Connecting rod
A connecting rod , also a connecting rod , push rod or drive rod , is the connection between the crankshaft or the crank pin and the reciprocating piston or cross head in a crankshaft drive of engines and machines . The connecting rod converts the linear movement of the power or working piston into the circular movement of the crankshaft (linear oscillating axial movement) or, conversely, a circular movement into a linear movement.
There are similar facilities in the skeleton of vertebrates, but of course only parts of the arc are converted into linear motion (see e.g. Os quadratum ).
According to Duden and other German dictionaries, the grammatical gender of "Pleuel" is male ( the Pleuel), in the technical field the word is also used as a neuter ( the Pleuel). This gender ambiguity can be avoided with “the connecting rod” . The term is etymologically derived from Bleuel .
Forms / construction
At both ends (the connecting rod head, connecting rod) of mostly double-T-shaped molded connecting rod shank are located end bearings . The piston pin is pushed through at the smaller connecting rod eye .
On the larger connecting rod eye (or connecting rod foot), the connecting rod is usually divided, with two screw connections; special expansion screws are usually used for screwing . The screw connection principle of the differential thread is described in the literature, but is almost never found in practice.
One-piece connecting rods without a removable connecting rod cover require crankshafts that can be dismantled for assembly. They are mostly found in engines with roller-bearing crankshafts, such as small appliance, moped and some motorcycle engines.
Link rods are occasionally used to shorten the overall length of V-engines and to prevent cylinder bank misalignment . Since the cylinder banks do not have exactly the same stroke due to kinematics , the compression may have to be compensated for with different piston heights per cylinder bank or with slightly different milled or planned bank heights.
Because of the changing loads, connecting rods must be designed for fatigue strength .
Materials
Commonly used materials for connecting rods today are C70 or micro-alloyed steels and sintered metals . For special applications such as the sports engine sector , special heat-treatable steels or, because of its lower weight, titanium are used. Cast iron connecting rods are also used .
production
Large series connecting rods are forged or sintered . Forged connecting rods have a better strength-to-weight ratio than sintered connecting rods at lower costs, but die production is very expensive and is only worthwhile for large series. In large engines, the connecting rods are forged or cast . In the case of small series, the connecting rods are machined from pieces of metal.
Fracture separation (cracking)
Separate fraction ( cracked ) connecting rods are produced in one piece, provided with breaking notches (Sinterpleuel) or laser scores (steel connecting rods) and then broken at the notched points into two parts. The fracture surfaces fit together exactly and the connecting rod and connecting rod cover can be screwed onto the crank pin of the crankshaft during assembly. The parting line is almost no longer visible on the assembled connecting rod. Due to the individual fracture geometry, both parts of a connecting rod belong together and cannot be replaced individually. Break-separated connecting rods offer advantages in terms of strength, manufacturing accuracy and manufacturing costs. An exact fit is guaranteed and the power transmission is better than with two separately manufactured components.
For the first time in 1977, Porsche used split connecting rods made of sintered steel in the Type 928 V8 engine . From 1992 BMW also used this technology - initially in the eight-cylinder BMW M60 engine - which has now been introduced worldwide.
Cutting / sawing
Other methods of separating the connecting rods (sawing followed by milling, possibly grinding the separating surfaces) and assembly (fitting screws or locating pins) are only used for small series or very large connecting rods (trucks, marine diesel engines, etc.).
use cases
The first known machine in which a rotary motion was converted into a linear motion with the aid of a connecting rod and crankshaft is the Roman sawmill of Hierapolis (3rd century AD). Since then, connecting rods have been used in a wide variety of machine types.
Bending punch
In the case of a bending punch , the connecting rod is used to convert the mostly overhead rotary unit to the bending or punching device, which is usually operated vertically. As a special feature, numerous articulated connecting rods have been established in this sector, which allow the force and path to be freely selected within certain limits and the machine stroke to be largely adapted to the workpiece and the processing.
Steam engine
Also in steam engines on steam ships or other steam engines there are connecting rods that convert the linear piston movement into a rotary movement of the crankshaft. In the drive system of steam locomotives , this component is called the drive rod and in most cases acts directly on the drive axle of the locomotive.
