30 Mar 21
CHECKING AND ADJUSTMENT OF THE EXHAUST CAM FOR TWO STROKE MAN BW MC ENGINES
For checking the exhaust cam lead, the engine-specific lifting height value must be known. This value can be found in Maintenance Manual Section 908-5.1 and/or in the engine shop test results included in the Operation Manual, with the shop test results being the preferred reference.
As a general guideline, the lifting height is typically around 14.00 mm for 70MC, 12.00 mm for 60MC, and 10.00 mm for 50MC and smaller bore engines.
The above values are for reference only. The engine-specific values stated in the shop test results shall always be taken into account.
Typical Procedure for Checking the Exhaust Cam Roller
1. Stop the engine and close the starting air.
2. Stop the lubrication oil pump.
3. Remove the exhaust valve hydraulic pipe.
4. Bring the NO:1 cylinder to TDC. Measure the crankshaft position with pin gauge and confirm NO:1 cylinder exactly at TDC. (Recommended). Measure the camshaft position by pin gauge and confirm camshaft is in correct position. You should read 0 degrees on the turning wheel.
5. Measure the height of the exhaust roller.
6. Turn the engine AHEAD direction until the roller is lifted to indicated value in the shop test or maintenance manual. (For example, 12 mm).
7. Read and note the new angle indicated on the turning wheel. (For example, 113.70 deg). This is the value of the angle, which exhaust cam turned from TDC to reach to lifting height which called A
8. Turn the engine until lifting height value became to same lifting height, which is 12,00 mm in our example. (Due to shape of the cam, measured value will be increased for a while and will start to decrease to previous measurement).
9. When same value 12,00 mm obtained, read and note the angle indicated on the turning wheel. (For example, 258,30 deg) This is the value of angle, which exhaust cam turned from TDC to reach lifting height during decreasing cycle which called B
10. Theoretical middle point of the cam 180°
11. A+B=360°
12. (A+B)/2=180°
13. Difference of (A+B)/2 value from the 180° indicate us exhaust cam Lead angle…
14. This value to be compared with shop test value.
In our example:
A=113,70° B=258,30°
(A+B)/2= 186… 180-186= -6° exhaust cam lead angle…
Same procedures to be repeated for remained cylinder.
EXHAUST ROLLER OIL CYLINDER | EXHAUST ROLLER AT DISMANTLED CONDITION(FOR REFERENCE ONLY) MEASUREMENT CAN BE TAKEN ON THE TOP OF THE PISTON WITHOUT REMOVING OIL CYLINDER AND ROLLER |
READ AND NOTE THE ANGLE ON THE TURNING WHEEL (NO:1 CYL AT TDC AT ILLUSTRATION)
Above table shows shop test inspection report for same engine(60MC). As can be seen, difference between A and B is same for all cylinder, i.e. 258,3-113,7= 114,6 deg for NO:1 cyl and
126,3(360-233,70) +18,3= 114,6 deg for NO:3 cylinder.
Above inspection report shows another engine’s(70MC) record. This table they do not indicated all A and B value for all cylinder, but they give only A and B value for NO:1 cylinder. Same values for other cylinders to be calculated based on firing order.
Another inspection report belongs to 35MC engine. Kindly note number of cylinders changed, which to be considered for firing order and TDC calculation for all cylinders
REMARK: The cam lead angle can be checked visually by reading directly the scale position of the exhaust cam over the marking scratch on the camshaft. For this check position of the camshaft to be confirmed by pin gauge.
Several incidents reported that some or all the exhaust or fuel cams lost their correct position due to roller damages. In that case, cams in questions to be adjusted back to correct position.
Conclusion and recommendations:
1. General
Experience from service and overhauls has shown that damage to exhaust and fuel cam rollers may occur due to several recurring causes. This Service bulletin summarizes the most common reasons for such damage and provides recommendations for inspection, overhaul, and spare parts to minimize operational risk.
2. Reversing System Malfunction
One of the most common causes of roller damage is malfunction of the reversing system. If reversing is incomplete or does not take place, excessive forces will act on the roller guide and reversing arm. These abnormal loads may result in damage to the roller, guide blocks, reversing arm, and related components.
During overhaul of the roller, the adjustment of the reversing arms shall always be checked and corrected if necessary. Reference is made to Service Letter SL1988-243 and the relevant section of the Maintenance Manual, Plate No. 909.
3. Water Ingress into Roller Housing
Another possible cause of roller damage is water ingress into the roller housing. In cases where effective sealing between the roller housing and the engine frame is missing, and where excessive leakage from cooling jackets is combined with blocked drain holes, water may enter the roller housing.
Water ingress may break down the lubricating oil film, resulting in roller seizure and subsequent damage. It is therefore essential to:
• Maintain effective sealing between roller housing and engine frame
• Eliminate any cooling water leakages
• Ensure that all drain holes are kept clean and free of foreign materials
4. Roller Overhaul and Inspection
Roller overhaul and inspection shall not be omitted and should always be carried out in connection with fuel pump overhauls.
Crack testing of rollers provides early detection of crack development, which may otherwise lead to severe damage during operation. Furthermore, loose guide blocks may cause misalignment, resulting in abnormal roller loading and damage.
For further guidance, reference is made to Service Letter SL1997-345.
5. Seals and O-Rings
Service experience has shown that roller overhauls are sometimes cancelled due to missing seals and O-rings for roller housings and umbrella seals. As roller overhaul and inspection are normally scheduled together with fuel pump overhaul during dry-docking, such cancellations should be avoided.
It is therefore strongly recommended to keep a complete set of seals and O-rings for the roller housing and umbrella seal system available on board.
6. Recommended Spare Parts on Board
To reduce the risk of operational delays, the following spare parts are recommended to be kept on board:
• 1 pc exhaust cam (two halves)
• 1 pc roller cam (two halves)
• 1 set fuel pump roller
• 1 set exhaust cam roller
• 1 pc fuel roller housing
• 1 pc exhaust roller housing
• 1 pc spring for fuel and exhaust roller
7. Remark
When checking fuel pump lead together with exhaust cam lead, it shall be ensured that the fuel pump roller is positioned in the ahead direction.
Instructions for changing the roller position can be found at: www.vitatr.com
For further information or service assistance, please contact:
info@vitatr.com
