With Blessings of Shreemad Bhagwat Geeta

A self-study guide on Crankpin

By: Uma Sankar Sahu, B.Tech. Mechanical Engineer

Encouraged By: Mr. Santosh Kumar Sahu

Dedicated to: Baba (My Ideal), Bou (My Love) & all my readers Content:

I. Introduction

II. Purpose of Crank Pin

III. Location of Crank Pin in the IC Engine

IV. Different types of holes and their significance

V. Manufacturing process of Crankpin

VI. Inspection parameters

VII. Load & stress on the Crankpin

VIII. Crankpin materials

IX. Reason of Crankpin failure

X. Manufacturing defects

XI. References

2 I. Introduction:-

A short cylindrical bearing surface fitted between two arms of the crank and set parallel to the main crank shaft is called a crankpin. In an engine, cranks consist of a main shaft which rotates, crankpin which revolves like a planet and a crank throw to solidly connect them. In a reciprocating engine, the crankpins (also called the crank journals) are the journals of the big end bearings at the end of the connecting rod opposite to the pistons. Crankpin is a separate pin and assembles with the crank (fig .A) or it may be machined in to the crankshaft as one piece (fig. B). Our topic is focused on separate crankpin (fig. A) i.e. Crankpin as an element.

Fig. A Fig. B

In a multi-cylinder engine, crankpin serves one or more connecting rods at a time. For e.g. in a straight or flat engine, each crankpin serves just one cylinder. In a V-engine, each crankpin usually serves two cylinders, one in each cylinder bank. In a radial engine, each crankpin serves an entire row of cylinders.

II. Purpose of Crankpin:-

∑ The main purpose of crankpin is helping to convert the reciprocating motion in to rotational motion. ∑ It provides lubrication to the big end bearing for smooth running of the engine as well as increases the life of bearing.

3 III. Location of Crankpin in the Crank shaft assembly and how it is assembled?

Before going through the above question, let’s understand a brief about connecting rod for better understanding of the prescribed answer.

Connecting rod (CR):-

The name itself explains the meaning and significance. It is simply a rod which connects the piston and the crank. The CR has two ends i.e. one small end & another is big end. One side of the CR is attached to the piston with its small end by a piston pin or gudgeon pin and another side of the CR is attached to the crank with its big end by a crankpin. Earlier the CR is used to convert rotational motion to reciprocating motion; but now in automotive engines, CR is used to convert reciprocating motion to rotational motion. From the design point of view, CR is of 3 types as shown in the figure.

Fig. C Fig. D

The connecting rod (refer fig.C) for endothermic engine has the modular head and the foot equipped with a bushing, the central rod has the oil drip rod equipped with parts. The connecting rod is very simple to use by passing over big end bearing and then closing another side of the crank.

Location of Crankpin at the Crankshaft assembly:-

Crank shaft is a shaft which is attached to the crank for rotational motion. Here we are going to discuss two types of assemblies.

4 A. Separate Crankpin at the crankshaft assembly: (refer to fig. A)

Attach the Crank shaft LH to crankpin, then the connecting rod with big end bearing is allowed to pass over the crankpin and then attach the crankpin to the Crank shaft RH.

Fig. E

B. Machined crankpin at the crank shaft assembly: (refer to fig. B)

Here the crankpin is already machined with crank shaft; hence there is no need of separate crankpin to use. In this type of crank shaft assembly, the connecting rod with modular head and foot equipped with bushing is used.

IV. Holes that are present in crankpin:-

o Face hole: It is used for locking purpose.

5 o Dimple hole / OD hole: If there is no inclined hole inside a dimple hole then, dimple buffing is not required and the dimple hole in this case is called OD hole.

o Inclined hole: It is always inside the dimple hole. Both dimple hole/ OD hole and inclined hole is responsible for allowing the oil within the crankpin.

o Cross hole: Some crankpin have single cross hole & some have double cross hole. Cross hole is the hole directed radially. The oil that comes through center hole or inclined/dimple/OD hole lubricates the big end bearing through cross hole, as the bearing sits on the cross hole.

o Centre hole: It is the hole directed axially. Sometimes the center hole is blocked with cap, if dimple/inclined hole is present and in some case the center hole is used to allow the oil through crankpin in the absence of dimple/inclined hole.

6 V. Manufacturing process of Crankpins:-

Following process flow is applied for manufacturing of crankpin.

A. Raw material inspection B. Move to raw material storage C. Material move to sawing D. Sawing: In sawing process the raw material rods are cut in to small required pieces. E. CNC turning-1: In this process, actual length of crankpin is arrived by turning. F. CNC turning-2: It is the process of reducing outer diameter by turning operation. G. Centre hole drilling H. Face hole drilling I. Face hole chamfering J. OD dimple hole drilling K. Inclined hole drilling L. OD cross hole drilling at 1st side M. OD cross hole drilling at 2nd side N. OD cross hole chamfering O. Punch marking P. Heat treatment: It includes a no. of process as follows; o Visual check( no damage/ rust) o Arranging in fixtures( layer wise in cast tray) o Pre-washing (rinsing 10 minutes, resting 1 minute & spraying 10 minutes) o Pre-heating (4000c., 30 minutes) o Carburizing/ boost diffusion (9400c.) o Vestibule o Hardening o Quenching

