General information
Bearing positions in the engine
The image of the six-cylinder engine shows the bearing positions in the engine. Seven main bearings are installed, one of which is designed as a thrust bearing. Between each of the main bearings you can find the connecting rod bearings – one connecting rod bearing per cylinder.
The other bearing positions, such as camshaft bearings, conrod bushes and bearings for balancer shafts, are generally not half shell bearings but plain bearing bushes.
This brochure focuses on the half shell bearings which are used as bearings for the connecting rod and crankshaft in the crank mechanism.
The other bearing positions, such as camshaft bearings, conrod bushes and bearings for balancer shafts, are generally not half shell bearings but plain bearing bushes.
This brochure focuses on the half shell bearings which are used as bearings for the connecting rod and crankshaft in the crank mechanism.
01 Connecting rod bearings
02 Thrust washers / main bearings or flanged bearings
03 Main bearings
04 Conrod bushes
Main bearings and connecting rod bearings In the crank mechanism
Connecting rod bearings connect the connecting rod with the crankshaft. The bearing shells can be divided into those on the rod side and those on the cap side; the bearing shells on the rod side are subject to much more strain than those on the cap side. The ignition force generated during combustion is transferred to the crankshaft through them. In petrol engines, the bearing shells on the cap side are also subject to a high stress as there are high inertial forces due to the higher speeds compared to a diesel engine. Connecting rod bearings are supplied with oil via bores from the main bearing via the crankshaft.
The bearings of the crankshaft are main bearings. Here, the bearing is also divided into an upper and lower bearing shell. For the main bearings, the lower bearing shell is subject to a higher strain by absorbing the ignition forces. The forces transferred from a connecting rod to the crankshaft are absorbed by several main bearings, meaning that these are subject to a lower strain than the conrod bearing shells on the rod side. The upper main bearing shell contains an oil groove which conveys the oil to the connecting rod bearings via bores in the crankshaft.
In order to also be able to absorb axial forces, which arise when the clutch is actuated, for example, thrust washers or composite bearings are installed as thrust bearings.
The bearings of the crankshaft are main bearings. Here, the bearing is also divided into an upper and lower bearing shell. For the main bearings, the lower bearing shell is subject to a higher strain by absorbing the ignition forces. The forces transferred from a connecting rod to the crankshaft are absorbed by several main bearings, meaning that these are subject to a lower strain than the conrod bearing shells on the rod side. The upper main bearing shell contains an oil groove which conveys the oil to the connecting rod bearings via bores in the crankshaft.
In order to also be able to absorb axial forces, which arise when the clutch is actuated, for example, thrust washers or composite bearings are installed as thrust bearings.
Functions of engine bearings
The main function of engine bearings is to absorb and transmit forces between components which are moving in relation to one another. Friction should also be minimised, thereby enabling a practically wear-free rotary motion. During operation, frictional forces are generated in every bearing which counteract the rotary motion and thereby generate heat. In order to reduce these forces and to dissipate the frictional heat, it is necessary to have a lubricating film between the bearing and the shaft journals. Without this lubricating film, direct contact causes dry friction, which in turn causes wear and abrasion on the bearing.
Hydrodynamic engine bearings, in which a stable lubricating film forms through the relative movement between the bearing shell and journal alone, pass through a mixed friction range up to a particular transition speed.
Hydrodynamic engine bearings, in which a stable lubricating film forms through the relative movement between the bearing shell and journal alone, pass through a mixed friction range up to a particular transition speed.
At low speeds, the hydrodynamic lift is not enough to keep the surfaces completely separate from one another. This results in partial contact between the sliding surfaces, creating the risk of bearing damage. It is only when the speed increases that thefrictional forces are reduced and a permanent lubrication film is formed. Liquid friction/fluid friction is generated, whereby both the sliding surfaces are completely separate from one another. Sothat the reliable functioning of the bearing can be ensured, theresulting lubricant pressure in the bearing gap must be largeenough to absorb the forces acting on the bearing without contact of the sliding surfaces. This is the ideal operating point for engine bearings. But this form of friction also generates heat, meaning that sufficient lubrication is also required for heat dissipation.
