Tracked vehicle
(are still considered a wheeled vehicle despite being basically related to the crawler track )
A tracked vehicle , also known as a crawler vehicle , is a land vehicle that uses crawler tracks with crawlers .
In contrast to wheeled vehicles , tracked vehicles are characterized by optimal off-road mobility and lane stability due to their higher traction as well as the lower specific ground pressure . Since the weight of the vehicle and the load (especially in the case of the crawler belt) is more evenly distributed over a larger area, the sinking of the vehicle is reduced on soft ground and, last but not least, very undesirable soil compaction is avoided in agriculture and forestry in particular . Conversely, the disadvantage is rather cumbersome driving behavior and maneuverability on paved roads with often high wear, both of the crawler chains themselves and, in part, of the road surfaces. In this area in particular, flexible rubber caterpillars are superior to crawlers. In addition, caterpillar vehicles require significantly more complex technical structures, especially for the suspension of the chassis and for steering, e.g. B. with a superposition steering gear for easier steerability .
In addition to applications such as snowmobiles , caterpillar tractors , bulldozers , excavators , bucket wheel excavators or conveyor bridges , heavy tracked vehicles are particularly common in the military sector for tanks , self-propelled guns, etc.
Tracked chassis
A caterpillar or crawler chassis usually consists of two caterpillar or chain drives . Among the earthmoving and opencast mining machines there are also multi-crawler tracks with several track tracks, which are arranged symmetrically or asymmetrically to the machine's longitudinal axis: depending on the design, the track tracks are single, paired or double paired and their support is on two or three points (results in two-track or three-track tracks) . In order for the multi-crawler tracks to move around bends, they need a steering caterpillar , which is used with a machine mass of 300 t or more . Each chain drive has its own drive, so drives that have sunk into the ground can free themselves on their own.
Chain construction
at the top
The original and basic structure consists of steel running surface links that are hinged to form a closed chain. In principle, this creates a separate route under the vehicle, which is expressed, for example, in the English term "crawler track" or "Caterpillar track". In the center of the inner side of the chain links are raised formations which, when lined up with the other links, form a track for the running and carrying wheels - they thus have a " track " function. Further formations on the inside serve as engagement for the drive wheel.
Crawler chains for faster vehicles such as battle tanks are usually used in the so-called "living" version, in which the chain links are given additional tension by means of spring force.
Today there are also designs with " rubber chains ", so-called treadmills, which are constructed in a similar way to a tire belt plus a tread. Several layers of fabric and steel are overmolded with tire rubber and profiled. At first these treadmills were used in so-called mini excavators, but today they are also used in agricultural tractors with caterpillar drives (caterpillar tractors), for example the John Deere Type 8000T and the CAT tractors of the AGCO Group . The advantages are a significantly smoother running of the treadmill, higher possible speeds and protection of the surface being traveled on. Rubber chains are usually less maintenance-free and cheaper in terms of variable costs, but are subject to greater wear and tear and a shorter service life. Small ski-doos use a single, relatively wide, rubber chain.
Chain drive
A chain drive consists of the following parts:
- the crawler track , made of metal or integral composite materials, often with rubber studs for road use ( chain pads )
- a drive wheel (A), which transmits the torque of the engine directly (mechanical clutch) or indirectly (hydraulic clutch) to the chain
- an opposite guide wheel (F) (which at the same time, mostly hydraulically, regulates the chain tension) and
- the mostly sprung wheels or rollers (L) (at least two)
- any support rollers that support the chain in the return strand
- the chassis or, in the case of tanks, the vehicle hull.
If the chain is fed back separately from the feed, supported by support rollers, it is referred to as a support roller drive. It is the most common form today. If the return of the chain runs at the top of the larger running wheels, it is called a roller drive. Both designs have specific advantages and disadvantages. In particular, the support roller drive is mechanically more complex, but has better driving properties, since the upper chain strand belongs to the sprung mass.
The number of running wheels is limited by their size, as they should not touch each other on the one hand, but should not be too small on the other. In order to be able to use more wheels, with which a more even load distribution, a finer suspension and a better contour adaptation is achieved, box drives with overlapping wheels were developed. Single rollers and double rollers, which have the same very large diameter, are used alternately, with the double rollers lying on the outside of the chain links and the single rollers moving in between. Alternatively, there were drives with alternately offset double rollers. This achieves a complex and difficult to maintain drive with almost twice the number of large rollers, which in turn have a large diameter and therefore roll with less resistance, but also with higher loads. This design remained essentially limited to the time of the Second World War .
The rollers or wheels of the military chain drive are spring-mounted in relation to the (civil) bogie or the housing tray in order to enable faster travel even off-road and to keep the load on the chain and chassis low. In the beginning, small spring travel and many small rollers were used on a large, rigid frame carrier, as in the "MK IV" and Churchill , but today military vehicles have large spring travel and comparatively large rollers. The Christie drive developed by John Walter Christie in the late 1920s enabled a particularly large spring deflection . In the original version, it was possible to remove the chains for driving on the road and drive directly on the large rollers; in later versions this option was again dispensed with. The best-known vehicle with a Christie drive was the Soviet T-34 . Later developments used torsion bar sprung rollers, in which the bars acting as torsion springs are arranged transversely to the direction of travel under the vehicle. On the other hand, in the case of low-speed crawler tracks, such as conveyor machines in opencast mines or construction machinery, the suspension is not required in some cases.
The technical limit is reached when the drive becomes too complex and too heavy for its practical application. The last generation of tanks in Germany during World War II therefore only used nested drives with steel rollers, the tanks had become too heavy, and rubber for the bandages was also limited. The tanks of the Tiger series needed a set of narrower crawler tracks for loading onto trains, otherwise they would have exceeded the width of the loading gauge of the transport wagons . More complex track drives have a lower rolling resistance, but because of their greater weight, they consume more fuel. Modern battle tanks are increasingly reaching the weight of this tank generation again, but due to the advanced technology of chains, damping and motorization, without the mobility restrictions of the drives of the types of that time.
