.

The bolt loosened after the test, indicating that it had experienced irreversible preload decay. The following is an analysis of the root cause of this problem:

I. Core Reason: Preload Reduction

The essence of bolted connections lies in the preload of the bolts generating a significant clamping force on the contact surfaces of the connected components (guide sleeve and cylinder). This clamping force generates friction to resist working loads (such as the force generated by hydraulic pressure). Any factor that causes a decrease in preload will lead to loosening of the connection. Pressure tests, especially high-pressure and pressure cycling, are “accelerated tests” that lead to preload reduction. The main reasons are as follows:

1. Plastic Collapse of Contact Surfaces (The most important and common reason)

• Mechanism: The lower end face of the bolt head/nut, as well as the contact surfaces of the connected guide sleeve and cylinder flange, are microscopically uneven. Under a large preload, these microscopic protrusions (crests) undergo plastic deformation (flattening). Furthermore, if the material is relatively soft, or if the surface has paint, burrs, etc., deformation is more likely to occur. This deformation process causes the small elongation of the bolt to recede, resulting in a decrease in preload.

Schematic diagram of microscopic changes on the material surface

• Effect of the test: Under the pressure pulsation of the pressure test, the mating surface is subjected to alternating loads, which exacerbates this plastic crushing process, leading to further attenuation of the preload. Loosening is an inevitable result after the test.

2. Plastic elongation (relaxation) of the bolt itself

• Mechanism: If the bolt strength grade is selected too low (e.g., using grade 4.8 instead of grade 8.8 or higher), or the preload torque is too large, the bolt may have approached or exceeded its yield strength. Under sustained load, the bolt will undergo a small amount of plastic elongation, resulting in permanent loss of preload.
• Effects of the test: The high pressure of the withstand test is equivalent to a continuous high load, which will accelerate the relaxation process.

3. Stiffness and Separation of Connected Components

• Mechanism: Under hydraulic pressure, the guide sleeve tends to separate from the cylinder body. If the flange design is insufficiently rigid (too thin or poorly structured), slight warping deformation will occur under pressure. This deformation directly leads to additional tension on the bolts. If the load is too large, it may cause complete separation of the mating surfaces, with the preload instantly dropping to zero. Even if complete separation does not occur, this alternating bending stress will exacerbate the plastic collapse mentioned in point 1 above.

4. The “Rotation” Effect of Threaded Pairs

• Mechanism: Under vibration and alternating loads, minute relative movements occur between threaded pairs, causing the bolts to tend to “rotate and loosen.” Although the pressure fluctuation frequency of a hydraulic cylinder is not high, it still falls under the category of alternating loads, which can trigger this effect.

II. Why is retightening after the initial test effective, but only a temporary fix?

• Effective: The first tightening has already achieved preliminary plastic compaction of the mating surfaces. Retightening establishes preload on a relatively “stable” contact surface, thus significantly reducing preload decay.
• Temporary Fix: This is only a remedial measure, indicating that the initially designed preload cannot be maintained after operating conditions. In practical use, it’s impossible to retighten bolts after every operation.

III. Systematic Solutions and Recommendations

To fundamentally solve this problem, a systematic approach is needed from the design, process, and management perspectives:

1. Design Level (Most Critical)

• Use high-strength bolts: Mandate the use of high-strength bolts of grade 8.8 or 12.9 and above. High-strength bolts have higher yield strength, can withstand greater preload, and are less prone to loosening.
• Increase the number or size of bolts: Where space permits, increasing the number of bolts or using bolts with larger diameters can reduce the load on individual bolts and increase the safety margin of the connection.

Optimize flange and contact surface design:

• Increase flange thickness to improve its rigidity and reduce deformation under pressure.
• Design a limiting stop to allow the stop to bear most of the radial and shear forces, while the bolts primarily bear the preload. This will greatly improve the stress state of the bolts.
• Ensure the flatness and smoothness of the contact surfaces.

2. Process and Assembly Aspects

2.1 Employing Correct Tightening Methods:

• Torque-Angle Method: This is a more precise method than the simple torque method. An initial torque is used to eliminate gaps, then a specific angle is rotated to precisely tighten the bolt to near its yield point, obtaining maximum and stable preload.

