Oil Clearance In Crankshaft Bearings: The Best Guide 24

Oil Clearance In Crankshaft Bearings

Oil clearance in crankshaft bearings is crucial for maintaining engine efficiency and preventing mechanical failures. Proper oil clearance ensures an adequate oil film, reducing friction and heat while providing lubrication to moving parts. Without the right clearance, bearings can suffer from excessive wear, leading to engine damage and costly repairs. This post will delve into the definition of oil clearance, methods to accurately measure it, recommended clearances for various applications, and the factors that influence these measurements. By understanding these elements, you can optimize your engine’s performance and longevity, ensuring smooth and reliable operation.

What is Oil Clearance?

Explanation of Oil Clearance in the Context of Crankshaft Bearings

Oil clearance is the small, precisely engineered gap between the crankshaft journals and the bearing surfaces in an internal combustion engine. This gap is crucial for creating a hydrodynamic oil film that lubricates the moving parts, ensuring they do not come into direct contact with each other.

In the crankshaft bearings, the oil clearance allows a thin film of oil to flow between the journal and the bearing surface. This oil film acts as a lubricant, reducing friction and wear, while also aiding in heat dissipation. Without adequate oil clearance, the crankshaft would make direct contact with the bearings, leading to increased friction, rapid wear, and potential engine failure.

Key Functions of Oil Clearance:

1. Lubrication:
   The primary role of oil clearance is to maintain a continuous oil film between the crankshaft and the bearings. This lubrication minimizes friction and wear, ensuring the smooth operation of the engine.
2. Heat Dissipation:
   The oil film helps in absorbing and dissipating the heat generated by the engine’s moving parts. Effective heat management prevents overheating and thermal expansion, which could damage the bearings and other engine components.
3. Shock Absorption:
   The oil clearance also allows the oil film to act as a cushion, absorbing shocks and vibrations from the crankshaft’s rotation. This reduces mechanical stress on the bearings, further preventing wear and tear.

Measurement and Importance:

• The oil clearance is measured using tools such as a dial bore gauge or Plastigauge. Accurate measurement is crucial as too much clearance can lead to oil leakage and insufficient lubrication, while too little clearance can cause metal-to-metal contact and excessive wear.
• Typical Oil Clearance Values:
  • In inches: 0.00075 to 0.0010 inches per inch of shaft diameter.
  • In millimeters: 0.019 to 0.025 mm per 25.4 mm of shaft diameter. For example, a 50.8 mm (2.000 inches) shaft diameter would typically require 0.038 to 0.050 mm of clearance.

Measuring Oil Clearance

Measurement Basics

Oil clearance in crankshaft bearings is measured as the vertical gap at 90 degrees to the split line of the bearing. This method accounts for the bearing’s eccentricity and ensures accurate measurement. Two primary tools used for measuring oil clearance are the dial bore gauge and Plastigauge.

Tools Needed

• Dial Bore Gauge: For precise measurement of the bearing’s inside diameter.
• Plastigauge: For a practical method to measure clearance directly on the assembled components.

Detailed Steps

Using a Dial Bore Gauge

Step-by-Step Process:

1. Preparation: Ensure the engine block and crankshaft are clean and free from debris. Install the bearings in their respective housings and torque the bearing caps to the specified settings without the crankshaft.
2. Setting Up the Dial Bore Gauge: Calibrate the dial bore gauge to a known standard or setting ring close to the nominal bearing diameter. Zero the gauge at the calibration point to ensure accurate measurements.
3. Measuring the Bearing Inside Diameter: Insert the dial bore gauge into the bearing bore. Rock the gauge gently to find the minimum reading, which indicates the true inside diameter. Take measurements at three locations: front, center, and rear of each bearing position, ensuring the gauge is perpendicular to the bearing split line.
4. Recording Measurements: Record the readings from the dial bore gauge. These measurements represent the inside diameter of the bearing when torqued to specification.
5. Calculating Oil Clearance: Measure the diameter of the crankshaft journal using a micrometer. Subtract the journal diameter from the bearing inside diameter to determine the oil clearance.

