A customer in the field recently asked, did a lubricant cause a crankshaft failure?
As most of you would suspect, the answer to the question was most likely “no,” because properly manufactured lubricants do not cause failure. The question, however, got me thinking, do we really understand crankshafts?
Not so much what they do, but other issues like, how are they made, what are they made of, keys to the design, and more?
So, this tip will take a closer look at the crankshaft – what it is, how it works, and how to properly maintain it to keep operational productivity high.
A crankshaft is a mechanical part able to convert reciprocating motion (linear) into rotational (circular) motion. They are most often found in a reciprocating engine, like and automobile or compressor, although in the latter, the “crank” is used to concert rotary motion to linear.
A crankshaft is composed of crank journals where the lower end of the piston connecting rod is fixed. This journal is usually separated from the connecting rod by a plain bearing. Crank journals are usually aligned in three ways, in-line, “V”, or radially and is dictated by the application or demand.
The crankshaft has a linear axis around which it rotates. During rotation there is a sideways load which much be supported, and this is accomplished by the main bearings, which again are also “plain” in design. The number of main bearings to support a cylinder typically exceeds the number of pistons supported, for example a six cylinder inline engine may have five to seven main bearings.
Crankshafts have counterweights to provide engine balance during rotation. These are cast into the unit and may be adjusted with bolt on pieces if load/thrust design changes.
Crankshafts are normally forged or cast and are typically one piece. Forged crankshafts are most common in today’s engines or compressors, with various micro steel alloys used. Crankshafts may be ground to remove excess material or to adjust for engine design changes. Most crankshafts are induction hardened or nitrided with the choice depending on application needs and cost considerations.
Stress on a Crankshaft
Did you know that if you try to twist a crankshaft by hand at opposing end nothing will happen? However, if you drop one from a height of three or four feet, the crankshaft will often shatter into two or three pieces. Why is that?
There are two main forces on a crankshaft; bending and twisting. Bending is most prevalent at the center and end positions and is controlled by the main bearing design, quantity and location. Twisting exists at each rod journal and is provided by the lateral movement of the connecting rod and journal. It is best handle by the design of the journal, rod bearings, and engine speed controls. In a properly designed and operated engine, bending stress is the lesser of the two forces.
Proper lubrication of a crankshaft is aided by a lubricant delivered to the load zone through both splash and pressure lubrication modes. Splash is provided by the turbulent motion of the crankshaft, and oil level within the crankcase.
Pressure lubrication is provided by an oil pump supplying lubricant to the holes and passages pre-drilled in the crankshaft axis and journals.
The types of oils used to lubricate a crankshaft are typically rust and oxidation inhibited circulating oils or engine oils that contain viscosity index improvers, anti-wear additives and oxidation inhibitors. Gear oils are typically avoided in this application.
Why did the crankshaft posed in the introduction fail?
More than likely, it had nothing to do with the oil but was instead related to excessive twisting stress, which in turn was most likely the result of a mechanical, operational, and/or design issues. However, only in-depth root cause failure analysis would be able to determine the true cause.
I hope this tip was helpful, and if you have any questions, please leave a comment in the section below!
Excellent information on the crankshaft. What would be the cause in case the crankshaft has scratching in the crankpins?, a customer asked me this and I did not know what to answer. Rick, do you have any idea of this situation given in a crankshaft?
I do not know what a "crankpin" is. Probably a poor translation. What could cause scratching of the crankshaft and journals is poor installation of the crankshaft or bearing, or debris in the oil.
Excellent article, as always, Rick! Everything is simple and clear.
Gracias por su respuesta Rick,es sobre los rayado del cigueñal,sobre las causas que lo provoquen,saludos.
My answer above applies.
In regards to the crank pin question, here's the google search results link for it
Thank you Sam. I have never heard the term "crank pin" but instead used the term "crank journal." Learn something everyday. My answer above addresses the question.
Failure of the crankshaft is often due to poor balancing of the shaft with the flywheel or their incorrect assembly after repair.