What is the function of the top bracing? The longitudinal vibration from the piston movement is transmitted to the crosshead guides and then to the engine structure ( a very stiff location in the ship’s hull supporting engine weight ).
Now to protect against the twisting forces it generates in the crosshead guides. Bracing or struts are fitted on the topmost part of the engine to provide support via the bracing shims and plates.
Because of the large size of marine diesel engines producing in between 100 to 900 horsepower. These engines produce lots of noise and vibration which if not checked or compensated can harm the engine and affects ship hull.
Thus effective vibration dampening systems such as the thrust pads, de-tuners, bracing and struts are used on a ship for the engine. For which bracings and struts are more concerned after the longitudinal vibration of the engine.
Cause of Longitudinal Vibration In Marine Engine
The vibrating tendency of an I.C Engine depends upon the degree of imbalance for its reciprocating and rotating parts. This can be better understood by considering the motion of connecting and piston rod.
At both TDC and BDC the piston and connecting rod are in straight line. Thus all the combustion forces is is transferred to the crankshaft directed in transverse direction.
But for any moment in between TDC and BDC, we have both longitudinal and transverse components of the acting force on the connecting rod. While the transverse forces are transmitted to the crankshaft. The longitudinal forces acting on the crosshead as the guiding force-generating vibration.
Such a force can cause rocking and twisting of the engine structure. These top bracing’s ( friction / hydraulic type ) are thus attached to the exhaust side of the engine structure to transmit these vibrations to the ship’s hull.
What If We Not Use Top Bracing Or Other Vibration Dampers
Its natural tendency of an engine to vibrate especially when it evolves high-speed motions working in sync. The high operating speed, moving parts, output power, and the combustion process creates vibration in an I.C engine.
The transverse reaction forces acting on the crosshead as a result of piston movement with different strokes create a guide force moment. Although not much harmful to the engine under normal condition
If they resonates with the annoying engine vibrations. These further excites the engine vibration and cause the engine to rock or twist based on the number of operating cylinders.
Without the presence of sufficient vibration dampers like the top bracing in the marine engine; the natural frequency of the engine, ship’s side, and its bottom resonates with the mean speed and guide force moment resulting in a subsequent cycle of never-ending vibration.
If this vibration surpasses a given threshold that the engine structure can not cope with its effects it simply results in severe engine damage and large cracks. Other than that we also see effects such as the:
- Subsequent cracks in various supporting points and attachments.
- Damage or negative impact to the turbocharger and other high speed supporting machinery at work.
- In the worst case, the engine may become inoperable.
- Severe damage to the engine foundation block and joints.
- damage to its trust and main bearings.
- Moderate to severe damage to the crosshead, intermediate shaft, and its bearings.
- The least it can do is to cause fatigue failure or drastically reduce the operating life span of the engine.
Function of Top Bracing in Marine Engine
Under normal condition of running engine for a short while there is not much risk of severe vibration levels thus damaging the engine. But when it is run for a longer time and with great power we need to be extra cautious.
The guide force acting on the engine elevates the vibration level. Now when the vibration frequency resonates with the engine natural frequency; it can lead to a number of unwanted events from large cracks, damaged bearings to the engine become inoperable.
The top bracing generally fitted on the exhaust side of the engine protect against the twisting forces it generates in the crosshead guides. Which means such a resonance will happens beyond the normal engine running speed; thus it reduces the effects of vibration.
Bracing or struts are fitted on the topmost part of the engine to provide support via the bracing shims and plates. The mechanical top bracing works on the the two friction shims and connect engine and the hull.
These friction stiffeners allows the bracing plates to move and adjust to the varrying load conditions. The other three dampers type fitted on the marine engine to fight against the axial, longitudinal and torsional vibration are:
- Axial dampers
- Torsional dampers
- Flexible coupling
- Side bracings
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