Best Tutorial On| Connecting Rod Of Marine Diesel Engine

Connecting Rod of Marine Engine

Introduction

In this post of Connecting Rod of Marine Engine, I will explain the following topics in detail.

1. Types of connecting rod
2. Types of a connecting rod in marine diesel engines
3. How do you make a connecting rod
4. Connecting rod material
5. Connecting rod maintenance
6. Connecting rod problems.

What is connecting Rod?

Connecting rod is an intermediate component that helps in connecting the piston and crankshaft of an Engine.

Connecting rod transmits the firing force. Connecting rod converts the reciprocating motion into rotary motion along with a crankshaft

Four Stroke Connecting Rod

Connecting Rod Material

Following is the list of materials for the connecting rod

1. Aluminum alloy
2. Alloy Cast iron
3. Mild carbon steel
4. Alloy steel
5. Graphite cast iron.

Connecting rod Lubrication

Both the ends have bearings, and the lubrication of bearings is of the following two types

1. Splash Lubrication
2. Pressure lubrication.

Splash lubrication; — Splash lubrication is done with the help of sprout installation at an angle. Sprout scoops the oil while in a downward stroke, and in the upward motion, sprout splashes the oil to bearing.

Pressure Lubrication;- In the pressure lubrication system, oil passes from the big End to small end bearings under pressure through the rifle hole drilled in the connecting rod. Oil is available to the big End at pressure from the crankshaft.

Types of a connecting rod in Four Stroke engine

Marine Head type Connecting rods shape

The Connecting rod illustrated in above image is the marine head type of connecting rod

The joint of the connecting rod is above the bearing.

Bearing shells are similar to main crankshaft bearings manufactured with a thin shell having AlSn running layer.

The bearing caps and bearing blocks are bolted together by waisted bolts.

Advantages of Marine head Connecting rod

1. Can pull the piston without dismantling the big-end bearing.
2. Ensures Operational safety since there is no change in position and no new adaption.
3. Marine head bearing reduces the height dimension for the piston dismantling.

Material;-

Connecting rod and bearing body is of CrMo steel. The modern material is drop forged with high tensile steel.

Carbon Manganese Molybdenum alloy steel.

Parts of Connecting Rod

1. Big End

2. Small End

3. Bearing Insert

4. Bearing Shell

5. Bearing Bush

6. Beam

7. Shank

8. Nut and Bolt

Big End; — The side of connecting rod connected with the crankshaft pin is Big-end

Small End;- The side of connecting rod connected to the piston via wrist pin /Gudgeon pin is a small End.

Bearing Insert;- Bearing insert is inserted in the big-end attached to the cap.

Shank; — Shank connects the big End and small End of the con- rod.

Bearing and bush: — Crankshaft will have bush and bearing at both ends — the bush inserts in the small End of the con- rod.

The piston pin connects the small End and piston.

Big End connects the con- rod with a crankshaft pin. The big End is in two halves for easy mounting over the big End bearing shell. Nut and bolts join both halves.

Bush bearing is of bronze or gunmetal.

The big End bearing is in two halves. The big End Bering shell is of thin-shell tri-metal in modern engines.

Marine Head Con– rod Maintenance

Inspection of Faces (Visual)

1. Inspect the con- rod faces for Damages, visible wearing marks, pitting, or even cracks because of the corresponding movement of faces.

Visible wear marks and cracks not felt by a fingernail. Pitting and impact marks perceptible with fingernails.

Handle the piston rod and marine head with utmost care. Mishandling will cause damages and will result in improper tightening without the ovalness of the big-end bore.

• Record the observed damages for future reference.
• Smoothen out the single raised spot in the faces caused by pitting impact marks by filing with a small file.

Inspection of Piston rod and Marine Head Bolts

1. Inspect the hydraulic nuts for seizure in the threads and pitting on contact faces of the threads.
2. Turn the hydraulic nuts on the bottom of the bolts

Inspection results ActionIf the bolts and nut have a seizure in threads and pitting on the surface renew the bolts and screwsHard/cant turn in to bottom position by handle new the bolts and screws

Big-end Bore measurement

1. Assemble the lower and upper bearing caps without the shells. The upper. While assembling the position of indent nos. Of bearing, caps and piston rod must match
2. Assemble the upper and lower bearing caps with hydraulic bolts.
3. Tighten the bolts with prescribed torque as per the torque datasheet.
4. Install the conrod
5. Measure different diameters

Record the gauging readings in the format.

1. Calculate the maximum ovality by subtracting the maximum and minimum readings.
2. Check the maximum ovality readings comparing the specified limits.

Inspection resultsActionMaximum ovality has exceededRenew the complete marine head unit, piston rod, screws, and bearing shellsMaximum ovality is within the limitsRe-use the marine head.

