Cam Effects.pdf

SERVICE LETTER Wärtsilä Italy, Service Subject 

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DLC

“Cam effect” on crankshaft pins

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WIT-S

03.05.2010

S-ALL.0003.GB 110

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“Cam Effect” Effect” On Crankshaft Pins 1

General The purpose of this Service Bulletin is to give basic knowledge about the so-called “cam effect” effect” phenomenon. This Service Letter is also intended to give advice for possible refurbishing work to be carried out on worn pins, and for countermeasures to be adopted in order to avoid undergoing such a kind of occurrence.

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Background Field experience reports a limited number of cases related to a particular wear phenomenon commonly known as “cam effect effect”, ”, which affects crankshaft pins on engines of different type and size. Such wear phenomenon may be found on medium-speed engines that have run for several tens of thousands running hours, including periods when fouled lubricating oil was used (i.e. oil with suspended abrasive particles, etc.). If not promptly and adequately remedied, it may lead soon to early bearing shells deterioration and even more serious consequential damages to the engine concerned (e.g. bearing seizure).

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Application This Service Letter concerns all W26, W38B, W64, all Sulzer Z40, ZA40, ZA40S and all GMT230, GMT320, GMT420, GMT550 4-stroke medium-speed engines provided with lower big end bearing shells with full oil groove.

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Description Of “Cam “Cam Effect” Effect” Phenomenon “Cam effect” effect ”  is a non-uniform wear phenomenon resulting as a deviation from the original geometry of crankshaft pins. It turns out as a pro truding band (“cam”) whose maximum projection typically comes across a particular angular position which corresponds to the portion of the pin pointing upward (to the cylinder head) when the pin itself is set at BDC, i.e. between the crank webs (see Figure 1). The typical “cam effect” protruding effect” protruding band is related with the use of partially-grooved bearing shells, which feature a through-groove for the oil feed to the connecting rod only on the lower half (see Figure 2).

Figure 1: c rankshaft rankshaft pin affected by “cam effect” .

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Figure 2: pair of partially-grooved partially-grooved big end bearing shells. The upper half (low in the picture) shows a central discoloured band due to the “cam” protruding on the affected pin.

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“Cam effect” on crankshaft pins

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“Cam effect” can affect crankshaft pins of engines that have run for many running hours (e.g. more than 50000 rhs), and its occurrence and significance is always closely connected to the fouling degree of the lubricating oil and/or inadequate care of lube oil system. In fact, the “cam”  protrudes because the surface of the pin at both sides of the “cam” is affected by a significant amount of wear. This is set off by the continuous action of tiny abrasive particles suspended in the lube oil film which may have been present in the lube oil system, when not constantly maintained in the proper way. Such wear mechanism typically takes place when the engine is running at low loads, and when the concerned driving unit moves across the TDC, at the beginning of the intake stroke. This is the only stroke of a whole engine cycle when the lower bearings half actually works: the crank pin begins to drag the driving unit components down to the BDC in opposition to their inertia (traverse reverse of piston and connecting rod), so the crank pin itself perceives a short and sudden counterstroke by the lower bearing shell half, which recovers the radial backlash at the same time. When fouled lubricating oil is used for prolonged operating periods, the above mentioned active phase for the lower bearing shell half can lead to cam effect development: the abrasive action of dirt particles emphasizes pin wear in line with the two running surfaces of the lower bearing shell at both sides of the oil groove, while the central band of the pin that faces the groove is not worn, thus protrudes out and the “cam” outline takes shape (see Figure 5). When a pin has developed “cam effect”, consequential  damage at the upper bearing shells may occur during the remaining strokes (compression, expansion and exhaust), which represent the active phases for the upper bearing shell halves. In fact, due to the protruding “cam”  developed on the pin, the stresses being borne by the upper bearing shell half (which features a solid running surface, i.e. do not include any oil groove) act on a limited contact zone corresponding to the protruding “cam” itself (see Figure 6). The resulting local overload on the upper bearing shell half can lead first to pronounced wear along the central band (early symptoms), then to abnormal deterioration (scuffing and smearing of the running layer  –  see Figure 3), and the risk to overcome the fatigue limit of the bearing material is increased (see Figure 4). As an ultimate danger, bearing shells failures may expose the engine to even more critical, sometimes catastrophic consequential failures, e.g. crankshaft seizures. Therefore, the occurrence of “cam effect” phenomenon  under certain conditions may also seriously compromise engine safety. Note:

