Crankshaft: 9 Important Facts You Should Know

Contents: Crankshaft

• What is Crankshaft?
• Material and manufacture of Crankshafts
• Crankshaft diagram
• Crankshaft Design Procedure
• Crankshaft Deflection
• Crankshaft Deflection Curve Plotting
• Marine Crankshaft Failure Case Study
• Failure Analysis of Boxer Diesel Crankshaft: Case Study
• Crankshaft Fatigue Failure Analysis: A Review
• Failure of Diesel Engine Crankshaft: A Case Study

What is crankshaft?

“A crankshaft is a shaft driven by a crank mechanism, involving of a series of cranks and crankpins to which the connecting rods of an engine is attached. It is a mechanical part able to perform a conversion between reciprocating motion and rotational motion. A reciprocating engine converts reciprocating motion of a piston to the rotational form, although in a reciprocating compressor, it translates opposite way means rotational to reciprocating forms. During this change in-between two motions, the crankshafts have “crank throws” or “crankpins” additional bearing surface which axis is offset from the crank, to which the “big end” of the connecting rod from each cylinder is attached.”

A crankshaft can be described as a component used to convert the piston’s reciprocating motion to the shaft into rotatory motion or vice versa. In simple words, it isa shaft with a crank attachment.

A typical crankshaft comprises of three sections:

1. The shaft section that revolves inside the main bearings.
2. The crankpins
3. The crank arms or webs.

This is categorized in two types as per position of crank:

1. Side crankshaft
2. Centre crankshaft

The crankshaft can be further categorized in Single throw crank-shafts and multi throw crank-shafts depending on the no. of cranks in the shaft. A crankshaft which possesses only center crank or one-sided crank is entitled as single-throw crankshaft. A crankshaft with 2 or multiple center cranks or ‘2’ side cranks,  ‘1’ on each end is recognized as “multi-throw crankshafts”. The Side crank configuration includes geometric simplicity, are comparatively simple to be manufactured and assembled. They can be used with simple slide-on bearings and are relatively cheaper than Center crankshaft.

The center crank configuration provides better stability and balancing of forces with lower induced lower stresses. Their manufacturing cost is high, and a split connecting rod bearing is required for assembly. Applications which require multiple pistons working in phase, a multi-throw crankshaft can be developed by placing several centers cranks side-by-side, in a specified sequence, along a common centerline of rotation. The throws are rotationally indexed to provide the desired phasing.

Multi-cylinder internal combustion engines such as Inline and V- series Engine utilizes Multi-throw crankshaft. All types of crankshafts Experience dynamic forces generated by the rotating eccentric mass center at each crank pin. It is often necessary to utilize counterweights and dynamic balancing to minimize shaking forces, tractive effort and swaying couples generated by these inertia forces.

Material and manufacture of Crankshafts:

The crankshaft often experiences shocks and fatigues loading condition. Thus, the material of the crankshaft must possess more toughness and better resistance to fatigue. They are usually product of carbon steel, certain steel or cast-iron materials. For Engines used in industry, the crankshafts are generally generated from carbon steel such as 40-C-8, 55-C- 8 and 60-C-4.

In transport engine, manganese steel i.e., 20-Mn-2, 27-Mn-2 and 37-Mn-2 are commonly utilized to prepare the crank shafts. In aero engines, nickel-chromium steel such as 35-Ni-1-Cr-60 and 40-Ni-2-Cr-1-Mo-28 are generally utilized for manufacturing the crankshaft. 

The crank shafts are commonly finished by drop forging or casting process. The surface hardening of the crankpin is finished through the case carburizing process, Nitriding or induction hardening process. The selected Crankshaft materials must meet both the structural strength requirements and the bearing-site wear requirements.

In the typical crankshaft application, soft, ductile sleeves are attached to the connecting rod or the frame, so the crankshaft material must have the ability to provide a hard surface at the bearing sites. Many materials may meet structural strength requirements, but providing wear resistance at the bearing sites narrows the list of acceptable candidates.

Because of the asymmetric geometry, many crankshafts have been manufactured by casting or forging a “blank,” to be finish-machined later. Built-up weldments are used in some applications. Traditionally, cast iron, cast steel, and wrought steel have been used for crank shafts. The use of selectively carburized and hardened bearing surfaces is also every day. 

Crankshaft Design Procedure

The subsequent procedure has to be followed for design.

1. Calculate the magnitude of the different loads acts upon the crank shaft.
2. According to the loads, calculate the distance between the support structures and positions.
3. For simplistic and safe design, the shaft has to be supported at the bearings’ center and all the forces and reactions has to be acts upon at those points. The distance between the supports be subject to on the length of the bearing, which usually depend on the shaft’s dia as of the tolerable bearing pressures.
4. The thickness of the webs is expected to be from 0.4ds to 0.6ds, wherever “ds” is the shaft’s diameter. It usually considers as 0.22*D to 0.32*D, where D is the cylinder’s bore diameter in mm.
5. Here and now estimate the distance between the support structures.
6. Assuming the acceptable bending and shear stresses for Crank shaft material, find the dimension of the crankshaft.

