CHAPTER 7 POWERPOINT powerpoint powerpoint .pptx

construction engineering

1. Dozers and Graders INTRODUCTION

2. Dozers may be either crawler (track-laying) or wheel-type machines. They are tractor units equipped with an upright front blade for pushing materials (dozing). These machines are designed specifically to provide high tractive power for drawbar work while operating at lower travel speeds. Consistent with their purpose as a unit for drawbar work, they are low center-of gravity machines. This is a prerequisite for an effective dozer. The larger the difference between the line-of-force transmission by the machine and the line-of-resisting force, the less effective the use of developed power.

3. Dozer Performance Characteristics Dozers are classified on the basis of running gear:

4. 1. Crawler type FIGURE 7.1

5. 2. Wheel type FIGURE 7.2

6. Rear sprocket drive wheel Front idler wheel

7. FIGURE 7.3 Track shoes

8. finish work Shaping the material to the final earthwork grade required by the specifications. FIGURE 7.4 Low-ground-pressure dozer operated on soft ground, note the wide tracks In the case of wheel machines, wider tires provide greater surface contact area and increase flotation. Two important things to consider when using larger tires: (1) they reduce the developed rimpull and (2) rimpull charts are based on standard equipment, including tires.

9. The crawler-type tractor is designed for those jobs requiring high tractive effort. No other piece of equipment can provide the power, traction, and flotation needed in such a variety of working conditions. A crawler dozer can operate on slopes as steep as 45 degrees. Both track- and wheel-type dozers are rated by flywheel horsepower( fwhp ) and weight. Normally the weight is an operating weight and includes lubricants, coolants, a full fuel tank, a blade, hydraulic fluid, the OSHA rollover protective structure (ROPS), and an operator. Dozer weight is important on many projects because the maximum tractive effort a tractor can provide is limited to the product of the weight times the coefficient of traction of the unit for the particular ground surface, regardless of the power supplied by the engine. Table 6.4 gives the coefficients of traction for various surfaces. An advantage of a wheel-type dozer when compared with a crawler dozer is the higher speed possible with the wheeled machines—in excess of 30 mph for some models. To attain a higher speed, however, a wheel dozer must sacrifice push/pull ability. Also, because of the lower coefficient of traction between rubber tires and some ground surfaces, the wheel dozer may slip its wheels before developing its rated pulling ability. Table 7.1 provides a comparison of crawler dozer and wheel dozer utilization

10. TABLE 7.1 Dozer-type utilization comparison

11. Internal combustion engines are used to power most dozers, with diesel engines being the common primary power unit. Gasoline engines are used in some smaller machines and electric- and air-powered dozers are available for tunnel work. Because the crankshaft rotation derived from the engine is usually too fast and does not have sufficient force (torque), machines have transmissions to reduce the rotational speed of the crankshaft and increase the force available to do work. Transmissions provide the operator with the ability to change the machine’s speed-power ratio to match the work requirements (force necessary to do work). Manufacturers provide dozers with a variety of transmissions, but primarily the options are Direct drive Torque converter and power-shift transmission Some less-than-100-hp dozers are available with hydrostatic powertrains. The small to medium, less-than-300-hp, diesel-powered machines are commonly available with either direct- or power-shift-type transmissions. Larger dozers are always equipped with power-shift transmissions.

12. Crawler Dozers with Direct Drive The term “direct drive” means the power is transmitted straight through the transmission as if there were a single shaft. This is usually what happens when the transmission is in its highest gear. In all other gears, mechanical elements match speed and torque. Direct-drive dozers are superior when the work involves constant load conditions. A job where full blade loads must be pushed long distances would be an appropriate application of a direct-drive machine. The specifications provided by some manufacturers list two sets of draw-bar pulls for direct-drive dozers: rated and maximum.