7 o Subzero o Post washing (as like pre washing) o Tempering o De-fixturing o Oiling Q. Face cleaning & heat code marking R. Rough OD grinding: It is done on center less grinding machine. Here we can remove up to 55-60 microns of material on the OD & the rest parameters are maintained according to the different control plans for different sizes. S. Pre final OD grinding: It is done on CNC center less grinding machine. Though it is the second pass we are removing less material but maintaining a little bit finishing and accuracy. Here we are removing up to 15-20 microns of material on the OD. T. End chamfers grinding: It is a type of forming process. The grinding wheel used here has many projections and that are used to form end chamfer. Here also we are using center less grinding machine. U. Deburring: It is also called buffing operation, here the sharp edges of the end chamfer, cross holes & dimple holes are removed. Here we are maintaining cross hole chamfer radius up to 0.15-0.3 mm. V. Crack detection: It is also called Magnetic Particle Inspection (MPI). It works on the principle that, if the job is passing through an electric current; then the magnetic field creates & thus magnetic flux occurs. The surface where some corrosion or crack is present, at that place magnetic flux leakage occurs. When it is immersed to oil bath, the oil/powder particles concentrate at the crack. By the UV light we can see the crack. Here we normally check the surface cracks & bar cracks. For detection of burn crack we can use another machine called Burn crack testing machine. W. Eddy current sorter: After the MPI occurs, demagnetization of material is important as we should not use any magnetic element/ parts in the IC engines. Then using eddy current we are checking the hardness of the material for each and every job. X. Finish OD grinding: It is the most important operation. Here we have to use better technology for better surface finishing & maintaining the perfect material removing rate. The customer gives different tolerance level so called grade. We have to maintain the grading system in small microns. The grinding wheel we are using is a coupled wheel having 3-4 layers throughout the length of the spindle having different grades which are responsible for material removing, surface finish etc.. The automated dressing of grinding wheel for 20-40 seconds & metallic regulating wheel

8 are one of the main sources to meet the tolerance level. The coolant concentration has to be maintained @ 3-4% as the grinding operations are not so rough. Y. Super finishing: As its name given its work also similar. During this process, in between 2 rotating metallic rollers crankpins are allowed to pass and from the upper side of the crankpins the force is given through different grade stones (i.e. at station-1 800 grade stone, at station-2 1000 grade stone and at station-3 1200 grade excel abrasive stones). Here maximum 2-3 microns of material is removed in the station 1 & 2. And in station 3 super finishing occurs. Z. Liquid honing: It is also called sand blasting. As per the customer requirement, the two sides of the crankpin should be rough enough to attach the crank tightly as it increases friction. The slurry (Aluminum Oxide powder of 100 grit properly mixes with water) coming from high pressure nozzle with high pressure air when sticks to the end sides of the crankpin, a layer of rough surface comes on the crankpin. To avoid sticking the slurry on the bearing area of crankpin, rubber is provided to that area.

AA. Ultrasonic cleaning: Cleaning & washing is very important process in crankpin production because rusty and burr in the crankpin are the biggest problem. Here crankpins are allowed to immerse in the oil bath where waves are generated. When these oil waves are passed through the crankpin; dirt, dust, burr comes out & cleaning occurs. Sometimes hot oils are also used for the cleaning purpose. BB.Jet cleaning/ vacuum cleaning/ air cleaning: Only ultrasonic cleaning is not sufficient to meet the specification (i.e. less than 0.8 milli gram of dirt, dust is allowable). CC.Soaking for temperature stabilization: After washing and cleaning of crankpins, those are stored in a place where temperature is maintained at

9 20-220c. As we know that at heating, material expands and at cooling, material compresses. As I have already cited in ultrasonic washing sometimes washing occurs by immersing the crankpins in hot washing liquid. So material expands, to regain its original dimension soaking is required. DD. Cap fitting: As per customer requirement, the centre hole sometimes blocked with cap.

EE. Relation checking for different type of holes: It is very important operation, where we use poka yokes to find out whether the holes are meeting the specifications. As face hole is used to lock with crank and inclined/ dimple holes use for supplying oil through alignment of the holes with crank holes, so the holes location and dimensions should be according to the specifications. FF. Visual inspection: Here we observe the materials for any different types of marks. GG. Grade wise sorting: As I have already cited that we have to meet the tolerance limit because crankpin is a part which is used for assembly. For example- 25.997 to 26 mm of OD is grade1 & 25.997 to 25.994 mm of OD is grade-2 as per customer requirement, so we have to sort out which crankpins belong to grade 1 & which is grade 2. HH. Grade etching on face: For easy recognition of grades, we use electro chemical process for grade etching. On an electrode etching paper is attached then electrolyte liquid is poured on that paper. To complete the electric circuit one earthing point is touched to the material and pressed the electrode on the crankpin and grade is etched on the face of the crankpin. II. Oiling: As crankpins are stored in inventory for long time, so oiling is required for rust prevention. JJ. Color coding: It is an alternate operation to grade etching. KK.Wrapping in VCI paper: Wrapping VCI paper is also a rust prevention method. LL. Tube packing MM. Box packing