Example:

• Bearing Inside Diameter (BID): 50.10 mm
• Crankshaft Journal Diameter (CJD): 50.00 mm
• Oil Clearance: 50.10 mm – 50.00 mm = 0.10 mm

Using Plastigauge

Step-by-Step Process:

1. Preparation: Clean the crankshaft journal and bearing surfaces. Cut a strip of Plastigauge slightly longer than the width of the bearing.
2. Applying Plastigauge: Place the Plastigauge strip on the crankshaft journal, parallel to the axis of rotation. Carefully install the bearing cap and torque it to the specified settings without rotating the crankshaft.
3. Removing the Bearing Cap: Gently remove the bearing cap to avoid smearing the Plastigauge. Observe the flattened Plastigauge strip on the crankshaft journal.
4. Interpreting Results: Compare the width of the flattened Plastigauge to the scale provided with the Plastigauge packaging. The width of the Plastigauge corresponds to the oil clearance in millimeters or thousandths of an inch.

Example:

• If the flattened Plastigauge measures 0.10 mm, this indicates an oil clearance of 0.10 mm.

Illustrations and Diagrams

To enhance understanding, visual aids can be invaluable. Here are some suggested diagrams:

1. Dial Bore Gauge Measurement:
   A diagram showing the insertion of the dial bore gauge into the bearing bore, highlighting the correct positioning at 90 degrees to the split line.
2. Plastigauge Application:
   A step-by-step visual guide showing the placement of Plastigauge on the crankshaft journal, torquing the bearing cap, and comparing the flattened Plastigauge to the measurement scale.

Clearance Guidelines for Marine Heavy Engines

Marine heavy engines, due to their larger size and higher operating loads, require different oil clearance specifications compared to smaller engines. These engines operate under more demanding conditions, which necessitate larger clearances to ensure proper lubrication and heat dissipation.

General Recommendations

For marine heavy engines, the standard clearance range is typically larger to accommodate the increased mechanical stresses and thermal expansion.

• Standard Clearance Range: 0.0015 to 0.0025 inches per inch of shaft diameter.
• Example Calculation:
  • Shaft Diameter: 10.000 inches
  • Minimum Clearance: 10.000 inches * 0.0015 = 0.0150 inches (0.381 mm)
  • Maximum Clearance: 10.000 inches * 0.0025 = 0.0250 inches (0.635 mm)

High-Performance Marine Engines

For high-performance marine engines, an additional clearance is recommended to handle the higher speeds and loads typical of these applications.

• Adjustment for High-Performance: Add 0.0010 inches to the maximum clearance.
• Example:
  • Shaft Diameter: 10.000 inches
  • High-Performance Clearance: 0.0250 inches (standard maximum) + 0.0010 inches = 0.0260 inches (0.660 mm)

Table of Standard and High-Performance Clearances for Marine Heavy Engines

Shaft Diameter (inches)	Standard Minimum Clearance (inches/mm)	Standard Maximum Clearance (inches/mm)	High-Performance Clearance (inches/mm)
5.000	0.0075 / 0.191	0.0125 / 0.318	0.0135 / 0.343
7.500	0.01125 / 0.286	0.01875 / 0.476	0.01975 / 0.502
10.000	0.0150 / 0.381	0.0250 / 0.635	0.0260 / 0.660
12.500	0.01875 / 0.476	0.03125 / 0.794	0.03225 / 0.819
15.000	0.0225 / 0.572	0.0375 / 0.953	0.0385 / 0.978

Considerations for Marine Heavy Engines

1. Load and Stress: Marine engines endure higher mechanical loads and stresses, making larger clearances necessary to ensure adequate oil film thickness and prevent metal-to-metal contact.
2. Thermal Expansion: The larger components and higher operating temperatures in marine engines lead to greater thermal expansion, which must be accommodated by the oil clearance.
3. Lubrication and Cooling: Proper clearance ensures sufficient lubrication and cooling, which are critical for maintaining engine performance and longevity in harsh marine environments.

Thumb Rule for Oil Clearance Calculation

Calculating oil clearance is crucial for ensuring the longevity and performance of engine bearings. A widely accepted thumb rule for oil clearance provides a good starting point for most applications.

