Industrial Wisdom of Cold Fastening: An Analysis of the Core Principle of Riveting Technology
I. Core of Riveting Principle: Cold Plastic Deformation and Engagement
The essence of riveting is to press specialized riveted fasteners (such as riveted nut pillars, riveted screws, and riveted nuts) into pre-drilled holes in the base material using the force from external pressure equipment (e.g., servo presses, manual riveting guns). Through localized plastic deformation of the base material and structural design of the riveted fasteners, an irreversible mechanical engagement is formed, ultimately achieving a tight connection between the two components. The entire process requires no high-temperature heating and remains in a "cold state" throughout, avoiding damage to the base material's properties caused by thermal processing methods like welding.
From the perspective of material mechanics, the key to riveting lies in "controllable deformation": Riveted fasteners typically feature special structures—such as knurled teeth, hexagonal edges, or annular grooves (e.g., the external knurled design of SOOS riveted nut pillars). When pressure is applied, the protruding structures of the fasteners squeeze the inner walls of the pre-drilled holes in the base material. At this point, the base material (mostly ductile metals like aluminum, iron, and stainless steel) undergoes localized plastic flow under pressure, gradually filling the grooves or gaps between the teeth of the riveted fasteners. Simultaneously, the riveted fasteners themselves undergo minimal elastic deformation due to the pressure, further enhancing their adherence to the base material. When the pressure reaches a preset value, the deformed parts of the base material fully engage with the structure of the riveted fasteners, forming an "interlocking structure"—similar to tree roots embedding in soil—preventing easy separation while transmitting stable fastening force.
The strength of this engagement depends on two key factors: first, the structural design of the riveted fasteners (e.g., tooth density, groove depth, which must match the thickness of the base material); second, the ductility index of the base material (excessively low ductility may cause the base material to crack, while excessively high ductility may lead to over-deformation). For example, for a 1mm-thick aluminum alloy base material, the knurled tooth depth of the riveted fastener is usually designed to be 0.3-0.5mm, ensuring sufficient deformation and engagement of the base material without cracking due to overly deep teeth.
II. Essential Elements for Implementing Riveting Principle: Three Core Conditions
The practical application of the riveting principle requires the coordinated matching of three components: "riveted fasteners, base material, and pressure equipment." Deviations in any link may lead to fastening failure.
1. Riveted Fasteners: "Active Components" with Built-in "Engagement Structures"
Riveted fasteners are the "core carriers" of riveting, and their structural design directly determines the engagement effect. Common structures of riveted fasteners include:
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Knurled tooth structure: Mostly used for riveted nut pillars and riveted nuts, with spiral or annular knurls on the outer surface. When pressed in, the teeth squeeze the inner wall of the base material, forming an engagement similar to "thread meshing."
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Hexagonal structure: The outer side of some riveted fasteners (e.g., hexagonal riveted nuts) is a regular hexagon. After being pressed in, the hexagonal edges fit tightly against the inner wall of the pre-drilled hole in the base material, preventing the fastener from rotating during subsequent use.
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Annular groove structure: The shank of riveted screws is often designed with annular grooves. After pressing, the base material deforms and fills the grooves, forming a "snap-fit" fixation, suitable for thin and lightweight base materials (e.g., metal middle frames of mobile phones).
In addition, the material of the riveted fasteners must match the base material—for example, if the base material is 304 stainless steel, the riveted fasteners are usually made of the same material or carbon steel (with nickel plating for rust prevention) to avoid electrochemical corrosion caused by material differences. For aluminum alloy base materials, lightweight aluminum alloy riveted fasteners can also be used to reduce overall weight.
2. Base Material: "Passive Components" with "Ductile Deformation Capacity"
The base material must meet the requirement of "being ductile and non-brittle." Common applicable base materials include: low-carbon steel (e.g., SPCC), aluminum alloy (e.g., 6061, 5052), stainless steel (e.g., 304, requiring grades with good ductility), and galvanized sheets. Brittle materials (e.g., cast iron, ceramics) cannot undergo plastic deformation and thus are not suitable for the riveting process.
