Double-End Stud, bm=2d

Double-End Stud bm=2d — Product Introduction

Double-end stud bm=2d (GB/T 900-1988) is a fastener with unequal thread lengths at both ends. The tap-end thread length bm is equal to 2 times the thread diameter (bm=2d). It is specially designed so that one end is screwed into the internal threaded hole of the connected part, and the other end passes through a through hole and is locked with a nut. The current standard is GB/T 900-1988, which specifies dimensions and technical requirements for thread diameters from M2 to M48. Its most typical design purpose is the connection between aluminum alloy connected parts and steel connected parts, using the extended tap-end thread to compensate for the insufficient strength of internal threads in aluminum alloy.

1. Product Features

• bm=2d extended tap end: The tap-end thread length is 2 times the thread diameter. It is longer than bm=1d (steel-to-steel) and bm=1.25d/1.5d (steel-to-cast iron), and is specially used for screwing into lower-strength aluminum alloy bodies, increasing thread engagement length to improve pull-out resistance.
• Unequal thread lengths at both ends: One end is the tap end (bm=2d), and the other end is the fastening end, used with a nut for locking. The tap end can be designed with a transition-fit thread, while the fastening end uses a fine-pitch ordinary thread, suitable for precision connections such as automobiles.
• Type A and Type B structures: Type A is the straight shank type (ds approximately equal to the thread pitch diameter), and Type B is the reduced shank type (ds approximately equal to the thread minor diameter). They can be selected according to assembly space and strength requirements.
• Complete specifications and materials: Thread diameters cover M2–M48. Materials include stainless steel, carbon steel and alloy steel (such as Q235, 45#, 40Cr, 35CrMoA, etc.). Strength grades cover 4.8–12.9.
• Various surface treatments: Options include no treatment, oxidation (blackening), zinc plating and passivation, zinc-nickel alloy coating, etc., meeting different anti-corrosion requirements.

2. Product Advantages

• Prevents damage to internal threads in aluminum alloy bodies: Internal threads in aluminum alloy have relatively low strength. Repeated assembly and disassembly with ordinary bolts can easily cause thread wear or damage. With bm=2d double-end studs, one end is permanently screwed into the main body. During disassembly, only the nut needs to be loosened, so the main body threads are not damaged, making maintenance and replacement very convenient.
• Suitable for accessory installation requiring frequent disassembly: For accessories that need frequent removal, such as sight glasses, mechanical seal seats and reducer frames, double-end studs avoid turning the base threads during every assembly and disassembly, significantly extending the service life of the main body threaded holes.
• Suitable for thick connected parts and long-distance connections: When the connected parts are very thick and ordinary bolts are insufficient in length, double-end studs provide a more flexible connection solution, especially suitable for anchor bolts, concrete roof trusses and similar applications.
• Strong load-bearing capacity and adaptability to complex working conditions: Made of materials such as 45#, 40Cr and 35CrMoA. Carbon steel tensile strength can reach over 700 MPa. Stainless steel versions have good corrosion resistance, and alloy steel versions can be used in high-temperature and high-pressure conditions.
• Good standardized interchangeability: As a national standard product, dimensions are uniform and marking is standardized (e.g., “Stud GB/T 900 M10×50”). Products from different suppliers can be used interchangeably.

3. Scope of Application

• Automobile and motorcycle engines: Connections between aluminum alloy cylinder blocks, transmission housings and steel cover plates or flanges are the most core application scenario for bm=2d double-end studs.
• Mechanical equipment and accessory installation: Connections between the main body and accessories requiring frequent disassembly, such as sight glasses, mechanical seal seats, reducer frames, pumps and valves.
• Mining and heavy machinery: Connections for mining machinery, crushers, rolling mills and other equipment subjected to relatively large loads and vibration.
• Bridges and steel structures: Locations requiring high-strength connections, such as bridges, towers, long-span steel structures and boiler steel structures.
• Pipelines and pressure vessels: Flange connections for oil pipelines, chemical reactors, heat exchangers, valves, etc. Alloy steel or stainless steel materials can adapt to corrosive or high-temperature and high-pressure environments.
• Power and railway: Connection locations such as transmission towers and railway bridges that require strict matching of design loads and service environments.