For an eccentric shaft, each recess pressure is different, as illustrated in Figure 1.3. Differences in recess pressures cause circumferential flow from one recess to another. Circumferential flow reduces bearing load support and also reduces film stiffness, particularly for L/D>1.
The width, b, of the inter-recess land must therefore be large enough to prevent excessive circumferential flow. Applying equation (2.9), inter-recess flow from recess 1 to recess 2 may be estimated from
Inter-recess land width b is often specified by bridge angle in the range 10 30 as in Figure 9.5a. Land widths are equal, b=a, when L/D=1, a/L=0.25, and =28.6 or roughly 30. The recommended value is 30 for a four-recess bearing.
This example of a single-rotor type pump makes use of an eccentric shaft on which are mounted several sliding vanes (Figure 3.3.14). The vanes are forced against the bore of the housing by springs and the centrifugal force of the rotor rotation. The vanes are usually made of materials that will not damage the surface of the bore (e.g., bronze and bakelite). This type of pump is useful for small and moderate-volume capacities and low pressure. This is due to the rather low speeds such pumps must be operated at. High speeds result in rapid wear of the vanes.
A schematic view of the Bleuler rotary mill is shown in Fig.9.2. In this mill, an eccentric shaft is driven by a heavy duty enclosed motor. The shaft actuates a flywheel, which makes the upper chamber assume a rotary motion. The sample container, C, is clamped to the chamber wall and contains a heavy cylindrical weight, W, and an annular ring, A. Because of the constant rotary motion of the sample container, the cylindrical weight and annular ring exert a constant pressure on the container wall, and coal particles caught in between are crushed to finer sizes. A fixed amount of energy was expended on a sample of given mass of uniformly sized material. This permitted the evaluation of the effect of the properties of coal alone on the production of respirable coal dust.
A Bleuler mill provides an efficient and rapid means for grinding samples and meets the requirements of reproducibility of particle size distribution for optical and X-ray fluorescence spectrographic studies. The time of grinding can be preset and is controlled electrically.
The particles perform successive jumps. When they leave the trough, they are at an angle to the horizontal axis, called the angle of attack. This angle is the same as that made by the flexible leaves to the vertical.
the product conveyed. The fall into the trough occurs by inelastic shocks. In addition, there is friction between the product and the bottom and sides of the trough, together with internal friction in the product itself.
The mechanical stress is proportional to the amplitude of these vibrations and the square of pulsation. This is why vibrating feeders, which are smaller, can tolerate high frequencies, which is not the case for transporters.
The operation of the rotary internal combustion engine is illustrated in Figure7.1. The intake process goes through (1), (2), (3), and (4); it is followed by compression at (5), (6), and (7); next is the combustion and expansion stage (8), (9), (10); finally the exhaust stroke takes place at (11) and (12). Accordingly, the rotary engine is a four-stroke engine. One of the specific features of this engine is that as the rotor makes one complete rotation, the output shaft accomplishes three revolutions. In terms of the rotating angle of the output shaft, the time required to complete one stroke is 270 degrees, that is, 1.5 times more than for a four-stroke reciprocating engine.
In addition, due to the longer intake stroke duration in the rotary engine, high volumetric efficiency can be achieved even at high speeds, resulting in flat torque versus engine speed curve . Also, the rotary engine is less prone to knocking and therefore more tolerable to the fuel quality. Another feature of the rotary engine is that its geometry is very suitable for charge stratification, as the rotor always moves the air charge past the stationary location of the spark plug and nozzles, and thus develops the necessary flow distribution for charge stratification within the chamber . Finally, in recent years there is an increased interest in development of the rotary engine fueled by hydrogen. While the reciprocating four-stroke engine fueled by hydrogen is prone to preignition and backfiring through the intake ports, in the rotary engine the intake ports are separated from the combustion zone, so there are no hot spots in the induction phase.