General Thumb Rule

• Standard Engines: For most standard automotive and light-duty engines, the recommended oil clearance is 0.00075 to 0.0010 inches per inch of shaft diameter.
• High-Performance Engines: For high-performance or racing engines, an additional clearance of 0.0005 inches is often added to the maximum value calculated using the standard rule.

Calculation Example

To illustrate, let’s use a 2.000-inch shaft diameter as an example:

1. Standard Engine Calculation:
   • Minimum Clearance: 2.000×0.00075=0.00152.000 \times 0.00075 = 0.00152.000×0.00075=0.0015 inches (0.038 mm)
   • Maximum Clearance: 2.000×0.0010=0.00202.000 \times 0.0010 = 0.00202.000×0.0010=0.0020 inches (0.051 mm)
2. High-Performance Engine Calculation:
   • Add 0.0005 inches to the maximum clearance:
   • High-Performance Clearance: 0.0020+0.0005=0.00250.0020 + 0.0005 = 0.00250.0020+0.0005=0.0025 inches (0.064 mm)

Factors Influencing Oil Clearance in Marine Engines

Understanding the factors that influence oil clearance in crankshaft bearings is crucial for maintaining the efficiency, reliability, and longevity of marine propulsion engines and diesel generators. Here we explore how different aspects such as engine application, lubricant selection, operating conditions, and manufacturing tolerances impact oil clearance. Additionally, real-world case studies will provide practical insights into these factors.

Engine Application

The type of marine engine significantly influences the required oil clearance:

• Slow-Speed Engines: Typically used in large vessels for long-distance travel, operating at low RPMs with high torque. They require larger clearances to handle the significant mechanical loads and thermal expansion. Typical clearances range from 0.0015 to 0.0025 inches (0.038 to 0.063 mm) per inch of shaft diameter.
• Medium-Speed Engines: Commonly used in smaller ships and ferries, operating at moderate RPMs. They need balanced clearances to ensure efficient operation under varying loads. Typical clearances range from 0.0012 to 0.0020 inches (0.030 to 0.051 mm) per inch of shaft diameter.
• High-Speed Engines: Used in high-performance marine vessels, such as patrol boats and yachts, operating at high RPMs. They require precise clearances to ensure adequate lubrication and cooling at high speeds. Typical clearances range from 0.0010 to 0.0018 inches (0.025 to 0.046 mm) per inch of shaft diameter.

Lubricant Selection

The type and viscosity of oil used in marine engines play a crucial role in determining the optimal oil clearance:

• Oil Viscosity: Thicker oils (higher viscosity) provide better film strength but may not flow as easily through tighter clearances. Thinner oils (lower viscosity) flow more readily but may not provide sufficient film thickness at larger clearances.
• Oil Type: Synthetic oils often have better flow properties and higher thermal stability compared to conventional oils, allowing for tighter clearances. They are particularly beneficial in high-performance and high-temperature applications in marine environments.

Operating Conditions

The operating environment of the marine engine affects clearance requirements:

• Operating Temperatures: Higher temperatures cause engine components to expand, which can reduce oil clearance. Engines operating at elevated temperatures require larger initial clearances to prevent metal-to-metal contact as the components expand.
• Engine Load: Heavier loads increase the mechanical stress on engine components, necessitating larger clearances to accommodate the additional forces and ensure sufficient oil flow.

Manufacturing Tolerances

Variations in manufacturing processes can lead to differences in the dimensions of engine components, affecting oil clearance:

• Crankshaft Journals: Tolerances in the diameter of crankshaft journals can affect the overall clearance. Precise manufacturing is required to maintain consistency.
• Housing Bores: Variations in the size of housing bores where bearings are installed can impact clearance. Accurate machining ensures that the housing bores are within specified tolerances.
• Bearing Walls: Differences in the thickness of bearing walls can also contribute to clearance variations. Quality control during manufacturing helps maintain uniformity.