The early Newcomen steam engines did not have this element; at most they used a lever deflection and could only drive linear consumers, e.g. B. reciprocating pumps. Only with the design of Watt found the connecting rod and later really (after the expiry of patent claims) the crankshaft moving into the technology of steam engines.
compressor
The connecting rod of a piston compressor transfers the rotating force components of the crankshaft to the linearly moving working piston to generate compression.
Artificial rods
The art linkage in mining is based on the invention of the curved pin (connecting rod), with the one for the first time rotating movements into linear transfer could.
sewing machine
In a sewing machine , connecting rods are used to convert the rotating movement of the drive into a vertical movement of the sewing needle. Historical sewing machines powered by human power continued to use a connecting rod to convert the tilting movement of the footplate into the rotational movement of the drive shaft.
Internal combustion engine
In the internal combustion engine based on the reciprocating piston principle , the connecting rod is the carrier of the kinetic energy between the piston and the crankshaft , to which it is movably connected by bearings. It then transfers the gas forces in the cylinder chamber to the crankshaft. In two-stroke engines , connecting rods with undivided lower bearings (connecting rod eyes) are used, which also swirl the sucked in mixture and fling oil onto the cylinder liner, into special catch pockets and onto the crankshaft bearings.
Each piston has a connecting rod as a coupling element between the oscillating movement of the piston and the rotation of the crankshaft. There were two connecting rods per piston in the V four-cylinder engine of the Honda NR models with super-elliptical pistons ("oval pistons"), where two parallel connecting rods were required due to the length of these pistons, and in diesel engines from the manufacturer Neander Motors with two counter-rotating crankshafts, whose two connecting rods act on a piston.
The use of push rods in the combustion engine in Albert Roder's ULTRAMAX control for the valve train of the motorcycle models NSU Max , Superfox and Maxi as well as for the two-cylinder engines of the passenger car model NSU Prinz is of a different kind ; two push rods (with a 90 ° phase offset) drive the overhead camshaft .
Other general and specific
In the past, especially in motorcycle engines, in order to save on a lubricating oil pump, rolling bearings in the connecting rod eyes were also widespread, as the upper connecting rod bearing, often in the form of needle bearings . In larger and modern designs, plain bearings dominate .
In combustion engines, two steel-lead-bronze or steel-aluminum half-shells are inserted into the large connecting rod eye on the connecting rod base. Today, the bearings are highly complex components. The upper and lower bearing shells are no longer symmetrical due to the different loads. The enormous progress made in material development has made it possible to have very thin-walled bearings with defined lubricating films in the µm range. Fixing lugs on the half-shells are used to position and fix the bearings during assembly. Contrary to the still widespread opinion, the fixing lugs do not serve as a safeguard against slipping out and twisting. The bearing shells are firmly seated through the surface pressure that the bearings receive when the bearing caps are screwed on. In the top end bearing is often in a one-piece Bronze - bush . These sockets consist of a steel jacket to which a sintered bronze is applied. Bushings in the upper connecting rod eye are only available for heavily loaded connecting rods, e.g. B. required in diesel engines. The use of lead-free bearings is currently advancing rapidly.
Bearings made of non-ferrous metals must be lubricated and cooled by oil during operation: a connecting rod bearing damage is almost always the result of a lack of oil. To supply oil, connecting rods used to be supplied with oil via the drilled crankshaft first at the large connecting rod bearing, from where lubricating oil was supplied to the piston pin bearing via an inner bore in the connecting rod. Newer simulation techniques have led to new designs of the engines, with which many oil wells could be omitted. In the case of two-stroke engines and older, small four-stroke engines, the bearings are supported by needle or roller bearings , whose lubricating oil is usually only supplied by centrifugal or mixed lubrication.
The fits in connecting rod bearings are dimensioned in such a way that in the pre-calculated hot condition there are small lubrication gaps from which the oil can escape. The desired lubrication technology in the connecting rod base is that of a hydrodynamic plain bearing , i.e. H. From the rotating movement of the connecting rod, the friction of the movement creates an oil cushion which, if dimensioned correctly, enables pure fluid friction and reliably prevents metal-to-metal contact. In the case of the piston pin mounting, however, no circumferential movement is possible; In this type of bearing, mixed friction between the piston pin and the connecting rod bearing is to be expected and the bearing surfaces, pressures and oil flow rates are accordingly more generous.