When coupled to pins with an already existing “cam effect”, brand new bearings shells suffer a deterioration rate higher than used ones. Indeed, while used bearings shells undergo a sort of gradual “adaptation” to the  altered pin geometry throughout several running hours, new bearings shells fit badly with pins whose running surface differs from their original cylindrical shape (see Figure 6). For this reason, in suspect conditions, particular attention must be paid with regard to the running-in procedure.

Figure 3: typical appearance of an upper BEB shell damaged by “cam”  Figure 4: running layer material detachment by fatigue damage from effect”: uneven wear at the central band of the bearing, where the upper half of a 3-metal big end bearing shell, located on

Wärtsilä Italia S.p.A. Services

Bagnoli della Rosandra, 334 34018 San Dorligo Della Valle (TS) ITALY

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+39 040 319 5000 +39 040 319 5647

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“Cam effect” on crankshaft pins

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WIT-S

03.05.2010

S-ALL.0003.GB 110

smearing of running layer material took place.

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the central band.

protruding “cam”

zone affected by wear caused by abrasive particles in the lubricating oil

crankpin BEB lower shell half

Figure 5 : crank at TDC position, at the beginning of the induction stroke. Solid particles suspended in the soiled lubricating oil lead to uneven wear of the pin, generating a sort of protruding “cam” due to the presence of the oil groove in the BEB lower shell half. Note the position of the “cam”: between the crank webs.

BEB upper shell half

crankpin

protruding “cam”

Figure 6: crank at BDC position. The BEB upper shell half is solid, i.e. does not feature an oil groove where the protruding “cam” can slip into. Consequently, the bearing shell undergo local overload along its central band, which lead to its early deterioration (uneven wear, scuffing, smearing, fatigue detachment of running layer material).

Wärtsilä Italia S.p.A. Services

Bagnoli della Rosandra, 334 34018 San Dorligo Della Valle (TS) ITALY

Telephone : Fax :

+39 040 319 5000 +39 040 319 5647

SERVICE LETTER Wärtsilä Italy, Service Subject 

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Document No.

“Cam effect” on crankshaft pins

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S-ALL.0003.GB 110

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Detection Methods Besides the above explained typical wear pattern that can be noticed on the concerned bearing shells, the occurrence of the “cam effect” phenomenon can be ascertained and evaluated by three different methods: - Prussian blue check. This qualitative check is performed making use of a couple of brand new bearing shell halves. After dismantling the complete driving unit (piston, connecting rod, BEB caps, etc.) and cleaning the pin from any trace of lubricating oil, the running surface of both upper and lower brand new bearing shell halves must be painted with a very thin and uniform Prussian blue layer. Afterwards, the complete driving unit must be assembled, all the screws or studs/nuts tightened according to the engine instruction manual, and the crankshaft must be turned slowly for at least one complete revolution. Then, the driving unit must be dismantled again in order to inspect the concerned crankpin and check for any uneven contact between the crankpin and the brand new bearing shells (in particular the upper one, which pushed against the pin, since it had to bear the weight of the complete assembled driving unit).  Any protruding “cams” on the crankpin should turn out as shown in Figure 7 and Figure 8.  An alternative way to carry out the Prussian blue check is to make use of calibrated tailor-made shells (bearing shell dummies) instead of brand new bearing shell halves, with no need to reassemble the complete driving unit. protruding “cams”

protruding “cams”

Figure 7: “Cam effect” detected by Prussian blue check on a crankpin.

Figure 8: detail of the protruding “ cams”  detected by Prussian blue check.

- Straightedge check. This is another qualitative check that can be performed quite quickly and easily on crankpins that are suspected to suffer “cam effect” phenomenon. It is performed evaluating   by naked eye possible macroscopic irregularity of the crankpin surface using a straightedge as reference, as shown on Figure 9 and Figure 10. Positioning a source of light behind the straightedge and the observation point could make easier the detection of contact and non-contact areas. Be aware that any non-axial (e.g. diagonal) positioning of straightedge leads to misinterpretation of results.

