The crankshaft is a component of thermal machines and alternative engines in general. The crankshaft is the power transmission shaft that acts as a crank for the crank-crank assembly. Its function is to transform alternative movements into rotational movements (explosion engines, for example), or vice versa (eccentric mechanical presses).
In thermal engines the crankshaft gathers the forces originated during the explosion and converts, by means of the connecting rod, the reciprocating linear movement of the piston into rotary motion. It transmits the movement and the driving force to the transmission elements that are coupled to it. It is subjected to torsional and flexural stresses, and has a strong and very resistant structure.
The crankshafts consist of supports, usually five for a four-cylinder in-line engine, which are attached to the block bench. They also carry elbows called stumps where the connecting rods are attached. In the extension of each elbow are the counterweights, which serve to balance the crankshaft. The timing pinion is mounted on one end of the crankshaft and, on the other, the flywheel.
Thermal motors are examples of engines that use a crankshaft to transform alternative movements into rotational movements. Eccentric mechanical presses, on the other hand, are examples of mechanisms that crankshafts use to transform rotational movements into alternative movements.
The crankshaft has an elbow for each connecting rod of the machine or engine. The elbow receives the connecting rod in the stump, which rotates offset around the axis of rotation of the crankshaft. The stump that constitutes the axis of rotation of the crankshaft is the neck or support of bed. The arm is the part of the crankshaft that joins the stump and neck. There is usually one arm on each side of the stump and one neck for each arm. Still there are compressor crankshafts that only have one neck, one arm and the stump, and two, four or six cylinder light engine crankshafts that have the stumps grouped two by two, with a single intermediate arm and no neck between two consecutive stumps.
To compensate for the imbalance effects produced by the off-center rotation of the stump and connecting rod, counterweights opposite the stump are installed on the limb of the arms. These weights can be of the same piece with the crankshaft or can be tightly pressed on the arms. In crankshafts of more than one elbow, the elbows are arranged in such a way that the off-center masses of one elbow are compensated with the off-center masses of another, in order to obtain a possible balance.
This arrangement of the elbows determines the position of the cylinders, the succession of the explosions and the entire distribution set. The crankshafts are obtained by forging, and further machining, of a single piece from separate, pressure-mounted necks, arms and stumps for hot coupling. In this way, the mounting of ball or roller bearings on the stump is allowed. The same technique is applied in very large crankshafts, of marine engines, for example, to avoid the difficulty of obtaining forged or castings that are too heavy.
The crankshaft is the engine part that must withstand the greatest efforts of fatigue and wear. In aviation engines, or in engines where the conditions of weight, safety, loading are very severe, heat treatments and surface hardening of nitriding, flame hardening or induction hardening are widespread.
Manufacturing material and technology for crankshaft parts
Materials and manufacturing technology are often closely linked. In the case of the manufacture of crankshafts, the steel shafts (to achieve the highest strength and toughness) are obtained by forging and casting.
The crankshafts are made of carbon, chrome-manganese, chrome-nickel-molybdenum and other steels, as well as special high-strength cast iron. The advantage of steel shafts is the greater resistance, the possibility of obtaining a high hardness of the necks by nitriding, cast iron shafts are cheaper.
The choice of steel is determined by the surface hardness of the necks to be obtained. The hardness of approximately 60 HRC (necessary for the use of roller bearings) can be obtained, as a rule, only by chemical-thermal treatment (cementation, nitriding, cyanidation). For these purposes, as a rule, low-carbon steels of chromium-nickel or chromium-nickel-molybdenum are suitable, and for medium and large size shafts, more alloy with expensive molybdenum is required. While maintaining the viscosity of the core, less hardness can be obtained for reliable operation of the plain bearings by turning off the HDTV such as Leu medium carbon and gray cast iron.
The medium-sized steel crankshaft keys in large-scale and mass production are manufactured by forging closed molds in hammers or presses, while the billet process goes through several operations. After the preliminary and final forging of the crankshaft in the dies, the flashing is trimmed in the edge press and hot straightened in the seal under the hammer.
Due to the high mechanical strength requirements of the shaft, the location of the fibers of the material when receiving the workpiece is of great importance to avoid cutting them during subsequent machining. For this, seals with special bending currents are used. After stamping before machining, the shaft blanks are subjected to heat treatment (normalization) and then decalcified by pickling or shot blasting.
Large crankshafts, such as ship crankshafts, as well as engine crankshafts with tunnel crankcases, are foldable and screwed. The crankshafts can be installed not only in sliding bearings, but also in rollers (connecting rod and main), ball (main in low power engines). In these cases, higher demands are imposed on both manufacturing precision and hardness. These shafts are always made of steel.
Cast iron crankshafts
Cast crankshafts are usually made of ductile iron modified with magnesium. Compared to "stamped" shafts obtained by precision casting (in housing molds), the shafts have several advantages, including a high metal utilization rate and good torsional vibration damping, which often makes it possible to abandon the External shock absorber at the tip of the front axle. In casting billets, several internal cavities can also be obtained during casting.
The tolerance for machining cast iron shaft necks is not more than 2.5 mm per side with deviations according to accuracy classes 5-7. Less stock fluctuation and less initial imbalance favorably affect the operation of the tool and the "equipment", especially in automated production.
Type of crankshafts
The crankshaft can be of two types:
- Composite / separable, these trees are separable, in the sense that the pin that houses the head of the connecting rod can slide, in order to accommodate the connecting rod with the head in one piece and its support, to In order to improve the reliability of means and reduce friction dispersions, however, this type of construction is very difficult to achieve, given the innumerable factors that must be respected so as not to fall into vibrations; So its use in civil vehicles is usually linked by the engine with two cylinders to the maximum.
- Monolithic, these trees are among the most used, because it will allow a smaller attention of the set and may have a lower weight than the separable models given by the fact that they are composed of a single element.
In addition, depending on the characteristics of the engine it can be simple when a single motor shaft is used for the engine, multiple, when there is a need for more trees, more trees may also be needed per engine cylinder, as in the case of engines of the opposing crankshafts or piston DUA, where they have two shafts rotate in the opposite direction by the piston to minimize the lateral forces of the piston.