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Crankpin
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Crankpin
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A crankpin or crank pin, also known as a rod bearing journal,[1] is a mechanical device in an engine which connects the crankshaft to the connecting rod for each cylinder. It has a cylindrical surface, to allow the crankpin to rotate relative to the "big end" of the connecting rod.[2][3][4]

The most common configuration is for a crankpin to serve one cylinder. However, many V engines have each crankpin shared by each pair of cylinders.

Design

[edit]
See also: Crankshaft and Connecting rod

The crankpin connects to the larger end of the connecting rod for each cylinder. This end of the connecting rod is called the "big end", as opposed to the "small end" or "little end" (which connects to the wrist/gudgeon pin in the piston).

The bearing which allows the crankpin to rotate around its shaft is called the "rod bearing".[5] In automotive engines, the most common type of rod bearing is the plain bearing, however bushings or roller bearings are also used in some engines.

Configurations

[edit]

In a single-cylinder engine, straight engine or flat engine, each crankpin normally serves just one cylinder. This results for a relatively simple design which is the cheapest to produce. Some V-twin engines use a single cylinder per crankpin.

Most V engines have each pair of cylinders sharing a crankpin. This usually requires an offset between the cylinders in each bank, resulting in a simple connecting rod design. If a cylinder offset is not used, then the connecting rods must be articulated or forked at the big end. Forked connecting rods are mainly used in V-twin motorcycle engines, but in the past were found on a number of automobile and aero engines, such as the Rolls-Royce Merlin aero engine of the WWII era. Articulated connecting rods consist of a "master" rod attached to the crank pin, with a "slave" rod connected to the big end of the master rod. This design was used in older or exotic V engines.

Radial engines use a more complicated version of articulated connecting rods, where a single "master" connecting rod attached to the single crankpin (one for each row in multi-row designs), and smaller bearings for each of the corresponding cylinders machined into the big end of the master rod.

The driving wheels (crank axle) of a German Reichsbahn Class 44. The crank pin can be seen on the right hand wheel.

Cylindrical crank pins were fitted onto the driving wheels of steam locomotives. They were connected to the driving rods that transmitted power from the cylinder to the wheel. The crank pin was usually made of high-quality steel because it had to withstand high forces.

The crank pin of a locomotive corresponds to the offset of a crankshaft in other crank drives. The distance from the centre of the crank pin to the centre of the wheel is also called offset and is exactly half the stroke of the pistons.

See also

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Crankpin

View on Grokipedia
from Grokipedia
A crankpin, also known as a connecting rod journal, is a cylindrical component integral to the in reciprocating and machinery, serving as the point of attachment for the big end of the to convert the piston's into the crankshaft's rotational motion. Positioned offset from the crankshaft's main journals and connected by crank webs, the crankpin experiences high cyclic loads and stresses as it transmits forces from the during the engine's power, compression, exhaust, and intake . Its design typically includes a smooth bearing surface lubricated by engine through internal passages to minimize and , with the center-to-center distance between the crankpin and main journal equaling half the piston's stroke length. Crankpins are commonly forged from high-strength alloy , such as manganese-molybdenum or nickel-chromium-molybdenum variants, to withstand and impact, though or may be used in specific applications like smaller engines or marine diesels. In internal combustion engines, they are critical for efficient power transfer, and failures—often due to inadequate , material defects, or excessive loads—can lead to catastrophic engine damage.

Fundamentals

Definition and Terminology

A crankpin, also known as a crank pin or rod bearing journal, is a cylindrical journal formed on the of a that serves as the attachment point for the big end of the . This precision-ground surface acts as a rotating pivot, facilitating the connection between the reciprocating motion of the via the connecting rod and the rotational motion of the . By enabling this interface, the crankpin plays a foundational role in the assembly, distinct from other components in the . In engineering terminology, the crankpin is synonymous with terms such as connecting rod journal, big-end journal, or , emphasizing its function as the bearing surface for the 's large end. It is differentiated from main journals, which are the coaxial cylindrical sections at the 's center that provide rotational support within the engine block's main bearings. Additionally, the crankpin is distinct from the wrist pin (also called the or ), which connects the small end of the to the , forming the upper pivot in the linkage. These distinctions ensure clear nomenclature in mechanical design, avoiding confusion in assembly and maintenance contexts. The basic geometry of a crankpin includes an offset from the crankshaft's main axis, known as the throw or crank radius, which equals half the piston's to achieve the full reciprocating . This offset creates the eccentric motion essential for motion conversion. The crankpin's diameter is engineered for load-bearing capacity, while its (bearing width) is designed to optimize hydrodynamic and distribute forces evenly across the interface.

