Crankshaft overlap
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Jerry Poole
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Crankshaft overlap
Could some of the members please explain the term or concept of "crankshaft overlap"? How does this effect the harmonics and the ability of the crank to rev? Can it be corrected with machine work? The crank in question is a late model Triumph Spitfire with a main journal dia. of 2.31" and a rod journal size of 1.875", stroke of 3.44". This crank is very heavy and can stand a lot of metal removal @ the counterweights. The term of "overlap" seems to be the "bugaboo" with this crank. Any insight would be appreciated> This is a fantastic forum. Thanks Jerry
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OldSStroker
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Crankshaft "overlap" can be visualized by looking along the axis of the crank. A simple sketch helps me:
Draw a 2.31 diameter (1.155 radius) circle to represent the main journal.
Extend a diameter line and draw a 1.875 diameter (.9375 radius) circle with its center one half the stroke, the "throw" or (3.44/2) 1.72 away from the main journal center. The area where the circles (journals) overlap is what most folks mean by crankshaft overlap.
You could do the math to get an overlap distance:
(main dia/2) + (rod dia/2) -(stroke/2) =Overlap
In this case: (2.31/2) + (1.875/2) - (3.44/2) = .3725
The metal in the crank journals doesn't really "overlap" because of the main and rod journals are offset, but intuitively, more overlap should equal a stronger crank.
The area of overlap is more important than the overlap distance. The simple drawing illustrates the area. Calculating it is somewhat more rigerous.
If you draw a few OEM and aftermarket overlap diagrams you find some negative overlaps on some very long stroke engines like Mountain Motors used in drag racing. These engines produce mountains of torque and power without breaking cranks, which usually scares the "overlap is essential" crowd. Here's where a very strong, often billet, crank shows its value.
If a crank has very thin "arms" connecting the main and rod journals, it can be weak, even with positive overlap. The placement of the metal and the type (forged carbon steel, forged alloy steel, billet steel or various cast (ductile) iron) is more important than overlap.
Obviously weak-armed cranks will flex more under the cyclic torsional and bending loads a crank withstands, and may fail sooner than properly designed crank arms. If folks are routinely breaking Spitfire cranks when the engines are modified and run at higher rpm it's probably the crank design rather than the overlap that is the "bugaboo". If these cranks fail at the ends of the journals where they meet the "arms", that is more likely insufficient or poorly executed fillet radii at the ends of the journals.
Does overlap effect harmonics and the ability to rev? I think that is more a factor of crank mass and rotating inertia than overlap. Look at good aftermarket cranks for 4-cylinder engines to see how to shape/reshape the crank.
Draw a 2.31 diameter (1.155 radius) circle to represent the main journal.
Extend a diameter line and draw a 1.875 diameter (.9375 radius) circle with its center one half the stroke, the "throw" or (3.44/2) 1.72 away from the main journal center. The area where the circles (journals) overlap is what most folks mean by crankshaft overlap.
You could do the math to get an overlap distance:
(main dia/2) + (rod dia/2) -(stroke/2) =Overlap
In this case: (2.31/2) + (1.875/2) - (3.44/2) = .3725
The metal in the crank journals doesn't really "overlap" because of the main and rod journals are offset, but intuitively, more overlap should equal a stronger crank.
The area of overlap is more important than the overlap distance. The simple drawing illustrates the area. Calculating it is somewhat more rigerous.
If you draw a few OEM and aftermarket overlap diagrams you find some negative overlaps on some very long stroke engines like Mountain Motors used in drag racing. These engines produce mountains of torque and power without breaking cranks, which usually scares the "overlap is essential" crowd. Here's where a very strong, often billet, crank shows its value.
If a crank has very thin "arms" connecting the main and rod journals, it can be weak, even with positive overlap. The placement of the metal and the type (forged carbon steel, forged alloy steel, billet steel or various cast (ductile) iron) is more important than overlap.
Obviously weak-armed cranks will flex more under the cyclic torsional and bending loads a crank withstands, and may fail sooner than properly designed crank arms. If folks are routinely breaking Spitfire cranks when the engines are modified and run at higher rpm it's probably the crank design rather than the overlap that is the "bugaboo". If these cranks fail at the ends of the journals where they meet the "arms", that is more likely insufficient or poorly executed fillet radii at the ends of the journals.
Does overlap effect harmonics and the ability to rev? I think that is more a factor of crank mass and rotating inertia than overlap. Look at good aftermarket cranks for 4-cylinder engines to see how to shape/reshape the crank.
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[i]"Being able to "think outside the box" presupposes you were able to think in it." [b]--Bob Lutz[/b][/i]