2. Torque/Angle Control Method

▶ The torque-angle control method is developed from the torque control method. This method involves first tightening the bolt to a small torque, and then, starting from that point, tightening it for a specified angle.

It is based on the relationship between a certain angle, causing a certain axial elongation of the bolt and compression of the connecting parts, resulting in a certain axial preload. When using this method, the purpose of setting the initial torque (Ts) is to tighten the bolt or nut to a tight contact surface and overcome initial unevenness factors such as surface roughness. The bolt axial preload is mainly obtained in the subsequent angles. As shown in Figure 5, the difference in frictional resistance (represented by the coefficient of friction in the figure) only affects the starting point of the measured angle and extends its influence to the end. After calculating the angle, the influence of frictional resistance no longer exists, so its impact on the bolt axial preload is minimal. Therefore, its accuracy is higher than the simple torque method. As shown in Figure 5, the torque-angle control method has the greatest impact on the accuracy of bolt axial preload at the starting point of the measured angle, i.e., point S1 (or S2) corresponding to TS in the figure. Therefore, in order to obtain higher tightening accuracy, attention should be paid to the study of this point.

The biggest difference between the torque-angle control method and the torque control method lies in the following:

The torque control method typically limits the maximum bolt axial preload to 90% of the bolt’s elastic limit, i.e., point Y in Figure 6; while the torque-angle control method generally uses the Y-M zone as the standard, ideally controlling it slightly beyond the yield point. The accuracy of the bolt axial preload using the torque-angle control method is very high. As shown in Figure 6, for the same angle error, the bolt axial preload error ΔF2 in the plastic zone is much smaller than the bolt axial preload error ΔF1 in the elastic zone.

• With the angle method, the bolt load can be within its elastic deformation range or enter the plastic deformation range. Most manufacturers use the angle method primarily in the plastic zone. If the bolt is to enter the plastic deformation range, rigorous testing or inspection is necessary.
• Advantages: Less affected by the coefficient of friction, can obtain relatively high preload with small preload dispersion.
• Disadvantages: It requires a lot of experimental and analytical work, and it is almost impossible to retest. If a torque wrench is used for retesting, the preload may exceed the original set value.
• Use a hydraulic torque wrench or tensioner: This is the best option for critical heavy-duty hydraulic cylinders, ensuring uniform and accurate preload on multiple bolts.

2.2 Controlling Contact Surface Quality:

• Ensure the mating surfaces are clean, free of paint, oil, and burrs.
• For soft materials (such as certain guide sleeve materials), consider using hardened washers to distribute pressure and prevent plastic collapse of the contact surfaces.

2.3 Using Anti-Loosening Solutions:

Effective types: such as nylon self-locking nuts, all-metal lock nuts, and DISC-LOCK washers.

• Auxiliary type: such as using threadlockers (such as Loctite 243 and other medium-strength adhesives), but attention should be paid to their removability and impact on preload.
• Use with caution: Ordinary spring washers perform poorly under vibration conditions and are generally not recommended for use in high-pressure hydraulic systems.

3. Quality Management Aspect

• Clearly define the bolt tightening sequence: A symmetrical, crisscrossing, and step-by-step tightening sequence should be adopted to ensure uniform pressure on the mating surfaces.
• Standardize the initial tightening and re-tightening:Include “re-tightening after the pressure test” as a mandatory process step in the work instructions. Although this is a temporary solution, it is a necessary means to ensure factory quality before design improvements are made.

The bolt loosening problem we encountered stems from the preload decay of the connection system under high-pressure alternating loads. The most fundamental solution is to upgrade the bolt grade, optimize the flange design, and adopt a more scientific tightening process. At the current stage, making re-tightening after testing a fixed procedure is a reasonable practice to ensure product quality.

It is recommended to work with design engineers to verify and calculate the strength and preload requirements of the bolt connections, and systematically improve the above aspects to fundamentally eliminate the problem.

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Contact: Nancy Zhu, Sales manager, JW GROUP.

Email: nancy@jwgroup.cc

Web: https://jwcylinder.com

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HYDRAULIC CYLINDER and CNC parts specialist

JW GROUP is an integrated steel product group, products include: hydraulic cylinder, hydraulic cylinder spare parts, drilling forging parts, pneumatic actuator-scotch yoke, CNC machinery parts etc.