The two main disadvantages of the rotary engines are the increased emission of unburned hydrocarbons and higher fuel consumption. Both disadvantages are related to the long and narrow shape of the combustion chamber, resulting in high surface-to-volume (S-V) ratio (large volume of the quenching layers), increased sealing perimeter (increased gas leakage from the combustion chamber), and long flame travel (lower thermodynamic efficiency).
At the same time, the content of NOx in the exhaust gases of rotary engines is lower than in reciprocating engines. Regarding carbon monoxide, the level of its emissions is similar to that from piston engines. Figure7.2 shows a comparison of the exhaust emission characteristics between rotary and reciprocating engines.
In the late 1970s, a thorough investigation of the sources of HC emissions was carried out at General Motors [3,4]. The investigation was vital to the future of rotary engines, for unless the base engine hydrocarbon emissions (and fuel consumption) could be cut to a level near that of a reciprocating engine, it would have been extremely difficult for rotary engines to enter the engine market. The bulk of this section is based on the results of those studies.
Beforehand, however, a short note on the low NOx emission rate in the rotary engines should be made. Plots shown in Figures7.3 and 7.4 illustrate a comparison between nitrogen oxides emissions from carbureted rotary and piston engines. The tested engines were a four-cylinder, in-line 2300-cc displacement, 8.44 compression ratio, water-cooled engine manufactured by Ford Motor Company and a two-rotor 1308-cc displacement, 9.4 compression ratio, water-cooled Model 13B engine manufactured by Mazda. The catalytic converter and exhaust gas recirculation apparatus were removed from both engines. The fuel used was a synthetic coal-derived gasoline. In the whole range of tested engine speeds and loads, the NOx emissions from rotary engines were about two to three times lower. As the engine speed increased, the measured NOx emissions from the rotary engine remained almost constant.
One of the reasons for the low NOx emission rate underlined in the study is the high concentration of burned gas in the fresh mixture. This is due to the leakage across apex seals, which creates a built-in exhaust gas recirculation system in the rotary engine. Another important reason is the relatively high rate of heat transfer into the walls of the combustion chamber caused by its high S-V ratio. Both phenomena lead to low maximum combustion temperatures and, therefore, a low rate of NOx generation.
The Wankel rotary type engine was first tested in 1957 (Heywood, 1988). In contrast to the standard reciprocating type IC engine with a crank-slider mechanism, the Wankel engine uses an oval-shaped housing with a triangular-shaped rotor on an eccentric shaft. The rotary engine is advantageous as it is compact, has high engine speeds to provide higher power to weight ratios, and is well-balanced. However, the engines downfalls include high heat transfer (higher surface-to-volume ratios) along with sealing and leakage issues, low efficiency due to smaller compression ratios, and poor emissions due to a non-optimal combustion chamber shape including large crevice volumes which can cause quenching (Heywood, 1988; Stone, 2002; Tartakovsky et al., 2012). In the rotary engine operation, there are three rotations of the eccentric shaft for one rotation of the rotor, with the rotor and housing forming three combustion chambers. This means that one working cycle in a Wankel engine occurs during 1080 degrees shaft rotation, whereas a standard four-stroke engine has 720 degrees of crankshaft rotation (Tartakovsky et al., 2012).
This steer angle dependent four wheel steering system provides dual steering characteristics enabling same direction steer to take place for small steering wheel angles. This then changes to opposite direction steer with increased steering wheel deviation from the straight ahead position. Both of these steer characteristics are explained as follows:
Opposite direction steer (Fig. 9.33)At low speed and large steering wheel angles the rear wheels are turned by a small amount in the opposite direction to the front wheels to improve manoeuvrability when parking (Fig. 9.32). In effect opposite direction steer reduces the car's turning circle but it does have one drawback; the rear wheels tend to bear against the side of the kerb. Generally there is sufficient tyre sidewall distortion and suspension compliance to accommodate the wheel movement as it comes into contact with the kerb so that only at very large steering wheel angles can opposite direction steer becomes a serious problem.
