Fun fact about suspension rings: Can the screws be reused?
Classification:
Time：
2024-10-14
The following reasons make it inadvisable to reuse screws:
1. Current technological level and product safety requirements
2. Process Capability in the Assembly Process
Current technological capabilities and product safety requirements
In the joining process, reference standards specifically addressing screw components under theoretical conditions have been established. General standard requirements are not mandatory; rather, they constitute non‑binding recommendations that, when applied, acquire greater enforceability precisely through adherence to the specified guidelines. When drafting orders, basic safety requirements can be refined in accordance with technical standards. In this way, existing technologies assume a pivotal role in the sales phase and also prove their worth in specialized application contexts.
Certain application standards confer a presumption of conformity with specific regulations (e.g., the CE marking). In the absence of applicable standards or relevant regulations, compliance must be ensured in accordance with generally recognized technical specifications.
When it comes to the reuse of screw‑type components, users or maintenance personnel are responsible for assessing their suitability. Component replacement is typically specified in the manufacturer’s instructions. According to the definition provided in the “Technical Instructions,” inspecting and replacing safety‑critical fasteners is also widely recognized as a necessary measure. This principle is grounded in years of practical experience and constitutes an essential requirement for ensuring reliable connections.
In addition, it is necessary to address post‑market responsibilities. To this end, essential health and safety requirements must be met, while also taking into account factors such as the assessment of knowledge levels*) and the current state of the art, as well as the identification and mitigation of any specific potential hazards associated with the product.
*) The comprehensive knowledge currently being implemented or soon to be implemented, as well as the usability of the product (screw assembly components) upon market launch.
Summary: From the perspective of adhering to relevant code‑based design requirements and complying with applicable regulatory provisions, redundant reuse of existing connection components should be avoided whenever possible.
Process capability in the assembly process
Assembly Safety – Friction:
The key factor governing connection safety is assembly preload. Assembly must be carried out in accordance with the assembly instructions and tribological boundary conditions. During assembly, the preload between the threaded fastener and the bearing surface (the mating component) significantly influences the process. Assembly efficiency is typically only 10–20%.
It is clear that the tensile force calculated in the design can only be achieved with a specified coefficient of friction. Moreover, safe process assembly should minimize the risk of friction‑induced failure. Therefore, for secure connections, it is necessary to use designated lubricants during operation. The commonly cited “anti‑friction coating” approach helps ensure proper lubrication and fully satisfies the relevant tribological boundary conditions.
Summary: Friction coatings are applied directly to the mating components, generating the preload necessary for secure assembly under specific conditions. Consequently, even when parts are replaced and reinstalled, assembly integrity remains critical. This necessitates the use of screws coated with a fresh layer of coating.
Solution: Coating. Dry‑friction coatings are a specialized system designed for fasteners—screws, nuts, and bolts—under mechanical stress. This non‑electrolytic coating forms a thin film on the component’s surface, combining lubrication and corrosion‑protection in a single layer. It consists of fluoropolymers and organic dry‑lubricant particles dispersed within a precisely engineered synthetic resin and its solvent. The so‑called AF coating (anti‑friction coating) creates a smooth film that fills surface irregularities, ensuring efficient friction even under saturated load conditions. Moreover, the synthetic resin enhances the coating’s corrosion‑protective performance.
Advantages of dry friction coatings:
An excellent and stable coefficient of friction is the foundation of any screw connection.
The coating is clean and environmentally friendly, and easy to use.
High installation safety during production and maintenance.
Based on a comprehensive cost analysis that takes into account the cost-effectiveness of assembly and disassembly processes, we help you reduce manufacturing costs.
Corrosion resistance – Appearance conditions
The surface coating on connecting components serves both protective and friction‑reducing functions. Typically, fluoropolymers such as PTFE enhance sliding performance, act as an additional protective layer, and increase the preload of assembled parts during initial installation.
Under the new conditions, connecting components equipped with this surface coating exhibit higher tensile strength and enhanced protective properties. Practical experience shows that these components can be reassembled up to five times while maintaining sufficient precision for repeated use. In exceptional cases where more than five reassemblies are required—due to variations in assembly preload—the decision should be based on the screw’s design and the intended application of the joint.
If the screw connection is already in use and its surface has been affected under operating conditions, the friction characteristics have likewise changed. In addition to reduced corrosion‑resistance, changes in the coefficient of friction must also be taken into account, as well as the additional time required for reassembly and installation. In practice, we observe that functionality gradually degrades over time, often attributable to industrial operating environments. Consequently, surface coatings may be compromised by aggressive media and/or degraded through chemical processing. These signs can be readily identified—such as iron oxide—and should be evaluated on a case‑by‑case basis. If corrosion resistance deteriorates more rapidly than anticipated, even within the specified shelf life, complaints may arise due to factors related to design, appearance, or other aspects.
Summary: In practical applications, surface coatings may degrade, discolor, and undergo changes in appearance. Moreover, depending on the screw material, residual failure risks may necessitate component replacement. It is recommended to conduct regular visual inspections and periodic assessments of connections involving safety‑critical screws.
Surface Treatment – Dismantling Capability
Surface coatings and/or additional coatings (protective coatings) can typically optimize certain specific functions. In addition to protective performance, frictional properties, and design requirements, ease of disassembly during maintenance is another key consideration.