13. Crawler Dozers with Torque Converter and Power-Shift Transmissions Power-shift transmissions can be shifted while transmitting full engine power. These transmissions are teamed with torque converters to absorb drive train shock forces caused by changes in gear ratios. A power-shift transmission provides an efficient flow of power from the engine to the tracks and gives superior performance in applications involving variable load conditions. Crawler Dozers with Hydrostatic Powertrains Confined hydraulic fluid under pressure is an effective means of power transmission. A hydrostatic powertrain offers an infinitely variable speed range with constant power to both of the tractor’s tracks. This type of powertrain improves machine control and increases operational efficiency. Hydrostatic powertrain transmissions are available on some lower horsepower dozers. Wheel Dozers Most wheel dozers are equipped with torque converters and power-shift transmissions. Wheel dozers exert comparatively high ground pressures, 25 to 35 psi (172–241 kPa). Comparison of Performance Usable rimpull force of a dozer depends on traction and weight of the dozer or the weight on the powered wheels of a wheel dozer. Even though the engine can develop a certain drawbar pull or rimpull force, all of the pull may not be available to do the work if the wheels slip on the travel surface. The caution is a restatement of Eq. [6.15]. If the project-working surface is dry clay loam, Table 6.4 provides the following coefficient of traction factors:

14. Using the 0.90 factor for tracks and considering a 45,560 lb track-type dozer with a power shift, the usable drawbar pull is found to be: 45,560 lb × 0.90 = 41,004 lb Now consider a 45,370 lb wheel-type dozer: 45,370 lb × 0. 60 = 27,222 lb The two machines have approximately the same operating weight and flywheel horsepower; yet, because of the effect of traction, the track machine can supply one and a half times the usable power. In the case of a track-type tractor and a wheel tractor operating on the same surface conditions, the coefficient of traction for wheel machine is much less. Therefore, a wheel-type dozer must be considerably heavier (by approximately 50%) than a crawler dozer to develop the same amount of usable force. As the weight of a wheel-type dozer is increased, a larger engine will be required to maintain the weight-to-horsepower ratio. As ballast weight is added to the wheel-type dozer, the speed and mobility of the machine is decreased and it loses its advantage over a track dozer.

15. GENERAL INFORMATION The vertical blade attached to the front of the tractor unit is used to push, shear, cut, and roll material forward. Dozers are effective and versatile earthmoving machines. On projects they are used both as support and as production machines. Typical tasks include: -Moving earth or rock for short push (haul) distances, up to 300 ft (91 m) in the case of large dozers -Spreading earth or rock fills -Backfilling trenches -Opening up pilot roads through mountains or rocky terrain -Clearing the floors of borrow and quarry pits -Helping load tractor-pulled scrapers -Clearing land of timber, stumps, and root mat

16. Blades A dozer blade consists of a moldboard with replaceable cutting edges and end (side) bits (Figure 7.5). Push arms and tilt cylinders or a C-frame connect the blade to the dozer (Figure 7.6). Blades vary in size and design based on specific work applications. The hardened-steel cutting edges and side bits are bolted on because they receive most of the abrasion and wear out rapidly. The bolted connection enables easy replacement. The design of some machines allows either end of the blade to be raised or lowered in the vertical plane of the blade, tilt. The top of the blade can be pitched forward or backward, varying the angle of attack of the cutting edge, pitch. Blades mounted on a C-frame can be turned from the direction of travel, angling. These features are not applicable to all blades, but any two of these may be incorporated in a single mount. Figure 7.7 illustrates tilt, pitch, and angling.

17. Blade Performance A dozer’s pushing potential is measured by two ratios: Horsepower per foot of cutting edge, cutting ratio Horsepower per loose cubic yard ( lcy ) of material retained in front of the blade, load ratio The horsepower per foot (hp/ft) cutting ratio provides an indication of the blade’s ability to penetrate and obtain a load. A higher cutting ratio indicates a more aggressive blade. The horsepower per loose cubic yard load ratio measures the blade’s ability to push a load. A higher ratio means the dozer can push a load at a greater speed. Many different special application blades can be attached to a tractor (Figure 7.8), but basically only five blades are common to earthwork: (1) the straight “S” blade, (2) the angle “A” blade, (3) the universal “U” blade, (4) the semi-U “SU” blade, and (5) the cushion “C” blade.