Main Engine Crankshaft | Types and Materials Explained
The main engine is responsible for moving a ship. It can turn the propeller at various speeds for manoeuvring and at constant speeds when the ship is out at sea. It is the main engine crankshaft that supplies this motion to the propeller. In this article, we shall discuss this critical part of the main engine.
We shall look at their various types, what makes each different and the material used for each of them. If you want an overview of all the parts in a marine engine, click here.
What is the main engine crankshaft?
The main engine crankshaft is a rotating part that sits on top of the engine bedplate. It connects to every unit of the main engine through a connecting rod arrangement that allows it to receive and transfer power between all the units and itself.
When a unit is in the power stroke, the pressure from combustion imparts linear motion to the piston. The piston rod transfers this power to the crosshead which helps the connecting rod convert it into rotational motion of the crankshaft. The propeller uses the bulk of this power and converts it into ahead or astern thrust for the ship.
A small portion of this power is also used to compress fresh air in other units that are in the suction stroke. Other power consumption avenues on the crankshaft are the camshaft (through chains or gears), shaft generator and frictional losses.
Types of main engine crankshaft
Crankshafts are often classified based on the type of construction. We can classify the common marine engine crankshaft into the following three types:
Fully built crankshaft
Crankshafts on large engines can weigh as much as 300 tonnes and are difficult to build as a single piece. As a result, engine manufacturers developed fully built crankshafts.
A fully built crankshaft is one that is built as separate parts that are then assembled using shrink fitting. The separate parts are the journal, the webs, and the crank pins. These parts are made using steel casting or forging. Shrink fit assembly ensures a firm grip without any slippage between individual parts.
Such a crankshaft offers several advantages such as easy part replacement and simple construction. Also, since the parts are machined before assembly, the crankshaft has a better build quality and surface finish.
This crankshaft was the preferred type in marine engines in older models. But the issues with uneven grain flow (lower fatigue strength) and the weakness imparted due to multiple shrink fits have led to reduced usage of this crankshaft type in modern engines. These crankshafts also require external piping for lube oil supply as it is not advisable to build lube oil channels in shrunk-fit parts.
Most large marine engines today use the semi-built crankshaft. The semi-built crankshaft consists of two main parts, the journal (main shaft) and the crank throw. The crank throw constitutes two webs and the crank pin and is built as a single piece. We shrink fit the forged journal into the crank throws.
Semi-built crankshafts are better than fully built crankshafts for several reasons such as fewer shrink fits, better grain flow, smaller and lighter webs, larger diameter journals and reduced slipping damage.
It also has the advantage of greater flexibility as compared to solid built crankshaft (next section) which allows it to sustain some misalignment.
Solid built crankshaft
When the entire crankshaft is made out of a single piece, it is known as a solid-built crankshaft. Main engines on ships are too large for this type of crankshaft and thus, it is typically used for small engines.
A solid-built crankshaft has much better grain flow characteristics compared to the previous two types which gives it much better fatigue resistance properties.
There are also no shrink fits which improves the overall strength while reducing the shaft size. A smaller shaft is also lighter allowing engineers to fit more crank throws per shaft if needed.
However, solid-built crankshafts are more rigid which renders them incapable of handling small misalignments. Another disadvantage is that there is not much possibility of repairs since it is not made of individual parts. When a defect occurs, the whole shaft has to be changed in most cases.
Welded crankshafts have become extremely popular in recent years and are steadily becoming the preferred crankshaft type in marine engines. There are two methods in which these are built.
In the first method, two crank arms are joined at the pin. The second weld is then made in the middle of the bearing journals as the crankshaft stresses are the lowest here. A crank arm consists of half main journal, half crank pin and one crank web.
In the second method, the crank throw is forged or casted as a single piece which is then welded at the main journal. The most suitable welding method is submerged arc welding carried out with a narrow gap and a low heat input.
A welded crankshaft provides the best combination of cost, space and weight without compromising on strength. The weld’s fatigue resistance is usually better than the welded material. These crankshafts also have low inertia which results in a higher natural frequency for the crankshaft. This is especially important in slow-speed engines.
The welded crankshaft also allows us to increase the journal diameter ultimately reducing the pressure on the bearing. To top it all, all the major classification societies have approved welded crankshafts making them a safe choice when it comes to compliance.
Main engine crankshaft material
The crankshaft is under continuous cyclic loads when in operation and under static load when at rest. The material for a crankshaft determines how effective and reliable its operation will be.
The demanding service environment is factored in when designing the crankshaft and selecting its material. The choice depends on factors such as the crankshaft design, geometry and the type of engine. In any case, it must have high strength.
Fully built and semi-built crankshafts are typically made from unalloyed carbon steel which has been normalized. We may also use low alloyed chrome-molybdenum steel for the same.
When it comes to the welded crankshaft, the preferred material is hardened and tempered low alloy Chrome-Molybdenum steel or normalized low carbon manganese steel.
Crankshafts are a critical part of the main engine. They bear tremendous loads for long periods of time as a ship engine may run continuously for weeks and even months.
It is important to understand the types of crankshafts before we delve into deeper topics such as crankshaft failure and crankshaft deflection. We shall be writing on these topics in the near future. We hope this article was able to improve your understanding of the different types of crankshafts in the main engine.