Inspection of Bush for Piston rod

1. Inspect the piston pin and piston rod bush surfaces
2. Measure the piston pin and bush clearance.
3. Check if the clearance is excessive. Change the bushes with new bushes if the measurement is more than the specified limit.

Big End Bearing shell Inspection.

Inspect the bearing shells and replace the shells as per the following conditions

1. Replace the bearings shells If the observation is not satisfactory as per the Equipment manufacturer’s recommendations.
2. Change the bearing shells as per the bearing changing schedule specified in manufacturer recommendations.
3. Change the bearings shells In case of accident/ failure of bearings.

Four stroke medium speed Engine Connecting rod

Design

H shape section to curtail mass and enhance strength

Description:-

The connecting rod is of die-forged technology.

The big End is with inclined joint for the easy withdrawal of piston and connecting rod assembly through cylinder liner.

The Connecting rod and bearing cap joint faces have serrations for exact location and arrest relative movement of parts.

Connecting has rifle bored whole to supply the oil from big End to small End.

The big-end bearing is tri-metal kind and is with a coating of running layer.

The design of the bearing shell is a precision type and to install it without scrapping.

The small end bearing is also tri-metal type and is shrinking fit into the connecting rod. The small end bearing has a circumferential groove for evenly distributing oil to bush and pin bosses.

Advantages of H shape design;-

• Increase in load carrying capacity by increasing the bearing Surface area.
• Reduces Shear stress
• Serrations Increased gripping size due to the serrations and also stressed withstanding capacity of connecting rod increases.

Inspection of the big-end bore

• Clean all the machined surfaces on the connecting rod.
• De-grease all the serrated joint faces and threaded holes with volatile solvent and blow with dry air.
• Inspect the serrated joint faces. The serration on the connecting rod may show damages due to the relative movement between the mating surfaces, may show cracks and pitting marks in the high-loaded zone.
• Record the observed damages in the con-rod inspection sheet.
• CarefullysSmoothen the sharply raised points with the help of the file.

Be careful not to damage the serrations on the connecting rod and bearing cap while dismantling the con-rod. File to rectify/smooth the dent mark.

• Reject the connecting rod in case of damage of serration.
• Carefully clean the threads of the antifatigue screws.
• Inspect the threads for any seizure. Test the threaded holes, and the test must confirm the turning of the screw by hand up to the last End of the thread. Change the screws if the test doesn’t prove re-usability.

Big-end bore measurement

• Assemble the con-rod with a bearing cap without the bearing shells. The indent no. on the con-rod and bearing cap should match.
• Tighten the con-rod Nut at prescribed torque/hydraulic pressure.
• Measure 30 mm from the serration point.
• Measure the bore with the help of an inside micrometer.
• Measure five different diameters as per the measurement points in the figure.
• Record all the measurements in the sheet for the con- rod measurement.
• Calculate the maximum ovality by the difference between the largest and smallest.
• Reject the complete con-rod if the difference in ovlty is exceeding the Manufacturer’s prescribed limits.
• Use a new complete con-rod with screws and new shell bearings.
• Fill all the observations in the squares of the measurement sheet.

Bush for Con-rod

• Inspect the surface of the piston pin and the small end bush.
• Measure the clearance between the bush and piston pin.
• Reject the bush in case the reading measurement exceeds the prescribed limits.

Two-Stroke Con–Rod

In a two-stroke, Marine Engine Conrod fits between the crankshaft and Crosshead. Conrod also has top and bottom end bearings.

Con-rod transmits the firing pressure and converts the reciprocating motion along with the crankshaft.

The material of the con-rod is forged steel. The shape in each end accommodates the suitable bearings.

Types of bearings in Two-stroke Engine

1. Old design;- Old design uses White metal thick wall bearings. In these bearings, adjustment of clearance is by adding or removing shims between the bearing halves.
2. Modern Engine;- thin wall tri-metal precision bearings. Tin-based, with a thin layer of white metal with a minimum of 88% tin ( Sn). 5–7% antimony ( Sb), 3–5 % copper ( Cu) is the material composition of these bearings. A tin aluminum alloy is consisting 20 to 40 % tin, remaining aluminum bounds with steel shell backing.

This type of bearings design doesn’t need clearance adjustment. Reject and change the bearing when the clearance exceeds the specified limits.

A rifle whole through the center of the rod allows passage for the lubricating oil flow between the bearings- in a downward position.

Conrod is with the smaller length for the limitation of the overall height of the Engine. Short con-rod provides greater angularity more enormous side thrust.

Conclusion

I hope that the post on Con-Rod of Marine Engine will be useful for marine engineering students starting their careers onboard ships.

Good Luck!!