Same direction steer (Fig. 9.33)At high speed and small steering wheel angles the rear wheels are turned a small amount in the same direction as the front wheels to improve both steering response and stability at speed (Fig. 9.33). This feature is particularly effective when changing lanes on motorways. Incorporating a same direction steer to the rear wheels introduces an understeer characteristic to the car because it counteracts the angular steering movement of the front wheels and consequently produces a stabilizing influence in the high speed handling of the car.
Front and rear road wheel response relative to the steering wheel angular movement (Fig. 9.33)Moving the steering wheel approximately 120 from its central position twists the front wheels 8 from the straight ahead position. Correspondingly, it moves the eccentric shaft peg to its maximum horizontal annular gear offset, this being equivalent to a maximum 1.5 same direction steer for the rear road wheels (Fig. 9.33).
Increasing the steering wheel rotation to 232 turns the front wheels 15.6 from the straight ahead position which brings the planetary peg towards the top of the annular gear and in vertical alignment with the gear's centre. This then corresponds to moving the rear wheels back to the straight ahead position (Fig. 9.33).
Further rotation of the steering wheel from the straight ahead position orbits the planetary gear over the right hand side of the annular gear. Accordingly the rear wheels steadily move to the opposite direction steer condition up to a maximum of 5.3 when the driver's steering wheel has been turned roughly 450 (Fig. 9.33).
Four wheel steer (FWS) layout (Fig. 9.34)The steering system is comprised of a rack and pinion front steering box and a rear epicyclic steering box coupled together by a central drive shaft and a pair of Hooke's universal end joints (Fig. 9.35). Both front and rear wheels swivel on ball suspension joints which are steered by split track rods actuating steering arms. The front road wheels are interlinked by a rack and transverse input movement to the track rods is provided by the input pinion shaft which is connected to the driver's steering wheel via a split steering shaft and two universal joints. Steering wheel movement is relayed to the rear steering box by way of the front steering rack which meshes with an output pinion shaft. This movement of the front rack causes the output pinion and centre drive shaft to transmit motion to the rear steering box. The rear steering box mechanism then converts the angular input shaft motion to a transverse linear movement. This is then conveyed to the rear wheel swivels by the stroke rod and split track rods.
Rear steering box construction (Fig. 9.35)The rear steering box is basically formed from an epicyclic gear set consisting of a fixed internally toothed annular ring gear in which a planetary gear driven by an eccentric shaft revolves (Fig. 9.35). Motion is transferred from the input eccentric shaft to the planetary gear through an offset peg attached to a disc which is mounted centrally on the eccentric shaft. Rotation of the input eccentric shaft imparts movement to the planetary gear which is forced to orbit around the inside of the annular gear. At the same time, motion is conveyed to the guide fork via a second peg mounted eccentrically on the face of the planetary gear and a slider plate which fits over the peg (Fig. 9.35). Since the slider plate is located between the fork fingers, the rotation of the planetary gear and peg causes the slider plate to move in both a vertical and horizontal direction. Due to the construction of the guide fork, the slider plate is free to move vertically up and down but is constrained in the horizontal direction so that the stroke rod is compelled to move transversely to and fro according to the angular position of the planetary gear and peg.
Adopting this combined epicyclic gear set with a slider fork mechanism enables a small same direction steer movement of the rear wheels to take place for small deviation of the steering wheel from the straight ahead position. The rear wheels then progressively change from a same direction steer movement into an opposite steer displacement with larger steering angles.
The actual steering wheel movement at which the rear wheels change over from the same direction steer to the opposite direction steer and the magnitude of the rear wheel turning angles relative to both conditions are dependent upon the epicyclic gear set gear ratio chosen.
Rear steering box operation (Fig. 9.36(ae))The automatic correction of the rear road wheels from a same direction steer to opposite direction steer with increasing front road wheel turning angle and vice versa is explained by Fig. 9.36(ae).