Figure 9: example of visual straightedge check for “cam effect”  detection on a crankpin .

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Figure 10: protruding “cam” (contact area) detected by straightedge check.

Bagnoli della Rosandra, 334 34018 San Dorligo Della Valle (TS) ITALY

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“Cam effect” on crankshaft pins

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- Hard resin replica of the crankshaft pin. This is the only method that allows to evaluate the severity of the “cam effect” phenomenon in a quantitative   manner. Apposite hard resin replicas (e.g. ®    and fig.12  ) Technovit   3040   type resin or similar) taken on a significant portion of crankpin (see fig.11  allow to draw up a comprehensive characterization of the “cam effect”. Lab precision feeler-pin machines can draw the replicated crankpin profiles along different generating lines, in a proper magnified scale (Figure 13). In this way, the extent of possible geometrical deviations can be evaluated with appropriate precision. Additionally, roughness measurements and profiles can also be obtained from such kind of hard replicas (see Figure 14).

Figure 11: Replication of the running surface of a crankpin affected by “cam effect” .

Figure 12: hard resin replica ready for being analysed.

Figure 13: example of surface profile drawn from a hard resin replica of a crankpin affected by “cam effect” .

Figure 14: example of surface roughness profile drawn from a hard resin replica of a crankpin affected by “cam effect” .

Wärtsilä Italia S.p.A. Services

Bagnoli della Rosandra, 334 34018 San Dorligo Della Valle (TS) ITALY

Telephone : Fax :

+39 040 319 5000 +39 040 319 5647

SERVICE LETTER Wärtsilä Italy, Service Subject 

Engine type

Ref.

Date

Document No.

“Cam effect” on crankshaft pins

See Application 

WIT-S

03.05.2010

S-ALL.0003.GB 110

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Corrective Actions For the reasons explained above, it is strongly advised to restore any pin affected by “cam effect” to its regular condition, according to the relevant crankshaft specifications, especially when new bearing shells are fitted. In presence of minor pin geometry deviations (ranging in the order of magnitude of 1/100 mm, and always to be judged case by case), it is recommend to perform a pin polishing in order to restore the proper pin geometry. Pin polishing activity can be carried out either manually, using a very fine grain-sized emery cloth or, alternatively, by means of a polishing machine. Pin polishing entails no pin under-sizing: besides, the original geometry of the pin should be preserved as much as possible, focusing the polishing action on the central band of the pin, where the “cam”  is located, paying attention always to remain within geometrical and dimensional limits of the crankshaft pin (wear and tear tolerances) established by Wärtsilä. When “cam effect” and the general crankshaft conditions are such that it is not possible to correct it by a mere polishing, machining of the crankshaft pin might be needed as an extreme solution, to reduce the diameter of the affected pin to the first available undersizing dimension.

Warning:  After any possible polishing or machining of any crankshaft pins, contact test (Prussian blue check, as per Chapter 5   of this document) must be always carried out, in order to be sure of the accuracy of the final results.

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Recommendations  As previously explained, the cause of cam effect is the presence of a period of unsuitable lubricating conditions (presence of abrasive particles in the lubricating oil) during the service history of the engine: a regime of pin wear set up such as to alter the original pin geometry. Under correct engine running conditions, no cam effect can be generated indeed. So, it is of utmost importance to use only Wärtsilä approved lubricating oils and to take particular care of the engine lubricating oil system, always following carefully the maintenance schedule as described in the Engine Instruction Manual.

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Distribution Wärtsilä Service Network

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Owners of engines concerned

Liability NO LIABILITY WHETHER DIRECT, INDIRECT, SPECIAL INCIDENTAL OR CONSEQUENTIAL, IS  ASSUMED WITH RESPECT TO THE INFORMATION CONTAINED HEREIN. THIS PUBLICATION IS CONFIDENTIAL AND INTENDED FOR INFORMATION PURPOSES ONLY.

10 Validity This general instruction is valid until further notice.

Wärtsilä Italia S.p.A. Services

Bagnoli della Rosandra, 334 34018 San Dorligo Della Valle (TS) ITALY

Telephone : Fax :

+39 040 319 5000 +39 040 319 5647