Role in Reciprocating Motion

The crankpin serves as the pivotal interface on the where the big end of the attaches, enabling the conversion of the piston's linear into the 's rotational motion in reciprocating engines. This linkage transmits the piston's up-and-down movement—driven by gas pressure and inertia—through the to the offset crankpin, which orbits around the crankshaft's main journal axis, producing that rotates the . During the engine's four- cycle, the crankpin accommodates varying forces: compressive loads predominate in the power from pressure pushing the downward, in the compression as the rises to compress the air-fuel mixture, and in the exhaust when the ascends to expel gases; tensile forces arise particularly during the at high engine speeds due to inertial effects pulling the downward against conditions. Kinematically, the crankpin's position defines the crankshaft throw, which is the radial offset distance rr from the main journal centerline to the crankpin centerline; this orbital path of the crankpin directly governs the piston's length, calculated as S=2rS = 2r, where SS is the full piston travel from top dead center to bottom dead center. As the crankshaft rotates, the crankpin traces a circular path with rr, pulling and pushing the to achieve the full , with the length influencing secondary motion effects like piston side but not altering the primary dimension. This geometric relationship ensures precise synchronization between piston displacement and crankshaft , fundamental to timing and displacement volume. Under operational loads, the crankpin endures high cyclic stresses from the alternating compressive and tensile forces transmitted via the , as well as torsional stresses from the generated across the ; in high-output engines, stresses can exceed 50,000 psi with torsional shear stresses up to around 7,000 psi under peak conditions, and higher peaks in finite element analyses. These dynamic stresses arise from the rapid cycling of events and inertial reversals, contributing to challenges where uneven firing across cylinders amplifies vibrations if not counteracted by design features like counterweights. Proper management of these loads is essential to mitigate and ensure smooth operation, as excessive vibration can propagate through the .

Design and Engineering

Materials and Construction

Crankpins are primarily constructed from high-strength forged steels to withstand the intense reciprocating loads and bending stresses encountered in engines. A common choice is 4340 , valued for its excellent tensile strength exceeding 100,000 psi and resistance, making it suitable for high-performance applications. For cost-effective production in standard engines, nodular or is often used, offering a tensile strength around 80,000-100,000 psi while being more machinable and economical than forged alternatives. Recent advancements as of 2024 include austempered (ADI) and microalloyed (MA) steels, which provide superior resistance, higher strength-to-weight ratios, and better compared to traditional nodular iron, enabling lighter designs in modern engines. In specialized racing engines, may be employed for their superior strength-to-weight ratio, reducing rotational mass and enabling higher RPMs, though their use remains limited due to high cost and manufacturing complexity. Construction typically involves integral forging, where the crankpin is formed as a single piece with the webs from a billet of , ensuring continuous flow and enhanced structural integrity under cyclic loading. This one-piece design is achieved through drop or closed-die processes, followed by precision machining to final dimensions. For larger or multi-piece assemblies, particularly in marine or heavy-duty engines, the crankpin may be separately forged and assembled via shrink-fitting into the webs, where and contraction create a secure to handle high torque without slippage. To enhance wear resistance and durability, crankpin surfaces undergo specialized treatments such as or induction hardening, which diffuse or rapidly and quench the outer layer to form a hard case while maintaining a tougher core. , often performed at 500-550°C in an atmosphere, produces a compound layer that improves life and resistance. These treatments typically achieve a surface of 50-60 Rockwell C on the journals, significantly reducing wear from bearings under operational stresses. Dimensional design of crankpins emphasizes balanced proportions to minimize stress concentrations and ensure stable bearing operation, with the length-to-diameter (l/d) generally ranging from 0.6 to 1.4 for optimal load distribution. Fillets at the transitions between the crankpin and webs are radiused to 0.04-0.06 times the pin , mitigating cracks from bending moments. tolerances are tightly controlled, with roundness and variations held to ±0.0005-0.001 inches to prevent uneven bearing wear and vibration.

