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Camshaft Bearings BY DR. DMITRI KOPELIOVICH Camshaft assembly 1

Clearance in Every Set

When speaking about bearings, most commonly, we focus on main and bearings and the roles they play in operation. Camshaft bearings are less loaded and generally have fewer problems. Therefore, innovative bearing 2 materials and new designs are mostly focused on crankshaft bearings, where probability of failure is greater. However, camshaft bearings are also hydrodynamic bearings. They suffer from the same operational conditions as do crankshaft bearings: overloading, oil starvation, too thin minimum oil film, misalignment, and contaminated oil. The possible failures are also similar: material fatigue, excessive wear, seizure, and corrosion. Bearings in Camshaft Mechanism Camshaft bearings (see Fig. 1) support the camshaft and provide for its rotation. There are three possible configurations of camshaft mechanism: 1) Overhead valve (OHV) designs (see Fig. 2) Overhead valve are also called pushrod engines. The camshaft of OHV is mounted in the block. The transmit the load through the pushrod to the which presses the valve and displaces it from the closed position The force required for opening the valve counteracts the cylinder pressure, the spring force and the dynamic forces of the

accelerating and decelerating (reciprocating) parts: valve, rocker arm and pushrod. Such force produces a load applied to the upper part of camshaft bearings. Thus, in the overhead valve-train design, the upper part of the bearings may potentially fail. However, at high rotation speed, the dynamic forces generated by the rotating cams may compensate for the load transmitted by the pushrods.

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SUPERIOR Load Capacity &Performance NEW Advanced Features: • 24% Stronger Overlay Achieving 18.1HV • 17% Greater Fatigue Resistance • Unique Geometric Design Maximizes Surface Load Area • Bull’s Eye Wall Tolerance™ Ensures Perfect Oil Clearance in Every Set

Bull’s Eye Tolerance Perfect clearance in every set ™

pMaxBlack U-Groove ElliptiX ™ ™ ™ Superior load capacity Greater load surface Improved oil ingress

For more about the NEW Features of King XP & HP visit: www.KingRaceBearings.com

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CAMSHAFT BEARINGS BY DR. DMITRI KOPELIOVICH 3 4

The inertia force of the may be 2) Overhead camshaft (OHC) design with which the dynamic (inertia) force expressed by the formula: rocker arms (see Fig. 3) developed by the cams is great and may In OHC design, the camshaft is placed in cause considerable loading of the upper F = MC * 2 * R the . bearing parts. ω The overhead camshaft configuration Where: has two versions: single camshaft (SOHC) Characterization of MC – eccentric mass of the cam; and double (twin) camshaft (DOHC). Camshaft Bearing Failures – rotation speed; The camshaft location in the OHC High performance engines, diesel direct ωR – eccentricity of the cam. design with rocker arms is different from injection engines, turbo and supercharged that in the overhead valve (OHV) engines are characterized by increased According to the formula, an increase mechanism. But the bearing loading is loading of the bearings. The cylinder quite similar. The rocker arm is pushed of rotation speed by three times raises the pressures are greater and the valve springs when the cam lobe is in upper position. It force by a factor of nine. are stronger. Therefore the loads applied to means that the upper parts of the bearings The dynamic forces produced by the the camshaft bearings are also higher than are loaded. The load decreases with an rotating cams decrease the load applied to in conventional gasoline engines. increase of the rotation speed due to the the upper parts of the bearings and create High load may cause two types of inertia force of the rotating eccentric cam. a load applied to the lower part. bearing failure: The effect of the inertia force is very 3) Overhead camshaft (OHC) design with direct 1) Fatigue of the lining (or overlay). If the similar to that in main bearings of the cam operation (see Fig. 4) load exceeds the fatigue strength of the crankshaft with its eccentric masses. In this design, the cam presses the valve lining, after a number of load cycles Overhead valve engines are not directly without any intermediate fatigue cracks form on the lining surface. commonly run at very high rotation rocker arm. Such configuration is The cracks propagate throughout the speeds. principally different regarding bearing lining thickness, reach the steel-lining This limitation (up to 10,000 RPM) is loading. interface and continue to advance along caused by too high an inertia force The valve is pressed when the cam lobe the boundary with the steel back. produced by the relatively heavy is in the lower position; therefore the Fragments of the lining detach from the reciprocating pushrods. lower part of the bearing is loaded, in steel surface. Greater rotation speeds may be contrast to designs with rocker arms. Soft thick linings are prone to fatigue. achieved in the engines with an overhead Overhead camshaft design is used in An additional factor deteriorating the load camshaft design (OHC). high speed engines (up to 20,000 RPM) in capacity of camshaft bearings is the

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5

Typical oil clearance of camshaft bearings is 0.0015-0.002” *D Where D – the bearing diameter.

