Bending results in the tensile, compressive & shear stresses in the material of the crank web.
Twisting results in the shear stresses.
Crankshafts are subject to a complex form of the loading which varies with time. In addition shrink fits, oil holes & fillet radii add to the complexity. Pure stress analysis & rules governing crankshaft dimensions are based upon a combination of theory & experience.
The main loading stresses are:
- Gas loads on the crankpin which produce alternating tangential bending of the webs alternating bending of the crankpin & elements of shearing of the crankpin at the inner web faces
- Torsional vibrations produce alternating twisting of the crankshaft, the journal of which is in any event torsionally loaded by the gas loads via the web
- Axial vibrations in conjunction with the alternating lengthening & shortening of the shaft combined with local bending. Crankshafts may, in addition be subject to misalignment due to bearing wear or poor chocking. This produces & alternating bending of the crankshaft
The forgoing alternating stress patterns produce fatigue, therefore the crankshaft material must have a built in resistance, which is of the equal importance to its U.T.S. (Ultimate Tensile Strength). Mild steel is usually the material used but in some cases alloying the steel with a small percentage of the Ni, Cr, Vanadium may take place.
Crankshafts generally fail due to the propagation of the cracks from stress concentration points.
Vibration
All components vibrate for example,. a weight on the spring; rotating components such as crankshafts can vibrate in a torsional manner. The systems may differ but the principals are the same. The operating frequency caused by the operating speed is called as the forcing frequency. All systems have natural frequencies where the vibration amplitude is excessive (consider out of balance wheels on a car). Resonance occurs when the forcing frequency & natural frequency coincide & the result is excessive vibration. If it is needed to keep the vibration amplitude below a certain value in order to limit stress to avert fatigue, then speeds coinciding to the natural frequency orders should be avoided. These speeds are known as the barred speeds (or critical speed ranges).
If the barred speed is coincidental to engine operating speed, say at half ahead, it will be essential to fit a detuner or vibration damper. These lower the vibration peak & move it slightly higher in the range. The barred speed is either removed or moved away from the area in which the engine is operated. A vibration damper consists of an additional rotating mass driven by the crankshaft & connected to it by spring or hydraulic fluid. The energy of vibration is used up in distorting the spring or shearing the fluid.
With constant speed engines using a CPP propeller, vibration dampers are sometimes needed because the natural frequencies of the engine & shaft system change with load due to the pitch of the propeller. In some cases there may even be a barred pitch.
Methods of forming a crankshaft
The ideal arrangement is that of the solid forged structure due to the continuity of the material grain flow which allows for smooth transmission of the stress.
Unfortunately, such crankshafts are limited to smaller engines because there is a limitation to the size of forging equipment & the size of steel ingot which can be produced.
Built up crankshafts with the shrink fits or welded sections allow very large units to be produced, but they tend to be heavier & less rigid than an equivalent solid forged items.
The grain flow method allows solid forged crankshafts to be produced with minimum energy & minimum need for post machining. A heated section of the ingot is held by 3 clamps which can be moved hydraulically. The 3 stages for forming the crank throws are illustrated. When one throw has been formed the next section of bar is heated, the shaft is held in the clamps again & the next throw formed.
Welded crankshaft
A form of the crankshaft construction recently developed is that of the welding. Cast web crank pin & half journal units are connected at the half journals by welding. These welds are stress relieved & the pins ground to give the correct finish. This type of construction is preferable for the large direct drive engines & it provides strength close to that of the solid forged crankshaft. Any number of units may be connected
Semi-built up
No dowels are fitted as these can act as stress raisers.
The usual form of the construction for direct drive engine crankshafts is the semi-built up type. This makes use of shrink fits between the journals & webs. Careful design is needed to assure the shrink fit is strong enough but does not impose excessive shrinkage stress.