Bearings, Lubrication, and Tolerances

In internal combustion engines, crankpin bearings primarily consist of plain journal bearings, often lined with babbitt alloy, which provide a low-friction sliding surface for the connecting rod's big end to rotate around the crankpin. These bearings distribute loads over a broad area and are standard in most automotive applications due to their ability to handle cyclic loading through hydrodynamic lubrication. In contrast, high-speed applications such as motorcycles frequently employ roller or needle bearings at the crankpin to reduce friction and support rapid reciprocation, where caged needle rollers fit within the connecting rod's big end for compact, high-load capacity. Over time, crankpin bearing design has evolved from solid bushings to advanced tri-metal constructions, featuring a backing, an intermediate copper-lead or layer for strength, and a thin babbitt overlay for enhanced performance. This progression improves embeddability—the capacity to absorb debris particles from the without scoring the crankpin—while maintaining resistance under varying loads. Tri-metal overlays, typically 0.0005 to 0.001 inches thick, offer superior conformability to minor crankpin imperfections compared to earlier solid designs. Lubrication of crankpin bearings relies on either splash methods, where oil is flung from the onto the assembly, or forced-feed systems that deliver pressurized oil through drilled passages in the to the crankpin journals. These mechanisms sustain a hydrodynamic oil film, typically 0.001 to 0.005 inches thick under loads at 2000 rpm, preventing direct metal contact and dissipating heat from forces. In forced-feed setups, oil flows from main bearings via cross-drilled channels to the crankpins, ensuring consistent film formation during operation. Tolerances for crankpin bearings emphasize precise clearances of 0.001 to 0.003 inches between the journal and bearing shell, verified using plastigage—a compressible strip placed on the crankpin, compressed under , and measured against a scale for oil gap assessment. This clearance allows oil film development while minimizing and . Common failure modes include scoring from oil starvation, where inadequate supply causes metal-to-metal contact, overheating, and surface grooves; remediation often involves regrinding to 0.010-inch undersize, paired with matching oversized bearings to restore geometry.

Configurations

Inline and V-Type Engines

In inline engines, each connects to a dedicated crankpin on the , with the pins positioned sequentially along the shaft to support balanced firing orders and minimize unbalanced forces. This one-to-one correspondence allows for straightforward conversion, where plain connecting rods attach directly to individual crankpins via two-piece bearings. For example, a four-cylinder inline employs a four-throw , with crankpins typically arranged in pairs at 180-degree intervals to achieve primary balance, as in standard multi-cylinder designs. In V-type engines, crankpins are shared by pairs of cylinders from opposing banks, optimizing crankshaft compactness; a typical V8 configuration features four such shared pins, each serving one cylinder from each bank to synchronize piston movement. To prevent rod interference on these common pins, forked or master-and-slave connecting rod assemblies are used, where the forked rod splits at the big end to accommodate the solid blade rod, allowing both to articulate on a single two-piece bearing around the crankpin. V angles between 60 and 90 degrees are selected to optimize crankpin loading, as narrower angles like 60 degrees in some V12 designs reduce side thrust on the pins by better aligning rod forces with the piston axis. Balance in both inline and V-type configurations relies on counterweights positioned adjacent to the crankpins to offset rotating masses and dampen vibrations from reciprocating components. In V8 engines specifically, flat-plane crankshafts arrange the four pins in a single plane with 90-degree firing intervals, providing even torque pulses and higher RPM potential but necessitating larger counterweights to mitigate inherent secondary imbalances, unlike cross-plane designs that offset pins at 90 degrees for superior primary and secondary balance through natural force cancellation, albeit with greater rotational . Proper of crankpin bearings via drilled oil passages is critical to sustain these dynamics under load.

Radial and Other Specialized Engines

In radial engines, the crankpin design centers around a single master connecting rod that attaches directly to the crankshaft's crankpin, while articulating rods from the outer cylinders connect to knuckles on the master rod rather than the crankpin itself, enabling the radial cylinder arrangement to drive a common output shaft. This configuration, as seen in the , a nine-cylinder air-cooled , uses a two-piece nickel-steel with a single-throw crankpin featuring a lengthwise oil passage for lubrication; the master rod, made of H-section chrome-nickel steel, slides onto the crankpin and is secured by a clamping cap screw, while eight articulated rods attach via bronze-bushed knuckle pins on the master rod's flanges. The master rod's bearing on the crankpin, often a steel-backed silver-lead type with floating clearance to reduce misalignment, handles peak loads from uneven firing orders—such as the 1-3-5-7-9-2-4-6-8 sequence in nine-cylinder radials—reaching up to 4,000-5,000 psi unit pressures during II-era operations at 2,500 rpm and 245 psi indicated . In , crankpins are mounted directly on the axles of the driving wheels rather than a central , with adjacent wheels' crankpins offset by 90 degrees (equivalent to half a stroke) to ensure continuous and prevent dead-center lockup, connected via side rods that couple multiple axles for synchronized and to distribute . These crankpins, typically forged from high-carbon for durability under high stresses, are sized based on bending formulas considering pressure and bearing length, with solid bearings providing the contact surface; in consolidation-class locomotives, the main crankpin must withstand compressive loads up to 70,000 pounds, while side-rod crankpins handle reduced forces calculated from wheel slip coefficients, often around 26,000 pounds for 40,000-pound wheel loads. Larger designs evolved to include roller bearings and side rods to manage increased loads from higher speeds and sizes, as configurations occasionally failed under excessive strain. Opposed-piston engines employ a dual-crankpin setup per , with each connected to its own —one at each end of the —geared together for synchronous operation, eliminating the need for a and enabling two-stroke cycles with improved . This configuration, as in historical designs like the diesel, uses separate crankpins to drive the opposing toward the center, where ports for and exhaust are uncovered sequentially, with the geared ensuring precise timing to avoid interference. Beam engines, suited for slow-speed applications like , feature a single central on the driven by a from the beam's pivot, converting the piston's into rotary output without complex multi-throw designs. In rotative beam engines, this solitary , often supported by a simple bearing, operates at low revolutions—typically under 50 rpm—to minimize wear in stationary setups, transmitting force from the overhead beam to a for steady, low-power tasks.