Structures and Materials of Camshaft Bearings The typical structures and designs of camshaft bearings are presented in Fig. 5. The most traditional design of camshaft bearings is a steel tube with a layer of lead based Babbitt alloy applied onto the inner surface (bush type camshaft bearing). The bearings of this type may be supplied in semi-finished (un-bored) circumferential oil groove located in the condition. Then the bearing housing of some engines. The bearings are bored after groove decreases the effective area of installation in the engine. contact between the bearing and the However, the precision housing. The bearing material that is not (bored) finished type is supported above the groove, strains under more popular. the alternating load and may fatigue. A relatively thick and soft Babbitt layer provides 2) Excessive wear. Under high load the good conformability of the thickness of the oil film decreases. If the bearing. The material minimum oil film thickness is lower than allows fitting its shape to the micro-asperities on the bearing and misalignments. Babbitt journal surfaces, the hydrodynamic also has very good lubrication is compromised and metal-to- embedability, which is metal contact between the surfaces is important for bearings established. Direct friction results in fast operating with wear of the bearing material. 6 contaminated oil. The main disadvantage Another factor causing camshaft of Babbitt bearings is their bearing failure is misalignment. The low load carrying capacity. bearing and the camshaft surfaces should Camshaft bearings with aluminum lining by Babbitt alloys are soft; therefore they have be aligned when the camshaft is installed King Engine Bearings. low fatigue strength. Also, the fatigue limit in the engine. However, distortions of the of the lining is directly dependent on its block induced by thermal or mechanical thickness: the thicker the layer the lower stresses cause misalignments of the oil path to the camshaft bearings is long in its fatigue limit. Since the Babbitt lining is bearings. Then some of the camshaft some engines. Therefore it takes some relatively thick, its fatigue strength is low bearings start to operate in a constant time for cold oil to reach the bearing (~2,000 psi). metal-to-metal contact with the journal surface. At each cold start the camshaft Bi-metallic camshaft bearings, with a surface and their lining wears fast. The bearings operate in the absence of oil, lining made of aluminum alloy, have a alignment of the distorted camshaft causing metal-to-metal contact. much greater fatigue strength of at least bearing housings may be reconditioned by Oil starvation may also result from an 5,800 psi. The bearings are split shells oversize boring. Oversized OD camshaft excessive leakage of oil due to the large type, rather than bush. bearings must be used in such engines. bearing clearance. In contrast to cold start, King Engine Bearings manufactures Excessive wear of camshaft bearings a reduction of oil pump pressure caused by camshaft bearings made of may also be caused by oil starvation leakage occurs mostly with hot low aluminum/silicon alloy: K-788 (see Fig. 6). conditions at cold start of the engine. The viscosity oils. Their load capacity reaches 8,000 psi.

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CAMSHAFT BEARINGS BY DR. DMITRI KOPELIOVICH

A bimetal structure with an aluminum excessive wear due to oil starvation are alloy lining is the best solution for typical causes of camshaft bearing failures, camshaft bearings. Aluminum alloy is not tri-metal construction is rarely used in the too hard, therefore it has good design of camshaft bearings.I conformability. Also, it is stronger and more wear resistant than Babbitt. In contrast to a tri-metal structure, aluminum alloy bimetal bearings have superior conformability, and can tolerate far greater wear since they do not have a thin overlay. The thickness of the aluminum lining is approximately 0.010”. If the load applied to camshaft bearings exceeds the fatigue strength of AERA Automotive aluminum alloys, tri-metal materials Dr. Dmitri Kopeliovich is regarded as one of having a copper based intermediate layer the foremost authorities on engine bearing Machining and very thin (up to 0.0008”) soft Babbitt research and development. He earned his Online Training overlay are used. Ph.D. in materials and serves as research and development manager at King Go online for more information Tri-metal materials have greater load Engine Bearings. As the manager and team capacity, but their conformability and about the AERA Cylinder Head and leader of King’s advanced materials research Machinist Certificate Program. maximum wear are limited by the very and development unit, Dr. Kopeliovich is To get started, call Karen at known as the “Engine Bearing Doctor” for low thickness of the overlay. Once the 815-526-7600, ext. 202 or overlay is locally worn out and the bronze his extensive investigations into the cause and prevention of premature engine bearing email [email protected]. intermediate layer is exposed, seizure of failure. To ask Dr. Kopeliovich a question, the bearing by the steel journal becomes visit www.kingbearings.com and click on www.aera.org/training very probable. Since misalignment and “Ask Dr. Dmitri”.

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