Description:The present invention relates to the storage of high-pressure compressed natural gas (CNG) and gaseous hydrogen (H₂) in composite cylinders. More specifically, it pertains to a robust mechanical interlocking mechanism, designed for Type 4 cylinders. The invention introduces an innovative V-shape protruding projection machined into the internal bore of the metallic boss to ensure a secure, leak-proof, and structurally stable mechanical coupling with a blow-molded polymeric liner.
BACKGROUND OF THE INVENTION:
Modern Type 4 CNG cylinders, widely utilized in the automotive and industrial sectors for the gas storage. It consists of polymeric liner used as permeation barrier for stored gas, metallic boss to hold the gas filling valve, and resin-fibre based composite provide strength to resist and hold the pressure of gases. These cylinders rely on metal bosses to interface with gas dispensing systems. A critical engineering challenge lies in achieving a reliable seal and mechanical joint between the polymeric liner and the metallic boss.
Traditional techniques rely on compression sealing, O-rings, or adhesive bonding. However, under extreme operating conditions such as high internal pressures (up to 300 bar), fluctuating temperatures, and long-term fatigue, these conventional sealing methods are prone to degradation, leading to leaks or liner displacement. This invention presents a transformative mechanical solution that overcomes the deficiencies of the existing methods.
By machining a sharp, precisely controlled-angle V-shape protruding projection on the inner face of the metal boss, and inducing a controlled deformation in the liner neck via torque during assembly, the invention creates a robust mechanical interlock. This eliminates relative motion, fortifies the sealing mechanism, and ensures long-term reliability even under adverse operational conditions.

SUMMARY OF THE INVENTION:
The invention discloses a highly efficient, precisely engineered V-shape feature machined on the internal bore of a metallic boss that interfaces with a blow-molded polymeric liner. This V-shape protruding projection is strategically positioned immediately downstream of an O-ring groove and is designed to induce a physical V-notch recessed groove into the polymer neck of the liner during torque-assisted installation. This deformation results in a permanent interlock that not only enhances sealing efficiency but also provides superior mechanical coupling. The enhanced design features enable precise alignment and a secure connection between the V-shaped protruding projection of the metallic boss and the induced V-notch recessed groove on the polymeric liner.
Background of the Invention
Type 4 cylinders comprise three critical components:
1. A polymeric liner serving as a gas barrier,
2. A metallic boss for valve integration,
3. A composite overwrap for structural reinforcement.
A major design challenge in these cylinders is achieving a robust and leak-proof mechanical connection between the polymer liner and the metallic boss. Conventional methods rely on adhesives, interference fits, or O-ring-based sealing techniques. These approaches are susceptible to mechanical degradation, thermal cycling fatigue, gas permeation, and internal pressure surges (up to 300 bar), which compromise reliability and safety.

PRIOR ART
Patent Literature
1. US20110203825A1 – “Composite pressure vessel with boss/liner assembly”
o Discloses a threaded boss-liner connection using adhesive bonding and O-rings. Lacks a mechanically embedded interlock feature.
2. EP2084011A1 – “Pressure vessel and method of making same”
o Shows a polymer liner fitted to a boss using heat and pressure. The bond relies heavily on thermal fusion and adhesive seals, without a positive mechanical engagement.
3. US7896166B2 – “Blow molded liner for a composite pressure vessel”
o Describes blow-molded liner shapes, focusing on uniform thickness and compatibility with bosses. No mention of deformation-induced interlocks.
4. WO2020151059A1 – “Boss configuration for gas cylinders”
o Suggests alternative grooves and sealing techniques inside the boss but lacks a projection-based mechanical locking mechanism.
5. IN202121045146A – “Advanced Type IV composite cylinder for hydrogen storage”
o Discusses composite layering and gas retention improvements, without addressing polymer-metal boss-liner interlocking.
Non-Patent Literature (NPL)
1. ISO 11119-3:2013 – “Gas cylinders — Refillable composite gas cylinders and tubes — Design, construction and testing — Part 3: Fully wrapped fibre reinforced composite gas cylinders with non-metallic liners”
o Specifies requirements for CNG cylinders but does not mandate or describe mechanical interlocking between liner and boss.
2. M. Farajpour et al., “Failure analysis of Type IV composite pressure vessels,” International Journal of Hydrogen Energy, 2020
o Indicates that boss-liner detachment and microleaks at the interface are primary failure modes in pressure vessels.
3. E. Lorenz et al., “Investigation of sealing performance in polymer-metal interfaces,” Journal of Pressure Vessel Technology, ASME, 2019
o Demonstrates challenges with O-ring-only or adhesive-based seals under pressure and thermal stress.
Therefore, none of the above art teaches or suggests the use of a precision-machined V-shaped protruding projection in the metallic boss to induce a matching V-notch in a polymer liner, forming a torque-driven mechanical interlock as claimed in the present invention. Therefore, the disclosed system demonstrates novelty and an inventive step.

