Bearings

Question: What Is A Bearing? 
Ans: Generally Speaking, A Bearing Is A Device That Is Used To Enable Rotational Or Linear Movement, While Reducing Friction And Handling Stress. Resembling Wheels, Bearings Literally Enable Devices To Roll, Which Reduces The Friction Between The Surface Of The Bearing And The Surface It's Rolling Over. It's Significantly Easier To Move, Both In A Rotary Or Linear Fashion, When Friction Is Reduced, This Also Enhances Speed And Efficiency.

Q. What Type Of Bearings Is Generally Used In Marine Diesel Engines?
Ans: Multi Layer Thin Wall Shell Type Bearings Are Used Widely In Marine Diesel Engines. These Are Usually Tri-metal Bearing. Babbitt Metal Lacks Fatigue Strength. It Breaks Down Under Load. The Durability Of Babbitt Greatly Increases As The Material Decreases In Thickness. The Common Solution Is To Apply A Thin Layer Of Babbitt Over A Supporting Layer Of Copper/lead Which Acts As A Cushioning Layer And Allows For Slight Misalignment.

Q. What Do You Mean By Thin Shell Bearing? Why Thin Shell Bearings Are Used Instead Of Cast Type, In Modern Marine Diesel Engine?
Ans: Thin Shell Bearings Are Most Common Bearings Used In Modern Main Engines. They May Be Bi-metal Or Tri- Metal Bearings. Typical Materials For Bearings Are Steel-babbitt, Steel-bronze Or Steel-tin/aluminum ( Tin-aluminum Has Slightly Greater Load Bearing Capacity Than White Metal And Maintains Its Fatigue Strength Over A Greater Range Of Temperatures. It Consists Of A Steel Backing Strip Coated With A Layer Of White Metal. The Bearing Metal Thickness Is 0.5 To 3mm. An Overlay Of 20 -40 Micrometers May Be Applied To Improve Conformity. This Is Generally A Ductile Coating Of Lead And Tin. In Addition New Bearings May Have A Flash Layer Of A Few Microns Of Tin To Prevent Oxidation. An Intermediate Layer May Be Used Between The Overlay And Main Bearing Metal To Avoid Diffusion. This Is Particularly Found Where Bearing Loads Are Very High Such As In The Lower Half Of The Cross Head Bearing.

Q. Which Material Is Used In The Bearing Of Marine Diesel Engines?
Ans: Materials Used For Bearing Of Large Marine Diesel Engines Are: 
Tin Based White Metals / Babbitt Metal: 
Tin-based White Metal Is An Alloy With Minimum 88% Tin (Sn), The Rest Of The Alloy Composition Is 8% Antimony (Sb), 4% Copper (Cu). Cadmium (Cd) And Small Amounts Of Other Elements That Are Added To Improve The Grain Structure, Homogeneity During The Solidification Process. This Is Important For The Load Carrying And Sliding Properties Of The Alloy. Lead (Pb) Content In This Alloy Composition Is An Impurity, As The Fatigue Strength Deteriorates With Increasing Lead Content, Which Should Not Exceed 0.2 % Of The Cast Alloy Composition. Tin Based White Metal Is Used In The Main Bearings, Crankpin Bearings, Crosshead Bearings, Guide Shoes, Camshaft Bearings And Thrust Bearings Because Of Its Excellent Load Carrying And Sliding Properties. 
Tin And Aluminum Based Bearings: 
Alloys Containing 20 To 40% Tin, Remainder Aluminum, Show Excellent Resistance To Corrosion By Products Of Oil Breakdown. The Sliding Properties Of This Composition Are Very Similar To Those Of Tin Based White Metal But The Loading Capacity Of This Material Is Higher Than Tin Based White Metals For The Same Working Temperature; This Is Due To The Ideal Combination Of Tin And Aluminum, Where Tin Gives The Good Embedability And Sliding Properties, While The Aluminum Mesh Functions As An Effective Load Absorber. The Higher-tin Alloys (40%) Have Adequate Strength And Better Surface Properties, Which Make Them Useful For Main And Crosshead Bearings For Heavy Marine Diesel Engines. Tin Aluminum Bearings Were Developed To Provide Bearings That Carry High Loads. Special Features Are Their Good Resistance To Corrosion, High Thermal Conductivity And High Fatigue Strength, But They Have The Disadvantages Of Only Moderate Embedding Properties, Poor Compatibility And High Coefficients Of Thermal Expansion. If Used As Solid Un Backed Bearings This Type Of Alloy Is Usually Too Weak To Maintain An Interference Fit And Too Hard To Run Satisfactorily Against An Unhardened Shaft. Considerable Improvement In Anti- Scoring Characteristics And Embedability Is Obtained By Using A Thin-lead Babbitt Or Electrodeposited Lead-tin Overlay. 
Lead Bronze Based Bearings: 
Lead Bronzes Basically Are Copper-tin-lead-alloys. They Are Used In Very Highly Loaded Bearings Because Of Their High Fatigue Strength; Their Drawback Is Poor Tribological Behavior. That Is Why They Require An Electroplated Overlay In Most Applications. Standard Composition For Conrod And Main Bearings Is 78 % Cu, 20 % Pb, 2 % Sn. The Alloy Is Used With Electroplated Overlay Or Cast Babbit Running Layer. These Bearings Can Be Found In Marine Medium Diesel Speed Engines.