Central positionWith the steering wheel in the straight ahead position, the planetary gear sits at the bottom of the annular gear with both eccentric shaft and planetary pegs located at bottom dead centre in the mid-position (Fig. 9.36(a)).
90 eccentric shaft peg rotationRotating the eccentric shaft through its first quadrant (090) in a chosen direction from the bottom dead centre position compels the planetary gear to roll in an anticlockwise direction up the left hand side of the annular ring gear. This causes the planetary peg and the stroke rod to be displaced slightly to the left (Fig. 9.36(b)) and accordingly makes the rear wheels move to a same direction steer condition.
180 eccentric shaft peg rotationRotating the eccentric shaft through its second quadrant (90180) causes the planetary gear to roll anticlockwise inside the annular gear so that it moves with the eccentric peg to the highest position. At the same time, the planetary peg orbits to the right hand side of the annular gear centre line (Fig. 9.36(c)) so that the rear road wheels turn to the opposite direction steer condition.
270 eccentric shaft peg rotationRotating the eccentric shaft through a third quadrant (180270) moves the planetary gears and the eccentric shaft peg to the 270position, causing the planetary peg to orbit even more to the right hand side (Fig. 9.36(d)). Consequently further opposite direction steer will be provided.
360 eccentric shaft peg rotationRotating the eccentric shaft through a fourth quadrant (270360) completes one revolution of the eccentric shaft. It therefore brings the planetary gear back to the base of the annular ring gear with the eccentric shaft peg in its lowest position (Fig. 9.36(e)). The planetary peg will have moved back to the central position, but this time the peg is in its highest position. The front to rear wheel steering drive gearing is normally so arranged that the epicyclic gear set does not operate in the fourth quadrant even under full steering lock conditions.
The expansion of the fluid drives the moving involute. However, the motion is not circular but it is actually orbital. To prevent the rotational movement of the moving involute and to convert this orbiting movement into a rotating shaft movement, anti-rotationmechanisms must therefore be used and associated with an eccentric shaft. Anti-rotation mechanisms exist in various ways and are adapted to different kinds of scroll machines. In refrigeration scroll machines, an Oldham ring is often used. However when compactness and high speed are required, the Oldham ring is not good enough and eccentric ball thrust bearings are used. A third mechanism is also used in scroll machines; it consists of three additional shafts that guide the mobile involute.
Scroll expanders can be compliant or kinematically rigid. Compliance allows for a degree of freedom in a given direction. This degree of freedom can be along the radial or axial direction, or even both. Such a mechanism allows the scroll expander to adapt itself to the operating conditions, leading to an increased efficiency. Kinematically rigid expanders cannot adapt when transient conditions happen, especially in the presence of liquid fluid or debris, which can cause damage when they are trapped between the involutes.
Finally, scroll machines are highly reliable components due to a very low number of moving elements. Maintenance is also rather low. Some scroll machines may need a tip seal (whose mechanism is explained in section 22.214.171.124) replacement but most of the time, the machine is designed to require no maintenance at all.
Non-standard parts are likely to damage the costly original parts of the machine. In emergencies we are forced to use non-standard parts as we cannot afford to keep a machine idle. It is suggested to paint the non-standard parts with a different colour so that one can always remember that a not standard part is being run. It should be replaced as early as possible, and kept as a spare for emergency.
Water contamination in the compressed air reduces the supply of compressed air to resulting in failure of critical mechanisms of feed control, doffing, splicing, drafting, etc. The water should be removed periodically.