Crankshaft Deflection

The crankshaft consists of the main shaft segments, individually reinforced by the main bearing, and then several web-shafts on which the specific piston connecting rod will rotate. The throw crank that is the crank pins and the connecting arms must be square with no deflection. If this is not the case, it causes unusual wear on the main bearings. A dial gauge detects the misalignment of the crank shaft between the crank arms. It is the uneven wear that occurs between the several segments of the crankshaft’s central axis.

Crankshafts Deflection Curve Plotting

• From the centerline of the crank shaft, A straight line is drawn parallel to it, and then perpendicular lines from each unit are drawn towards this parallel line.
• After taking the crank shaft deflection of each unit, the values derived are noted above every unit of the crank web in the above graph.
• Plot the distance -5.0 mm, which is the first deflection reading, downwards (for negative value and upwards for positive value) from the reference line on the center line of the unit and have the line “a-b” that is at an angle proportionate to the deflection at ‘a’.
• This line is extended to intersect the center line of the next unit. The subsequent step is to calculate the deflection from this point of joint and join the point from the preceding point, which will escalate to the line “b-c”. The steps have to be repeated again till completion.
• Plot a smooth curve between these points and compare this curve’s position with respect to the baseline XY. In the above graph, the curve drawn from the readings of unit 1 and 2 is being too far away from the baseline compared to the rest of the curve and hence need attention.

Marine Crankshaft failure Case Study

The case study done is about the tragic failure of a web marine crankshaft.  The crank shaft is subjected to high bending and torsion, and its combined effect on failure of the crank shaft is analyzed.  The microscopic observation suggested that the crack initiation began on the crankpin’s filet due to rotary bending, and the propagation was a combination of cyclic bending and steady torsion.  The number of cycles from crack initiation to the crank shaft’s final failure was found by readings of the main engine operation on board. Benchmarks left on the fatigue crack surface are taken into consideration.

By using the linear elastic fracture mechanics, the cycles calculated depicted that the propagation was quick. It also shows that the level of bending stress was quite high compared with total cycles of the main engine in service. Microstructure defects or inclusions were not observed; thus, it indicates that the failure was due to external cause and not the internal intrinsic defect.

The crank shaft material had configuration (42CrMo4 + Ni + V) (chemical composition, %: C = 0.39; Si = 0.27; Mn = 0.79; P = 0.015; S = .014; Cr = 1.14; Mo = 0.21; Ni = 0.45; V = 0.10). The crank shaft of the main engine has damaged. The crank-web no. 4 has broken. Material near the crack initiation region was analyzed, and it showed bainitic microstructure. The material had hardness vickers285.

The fatigue looks as if in two different surfaces, one vertical to the crank shaft and the other in the horizontal plane with the crank shaft with changeover zones among two planes. Thus, the tragic failure of the above marine crank shaft was by fatigue and a combined with the rotating-bending with the steady torsion. The research and observation and development of new crank shafts are in progress to avoid this type of failure.

Reference:

Fonte MA, Freitas MM. Marine main engine crank shaft failure analysis: A case study, Engineering Failure Analysis 16 (2009) 1940–1947

Failure Analysis of Boxer Diesel Crankshaft: Case Study

The report is about the failure mode analysis of boxer diesel engine crank shaft. Crank shaft is the component that experience a higher complex dynamic loading because of rotating bending supplemented with torsion and bending on crankpin. Crank shafts are subjected to multi-axial loading. Bending-stress and shear-stress due to twisting and torsional-loading because of power-transmissions. Crank shafts are manufactured from forged steel, nodular cast iron and aus-tempered ductile-iron.

They should possess adequate strength, toughness, hardness, and high fatigue strength. They must be easy to machine and heat treat and shaped. Heat treatment increases wear resistance; thus, all diesel crank shafts are heat treated. They are surface hardened to enhance fatigue strength. High-level stresses are observed on critical zones like web fillets and the effects of centrifugal force due to power transmission and vibrations. The fatigue fracture near the web fillet region is the major cause of crank shaft failure since the crack generation, and propagation occurs through this zone. 

The specifications of the crankshaft of a box motor are: displacement = 2000 cu. cm, diameter cylinder = 100 mm, max power = 150 HP, max torque = 350 N m. It has been observed that after 95,000 km in service, the failure of crank shaft takes place. Fatigue failure has occurred at nearly 2000 manufactured engines. After analysis, it has been noted that the weakness of two central steel shells and the yielding of bedplate bridges due to cracking were the main culprits of failure of crank shaft.

