Exhaust valves are subjected to high temperature and shoot. This can sometimes leads to valve seizure, heavy shoot deposits and local Overheating. A Rotocap is used to protect the exhaust valve from above concerns extending its overall life. It is fitted on the exhaust valves of auxiliary or medium speed engines. Main engines have rotating vanes fitted to them instead of Rotocap to help them rotate.
Function Of Rotocap
A Rotocap is a simple device used for forced valve rotation. A Rotocap is fitted on the exhaust valves of medium speed engines; they are also being used in automobile industry for vehicles with high power and low speed. It turns the valve spindle during opening of the valve to ensure even heat transfer around the valve seat. Thus helps avoid local hot-spots, pitting and distortion due to uneven heating.
It also help reduce abrasive wear and shoot deposits on the valve seat and valve guide. It avoid uneven or unintentional rotation of exhaust valves by rotating the valve by 8 degree on each operation; increasing overall life of the valves. It avoids exhaust valve from a situation called burning. When heavy shoot deposits on valve seat due to Rotocap failure; exhaust valve stops to seat correctly. This allows the hot flue gas to pass through the valve face.
Such condition leads to local overheating, weakening the valve material. This helps further weakening of the valve producing a chain reaction making more and more gap for gas to pass overtime. This will only be noticed once the damage had a;ready been done, by high exhaust temperature accompanied by power loss. If it not be the Rotocap, such condition would be part of regular maintenance which is ideally nor required.
A Rotocap is a self contained assembly of balls and ramps. It is a positive type of valve rotor structure which consists of a valve plate with six slots machined into it for balls to roll. Each slots have a separate set of spring and ball placed into it; where each ball is pressed upon by these small springs. While the valve is in closed position; the belville springs locks the spring carrier and valve cover together. This contains a complete set of spiral / Helical coil seat.
When valve is in closed position, the balls are free to move over the grooves. Each ball race works as a path or track for these balls to move / roll while the spring washer sits against the rim of Rotocap. It is constructed to retain the valve rotor when the retaining rings are removed.
Working of Rotocap
Increasing load on the valve spring during start of exhaust ( Opening of exhaust valve ) make the spring washer flat. The force applied on the balls placed on race / track leads the ball to move freely on the inclined tracks. This transfer the reactive force of ball to the spring washer; while the reactive force of spring washer being transfered to inner rim of the valve body.
During these process, spring loaded seat and valve assembly rotates to the same degree. This ensures spring washer, valve spring, ball race / track and collar locked together on a friction grip. As the valve start to close, the belville washer starts to return to its original position; lifting the ball against the small compression spring. This turns the valve assembly and spring holder to its natural position ensuring no further movement of the balls.
During preventive maintenance of cylinder head; Individual components are to be dismantled and checked for any signs of crack or damage. This is applicable for all valves, its seat, their guides and valve springs. If Rotocap is fitted ( generally for auxiliary and medium speed engines ) then strip it out, clean and then inspect.
If there is signs of defect or irregularity in the device, change it with a spare if available. Under the condition of lack of spare available or not much signs of defects; then disassemble the Rotocap for maintenance. Make sure to renew all the springs, clean all tracks and check ball condition. It is to be ensured that the balls are perfectly round with spring washers in good condition.
Do not and I mean, Do not try to fix any defects if any on spring retaining plate; as its much easier and cheaper to just replace with a new Rotocap then to fix it. Use special tools provided by the manufacturer for the whole process; at last assemble all units and test the engine.
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