Jaw crushers are mainly used as primary crushers to produce material that can be transported by belt conveyors to the next crushing stages. The crushing process takes place between a fixed jaw and a moving jaw. The moving jaw dies are mounted on a pitman that has a reciprocating motion. The jaw dies must be replaced regularly due to wear. Figure 8.1 shows two basic types of jaw crushers: single toggle and double toggle. In the single toggle jaw crusher, an eccentric shaft is installed on the top of the crusher. Shaft rotation causes, along with the toggle plate, a compressive action of the moving jaw. A double toggle crusher has, basically, two shafts and two toggle plates. The first shaft is a pivoting shaft on the top of the crusher, while the other is an eccentric shaft that drives both toggle plates. The moving jaw has a pure reciprocating motion toward the fixed jaw. The crushing force is doubled compared to single toggle crushers and it can crush very hard ores. The jaw crusher is reliable and robust and therefore quite popular in primary crushing plants. The capacity of jaw crushers is limited, so they are typically used for small or medium projects up to approximately 1600t/h. Vibrating screens are often placed ahead of the jaw crushers to remove undersize material, or scalp the feed, and thereby increase the capacity of the primary crushing operation.
Both cone and gyratory crushers, as shown in Figure 8.2, have an oscillating shaft. The material is crushed in a crushing cavity, between an external fixed element (bowl liner) and an internal moving element (mantle) mounted on the oscillating shaft assembly. An eccentric shaft rotated by a gear and pinion produces the oscillating movement of the main shaft. The eccentricity causes the cone head to oscillate between the open side setting (o.s.s.) and closed side setting (c.s.s.). In addition to c.s.s., eccentricity is one of the major factors that determine the capacity of gyratory and cone crushers. The fragmentation of the material results from the continuous compression that takes place between the mantle and bowl liners. An additional crushing effect occurs between the compressed particles, resulting in less wear of the liners. This is also called interparticle crushing. The gyratory crushers are equipped with a hydraulic setting adjustment system, which adjusts c.s.s. and thus affects product size distribution. Depending on cone type, the c.s.s. setting can be adjusted in two ways. The first way is by rotating the bowl against the threads so that the vertical position of the outer wear part (concave) is changed. One advantage of this adjustment type is that the liners wear more evenly. Another principle of setting adjustment is by lifting/lowering the main shaft. An advantage of this is that adjustment can be done continuously under load. To optimize operating costs and improve the product shape, as a rule of thumb, it is recommended that cones always be choke-fed, meaning that the cavity should be as full of rock material as possible. This can be easily achieved by using a stockpile or a silo to regulate the inevitable fluctuation of feed material flow. Level monitoring devices that detect the maximum and minimum levels of the material are used to start and stop the feed of material to the crusher as needed.
Primary gyratory crushers are used in the primary crushing stage. Compared to the cone type crusher, a gyratory crusher has a crushing chamber designed to accept feed material of a relatively large size in relation to the mantle diameter. The primary gyratory crusher offers high capacity thanks to its generously dimensioned circular discharge opening (which provides a much larger area than that of the jaw crusher) and the continuous operation principle (while the reciprocating motion of the jaw crusher produces a batch crushing action). The gyratory crusher has capacities starting from 1200 to above 5000t/h. To have a feed opening corresponding to that of a jaw crusher, the primary gyratory crusher must be much taller and heavier. Therefore, primary gyratories require quite a massive foundation.
The cone crusher is a modified gyratory crusher. The essential difference is that the shorter spindle of the cone crusher is not suspended, as in the gyratory, but is supported in a curved, universal bearing below the gyratory head or cone (Figure 8.2). Power is transmitted from the source to the countershaft to a V-belt or direct drive. The countershaft has a bevel pinion pressed and keyed to it and drives the gear on the eccentric assembly. The eccentric assembly has a tapered, offset bore and provides the means whereby the head and main shaft follow an eccentric path during each cycle of rotation. Cone crushers are used for intermediate and fine crushing after primary crushing. The key factor for the performance of a cone type secondary crusher is the profile of the crushing chamber or cavity. Therefore, there is normally a range of standard cavities available for each crusher, to allow selection of the appropriate cavity for the feed material in question.