Structure of engine bearings
In accordance with the standard DIN 50282 („The Tribological Behaviour of Metallic Antifriction Materials – Significant Definitions“), the tribological behaviour of an anti-friction material can be characterised by terms such as running-in behaviour, embedding properties, dry-running behaviour, wear resistance and adaptability. The requirements made of the engine bearing are therefore crucial when it comes to the choice of material.
There are two different anti-friction material families.
TWO-COMPONENT BEARINGS
Two-component bearings consist of a steel back, an intermediate layer made of pure aluminium and the plated bearing material. In the majority of cases, an aluminium alloy with tin, copper and silicon additives is selected for the material.
01 Steel back
02 Intermediate layer (if required)
03 Bearing material
There are two different anti-friction material families.
TWO-COMPONENT BEARINGS
- Steel-aluminium composites
Two-component bearings consist of a steel back, an intermediate layer made of pure aluminium and the plated bearing material. In the majority of cases, an aluminium alloy with tin, copper and silicon additives is selected for the material.
01 Steel back
02 Intermediate layer (if required)
03 Bearing material
THREE-COMPONENT BEARINGS
Depending on the area of application and its specific requirements, the overlay of the three-component bearings is applied as an additional sliding layer in the form of a sputter, galvanic or bonded coating layer. The bearing metal (aluminium, bronze or brass alloy) is plated, cast or sintered onto the steel back. If required, an intermediate layer made of nickel or a nickel alloy is applied between the bearing material and the sliding layer (overlay) as a diffusion barrier.
Different materials can therefore be used for engine bearings depending on the requirements. Often a different material is chosen for the bearing shell subject to the higher stress than for the opposing bearing shell. In a V engine, the conrod bearing shells are produced, for example, with a half shell bearing with a sputter coating on the rod side, and with a half shell bearing made from a steel-aluminium composite without coating on the cap side.
01 Steel back
02 Bearing material
03 Intermediate layer (if required)
04 Sliding layer (overlay)
- Sintered / cast steel-bronze or steel-brass composites with an overlay
- Steel-aluminium composites with an overlay
Depending on the area of application and its specific requirements, the overlay of the three-component bearings is applied as an additional sliding layer in the form of a sputter, galvanic or bonded coating layer. The bearing metal (aluminium, bronze or brass alloy) is plated, cast or sintered onto the steel back. If required, an intermediate layer made of nickel or a nickel alloy is applied between the bearing material and the sliding layer (overlay) as a diffusion barrier.
Different materials can therefore be used for engine bearings depending on the requirements. Often a different material is chosen for the bearing shell subject to the higher stress than for the opposing bearing shell. In a V engine, the conrod bearing shells are produced, for example, with a half shell bearing with a sputter coating on the rod side, and with a half shell bearing made from a steel-aluminium composite without coating on the cap side.
01 Steel back
02 Bearing material
03 Intermediate layer (if required)
04 Sliding layer (overlay)
Removal of engine bearings in the event of damage
The following should be noted during the removal of bearing shells in the event of damage:
- The bearing shells should be labelled according to seat and position in the main bearing centre line so that the events leading to the damage can be better understood. In addition to the appearance of the bearing, the seat can often provide information on the events leading to the damage. In the event of bending of the crankshaft, above all the first and last main bearing along the centre line demonstrate one-sided wear marks, for example.
- Operating conditions (duration, type of stress) and other influences, such as the oil used, must be documented so that it is possible to have a better assessment of the damage.
- Issues with other engine components, for example the crankshaft, must also be documented. In the majority of cases, damage to the interacting sliding partner of the engine bearing can be recognised. Often, damage to the bearing is also the result of damage to other engine components.
- In order to allow subsequent analyses, a sample should be taken of the oil used and the oil filter retained. Particle residues can be documented and analysed, providing information about possible causes of damage.
- The torques required to loosen the engine bolts must be documented. If the bolts are not fastened with the right torque, relative movement between the bearing shell and the housing bore may be the result.