With regard to kinematic calculations, a drive chain can easily be viewed as a degenerate wheel.
Drive and steering
Initially, steam engines and various motors were used as motor drives ; today's tracked vehicles have predominantly diesel engines .
The drive wheel of the drive transports the chain links forwards in the direction of travel and deposits them on the ground below, over which the rest of the vehicle is moved forward on the running and guide wheels. While the lower part of the chain lies motionless on the floor, the chain links behind the last roller are lifted off the floor again. The chain drives on each side can be driven independently of one another, in some designs even in opposite directions. This makes it possible for steering types 2 and 3 to turn on the spot.
Steering designs:
- Central gearbox and combination of clutches and brakes, right and left. Used in light and medium-weight civil tracked vehicles, previously also in tanks. In most cases, turning around on your own axis is impossible with this design.
- Two-strand drive with two continuously variable gears, mostly hydrostatic . Used in civil tracked vehicles, for example from Liebherr and Case Corporation . The Porsche Tiger had a two-strand electric drive, which, however, did not prove itself; the Henschel tiger , which was mass-produced instead of the Porsche tiger, dispensed with this technology (see below). The Soviet light tank T-70 and the self-propelled gun SU-76 based on its chassis used two separate engines and transmissions; the vehicle was steered by regulating the speed, which made the vehicle rather unpopular, hence the nickname "Suka" (dt. bitch). The main problem with a two-strand drive with one motor is that each strand must be designed to transfer more than 100% of the engine power, because the drive on the inside of the bend brakes and the braking power is not lost, but is transferred to the drive on the outside of the bend via the transfer case.
- Drive with central gearbox and a superimposed steering gear . Today, the steering speed is usually superimposed with a hydrostatic transmission. Used on tanks and some civil tracked vehicles from Caterpillar and Komatsu .
The braking distance of a modern battle tank is often the same or shorter than that of a sports car with the same speed , mainly due to the force distribution over a large area . Conversely, the tractive force that can be transmitted increases , especially when the ground conditions are poor. A similar effect is achieved with the Lauster axis tested by the Bundeswehr in the 1960s .
history
A suitable track chain was invented by Alvin Lombard for steam tractors for transporting tree trunks in Maine (USA) and patented in 1901. With a large number of steam tugs produced, Lombard was the first commercial manufacturer of tracked vehicles. Another early producer was the Californian Benjamin Holt , who also designed a chain on November 24, 1904. His company, The Holt Manufacturing Company , merged with Best in 1925 to form Caterpillar, which still exists today . The first tracked vehicles were steered exclusively by one-sided braking intervention or separate speed control by two traction motors, which led to poor driving characteristics and high wear . A significant advance in steering technology was achieved with the so-called double differential, which was developed by the manufacturer Cletrac . Before the hydrostatic transmission era, this technology made it possible to control the crawler tracks separately without brake intervention. The German tanks Tiger and Panther in World War II had so-called two-wheel steering gears, which made it possible to negotiate a curve in two specific radii by delaying the chain in each gear.
Well-known German manufacturers of tracked vehicles were or are: Demag , Hanomag (today Komatsu ), Stock, LHB , Lanz , Deutz AG , Famo , Henschel , Krupp , Liebherr .
Modelling
There are radio-controlled models of tracked vehicles, especially excavators, tanks and bulldozers, both electrically powered and powered by an internal combustion engine.
Examples of tracked vehicles
- Snowmobile and especially snowcat (for leveling )
- general caterpillar tractor or chain tractor ( tractor with caterpillars )
- Agricultural machinery : combine harvesters and Ä.
- Special forestry machinery such as wood harvesters or forwarders
- Construction vehicles : bulldozer , excavator , crawler loader , dump truck
- Mining (especially in open-cast mining ): reverse machine (open-cast mining) , bucket wheel excavators , spreaders
- Military vehicles : tanks , battle tanks , Kettenkrad , half-track vehicles , " caterpillar tractors east "
See also
literature
- The development of the crawler tractor over three decades In: Motor vehicle technology 4/1954, p. 102-107 (including an overview table of all worldwide crawler tractor types 1954).
- Wolfgang Merhof: Driving mechanics of tracked vehicles . Ed .: Ernst-Michael Hackbarth. 2015, ISBN 978-3-943207-13-2 ( link to PDF document [accessed February 28, 2020]).
Web links
Individual evidence
- ^ Günter Kunze, Helmut Göhring, Klaus Jacob: Construction machinery: Earthworks and opencast mining machines . Springer-Verlag, 2013, ISBN 978-3-663-09352-7 , pp. 130 ( google.de [accessed April 20, 2019]).
- ↑ Patent US674737 : Logging Engine. Registered November 9, 1900 , published May 21, 1901 , inventor: Alvine O. Lombard.
- ↑ Patent US874008 : Traction Engine. Registered February 9, 1907 , published December 17, 1907 , inventor: Benjamin Holt.
- ↑ Patent US945538 : Traction Engine. Registered December 24, 1908 , published January 4, 1910 , inventor: Benjamin Holt.
- ↑ Patent US970503 : Traction Engine. Registered on August 20, 1909 , published September 20, 1910 , inventor: Benjamin Holt.
- ↑ Patent US1026090 : Traction Engine. Registered on August 17, 1909 , published May 14, 1912 , inventor: Benjamin Holt.
- ↑ Patent US1456349 : Direction-Changing Transmission Mechanism. Registered April 14, 1921 , published May 22, 1923 .