SUMMARY OF THE INVENTION
This invention presents a mechanically interlocked joint between a metallic boss and a polymeric liner in a Type 4 cylinder. A V-shaped protruding projection is machined into the boss's internal bore, immediately following the O-ring groove. When torque is applied during assembly, the projection deforms the liner neck to create a permanent V-notch groove, resulting in:
• A tight, leak-proof mechanical seal
• Enhanced resistance to displacement, fatigue, and thermal cycling
• Simplified manufacturing by eliminating adhesives or chemical bonding

BRIEF DESCRIPTION OF THE DRAWINGS
1. Figure 1 – Exploded view of the Type 4 cylinder showing liner and boss.
2. Figure 2 – Longitudinal section of boss showing the V-projection and O-ring groove.
3. Figure 3 – Enlarged cross-section of boss-liner interface with induced groove.
4. Figure 4 – Assembly sequence depicting torque-induced liner deformation.
5. Figure 5 – FEA results highlighting stress concentration and interlock behavior.
6. Figure 6 – Photograph of fabricated interlock showing V-notch in liner.

DETAILED DESCRIPTION OF THE INVENTION
1. Metallic Boss Design:
Machined from Aluminum 6061-T6 or Stainless Steel AISI 316, the boss includes a standard O-ring groove, followed by a precision V-shaped protruding projection. The projection is positioned at a defined axial distance to prevent interference with sealing.
2. Polymeric Liner Material:
Manufactured via blow molding using HDPE, PA6, or PA12, the liner neck is sufficiently ductile to deform plastically under torque, forming a permanent interlock with the boss projection.
3. V-Projection Geometry:
• Internal angle: 60°–90°
• Depth: 0.5 mm–1.5 mm
• Location: Downstream of O-ring groove
• Machining Method: CNC lathe with tight tolerance control
4. Assembly & Interlocking Process:
Torque (20–60 Nm) is applied to the liner during insertion into the boss. The liner material plastically deforms into the V-shaped groove, forming a self-locking interlock and creating a mechanical V-notch groove in the liner.
5. Mechanical and Sealing Benefits:
• Axial Retention: Prevents pull-out under internal pressure and shock.
• Enhanced Gas Sealing: Minimizes micro-movements, improving O-ring efficiency.
• Thermal Tolerance: Withstands expansion differentials between metal and plastic.
• No Adhesives: Simplifies production and avoids long-term degradation.
• Alignment Integrity: Maintains concentricity with composite wrapping.
The present invention relates to a unique mechanical interlock system between a polymeric liner and a metallic boss designed for Type 4 composite pressure vessels. These cylinders are widely used in the storage and transportation of high-pressure gases, such as Compressed Natural Gas (CNG) and hydrogen. A major innovation introduced by this invention lies in a novel V-shaped mechanical projection that is precision-machined into the inner circumference of the metallic boss. During assembly, this projection permanently embosses a V-notch groove into the neck of the polymeric liner through torque-induced plastic deformation. This physical engagement acts as a mechanical lock, preventing axial displacement or rotational slippage of the liner under high internal gas pressures. The described system thus ensures enhanced structural integrity, long-term sealing reliability, and ease of quality assurance — without resorting to adhesives or chemical bonding techniques.
Figure 1 illustrates an exploded isometric view of the full composite pressure cylinder assembly. The major components of the system include a polymeric liner and a metallic boss. The polymeric liner is typically blow-molded and thermoplastic in nature, forming the internal gas containment barrier of the cylinder. This liner is designed to endure the full working pressure and ensure gas impermeability. The metallic boss is a machined insert, usually made of high-strength aluminum or stainless steel, that interfaces with the valve and serves as the inlet/outlet port for gas filling and withdrawal. During assembly, the liner neck is inserted into the bore of the metallic boss, forming a tight interface that becomes the foundation for further sealing mechanisms. This boss-liner connection is critical to maintaining gas-tightness during cyclic pressure loads, thermal excursions, and mechanical vibrations. The illustrated layout shows the alignment of components prior to torque-induced interlock installation.
Figure 2 provides a detailed longitudinal cross-sectional representation of the metallic boss, revealing two important internal geometrical features. First, there is a circumferential O-ring groove, carefully machined to standard tolerances. This groove accommodates an elastomeric O-ring that functions as the secondary sealing interface between the boss and liner. The second, and more innovative feature, is a V-shaped protruding projection located axially downstream of the O-ring groove. This projection is sharply machined into the inner surface of the boss and designed with a precise angular profile. Typically, the included angle of the V-projection ranges between 60 to 90 degrees, while the depth is maintained in the range of 0.5 mm to 1.5 mm, depending on the thickness and material properties of the liner. This projection is instrumental in creating a mechanical interlock, which securely fastens the liner within the boss during the torque-driven assembly process. The depth and geometry of this protrusion ensure a uniform, symmetrical engagement across the liner circumference, facilitating predictable and repeatable deformation.
