Cranking it up!February 25, 2021
Little thought is given to the crankshaft. It goes about its business only demanding that it be kept well oiled. Still, if something does go wrong, the result is usually catastrophic.
To accomplish that sterling record of reliable performance requires a great deal of engineering.
A hard life
To understand a crankshaft, it is necessary to study the events that create its movement.
Many of us have had the opportunity to spin a crankshaft over in an engine block with no connecting rods and pistons attached.
It did not take much for it to move — simply grab one of the counterweights and give it a push while moving your hand in an arc.
The resultant motion is deceiving since your hand is rotating to turn the crankshaft.
When the engine is running, the energy imparted into the crankshaft to make it move is violent.
In an engine, the fuel and air mixture is ignited, which creates a flame in the bore.
As the flame travels across the cylinder, the pressure rises since it expands and generates heat.
The expansion of the flame then drives the piston down in the bore.
The piston is attached to the connecting rod small end (pin bore) with the big end on the crankshaft journal.
At that point, the expanding flame’s motion is converted through an awkward angularity into the rotation.
Cylinder pressure during the flame’s expansion is the key to power– a high horsepower engine produces more cylinder pressure. In turn, more of that transfers to the crankshaft.
This was known early on, but it was not until after WWII and the refinement of high-speed photography was the gyrations of the crankshaft and its movement under cylinder pressure discovered.
When under high pressure, the crankshaft walks back- and -forth, twists, moves up and down, and does things that would seem to defy its ability to last in an engine.
Then to make matters worse, the force is reversed when the piston changes direction abruptly.
The making of a crankshaft
A crankshaft can either be one-piece or a design comprised of many pieces joined together.
It consists of a series of throws connected by the main bearing journals.
Each crank throw is formed by a pair of webs. These being united by the crankpins to which the big end of the connecting rod attaches.
Adequate rigidity is required for the crankshaft to resist and limit both bending and twisting.
It must also be noted that the crankshaft has points of heavy loading and a concentration of stresses, especially where the big end of the connecting rod attaches.
This is further exacerbated by the need to drill oil holes in the cranks pins that reduce the amount of material to handle the loading.
When the crankshaft is rotating, the centrifugal force acting upon each crank throw along with the lower part of the connecting rod tends to want to deflect the crankshaft.
Since the deflection is resisted by the main bearings, their loading is increased dramatically.
To reduce this load, counterbalance weights are employed at the crank throw webs.
When designing a crankshaft, many follow the rule that the crankpin needs a diameter of at least 0.60 of the cylinder bore dimension. The length is not less than 0.30 of the pin diameter.
Web thickness of the crank throw is traditionally in the area of 0.20 of the cylinder bore dimension.
The main bearing journal is larger than that of the crankpin with up to 0.75 of the cylinder bore dimension and a length of approximately 0.50 of the journal diameter.
Until the early 1960s, automotive crankshafts were traditionally forged from high-strength, low-alloy steels.
Since then, the industry has gone to iron castings with a material known as spheroidal graphite or S.G. iron.
High strength cast iron of this composition was developed in America in the late 1940s.
Its distinguishing feature is the graphite structure takes the form of spheroidal nodules.
This produces higher strength, better ductility, and greater toughness than a flake-like graphite structure of normal gray cast iron.
It results from injecting a trace of magnesium into the iron melt, which causes the graphite flakes to gather into little balls or, in technical terms, spheroidal nodules that significantly strengthen the material’s grain structure.
A crankshaft made from this material is known as nodular iron.
The Brinell hardness of the spheroidal nodule material used for a crankshaft is usually in the 217 to 286 range.
The Brinell hardness test is named after its founder, J.A. Brinell, who introduced the procedure in 1900.
It measures the resistance to penetration of a material by a harder one in the form of a steel ball. It is useful to determine the tensile strength of a material.
There are casting and forging processes to produce a crankshaft. The material employed, in most instances, is not associated with the procedure.
A forged crankshaft is manufactured by a process in which the metal is a plastic state instead of molten.
It is forced to flow, employing hammering, squeezing, and bending.
A cast crankshaft is made by a process where molten metal is poured into a mold and allowed to solidify.
Regardless of the process (forged or cast), the crankshaft is then heat-treated for a specific tensile strength.
A standard specification is 40 tons per square inch.
The crankshaft is machined to its final dimensions. This entails rough turning, finish grinding, and final lapping of the main journals and the crank pin.
Making it last
The foundation of a reliable crankshaft lies in its design, material, and manufacturing process, but other factors come into play.
The harmonic damper that is mounted on the snout is used to absorb and cancel out the vibrations imparted to the crankshaft during its movement.
A well-designed damper will be a real asset in allowing the crankshaft to live in the conditions that it must.
When designing a crankshaft for an engine, the peak power and use must be considered.
For this reason, an engine that is being placed in a truck that will see more continuous loading (cylinder pressure) than a passenger car will traditionally be fitted with a different crankshaft not only in material or manufacturing process, but in journal size and web thickness in relation to the bore.
The crankshaft now has finally received its due!