Heat engines

What is the crankshaft of an engine? Operation and types

The crankshaft is a component of thermal machines and reciprocating engines in general. The crankshaft is the power transmission shaft that acts as the crank for the connecting rod-crank assembly. Its function is to transform alternative movements into rotational movements (combustion engines, for example), or vice versa (eccentric mechanical presses).

In thermal engines, the crankshaft collects the forces generated during the explosion and converts, through the connecting rod, the reciprocating linear movement of the piston into rotary movement. It transmits the movement and the driving force to the transmission elements that are coupled to it. It is subjected to torsional and bending stresses, and has a strong and very resistant structure.

The crankshafts consist of supports, normally five for an in-line four-cylinder engine, which are attached to the block mount. They also have 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 the flywheel on the other.

Heat engines are examples of engines that use a crankshaft to transform reciprocating motion into rotational motion. Eccentric mechanical presses, on the other hand, are examples of mechanisms used by crankshafts to transform rotational movements into reciprocating movements.

How does a crankshaft work?

The crankshaft has one elbow for each connecting rod of the machine or engine. The elbow receives the connecting rod on the journal, which rotates off center about the crankshaft's axis of rotation. The journal that constitutes the axis of rotation of the crankshaft is the neck or main support. The arm is the part of the crankshaft that joins the journal and the neck. There is usually an arm on each side of the stump and a neck for each arm. Even so, there are compressor crankshafts that only have a neck, an arm and the journal, and crankshafts of light engines with two, four or six cylinders that have the journals grouped two by two, with a single intermediate arm and no neck between two. consecutive stumps.

To compensate for the unbalance effects produced by the off-center rotation of the journal and connecting rod, counterweights opposite the journal are installed at the ends of the arms. These counterweights may be integral with the crankshaft or may be clamped tightly onto the arms. In crankshafts with 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. The crankshafts are obtained by forging, and subsequent machining, in one piece from necks, arms and journals separated and assembled by pressure for hot coupling. This allows the mounting of ball or roller bearings on the journal. The same technique is applied to very large crankshafts, for marine engines, for example, to avoid the difficulty of obtaining too heavy forgings or castings.

The crankshaft is the part of the engine that must withstand the greatest efforts of fatigue and wear. In aviation engines, or in engines where weight, safety, and load conditions are very severe, heat treatments and surface hardening by nitriding, flame hardening, or induction hardening are widespread.

Characteristics of the crankshafts: Material and technology

Materials and manufacturing technology are often closely linked. In the case of crankshaft manufacturing, steel shafts (to achieve the highest strength and toughness) are obtained by forging and casting.

steel crankshafts

Crankshafts are made from carbon, chrome-manganese, chrome-nickel-molybdenum and other steels, as well as special high-strength castings. The advantage of steel shafts is greater strength, the possibility of obtaining 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 about 60 HRC (necessary for the use of roller bearings) can be obtained, as a rule, only by chemical-thermal treatment (carburizing, nitriding, cyanidation). For these purposes, as a rule, chromium-nickel or chromium-nickel-molybdenum low-carbon steels are suitable, and for shafts of medium and large sizes, further alloying with expensive molybdenum is required. While maintaining core viscosity, less hardness sufficient for reliable journal bearing operation can be obtained by quenching HDTV as medium carbon Leu and gray cast iron.

Medium-sized steel crankshaft keys in large-scale and mass production are made by forging closed molds in hammers or presses, while the billet making 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 at the stamp under the hammer.

Due to the high mechanical strength requirements of the shaft, the location of the material fibers when receiving the work piece 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 heat treated (normalizing) and then descaled by pickling or shot blasting.

Large crankshafts, such as boat crankshafts, as well as tunnel crankcase engine crankshafts, are collapsible and bolted. Crankshafts can be installed not only on sliding bearings, but also on roller (connecting rod and main), ball (main in low-power engines). In these cases, higher demands are placed on both manufacturing precision and hardness. Such shafts are always made of steel.

Cast iron crankshafts

Cast crankshafts are typically made of magnesium-modified ductile iron. Compared to "stamped" shafts obtained by precision casting (in housing molds), 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 cast billets, various internal cavities can also be obtained during casting.

The tolerance for machining the necks of cast iron shafts 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 "equipment", especially in automated production.

Types 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 bar can slide, in order to accommodate the connecting bar with the head in one piece and its support, to in order to improve media reliability and reduce friction dispersion, however, this type of construction is very difficult to achieve, given the innumerable factors that must be respected in order not to fall into vibrations; so its use in civil vehicles is generally joined by the engine with two cylinders at the maximum.
  • Monolithic, these trees are among the most used, because they will allow a smaller attention to the whole and can have a lower weight than the separable models given by the fact that they are made up of a single element.

In addition, depending on the characteristics of the engine, it can be simple, when a single engine shaft is used for the engine, multiple, when there is a need for more shafts, more shafts per engine cylinder may also be necessary, as in the case of engines. Opposing crankshafts or piston DUAs, where they have two shafts rotated in opposite directions by the piston to minimize lateral forces on the piston.

Data de publicació: November 18, 2016
Última revisió: February 27, 2020