Meanwhile, the size of the pre-drilled hole in the base material is a key parameter: The diameter of the pre-drilled hole must be slightly smaller than the maximum outer diameter of the riveted fastener (usually with a difference of 0.1-0.3mm). If the hole diameter is too large, the base material cannot fully squeeze the riveted fastener, making it difficult to form effective engagement; if the hole diameter is too small, the base material may crack during riveting. For example, for an M4-sized riveted nut pillar used with a 1.5mm-thick cold-rolled steel plate, the diameter of the pre-drilled hole is usually designed to be 4.2mm, which not only reserves space for the deformation of the base material but also ensures extrusion strength.
3. Pressure Equipment: "Power Source" for Precise Control of "Pressure and Stroke"
The core function of pressure equipment is to provide "stable and controllable" pressure and pressing stroke, ensuring that the deformation of the riveted fasteners and base material remains within a reasonable range. Pressure equipment for different scenarios has significantly different precision requirements:
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Manual riveting guns: Suitable for small-batch, small-sized riveted fasteners (e.g., M3 or smaller). Pressure is controlled manually, with a short stroke, making them suitable for on-site maintenance.
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Pneumatic riveting machines: Pressure is driven by compressed air, with a pressure range typically of 0.5-5kN and adjustable stroke, suitable for medium-batch production.
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Servo electric presses: Offer the highest precision, with pressure control accuracy up to ±1N and stroke accuracy up to ±0.01mm. They can also real-time monitor the pressure-stroke curve to determine whether riveting is qualified (e.g., a sudden drop in pressure may indicate base material cracking; excessively high pressure may cause deformation of the riveted fastener). They are widely used in high-precision scenarios such as automotive battery packs and aerospace components.
The pressure setting must be calculated based on the specifications of the riveted fasteners and parameters of the base material—for example, pressing an M5 riveted nut pillar into a 2mm-thick 304 stainless steel plate requires a pressure of approximately 8-12kN. Insufficient pressure will result in insufficient engagement depth, while excessive pressure may damage the riveted fastener or dent the base material.
III. Process Flow of Riveting Principle: Four Steps from "Preparation" to "Engagement"
The practical application of the riveting principle is realized through a standardized process flow, ensuring consistency for each connection point:
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Pre-drilled hole processing: Pre-drilled holes are created in the base material via punching, drilling, or other methods, ensuring the accuracy of hole diameter and position (positional deviation must be ≤±0.1mm to avoid fastener eccentricity).
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Positioning and clamping: The base material is fixed on a tooling fixture, and the position of the riveted fastener is adjusted to align the axis of the fastener with the axis of the pre-drilled hole (eccentricity must be ≤±0.05mm).
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Pressurization and engagement: The pressure equipment is activated, and the press head slowly presses the riveted fastener until the positioning step of the fastener is flush with the surface of the base material (at this point, the pressure reaches the preset value). The base material undergoes plastic deformation and engages with the riveted fastener.
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Inspection and acceptance: Qualification is confirmed through tensile testing (to check connection strength—e.g., the pull-out force of an M4 riveted nut pillar must be ≥1500N) and visual inspection (no base material cracking or fastener deformation). Unqualified parts must be disassembled and re-riveted (disassembly damages the base material, making riveting an irreversible process).
IV. Advantages and Limitations of Riveting Principle: Why It Is the First Choice for Precision Manufacturing
Based on the principle of "cold plastic deformation and engagement," riveting offers significant advantages over traditional processes such as welding and threaded connection:
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No damage: No high temperature or cutting is required, avoiding base material deformation and annealing (e.g., the strength of aluminum alloy decreases by 10%-20% after welding, which does not occur with riveting).
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High precision: The positional accuracy of connection points depends only on the tooling and equipment, meeting the batch requirements of automated production lines.
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High strength: The pull-out force and torque resistance of the interlocking structure are stable. For example, the torque resistance of a 304 stainless steel riveted nut pillar on a 1.5mm-thick cold-rolled steel plate can reach 10N·m, far exceeding that of self-tapping screws of the same specification.
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High efficiency: A single riveting operation takes only 3-5 seconds, which is 3-5 times more efficient than welding.
However, the riveting principle also has limitations: It is only applicable to ductile metal base materials and cannot be used for brittle materials or non-metals (e.g., hot-melt riveting is required for plastics). Additionally, the thickness of the riveted fasteners and base material must be strictly matched—excessively thin base material may be penetrated, while excessively thick base material may result in insufficient pressing.