If the connecting rod is not designed in a split manner, i.e. the connecting rod consists of only one part, the crankshaft must consist of several assemblable components ("be built") to enable assembly, i.e. the crankshaft journal must be screwed or dismantled and in another way can be mounted again (pressing, heat shrinkage) ( Hirth coupling ), or a one-sided offset comprise, with only one crank arm instead of on both sides of the connecting rod.
The connecting profile of the two connecting rod eyes is usually an H or double T profile. In racing engine construction in the 1950s and 1960s, there were also so-called knife connecting rods with a slim diamond cross-section, thick in the middle (connecting line of the two eyes) and sharp-edged on the sides: these connecting rods were advantageous in the gas transport movements of two-stroke engines attributed. In two-stroke engines , the pumping movement for the gas supply is usually given up on the underside of the piston and the crankcase (see gas exchange ), which is why the crankshaft, the connecting rods and the undersides of the pistons are in the fresh gas flow, with the fresh gases usually also transporting the added quantities of lubricant.
In many V engines - some like the Ford V-4 engine have the same number of pistons, connecting rods and crank pins - either two identical connecting rods act on a crank pin in succession in the crankshaft direction (the result: a slight displacement of the cylinders ), or one of the two The connecting rod is designed as a fork and includes the second, so that the second connecting rod engages the crankshaft between the fork opening. Then there is no longitudinal offset of the cylinders one behind the other, and therefore no additional tilting moments. This more complex design is z. Found on Harley-Davidson motorcycles ; the front V-cylinder is angled towards the rear cylinder, but not offset to the side.
There is also another type of fork connecting rod: a connecting rod on a crankshaft offset, but the connecting rod forks upwards in a y-shape to guide two pistons: one piston of normal design, piston and connecting rod eye each with a round bore, the other Eye also round, but the piston with a slot for freedom of movement for the second piston pin. This design was used by Puch from the 1920s for a so-called double - piston engine based on the two-stroke principle, but was later replaced by a linkage or auxiliary connecting rod design. The advantage of the double piston lies in the asymmetry of the slot controls for the gas exchange, which is made possible by the offset stroke movements of the two pistons . On the other hand, there are considerable disadvantages: cooling problems, the construction costs, and an unfavorably shaped double common combustion chamber with long flame paths and relatively high consumption. Both fork and connecting rods using asymmetrical valve timing can be found on Puch motorcycles from the early twenties to the late sixties. Between the end of the forties and the middle of the fifties, connecting rods were forked parallel to the crankshaft axis in motorcycles from the Triumph factory in Nuremberg , which, however, did not allow asymmetrical timing to be achieved.
Another alternative is to use a main connecting rod with a third eye on the side next to the crank pin bore, which is attached to a shorter secondary connecting rod. Disadvantage of this technology: in terms of vibration mechanics, this is a coupling gear with a highly complex geometric description of the movement of the secondary connecting rods.
In radial engines , depending on the number of cylinders, up to eight auxiliary connecting rods act on the side of the main connecting rod, for example in the nine-cylinder engine.
See also
literature
- Tullia Ritti, Klaus Grewe, Paul Kessener: A Relief of a Water-powered Stone Saw Mill on a Sarcophagus at Hierapolis and its Implications. In: Journal of Roman Archeology. Vol. 20 (2007), pp. 138-163.
Web links
Individual evidence
- ↑ This form is also known as I-form and must be doing the "I" in a serif imagine: I .
- ↑ Roloff / Matek: machine elements. ISBN 978-3-658-26280-8 .
- ↑ T. Ritti, K. Grewe, P. Kessener: A Relief of a Water-powered Stone Saw Mill ... 2007, p. 161.
- ^ Neander Motors , accessed December 1, 2015
- ↑ Archived copy ( Memento from April 2, 2015 in the Internet Archive )