Q. Why Thin Shell Bearings Are Used Instead Of Cast Type, In Modern Marine Diesel Engine?
Ans: Compared With The Traditional Cast Bearing, Thin Shell Bearings Have A Number Of Advantages. Shells Are Prefinished Thus Allowing For Quicker And Easier Replacement. The Bearings Are Made Under Strict Controlled Conditions Giving Consistent High Quality Products In Many Cases The Top And Bottom Halves Are Interchangeable In An Emergency. Thin Layer Of White Metal Cools Quickly Giving Fine Grain Structure Which Has High Strength And Fatigue Resistance.



Q. What Are The Different Layers Found In Modern Tri-metal Bearings? Give Brief Description?
Ans: A Modern Tri-metal Bearing In Fact Have Multi Layers And A Steel Back Supports Bearing Structure, Provides Its Rigidity And Press Fit Under Severe Conditions Of Increased Temperature And Cycling Loads. 
The Main Layers In Modern Diesel Engine Bearings: 
1. Flash Layer: A Flash Layer Is A 100% Tin (Sn) Layer Which Is Applied Galvanically. The Thickness Of This Layer Is From 2 Microm To 5 Micro M. The Coating Of Tin Flash Is Applied All Over And Functions Primarily To Prevent Corrosion (oxidation) Of The Bearing. The Tin Flash Also Functions As An Effective Dry Lubricant When New Bearings Are Installed. 
2. Overlay/ Tri-metal : An Overlay Is A 20 Micro Meter Thick White Metal. The Overlay Is A Soft And Ductile Coating. Its Main Objective Is To Ensure Good Embedability And Conformity Between The Bearing Sliding Surface And The Pin Surface Geometry. 
3. Interlay: An Intermediate Layer May Be Used Between The Overlay And Main Bearing Metal To Avoid Diffusion. This Is Particularly Found Where Bearing Loads Are Very Such As In The Lower Half Of The Cross Head Bearing. It Is 5 Micro Meter Thick Nickel Diffusion Barrier (nickel Dam) Is Deposited Between The Overlay And Main Bearing In Order To Prevent Corrosion Of White Metal. 
4. Bonding Layer: Is Used For Good Adhesion Between The Steel Back And Aluminum-tin Alloy. Presence Of Tin Particles On The Surface Of An Aluminum-tin Alloy Bonded To A Steel Strip Weakens The Adhesion Strength Between The Materials. A Bonding Layer Of Pure Aluminum Between The Aluminum -tin Alloy And Steel Allows Achieving Strong Adhesion. Pure Aluminum Is Commonly Bonded To The Aluminum-tin Alloy And Then The Aluminum Strip Is Bonded To Steel Back. The Thickness Of The Aluminum Bonding Layer In A Bearing Is About (25-50 Micro Meter). Another Type Of Bonding Layer Is A Nickel Layer Deposited On The Steel Surface Prior To Bonding With Aluminum-tin Alloy. 
Another Type Of Bonding Layer Is A Nickel Layer Deposited On The Steel Surface Prior To Bonding With Aluminum-tin Alloy.