What Are Crankshaft Bearings?
Crankshaft bearings, also known as main bearings, are essential parts of an internal combustion engine. They support the crankshaft and allow it to rotate smoothly within the engine block.
Function of Crankshaft Bearings
Crankshaft bearings help turn the pistons’ up-and-down motion and connect rods into rotational motion. They guide the crankshaft, reduce friction, and keep the engine running smoothly.
Types of Crankshaft Bearings
There are two main types of crankshaft bearings:
Plain bearings: The most common and simple type.
Roller bearings: These are used in some engine designs for better performance.
Both types are typically made from strong alloy steel to handle high-speed engines and long-term use.
Lubrication and Maintenance
Crankshaft bearings are lubricated by either pressurized oil or oil mist. Proper lubrication stops the crankshaft from directly touching the bearing shells, which reduces friction and wear. Regular maintenance includes checking the oil and making sure the bearings are properly aligned during installation.
Structure of Crankshaft Bearings
Crankshaft bearings are crucial parts that support the crankshaft, allowing it to rotate smoothly while reducing friction. These bearings are designed to withstand high pressures, wear, and temperature changes during engine operation.
Material Composition
Crankshaft bearings are made from a variety of durable materials, such as: – Copper alloy – Aluminum alloy – Steel – And polymer.
These materials are selected for their: – High strength and hardness – Good resistance to temperature and wear – Resilience to fatigue and corrosion – Effective thermal conductivity.
Layered Design
Crankshaft bearings often use multiple layers to boost their durability and performance: – Bi-metal bearings: These feature a strong backing material (often metal or aluminum) and a soft overlay. The backing ensures structural stability, while the overlay, usually an aluminum alloy with tin and silicon, reduces friction and helps trap foreign particles. – Tri-metal bearings: These have three layers. The middle layer, made from copper alloy with lead and tin, serves as a lubricant and strengthens the bearing. It sits between the steel backing and the soft Babbitt overlay.
Thickness and Overlay
- Bi-metal bearings: The overlay is about 0.012 inches thick, which allows for more tolerance for misalignments and shape irregularities.
- Tri-metal bearings: The overlay is thinner, ranging from 0.0005 to 0.0007 inches, providing better precision and performance under higher loads.
Bearing Caps and Bolts
The lower part of the crankshaft bearings is secured with bearing caps that are bolted to the engine block. In high-performance engines, you may find four-bolt mains or six-bolt mains, which offer extra strength: – Four-bolt mains: Four bolts provide added stability. – Six-bolt mains: Four vertical bolts and two lateral cross-bolts offer better lateral strength, ideal for high-power or high-RPM engines.
Lubrication and Clearance
Crankshaft bearings need a hydrodynamic lubrication system to maintain a continuous film of lubricant between the bearing and journal surfaces. Proper clearance between the bearing and the crankshaft journal is vital for optimal performance. This clearance is usually measured with tools like plastinate or a micrometer during installation.
Installation Requirements
Before installing, ensure the bearings are clean and well-lubricated. Check the crankshaft journals for any signs of wear or damage. Place the bearings into the engine block and bearing caps, then torque the cap bolts to the specified values from the manufacturer.
Examples and Applications
- The Toyota VZ V6 engine uses a “four bearing crankshaft.”
- The Jaguar XK6 straight-six engine features a “seven-bearing crankshaft.”
- High-performance engines might use special bearings, like King Engine Bearings’ XP bearings, which can improve load capacity by up to 17% compared to standard bearings.
Function of Crankshaft Bearings
Crankshaft bearings are vital parts that ensure the engine runs well and lasts longer. They perform key functions that keep the engine operating smoothly and efficiently.
Minimizing Friction
Crankshaft bearings reduce friction between the crankshaft and the engine block by providing a smooth, lubricated surface. This helps improve engine efficiency and fuel economy. Without good lubrication, friction can cause damage and wear to the engine.
Absorbing Shock
These bearings act as shock absorbers. They reduce the impact of the intense forces created during combustion. By absorbing these forces, crankshaft bearings protect other engine parts, preventing damage and ensuring smooth engine operation.