Figures 3 and 4 collectively depict the operational engagement process between the polymeric liner and the metallic boss. In Figure 3, a sectional isometric view of the boss displays the relative positioning of the O-ring groove and the V-shaped projection. Upon insertion of the liner, and during controlled torque application, the liner's thermoplastic neck begins to conform to the internal geometry of the boss. Figure 4 visually captures the transformation that occurs during this torque-driven deformation process. As the torque is applied — typically in the range of 20 Nm to 60 Nm — the liner’s neck undergoes plastic deformation. The V-shaped projection mechanically embeds into the thermoplastic wall, thereby creating a mirror-imaged recessed V-notch groove in the liner neck. This groove becomes a permanent feature of the liner post-assembly and functions as the primary mechanical anchor. The interlocked groove resists axial pull-out forces caused by internal pressure and prevents relative rotational movement. Moreover, it minimizes any liner displacement in the sealing zone, thus contributing to consistent O-ring compression and improved sealing performance.
Figure 5 provides a sequential breakdown of the interlocking process, demonstrating the transition from component placement to full mechanical engagement. The first step involves axial insertion of the polymeric liner into the bore of the metallic boss. In the second step, torque is applied to either rotate or press-fit the liner further into the boss. As torque increases, the V-shaped projection starts to penetrate and deform the softer polymeric material. In the final step, the liner conforms fully to the internal geometry, forming the recessed V-notch groove and thus creating a non-reversible mechanical lock. The process requires no adhesives or fasteners and can be performed using conventional mechanical fixtures. Also included in Figure 5 are results from Finite Element Analysis (FEA), which validate the stress distribution during and after the assembly. The simulation results confirm that stress is symmetrically distributed around the V-notch zone, avoiding any localized concentrations that could induce fatigue cracking. Furthermore, the O-ring groove remains isolated from the deformation zone, ensuring its sealing integrity remains uncompromised.
Figure 6 presents a close-up photographic image of the final interlocked assembly. The V-notch groove formed in the polymeric liner is clearly visible, demonstrating successful torque-induced deformation. This visible groove provides two significant advantages: first, it serves as a mechanical key, ensuring irreversible liner orientation; second, it enables visual inspection and quality assurance during post-assembly verification. The assembly maintains high concentricity and sealing reliability, even under internal pressures up to 300 bar. The absence of adhesives eliminates potential issues related to chemical incompatibility, aging, or rework difficulty. Moreover, this mechanical locking system allows for easier disassembly and recycling, should the need arise. The design’s simplicity enables automation and facilitates higher manufacturing throughput with reduced production variability.
Following the boss-liner interlock, the entire cylinder assembly undergoes composite overwrapping, using high-performance fibers such as carbon fiber or glass fiber, impregnated with epoxy resin. The interlocked liner neck acts as a stable fixture during this wrapping process, preventing any radial or axial slippage. This fixed alignment ensures uniform distribution of hoop stresses during filament winding, which is critical to maintaining burst strength and fatigue life. The mechanical interlock thus functions as a structural anchor, ensuring that the fiber-wrapped layers remain evenly tensioned and securely bonded. This aids in automated winding by establishing predictable boundary conditions and reduces the risk of localized stress concentrations or wrinkles. As a result, the final composite cylinder exhibits superior mechanical properties, improved durability, and consistent dimensional stability.
To ensure optimal mechanical performance and chemical resistance, the invention relies on judicious material selection for both the metallic boss and the polymeric liner. The metallic boss may be constructed from:
• Aluminum 6061-T6: for applications requiring a lightweight and corrosion-resistant structure, especially suitable for hydrogen.
• Stainless Steel AISI 316: for applications requiring higher strength and exposure to aggressive chemicals.
The polymeric liner materials include:
• HDPE (High-Density Polyethylene): Excellent gas barrier and fatigue resistance.
• PA6 (Polyamide 6) and PA12 (Polyamide 12): Superior thermal stability and crack resistance.
These material combinations are carefully selected to provide:
• Compatibility with CNG and hydrogen environments
• Excellent mechanical deformability for interlock formation
• Long-term aging resistance and low permeation rates