The crank shaft’s bending amplitude increases from cracked steel shells’ weakness and the bridges of the bedplate, which are beneath them. There was certainly no evidence of material defects or misalignment of main journal bearings. The devastating failure of the crank shaft was due to flawed design of steel support shells and bedplate bridges. The improved design from the manufacturer will solve this problem.

Reference:

M. Fonte et al., Crankshaft failure analysis of a boxer diesel motor, Engineering Failure Analysis 56 (2015) 109–115.

Crankshaft Fatigue Failure Analysis: A Review

In this paper, the root cause of fracture of the air compressor’s crank shaft is being analyzed using various methods and parameters like chemical composition, mechanical property, macroscopic, microscopic characteristics, and theoretic calculations. This paper also aims in improving the design, fatigue strength and work reliability of the crank shaft. The crank shaft used in this study is 42CrMo steel which is forged and heat-treated and nitridated to increase the fatigue strength of crank shaft.  The analyzing procedure for the cause of crank shaft fracture is carried out in three parts:

• Experimental analysis of crank shaft
• Macroscopic features and microstructure analysis
• Theoretical calculations

The chemical element analysis is being done to accurately determine the crank shaft material’s chemical composition and check if they are under the standard permissible values. It is done with the help of spectrometer. The fractured surfaces are classified into three regions: (1) fatigue crack initiation region, (2) fatigue expansion region and (3) static fracture region.

During the analysis, its w found that the fatigue crack growth rate is high due to high bending. The misalignment of main journals and small fillet to lubrication hole are the leading causes of high bending. The fatigue crack was initiated on the edge of the lubrication hole and thus led to the fracture. The beach marks produced due to small overloads because of starting and stopping the compressor were not visible. In a particular rotating cycle after a period of standard work, micro-cracks due to high bending stress concentration appeared on the lubrication hole’s fillet. However, the crank shaft can still close to normal working condition.

As the operating time went on increasing, the fluctuation also increased, leading the cracks to propagate to the static fracture region, leading to complete failure. The microscopic observation of the fracture surface measured utilizing Scanning Electron Microscopy (SEM), which showed that crack at the edge of the lubricating hole was the reason to fracture the crank shaft. According to the theoretical calculation, the curve for safety for the lubrication hole and fillet region is obtained, which helps identify the weakest sections.

By improving the surface quality and reducing surface roughness reliability of the crank shaft can be increased.  Proper alignment of main journals will reduce induced bending stress and increase the fatigue life of crank shaft.

Reference:

W.Li et al., Analysis of Crankshaft fatigue failure, Engineering Failure Analysis 55 (2015) 139–147.

Failure of Diesel Engine Crankshaft: A Case Study

In this paper, the failure analysis, modal, and stress analysis of a diesel engine’s crankshaft is conducted. To evaluate the fracture of crankshaft material, both the visual inspection and investigation were done. The engine used was S-4003, and its crank shaft was ruptured near the crankpin four after 5500 hours of operation.  The crankshaft was broken after about 30h to 700h of engine operation. The additional analysis showed the presence of micro-cracks near the 2nd crankpin and 2nd journal. The study showed that the primary reason behind the failure was a faulty grinding process.

For further experimental analysis, the specimen was cut from the damaged part. Non-linear Finite element analysis was used to identify the reasons for the abrupt failure of crankshaft. The analysis was performed for determining the stresses induced in the shaft due to cyclic loading conditions when the engine runs at maximum power.

Numerical analysis is used to find the relation between the connecting rod and the crankshaft by applying complex boundary conditions. For the determination of modes and frequency of free vibration, numerical modal analysis of the crankshaft was performed.

After the analysis, it was observed that the stress value in the fillet of the crankpin no.4 was about 6% of the yield stress of crankshaft material. The modal analysis gave the result that during the second mode of free vibration, the high-stress area was found in the area where the crack generation took place (critical zone).

On further observation, it was discovered that crankshaft failure occurred by resonant vibration generated due to unbalanced masses on the shaft, which induced high cyclic stress conditions, causing it to decrease crankshaft’s fatigue life.

Reference:

Lucjan Witek et al., Failure investigation of a crankshaft of diesel engine, Procedia Structural Integrity 5 (2017) 369–376

To know about Strength of material click here

Hakimuddin Bawangaonwala

I am Hakimuddin Bawangaonwala , A Mechanical Design Engineer with Expertise in Mechanical Design and Development. I have Completed M. Tech in Design Engineering and has 2.5 years of Research Experience. Till now Published Two research papers on Hard Turning and Finite Element Analysis of Heat Treatment Fixtures. My Area of Interest is Machine Design, Strength of Material, Heat Transfer, Thermal Engineering etc. Proficient in CATIA and ANSYS Software for CAD and CAE. Apart from Research.