Supporting Load
Crankshaft bearings support the crankshaft and keep it aligned properly. They help absorb the forces transferred from the connecting rods. This support is crucial for preventing engine failure and ensuring the crankshaft works correctly.
Transmitting Forces
Crankshaft bearings absorb and transmit forces between moving components. This includes the forces created during combustion, transferred to the crankshaft through the connecting rods. Crankshaft bearings help manage this force transmission, allowing the engine to run efficiently.
Maintaining Lubrication
The bearings maintain a stable lubricating film between the crankshaft and shaft journals. This reduces friction and helps dissipate heat generated during engine use. Proper lubrication prevents overheating and protects the bearings and crankshaft.
Stabilizing Crankshaft Movement
In heavy-duty engines, additional components like thrust washers or composite bearings are used to absorb axial forces, such as those when the clutch engages. These parts stabilize the crankshaft’s movement and prevent misalignment or failure.
Additional Key Features for Durability and Performance
Crankshaft bearings need certain qualities to ensure long-lasting performance:
Fatigue Strength: They need to withstand repetitive stresses and loads.
Seizure Resistance: Bearings must resist damage from overheating or lack of lubrication.
Wear Resistance: They must resist wear to stay functional throughout the engine’s life.
Conformability: Bearings must adapt to the crankshaft and connecting rods for a proper fit.
Corrosion Resistance: Protection against corrosion is vital to avoid damage from contaminants and engine fluids.
Cavitation Resistance: Bearings must resist cavitation, which occurs when air bubbles form in the lubricant.
Embedability: Bearings should be able to embed small particles without damaging the surfaces.
In short, crankshaft bearings are critical for an engine’s performance, longevity, and overall efficiency. They help minimize friction, absorb shock, and maintain lubrication, ensuring smooth operation and reducing wear and risk of failure.
Importance of Crankshaft Bearings
Crankshaft bearings are crucial for engine operation and longevity. They help the crankshaft rotate smoothly, reducing friction and wear. Here are some important reasons why crankshaft bearings matter:
Reduce Friction and Improve Performance
- Crankshaft bearings lower friction between engine parts, allowing the crankshaft to rotate smoothly.
- Reducing friction improves power transfer, boosting engine performance and fuel efficiency.
Increase Longevity and Support Engine Loads
- Well-functioning crankshaft bearings extend the life of engine parts by keeping engine conditions optimal.
- They also help support the engine loads, allowing the crankshaft to rotate smoothly under different conditions.
Types of Crankshaft Bearings
- Plain Bearings: Simple and affordable, great for handling high loads.
- Roller Bearings: Use rolling parts to reduce friction, good for moderate loads but more expensive.
- Needle Bearings: Long, thin rollers, ideal for tight spaces with high load capacity. They provide smooth operation with minimal resistance.
Impact of Bearing Failure
- A broken crankshaft bearing can cause severe engine damage, including destroying the rotating assembly. Bearing debris can spread through the oil system, damaging critical parts like the camshaft and pistons.
Misalignment and Wear Performance
- Misalignment in journal bearings increases friction and reduces the lifespan of bearings by lowering their load capacity.
- Engine main bearings account for about a quarter of the engine’s friction losses. Understanding bearing coatings and shaft textures is key to improving efficiency and reducing fuel use.
Number and Placement of Crankshaft Bearings
The number and placement of crankshaft bearings depend on the engine design and load requirements. For example, the BMW N55 engine has 7 main bearings, each positioned to ensure smooth engine operation.
Number of Bearings
- The number of crankshaft bearings in an engine varies based on its design. The BMW N55 engine has 7 main bearings.
- Other engines, like the Honda Civic EG, may use a different number of bearings, each coded for identification.
Placement Coding
- The upper crankshaft bearings are placed in specific positions on the engine block. In the BMW N55, these positions are marked with letters such as A, B, and C. Each letter represents a type of bearing with a specific color: A (yellow with lubricating groove), B (green with lubricating groove), and C (red with lubricating groove).
- The lower main bearings are coded directly on the crankshaft. Numbers 1, 2, and 3 are used to indicate different grinding stages, each associated with specific colors and part numbers.
