Understanding Crankshaft Dimensions: Crank Throw Explained

The question asks about a specific distance found within a crankshaft assembly: the distance measured between the centerline of the main journal and the centerline of the crank shaft pin. This distance is a fundamental dimension in the design and operation of an internal combustion engine.

Let's define the key terms involved:

• Main Journal: These are the parts of the crankshaft that are supported by bearings in the engine block. The crankshaft rotates about the centerline of the main journals.
• Crank Pin (or Crank Shaft Pin): This is the journal to which the connecting rod is attached. The connecting rod rotates or oscillates around the centerline of the crank pin.

The distance between the centerline of the main journal and the centerline of the crank pin is crucial because it determines the radius of the circle that the crank pin traces around the main journal axis. This radius is directly related to the engine's stroke.

Analyzing the Options

Let's look at the provided options and see which one correctly identifies this distance:

1. Balance weight: Balance weights are masses added to the crankshaft to counterbalance the inertial forces created by the rotating crank pins and connecting rods. They are used to reduce vibration but are not a measure of distance between journals.
2. Crank throw: The crank throw is defined precisely as the distance between the centerline of the main journal and the centerline of the crank pin. This distance is half of the engine's stroke length (\(\text{Stroke} = 2 \times \text{Crank Throw}\)).
3. Web: The web is the connecting part of the crankshaft that links the main journal to the crank pin. It's a structural component, not a distance measurement between the centerlines of the journals.
4. Pitch: In the context of mechanical components, pitch can refer to various things like the distance between threads, teeth, or structural elements. While a crankshaft has specific dimensions and spacing, "pitch" is not the standard term for the distance between the main journal and crank pin centerlines.

Based on the definitions, the distance between the centerlines of the main journal and the crank shaft pin is indeed known as the crank throw.

Here's a summary:

Therefore, the correct term for the distance between the centerlines of the main journal and the crank shaft pin is the crank throw.

Revision Table: Key Crankshaft Terms

Additional Information: Role of Crank Throw in Engine Design

The crank throw is a fundamental parameter in engine design as it directly determines the engine's stroke length. A longer crank throw results in a longer stroke. Stroke length, along with the cylinder bore (diameter), determines the engine's displacement volume (the total volume swept by the pistons in all cylinders).

Engines with a longer stroke relative to their bore are often called "undersquare" or "long-stroke" engines. They tend to produce more torque at lower engine speeds. Engines with a shorter stroke relative to their bore are called "oversquare" or "short-stroke" engines, and they typically favor higher engine speeds and power output.

The crank throw distance, therefore, is not just a measurement; it's a critical design choice that significantly influences an engine's performance characteristics.