Case Study Examples

Example 1: Slow-Speed Marine Engine

• Engine: MAN B&W 6S70ME-C
• Clearance: 0.0020 inches (0.051 mm) per inch of shaft diameter
• Operating Conditions: High torque, low RPM
• Outcome: Larger clearances accommodate thermal expansion and heavy loads, ensuring reliable operation under varying conditions

Example 2: Medium-Speed Marine Engine

• Engine: Wärtsilä 32
• Clearance: 0.0015 inches (0.038 mm) per inch of shaft diameter
• Operating Conditions: Moderate RPM, varying loads
• Outcome: Balanced clearances provide efficient operation and longevity under diverse operational conditions

Example 3: High-Speed Marine Engine

• Engine: MTU 16V 4000 M93
• Clearance: 0.0012 inches (0.030 mm) per inch of shaft diameter
• Operating Conditions: High speeds, extreme loads
• Outcome: Sufficient clearance allows for better lubrication and cooling, preventing wear and failure at high performance levels

Impact of Clearance on Engine Performance

Understanding the impact of oil clearance on engine performance is essential for optimizing engine efficiency, reliability, and longevity. The clearance between crankshaft bearings and journals affects noise and vibration, oil flow and pressure, friction and wear, and serves as a crucial safety margin.

Noise and Vibration

Relationship Between Clearance, Noise, and Vibration:

• Noise: Tighter clearances can lead to increased noise, especially during engine startup or under high load conditions. This is because there is less room for the oil film to absorb vibrations, resulting in more direct contact between metal surfaces.
• Vibration: Proper clearance helps to dampen vibrations by providing a cushion of oil. Inadequate clearance can lead to higher vibration levels, which can cause component fatigue and failure over time.

Why Tighter Clearances Might Be Noisier:

• In certain engines, particularly those with high precision and operating at high speeds, tighter clearances can reduce the oil film’s effectiveness in dampening noise. This can make the engine noisier as the metal components have less room to move and absorb vibrations through the oil film.

Oil Flow and Pressure

How Clearance Affects Oil Flow and Pressure:

• Oil Flow: Larger clearances allow for greater oil flow, which can improve cooling and lubrication. However, if the clearance is too large, it can lead to oil leakage and insufficient lubrication.
• Oil Pressure: Smaller clearances increase oil pressure, which can ensure a more consistent oil film. Conversely, larger clearances can reduce oil pressure, potentially leading to inadequate lubrication.

Differences Between Light and Heavy Oils:

• Light Oils (Lower Viscosity): These oils flow more easily through tighter clearances, ensuring that lubrication reaches all necessary parts. However, they may not provide sufficient film strength under high load conditions.
• Heavy Oils (Higher Viscosity): These oils provide better film strength and are more effective in larger clearances, offering better protection under heavy loads and high temperatures. However, they may struggle to flow through very tight clearances, potentially leading to lubrication issues.

Friction and Wear

Balancing Minimizing Friction and Preventing Wear:

• Minimizing Friction: Proper oil clearance ensures that a continuous oil film reduces friction between moving parts. Insufficient clearance can increase friction and heat generation, leading to component wear and potential failure.
• Preventing Wear: Adequate clearance allows the oil film to act as a barrier, preventing metal-to-metal contact and reducing wear. Too much clearance can cause excessive oil flow, washing away the lubricating film and increasing wear rates.

Safety Margins

Importance of Clearance as a Safety Factor:

• Alignment and Component Geometry: Variations in manufacturing tolerances and operational conditions can affect the alignment and geometry of engine components. Proper oil clearance provides a safety margin that accommodates these variations, ensuring reliable engine performance.
• Preventing Mechanical Failures: Adequate clearance helps absorb operational stresses and misalignments, preventing mechanical failures. This safety margin is crucial for maintaining engine integrity under various operating conditions.

FAQ on “Oil Clearance In Crankshaft Bearings”

Conclusion

Understanding and optimizing oil clearance in crankshaft bearings is crucial for ensuring engine performance, reliability, and longevity. Whether for marine propulsion engines or diesel generators, proper oil clearance balances lubrication, heat dissipation, and wear prevention. Using thumb rules for initial calculations, adjusting for specific engine types, and considering factors like operating conditions and lubricant selection can help achieve optimal results. High-performance engines, in particular, benefit from larger clearances and advanced solutions like synthetic oils and friction-reducing coatings. By paying close attention to oil clearance, you can enhance engine efficiency and durability, ensuring smooth and reliable operation in demanding environments.