Historical Development

Origins in Early Engines

The crankpin, serving as the pivotal connection between the crankshaft and the connecting rod in reciprocating engines, traces its origins to ancient mechanical innovations that harnessed rotary motion for practical work. Archaeological evidence reveals the use of hand-cranked mechanisms as early as the 1st century AD in various milling devices, but the first documented integration of a crankpin with a connecting rod appeared in the 3rd century AD at the Hierapolis sawmill in Roman Syria. This water-powered facility employed the crank to convert the rotational energy of a water wheel into the linear motion needed to drive saw blades through stone, marking an early advancement in force transmission that influenced later engineering designs. By the 17th and 18th centuries, these foundational concepts evolved into the beam engines pioneered by , whose atmospheric engine of represented a breakthrough in steam-powered pumping for operations. Although initial Newcomen designs focused on vertical reciprocation via a rocking beam, subsequent rotative adaptations incorporated crankpins on a to transform the piston's up-and-down motion into continuous rotary output, enabling applications beyond mere pumping, such as driving machinery in mills. These early crankpins were typically forged from , a material chosen for its malleability in shaping complex forms, though it proved susceptible to under repeated cyclic loading from steam pressure variations. James Watt's refinements in the 1780s further embedded crankpins into architecture, supplanting the less efficient atmospheric engines with his separate condenser and double-acting designs. In Watt's rotative beam engines, the crankpin linked the —often guided by his innovative —to the , allowing efficient conversion of reciprocating motion to rotary power for industrial use, such as powering textile mills. This integration not only boosted by up to 75% over Newcomen models but also standardized the crankpin's role in balancing the offset geometry required for stroke, despite ongoing challenges with wrought iron's vulnerability to cracking under high-stress operations. The 19th century saw crankpins gain prominence in mobile applications through locomotive development, exemplified by George Stephenson's Rocket in 1829. This pioneering engine mounted crankpins directly on the driving wheels, connecting them via side rods to the pistons, which standardized the offset crank geometry to achieve a consistent 10-inch stroke while distributing power across multiple axles. Built primarily from wrought iron to withstand the locomotive's dynamic loads, these crankpins endured vibrational stresses but highlighted the material's fatigue limitations, prompting iterative design tweaks for reliability in rail transport.

Advancements in the 20th Century

In the early , crankpin designs in radial engines advanced significantly with the adoption of split crankpin configurations, as seen in the Wasp engine introduced in 1926. This design featured splined and bolted crankpins that replaced earlier clamp-type assemblies, improving reliability under high torsional loads and enabling higher engine RPMs while supporting lighter overall weights. By the 1930s, bored crankpins became standard, with demonstrating that internal boring doubled the strength of journals compared to solid designs; for example, the engine used undercut fillets and bored pins to achieve over a 2:1 improvement in fatigue life. Mid-century innovations focused on materials and heat treatments to withstand increasing combustion pressures exceeding 120 bar in piston engines. Forged alloy steels, such as SAE 4340 with additions of chromium, nickel, and molybdenum, replaced cast iron for crankpins, offering tensile strengths of 140,000–145,000 psi and enhanced fatigue resistance up to 165,000 psi after processing. Nitriding and shot peening emerged as key treatments; in the Rolls-Royce Merlin engine of 1933, nitriding after heat treatment raised crankpin surface hardness to 60 Rockwell C, boosting endurance by 25% and supporting power outputs up to 2,000 hp. Similarly, the Allison V-1710 engine in the 1940s optimized bored crankpin dimensions through strain gauge analysis, reducing weight to 125 lbs while maintaining balance via 12 counterweights. Later 20th-century developments emphasized precision manufacturing and reduction for high-performance automotive and aviation applications. Induction hardening and plasma nitriding of crankpins improved wear resistance under loads from accelerations, with microfinished surfaces achieving 5 microinches to minimize . In the R-2800 engine, revisions to two-piece crankshafts with face-splined crankpins and torsional dampers yielded over 32 lbs in weight savings alongside better control, addressing early failures and enhancing overall . These advancements collectively enabled crankpins to handle greater reciprocating forces in multi-cylinder configurations, such as cross-plane V8 designs, where optimized crankpin overlap ratios improved without excessive mass.

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