Particularly in power supply and transportation systems—covering energy generation and vehicles—the safety of the power‑supply process and the suitability of machinery and systems are at the cutting edge of technology. Effective maintenance, coupled with ease of disassembly during operation, is of paramount importance—its significance approaching that of ensuring proper assembly under preload. If, for instance, a screw fastened to stainless steel can no longer be loosened, we are dealing with a “seized” joint—also known as cold welding. What causes this seizing? When the mating surfaces (the thread flanks) reach the limit of adhesive friction, mechanical resistance (excessive friction) arises, preventing any relative movement between the overlapping contact areas.
Some factors that can cause a deadlock:
Excessive Stress: Overstretching or excessive torque can cause plastic deformation of the threads.
Excessive assembly speed: Assembly performed using a pneumatic impact screwdriver.
Excessive surface roughness: threads/grooves were cut using an improper method.
Impurities: such as debris, dirt, or fine particles like sand.
Defects: for example, center offset or deviations in thread tolerance.
Assembly process under additional pressure or tensile load: Contraction of the sealing flange surface with soft sealing material.
Mounting nuts with locking elements—such as self-locking nuts or polyamide‑coated locknuts—typically induce coaxial displacement, thereby generating localized pressure on the thread flanks.
Summary: It can thus be concluded that, depending on the specific circumstances encountered during each operational phase, the designer has developed an appropriate connection scheme. Ensuring the product’s requisite safety, providing for the maintenance and servicing of all functions, and carefully assessing the feasibility of such maintenance activities together constitute a comprehensive, integrated approach. Spare parts and replacement components—including fasteners—must be clearly specified, and the replacement procedures should be fully documented alongside the installation and disassembly instructions.
Defining surface coatings and lubrication conditions is critical to preventing cold welding and ensuring proper assembly. Therefore, it is strongly recommended to replace screw components promptly and apply appropriate lubricants. Professional assembly practices require the use of new fasteners in accordance with established assembly specifications during actual operations.
Durability – Surface Damage
In high‑stress joint configurations, screw fastening requires appropriate preload. Both settlement and dynamic stress conditions can compromise the screw’s durability. For screws subjected to dynamic loading, surface condition is another critical factor. Therefore, during manufacturing and other operational steps, measures should be taken to prevent damage to the screw surface.
Industrial manufacturing and assembly are primarily based on quality management systems compliant with ISO 9001. To ensure safe joining processes, it is essential to establish appropriate inspection plans for both manufacturing and assembly, as well as for actual field use. Although each joined component must meet the relevant production standards or technical specifications, this requirement is often difficult to achieve in mass production. This is precisely why ISO 3269 was developed; it should be noted, however, that even such quality inspections cannot absolutely guarantee zero defects across an entire production batch.
Defects in manufacturing, operation, maintenance, and repair typically arise from the complex interplay of multiple factors. Drawing on practical experience, this work primarily summarizes the relationships among screw components, their mutual fit, and the assembly methods employed. A thorough understanding of the parameters of various screws, along with specialized knowledge of design and assembly, is essential for achieving secure connections.
Summary: Whether a screw component meets its functional requirements is of greater importance than its aesthetic appearance. The assessment of surface defects is addressed differently in ISO 6157‑1 and ISO 6157‑2, and the use of fastening components should be approved on a case-by-case basis according to the specified conditions. Consequently, screws intended for reuse must undergo re‑evaluation, and each time they are reassembled, they should be treated as newly manufactured parts. Therefore, the decision to permit reuse rests with the distributor or their authorized representative, who shall approve it based on the specific operational conditions during maintenance.
In summary, to comply with the originally specified design requirements, it is recommended to use entirely new connecting components.
Cost-effectiveness of production – Assembly costs
The demand for cost‑effective production is steadily increasing. In a challenging market environment and with development cycles growing ever shorter, maintaining competitive edge is essential. At the same time, ecological considerations are shaping the future of connection‑assembly solutions. By optimizing resource use to reduce production costs, we can deliver maximum value to our customers. Moreover, when all quality requirements are rigorously met, customer satisfaction can be further enhanced—empowering customers to take an active role and assume corresponding responsibilities.
Although we typically maintain a comprehensive set of control documents and certifications for systematic management, customer complaints still persist. Customer dissatisfaction may stem from delivery services, logistics operations, collaboration and communication, or product quality. Experience shows that these factors often intertwine, giving rise to irreconcilable conflicts. Consequently, the design of components with specific functional characteristics, the assembly process, and the selection of fasteners are critical prerequisites for achieving the intended performance.
Therefore, assembly process performance is a critical prerequisite for ensuring on-time delivery. In practice, quality variations and even component shortages can lead to unexpected downtime. To prevent such issues, it is essential to use the correct fastener components and employ appropriate assembly methods. Consequently, superior process performance means being able to implement the right decisions on schedule.
Summary: The cost-effectiveness of an assembly approach stems from straightforward assembly processes and appropriately selected assembly equipment. In this context, screw components with specific frictional properties establish critical specifications for achieving assembly preload. Lean manufacturing emphasizes the importance of optimizing value-added steps and eliminating non-value-added activities, such as replacing “wet” lubricants with dry coatings.
Therefore, assembly personnel should use brand‑new screws with a specified treatment from the outset; if necessary, an appropriate lubricating protective coating may be applied. Used screws, having been subjected to operating conditions, no longer retain their original properties as when first installed and must therefore be replaced.
Accordingly, safety‑related connections should always be assembled using brand‑new fasteners, and, when necessary, labeled to facilitate traceability (e.g., by production batch).
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