Importance of Correct Placement
- Correct bearing placement is vital for aligning oil holes and avoiding interference between the bearing edge and the crankshaft fillet radius. Incorrect placement can cause serious engine damage.
- Each bearing is installed with a slight crush of about 0.004” per shell. This helps with heat transfer and ensures the bearing stays in place, supporting the engine’s proper function.
General Placement Guidelines
- The bearing codes and colors must be followed carefully to prevent mix-ups during assembly. For example, in the BMW N55, the upper bearing placement might look like “BBABABB,” while the lower bearings could be coded “2222233.”
Proper placement ensures all parts work together smoothly, reducing wear and maintaining engine performance.
Bearing Caps and Additional Bolts
Bearing caps are essential for the crankshaft’s proper functioning. They hold the lower half of the main bearings in place, ensuring the crankshaft stays aligned and reducing friction during engine operation.
Number and Type of Bolts
Two bolts are generally used per bearing cap, but some engines may have four or six bolts per cap. These are known as “four-bolt mains” or “six-bolt mains” and provide extra strength to the engine.
Extra bolts help the engine handle more power output or RPM without compromising structure.
Four-Bolt Main
The V12 Maybach Zeppelin from 1928 was the first car engine to use four-bolt main bearings on three of its eight main bearings.
Six-Bolt Main
In a six-bolt main system, there are usually four vertical bolts (two on each side of the crankshaft) and two lateral cross-bolts. These lateral bolts strengthen the bearing caps by extending from the pan rails into the sides of the caps.
Function and Importance
Bearing caps help maintain crankshaft stability, ensuring smooth rotation and protecting the crankshaft from the forces inside the engine.
They also aid in lubrication and cooling, working with the crankshaft bearings to maintain an oil film that reduces friction and prevents wear.
Materials and Construction
Bearing caps are made from strong materials like cast iron, steel, or aluminum to withstand harsh engine conditions.
New materials like powder metallurgy (PM) improve durability while reducing production costs.
Installation and Maintenance
When installing bearing caps, make sure all parts are clean to avoid dirt-causing wear.
Proper alignment and following the manufacturer’s torque specifications are crucial to prevent damage and ensure the engine runs smoothly.
Service Life and Maintenance
Crankshaft bearings are crucial for engine smoothness, and their lifespan depends on several important factors. Regular maintenance helps extend their life and keeps the engine running well.
Key Factors Affecting Crankshaft Bearing Life
- Engine Type, Load, and Speed: The life of crankshaft bearings varies based on the engine type, the load it carries, and its speed. Engines that run faster or handle heavier loads tend to wear out bearings more quickly.
- Lubrication: Proper lubrication is crucial. Clean oil helps prevent bearing damage. Contaminated oil can cause corrosion or scratching, which reduces the bearing’s performance.
- Temperature Control: Keeping the engine at the right temperature avoids thermal expansion and reduces stress on the bearings. Overheating can cause damage and shorten their lifespan.
- Driving Habits: Aggressive driving, such as frequent idling, fast acceleration, and hard braking, can speed up bearing wear.
Maintenance Tips for Extending Bearing Life
- Periodic Inspection: Regularly check for unusual noises, vibrations, or metal particles in the oil to detect early signs of bearing wear.
- Lubrication Management: Always use high-quality lubricants that suit your engine and follow the recommended oil change intervals to maintain proper lubrication.
- Filter Maintenance: Regularly clean or replace the oil filter to prevent contaminants from damaging the bearings.
- Alignment Checks: Ensure components are properly aligned to avoid uneven load on the bearings.
Under normal conditions, crankshaft bearings last between 100,000 and 200,000 kilometers. Poor maintenance or the use of low-quality lubricants can significantly reduce their lifespan.
If bearings are not properly maintained, they may fail early. This can lead to engine seizure due to excess heat, pressure, and metal-to-metal contact, often requiring a full engine rebuild.
Modern Bearing Materials
Some manufacturers are switching to aluminum bearings instead of traditional copper/lead ones. Aluminum bearings are more durable, tolerate higher temperatures, and are cheaper.