10 NN. PDI (Pre Dispatch Inspection): Here quality inspector inspects different parameters randomly from one lot.

VI. Which parameters to be inspected at the time of manufacturing:-

o Length o Cross hole diameter o Cross hole surface finish o Center hole chamfer length o Parallism o Center hole depth from face hole o Face surface finish o Cross hole diameter o Cross hole location o Cross hole chamfer angle o Cross hole chamfer diameter o Face hole diameter o Face hole drill depth o Face hole location o Face hole chamfer length o End profile chamfer length o End chamfer finish o End chamfer angle o OD surface finish o Liquid honing surface roughness o Liquid honing length o Cap depth o Circularity o Cylindricity o Face hole relation angle o Surface hardness o Effective case depth

VII. Load & stress on the Crankpin:-

The performance of any automobile largely depends on its size & working in dynamic conditions. The design of the crankpin considers the dynamic loading. The optimization can lead to, a pin diameter satisfying the requirements of automobile specifications with

11 cost and size effectiveness. The piston and the connecting rod transmit gas pressure from the cylinder to the crankpin. It also exerts forces on the crankpin, which is time varying.

A. Forces on Crankpin:

In the process of converting the linear reciprocating motion of the piston into a rotational output, the crank shaft undergoes both bending & torsion. As these forces are transmitted through the crank shaft, it becomes highly stressed, particularly at the crankpin/ web and the web/ journal intersections at the crank shaft. Radial, Tangential & Inertia forces are acting on crankpin as shown in the fig. F.

Fig. F

B. Stresses in the Crankpin:

Due to high dynamic load, the crankpin is subjected to bending. Each web is subjected to both bending & torsion whereas the journals with twisting.

o Bending causes both tensile & compressive stress o Twisting causes shear stresses o Due to the shrinkage of the web on to the journals, compressive stresses are set up in journals & tensile hoop stresses in the webs.

Fig. G

12 VIII. Crankpin Materials:-

The crankpins are subject to shock & fatigue loads. Thus materials of the crankpin should be tough & fatigue resistant. The crankpins are generally made of carbon steel, forged steel & alloy steel. It also improves the strength of the component.

IX. Reasons for failure of crankpin:

Crankpin fails due to the following reasons- o Crankpin materials & its chemical composition o Manufacturing defects o Improper lubrication o Incorrect geometry (stress concentration) o High engine temperature o Overloading o Pressure acting on the piston

In order to manufacture a cost effective component with the minimum weight possible, proper fatigue strength, Design & developments & other functional requirements have always been an important issue in the crankpin industry. These improvements result in lighter & smaller engines with better fuel efficiency & higher output.

X. Types of defects and its effect on the engine:-

A. Grinding mark: It is one type of surface marks which can be observed in naked eyes formed at the time of grinding operations. The grinding mark occurs for the following reasons, if the coolant strainer or magnetic separators are not enough to minimize the grinding sludge in circulating coolant then the chips stick to the grinding wheel and produces different types of marks (i.e. scratch marks, grinding marks, chatter marks etc.). Another reason we can say that if grinding wheel is not properly dressed, then also grinding marks comes. If the crankpin having grinding mark used in engine then due to rough surface more rolling friction will be there and bearing life will be less.

B. Burning mark: Due to less coolant flow & less coolant concentration, burning marks occurs on the OD surface and chamfer. Burning mark leads to crankpin failure.

13 C. Rusty: The initial stage of rust is pitting mark (i.e. small black spots). It happens due to less coolant concentration, improper washing and cleaning, dust or dirt present in the environment, moisture contact and prolonged storage of material. As we all know junk or rust spreads with use & time, so with time the rust spreads through all engine components through oil.

D. Dent marks: Due to improper handling of materials, metal to metal contact; dent marks produced on the end chamfers, OD surface, on face etc. if the crankpin having dents used in the crankshaft assembly, the bearing will not rotate properly and the most important thing is that maximum stress will concentrate there and premature failure of crankpin will occur.

E. Turning marks: At the time of face turning and OD turning some chips adhere to the tool and sometimes on the materials also, that adhering produces turning marks. Low quality tool, undesired speed, unskilled operators are also responsible for these marks. Due to this mark, bearing will not rotate properly and bearing failure will occur in no time.

F. Burrs: Today it is a challenge for the crankpin production industry to reduce the burr problem in crankpins. Burrs are of various types like transient burr, folded burr etc. produce at the time of drilling operation. The burr in the crankpin can be a stress raiser during rolling contact phenomenon and reduces lubrication to the bearings.

Some other defects like chatter mark, scratch mark which don’t have any adverse effect on the engine or engine components, but aesthetically these are not ok. So the production engineer always tries to reduce these type of defects.

XI. References:

o Wikipedia ( Crankpin, Crank shaft, Connecting rod)

o A study of crankpin failure in IC engine., Mechanical Engineering: An International Journal ( MEIJ), Vol. 1, No. 3, November 2014

o Working experience with one of the leading crankpin manufacturer.

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