Geometry and Dimensional Tolerances
In my last article for Engine Professional, I examined the causes for engine bearing failure and the importance of selecting the right materials to achieve maximum bearing life, even under the harshest conditions.
In this article we will look at engine bearing tolerances in relationship to the entire engine bearing assembly. To achieve maximum bearing operation success, it is important to understand the dimensions and tolerances of each part of the bearing assembly. We will look at the phenomenon of hydrodynamic lubrication, the importance of proper oil clearance, the difference between maximum and minimum wall thickness (eccentricity), an explanation of crush height, engine bearing damage that occurs because of defects in the crankshaft journal, and issues related to the crankcase and connecting rod.
Engine bearings operate mostly in the hydrodynamic regime of lubrication, in which the bearing surface is separated from the journal surface by the pressurized lubricant film generated by the journal rotation.
Normally, the rotating journal is displaced from the concentric position and forms a converging gap between the bearing and journal surfaces (Fig. 1). The pumping action of the journal forces the oil to squeeze through the wedge shaped gap generating a pressure. The oil pressure creates a supporting force separating the journal from the bearing surface.
The minimum value of the oil film thickness may be as little as 0.00002″ (1/100 of the diameter of a human hair). Such a minor gap between the bearing and the journal surfaces demonstrates the importance of keeping the dimensions, shapes and the surface quality of the parts at very tight tolerances.
An engine bearing assembly includes three parts: the bearing housing (either big end of the connecting rod or the crankcase main bearing housing), the engine bearing itself (the shells) and the journal (the crank pin or the main journal). Dimensions and tolerances of each of them affect the bearing operation.
The basic geometrical parameter of an engine bearing is the oil clearance – the difference between the inside diameter of the bearing installed in the housing and the diameter of the journal (the inside bearing diameter is measured at 90° to the parting line).
Oil clearance should have an optimal value providing the desirable combination of the lubrication parameters.
Higher oil clearance causes an increase of the oil flow passing through the bearing and resulting in a lower oil temperature rise. However higher clearance produces less uniform distribution of oil pressure and greater peak pressure, which increases the probability of bearing material fatigue. Minimum oil film thickness decreases at higher pressure and may allow direct metal-to-metal contact between the mating surfaces. Too much clearance also produces excessive vibration and noise.
Lower oil clearance results in a more uniform distribution of pressure through the oil film as well as greater oil film thickness. However, too small a clearance causes overheating of the oil and a sharp drop in its viscosity.
Typical values of oil clearance C:
Passenger car engines:
Cmin = 0.0005*D
Cmax = 0.001*D
High performance engines:
Cmin = 0.00075*D
Cmax = 0.0015*D
Where D – the journal diameter.
The inside bearing surface is not round. It has a lemon shape due to the varying thickness of the bearing wall — having maximal value at the centerline (T) and gradually decreasing towards the parting line. It is accepted to measure the minimal value of the bearing wall thickness (Te) at a certain specified height h (Fig. 2) in order to exclude the crush relief area.
The difference between the maximum and minimum wall thickness is called eccentricity:
eccentricity = T – Te
Eccentricity, produced by the varying wall thickness, is added to the eccentricity “e” caused by the displacement of the journal from the concentric position (Fig. 1).
This increased total eccentricity allows the formation of a more stable regime of hydrodynamic lubrication.
With regards to hydrodynamic conditions, a bearing with eccentricity is equivalent to a bearing of increased diameter (or increased oil clearance). The oil wedge of a bearing with eccentricity is the same as the wedge formed by a bearing with increased diameter (“effective bearing diameter”). However, since the actual bearing diameter is not changed, the adverse effect that excess bearing clearance has on vibration and noise is prevented.
Eccentricity also compensates for rod bore distortion caused by the force applied to the connecting rod. These forces cause the housing bore to be stretched in the vertical direction. As a result, the bearing diameter measured along the parting line decreases (closes-in) changing the shape of the oil wedge. Despite this change, the wedge shape nature of the oil gap is retained due to the bearing’s eccentricity. This wedge shape is required to produce a hydrodynamic regime of lubrication.
King’s performance bearings feature increased eccentricity enabling proper oil wedge formation at high rotation speeds and high loads.
The automated quality control of King’s machining operations allows us to produce bearings with an exceptionally tight +0.0001″ tolerance of wall thickness (“Bull’s Eye Tolerance”). Such tight tolerances result in superior consistency of the oil clearance and eccentricity.
Recommended values of eccentricity:
For passenger car engines: 0.0002 – 0.0008″
For high performance engines: 0.0006 – 0.0012″
Location of the eccentric wall measurement (h) is within the range
1/4-5/8″ depending on the journal diameter. h=3/8″ for 1.6-3.4″ journals.
The outside diameter of an engine bearing is always greater than the diameter of its housing.
The difference between the diameters affects the amount of elastic compression of the bearing installed in the housing. A firmly tightened bearing has uniform contact with the housing surface, preventing the bearing’s displacement during operation, and providing maximum heat transfer through the contacting surfaces. This also increases the rigidity of the housing.
Since the direct measurement of the bearing circumference is a difficult task, another parameter characterizing the bearing press fit is commonly measured — crush height.
Crush height is the difference between the outside circumferential length of a half bearing (one half shell) and the half of the housing circumference.