The mechanical interlock system offers several functional and operational advantages over conventional bonding methods. First, it delivers leak-proof performance, even under harsh conditions involving thermal cycling, pressure pulsation, and vibration. Second, the absence of adhesives simplifies the manufacturing process, reducing cycle time and material handling complexity. Third, the design allows for reusability and ease of disassembly, making it more environmentally friendly and adaptable for maintenance. Additionally, the mechanical integrity of the interlock ensures long-term sealing without material fatigue, even in cyclic applications. This results in reduced lifecycle costs, longer operational life, and enhanced safety margins. Collectively, these advantages make the system suitable for next-generation lightweight composite pressure cylinders for clean mobility, industrial gas distribution, and hydrogen storage.
 
, Claims:1. A mechanical interlocking system for Type 4 gas cylinders, comprising:
a) A metallic boss with a machined V-shaped protruding projection on its inner wall;
b) A polymeric liner with a neck portion configured to deform under torque;
c) Wherein the projection forms a V-notch groove in the liner during assembly, resulting in a permanent mechanical interlock.
2. The system of Claim 1, wherein the projection is located axially after an O-ring groove.
3. The system of Claim 1, wherein the V-shape has an internal angle between 60° and 90°.
4. The system of Claim 1, wherein the V-shape has a depth between 0.5 mm and 1.5 mm.
5. The system of Claim 1, wherein torque between 20 Nm and 60 Nm is applied to deform the liner.
6. The system of Claim 1, wherein the liner is made from HDPE, PA6, or PA12.
7. The system of Claim 1, wherein the boss is made from aluminum 6061-T6 or stainless steel AISI 316.
8. The system of Claim 1, wherein the interlock prevents axial movement under internal pressures up to 300 bar.
9. The system of Claim 1, wherein the interlock maintains alignment during composite overwrapping.
10. The system of Claim 1, wherein the V-notch improves sealing and co-axial retention without adhesives.