Twisting results in shear stresses.Crankshafts are subject to a complex form of loading which varies with time. In addition shrink fits, oil holes and fillet radii add to the complexity. Pure stress analysis and rules governing crankshaft dimensions are based upon a combination of theory and experience.
The three main loading stresses are;
- Gas loads on the crankpin which produces alternating tangential bending of the webs alternating bending of the crankpin and on elements of shearing of the crankpin at the inner web faces
- Torsional vibrations producing 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 and shortening of the shaft and in conjunction with local bending. Crankshafts may, in addition be subject to misalignment due to bearing wear or poor chocking. This produces and alternating bending of the crankshaft
Crankshafts fail usually because of cracks propagating from a stress concentration point.
VibrationAll components vibrate e.g. a weight on a spring, rotating components such as crankshafts can vibrate in a torsional manner. The systems will differ but the principals are the same. The operating frequency caused by the operating speed is known as the forcing frequency. All systems have natural frequencies were the vibration amplitude is excessive (consider out of balanced wheels on a car). Resonance occurs when the forcing frequency and natural frequency coincide and the result is excessive vibration. If it is required to keep the vibration amplitude below a certain value in order to limit stress to prevent fatigue, then speeds coinciding to the natural frequency orders of it must be avoided. These speeds are referred to as the barred speeds (or critical speed ranges).
If the barred speed is located where it is required to operate the engine, say at half ahead, it will be necessary to fit a detuner or vibration damper. These lower the vibration peak and 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 essentially of an additional rotating mass driven by the crankshaft and connected to it by a spring or a hydraulic fluid. The energy of vibration is used up in distorting the spring or shearing the fluid.With constant speed engines employing a CPP propeller, vibration dampers are sometimes required because natural frequencies of the engine and shaft system changes with load due to the pitch of the propeller. In some cases there may even be a barred pitch.
Methods of forming a crankshaftThe ideal arrangement is that of the solid forged structure because there is continuity of material grain flow which allows for smooth transmission of stress. Unfortunately, such crankshafts are limited to the smaller engines because there is a limit to the size of forging equipment and the size of steel bar which can be produced.
Built up crankshafts with shrink fits or welded sections allow very large units to be produced, but they tend to be heavier and less rigid than an equivalent solid forged.
The grain flow method allows solid forged crankshafts to be produced with minimum energy and minimum need for post machining. A heated section of bar is held by three clamps which can be moved hydraulically. The three stages for forming the crank throws are shown. When one throw has been formed the next section of bar is heated, the shaft is held in the clamps again and the next throw formed.
Semi-built upNo dowels are fitted as these can act as stress raisers.
Welded crankshaftA form of crankshaft construction recently developed is that of welding. Cast web crank pin and half journal units are connected at the half journals by welding. These welds are stress relieved and the pins ground to give the correct finish. This form of construction is suitable for large direct drive engines and it provides strength close to that of the solid forged crankshaft. Any number of units may be connected
The usual form of construction for direct drive engine crankshafts is the semi-built up type. This makes use of shrink fits between the journals and webs. Careful design is required to ensure the shrink fit is strong enough but does not impose excessive shrinkage stress.
The shrink fit must provide sufficient strength to allow necessary torque to be transmitted. 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 large torque transmission without high stress the area of contact at the shrink fit should be increased.
This is usually by means of an increased diameter (over increase length as this increase the engine length) which allows the fillet radius to be used, as the journal part of the pin does not need to be of the same large diameter. The fillet allows a smooth transmission and is rolled because this produces a compressive stress which provides safe guard against fatigue. The fillet is undercut allowing the web to be positioned against the bearing reducing the engine length and oil loss from the ends of the bearing.
Slippage of shrink fitsSlippage can occur at the shrink fits and this can be noticed by consideration of the reference mark at the end of the web and pin.
For slippage above 5o there may be problems of loading on the crankshaft 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 carried out. This consists of cooling the pin with liquid nitrogen and heating the web to give a temperature difference of about 180oC. The web may then be jacked back into position. In both cases the slip fit will have been damaged, the contact faces which originally should be as smooth as possible to give maximum contact area. The engine should be run at below the max. rating until the parts can be replaced.
Most slipped fits are caused by starting the engine with water in the cylinder. But any overload can result in this problem.
Post machiningModern engines designed for high power and weight should 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 avoid the use of balance weights.
Crankshaft alignment checkIf 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 bent attitude causing the crankwebs to be subjected to an oscillatory bending action and so fatigue. It is therefor necessary to check the alignment of crankshafts by the use of special gauges.
The crankweb will often have a light center punch mark to ensure that the gauge is fitted in the same position at each reading. The trim of the ship, whether loaded or unloaded, whether hogged or sagged are all important factors which can effect the reliabililty of the readings. Ideally the readings should be taken when the ship is drydocked.
Medium speed vee-type crankshaft layoutsWith vee-type engines it is necessary to connect two con rods too each bottom end. Three basic arrangements are available as shown. The side by side 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 length. The fork and blade type 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. But 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 arrangement has cylinders in line across the engine and a single bottom end is used. On con 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 ends.
Modern trends in materialsFor a long period most crankshafts were made out of a material known as CK40. This had very good ability to withstand the damage caused by bearinf failure such as localised hardening and cracking. Undersizing by grinding was possible.
The modern trend is to move the 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 ( tends to loose its hardeness and due to changes in the molecular structure will crack). In both these cases grinding is generally nnot an option for repair.
For modern material cranks subject to normal wear grinding may be carried out.