Fig. 3 illustrates a method for measuring crush height.
The tested bearing is installed in the gauge block and pressed with a predetermined force F. The force is proportional to the cross-section area of the bearing wall.
The value of the crush height is dependent on the bearing diameter, housing material (modulus of elasticity and thermal extension), housing structure (rigidity) and the temperature.
Since King performance bearings work at high loads and increased temperatures, they are designed with increased crush height. This provides better heat transfer and a greater press fit in the bearing housing.
Typical values of the crush height of 1.5-2.5″ diameter bearings:
For passenger car engines: 0.001-0.002″
For high performance engines: 0.002-0.004″.
A significant cause of engine bearing damage is defects in the crankshaft journal.
The basic geometrical parameter of a journal is its diameter. The required relationship between the housing bore diameter, the bearing wall thickness, and the journal diameter determine the value of the oil clearance within the specified tolerances that will provide a reliable hydrodynamic lubrication.
Commonly, crankshafts have a diameter tolerance of 0.0005-0.001″.
Ideally, a journal has a cylindrical shape. However the actual journal shape may deviate from the perfect cylinder.
If the journal pin diameter varies in the axial direction the journal shape forms one of the following patterns: taper (conical), barrel (convex) or hour glass (concave). (See Fig. 4.)
The taper/barrel/hour glass journal diameter deviation should be not greater than:
1/10,000 of the journal length (for tri-metal bearings),
2/10,000 of the journal length (for bi-metal bearings).
Variations of the journal diameter in the tangential direction produce roundness defects such as ovality or waves along the circumference of the journal (grinding chatter marks).
Chatter marks produce an undesirable shape of the oil gap breaking the oil film between the bearing and journal surfaces.
Out-of-round deviations of a journal should be maximum 0.00004″.
The main pins of a crankshaft should be aligned (concentric). Misalignments (deviations from concentricity) may cause direct contact between the bearing and the misaligned journal pin. Misalignments are particularly dangerous for tri-metal bearings because they have a thin overlay, which may be removed by contact with the journal.
Recommendations for maximum misalignment of the main pins:
For tri-metal bearings: 0.001″ overall value; 0.0005″ on adjacent journals.
For bi-metal bearings: 0.002″ overall value; 0.001″ on adjacent journals.
Crankshaft pins should be parallel to each other.
Maximum deviations from parallelism: 0.0005″ for tri-metal bearings and 0.001″ for bi-metal bearings.
Excessive wear of the bearing surface may also be caused by direct metal-to-metal contact due to journal surface roughness. Surface quality is particularly important for bearings operating with low oil film thickness (highly loaded bearings, low viscosity oils).
Reliable hydrodynamic lubrication is guaranteed if two surface quality characteristics are controlled: Ra (average roughness) and Rz (average maximum height of the profile):
For low and medium loaded bearings:
Ra = 15 microinch max.
Rz = 60 microinch max.
For highly loaded bearings:
Ra = 10 microinch max.
Rz = 30 microinch max.
Bores have a diameter tolerance 0.001″. Surface finish of bores: 60-90 micro inch.
Out-of-round (ovality) is allowed only if the diameter in the horizontal direction (along the parting line) is larger than that in the vertical direction. Otherwise the bearing eccentricity required for establishing stable hydrodynamic lubrication may be too low. The maximum out-of-round is 0.001″.
Recommendations for maximum misalignment of the bores:
For tri-metal bearings: 0.001″ overall value; 0.0005″ on adjacent bores.
For bi-metal bearings: 0.002″ overall value; 0.001″ on adjacent bores.
Bores have a diameter tolerance 0.0005″. Surface finish of bores: 60-90 micro inch. Out-of-round (ovality) is allowed only if the diameter in the horizontal direction (along the parting line) is larger than that in the vertical direction.
The maximum out-of-round is 0.001″.
The taper/barrel/hourglass journal diameter deviation should be not greater than:
1/10,000 of the bearing length (for tri-metal bearings),
2/10,000 of the bearing length (for bi-metal bearings).
Parallelism between rod bore and wrist pin hole: 0.001” max.
Twist: 0.001” max.
King Engine Bearings is known for its extensive line of high performance engine bearings for circle track, drag racing, monster trucks, tractor pulls, off-road and off-shore racing applications. King manufactures its automotive, aviation and high performance engine bearings to “Bull’s Eye Tolerance,” which is the company’s commitment to ultra-precise control of the critical wall thickness tolerance.
Through its Bull’s Eye Tolerance standard, King is able to provide a level of product consistency that is unmatched by any competitor. Because of King’s bearing consistency, engine builders can more easily achieve their targeted rod and main clearances. This achievement allows engine building shops to be more efficient and their engines more dependable. King is proud to bring added-value to its customers through the Bull’s Eye tolerance advantage.
Dr. Dmitri Kopeliovich is regarded as one of the foremost authorities on engine bearing research and development. He earned his Ph.D. in materials engineering and serves as research and development manager at King Engine Bearings. As the manager and team leader of King’s advanced materials research and development unit, Dr. Kopeliovich is known as the “Engine Bearing Doctor” for his extensive investigations into the cause and prevention of premature engine bearing failure. To ask Dr. Kopeliovich a question, visit www.kingbearings.com and click on Ask Dr. Dmitri.
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