The shrink fit must provide sufficient strength to allow the required torque to be transferred. The actual allowance is about 1/500-1/600 of the diameter. Too large an allowance produces a high stress which can result in yielding when the working stress is added. Too small an allowance can lead to slippage.
In order to provide for the large torque transmission without high stress the area of contact at the shrink fit should be increased. Increasing the diameter rather than an increase in the length generally achieves this, an increase in length would result in an over-long engine. Additionally it allows a fillet radius to be used as the journal part of the pin does not require to be of the same large diameter. The fillet permits a smooth transition & is rolled as this produces a compressive stress which provides a safe guard against the fatigue. The fillet is undercut allowing the web to be positioned against the bearing reducing the engine length & oil loss from the ends of the bearing.
Slippage of shrink fits
Slippage may occur at the shrink fits and this can be checked examination of the reference mark at the end of the web and pin.
For Slippage up to about 5o retiming of the effected cylinder may be achieved provided that the oil holes passing through the shrink fit have not become obstructed.
If slippage is >5o problems of crankshaft loading are likely to be present due to firing angles and the relative position of the cranks; this can lead to excessive vibrations and stress. The ideal solution is the replacement of the effected parts; a temporary repair may be done. This consists of cooling the pin with liquid nitrogen & heating the web to give a temperature difference of about 180ºC. The web may then be jacked back into the position. Slippage will have caused damage to the contact surfaces, which originally would have been smooth to provide maximum contact surface area, due to the micro-seizure. The engine should be operated below the maximum rating until a permanent repair can be achieved.
Starting the engine with water in the cylinder causes most slips, but any overload can result in such an occurrence.
Post machining
Modern engines, designed for high power, will have a well balanced crankshaft with a minimum of material. Post machining allows the tapering and chamfering of webs and the counter boring of pins, thereby removing all unnecessary metal. A modern well-balanced engine using higher strength steels can preclude the use of balance weights.
Crankshaft alignment check
If a main bearing has suffered wear then the journal supported by the bearing will take up a lower position, if adjacent bearings have not worn to the same degree then the shaft will take on a deformed attitude causing the crank webs to be subjected to an oscillating bending action and therefore fatigue.
It is necessary to check the alignment of crankshafts on a regular basis, this is achieved by the use of special gauges.
The crank webs will often have a light centre punch mark to ensure that the gauge is fitted in the same position on each occasion that readings are taken. The trim of the ship, whether loaded or unloaded, whether hogged or sagged are all important factors which can effect the accuracy of the readings taken. Ideally the readings should be taken when the ship is dry-docked.
Medium speed V-type crankshaft layouts
With V-engines it is necessary to connect two con rods to each bottom end. Three basic arrangements are available as indicated.
The side by side method is the simplest with each bottom end being positioned alongside each neighbour on the crankpin. This requires cylinders to be offset across the engine thus giving a slight increase in engine length.
The fork and blade arrangement allows cylinders to be in line across the engine but the bottom end arrangement is more complicated. The fork may have two bottom end shells with the blade positioned between them. Alternately the arrangement as shown may be used, however, in this case the fork shell runs the whole length of the crankpin and the blade shell runs on specially ground outer face of the fork shell.
The articulated layout has cylinders in line across the engine and a single bottom end is used. One connecting rod is connected rigidly but because of piston motions the other rod is connected by means of a gudgeon pin arrangement. Both pistons and con rods can be removed without disturbing the bottom end bearings.
Modern trends in materials
For a long period most crankshafts were made out of a material known as CK40.
This material had a good ability to withstand the damage caused by bearing failure, such as localised hardening and cracking. Undersizing by grinding was possible.
The modern trend is to move toward chrome-molybdenum alloyed steel of high tensile stress. These may be non-surface hardened (which tend to bend and have localised hardening when reacting to an overheated bearing), or hardened (tend to loose hardness when overheated and due to changes in the molecular structure will crack).
In both of these cases, grinding is generally not an option for repair.
For modern material cranks subject to normal wear grinding may be carried out.
