Whether the working equipment moves on tracks or tires has a major influence on productivity (how much dirt can be moved or excavated in a certain amount of time or how fast material can be transported). Both types of movements offer advantages and disadvantages based on working and surface conditions.
Usable force available to perform work depends on the coefficient of traction of the work surface and the weight (lbs) carried by the running gear or wheels. The amount of tractive force necessary to push or pull a load is important for sizing the right machine. Manufacturers provide rimpull or drawbar pull tables for most of their equipment models showing tractive power that can be delivered at specified operating speeds. This information can be used to verify a machine’s ability or capacity to work in specified job conditions (primarily rolling or surface resistance and grade resistance) and achieve the desired production.
Usable force available to perform work depends on the coefficient of traction of the work surface and the weight (lbs) carried by the running gear or wheels. The amount of tractive force necessary to push or pull a load is important for sizing the right machine. Manufacturers provide rimpull or drawbar pull tables for most of their equipment models showing tractive power that can be delivered at specified operating speeds. This information can be used to verify a machine’s ability or capacity to work in specified job conditions (primarily rolling or surface resistance and grade resistance) and achieve the desired production.
Coefficients of traction vary based upon the travel surface. They measure the degree of traction between the wheel or track and travel surface. Slick surfaces have lower coefficients of traction than rougher surfaces (assuming both surfaces are relatively level and flat). Coefficients of traction for rubber-tired vehicles range from 0.90 for a concrete surface, 0.20 for dry sand to 0.12 for ice. Typically, coefficients of traction tables are available in equipment performance handbooks. The better the traction generated by the piece of equipment on the travel surface, the shorter the travel time and less wear and tear on the piece of equipment. Simply stated, maximum tractive effort (drawbar or rimpull in pounds) equals the equipment weight multiplied by the coefficient of traction of the travel or work surface.
This formula calculates the maximum amount of force that can be generated for a load on a surface. Excess tractive effort generated by the equipment will cause the tires or tracks to spin. Overloading will cause this result. The machine’s engine provides the power to overcome the resistances and move the machine. The engine must be sized or matched to meet the tractive effort required to the capabilities of the machine. The model selected would be appropriate if it can generate enough tractive effort to perform the specific task without overburdening the machine.
Tracked equipment is designed for work activities requiring high tractive effort (drawbar) or the ability to move and remain stable on uneven or unstable surfaces. Tasks such as pushing over trees, removing tree stumps, or removing broken concrete flatwork require a very high pushing force. The tracked bulldozer is ideal for this type of work. Tractive effort results from the track cleats or grousers gripping the ground to create force necessary to push or pull dirt, material, or any other piece of equipment. Tracked equipment is most efficient when used for short travel distances less than 500 ft. Figure 1 shows a typical piece of heavy construction equipment running on tracks. Most loaders on construction sites run on tires.
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| Figure 1 Tracked loader at work |
Tracks can be metal or rubber. Metal tracks are more durable and can withstand much greater abuse than rubber tracks. Heavy-duty dirt moving equipment will almost always run on metal tracks. Rubber tracks are lighter and best for smaller equipment working in organic matter and surfaces requiring minimal disturbance. Tracks come in varying widths and thicknesses. The width of the track shoe determines the ground pressure. The wider the track the more surface area covered and the wider the load distribution. Wider track shoes have greater flotation on the work surface. The heavier the track, the more power required to make it move. Narrow track shoes are better for harsh irregular hard work surfaces. Shoes are typically designed with single or double grousers. Single grouser shoes are better for developing traction and double grouser shoes typically are less damaging to travel or work surfaces.
It should be noted that tracked equipment typically marks or gouges the surface on which it is operating. Skid-steer types of equipment (bulldozers and loaders) will gouge the surface with the track cleats when they turn. To avoid ‘‘customizing’’ a parking lot surface, plywood can be laid, on which the tracked equipment can maneuver, and rubber or padded tracks or use a tired piece of equipment could be used. A hot asphalt surface typically will mark or rip with tires or tracks unless the surface is protected.
Tired equipment is more mobile and maneuverable than tracked equipment. Machines can achieve greater speed and therefore are better for hauling. However, pulling ability is reduced to reach a higher speed. Tired equipment is more efficient than tracked equipment when the distance is greater than 500 ft. The tire diameter and width, tread design, and inflation pressure influence the ability to roll. The larger the tire, the more power required to make it roll. Tread and track design influence the ability to grip the travel surface. A more pronounced deeper tread grips better. The inflation pressure also influences how much resistance the tire has on the travel surface. The less the inflation pressure, the greater the surface area covered by the tire, the harder it is to roll and more buoyant the equipment.
Rolling resistance is the resistance of a level surface to a uniform velocity motion across it. It is the force required to shear through or over a surface and is also termed wheel resistance (e.g., a truck tire developing friction on the road surface as it turns). Rolling resistance has two components: surface resistance and penetration resistance.
Surface resistance results from the equipment trying to rollover the travel surface material. Penetration resistance results from the equipment tires sinking into the surface. Obviously, this resistance will vary greatly with the type and condition of the surface over which the equipment is moving. Simply put, soft surfaces have higher resistance than hard surfaces.
On a hard surface, a highly inflated tire has less rolling resistance than a less inflated tire, primarily because of less tire surface area coming in contact with the road surface. A highly inflated tire has greater rolling resistance in sand than a less inflated tire because it will sink deeper into the rolling surface. The rolling resistances shown in Table 1 are adapted from John Schaufelberger’s book, Construction Equipment Management. The table shows several surfaces and their rolling resistances. Rolling resistance is expressed in pounds of resistance per ton of vehicle or equipment weight. The rolling resistance is greater for a loaded piece of equipment than when it is unloaded. Use the loaded weight of the equipment (equipment including fuel and lubricants plus load) in tons when calculating resistance.
When there is no real penetration into the travel or operating surface, the rolling resistance is about 40 lbs/ton. The weight of the equipment should include the load. When a tire sinks in the mud until it is stable, the rolling resistance as it tries to climb out of the rut increases about 30 lbs/ton (2000 lbs) for each inch of penetration.
Example
92,000 lbs/2000 lbs/ton = 46 tons.
Rolling resistance = 46 tons (50 lbs/ton) = 2300 lbs.
Penetration resistance = 200(46 tons) (30 lbs/ton/inch) = 2760 lbs.
Total tractive effort = 2300 lbs + 2760 lbs = 5060 lbs.
With this number, the equipment manager can refer to the manufacturer’s performance specs to select a piece of equipment that can generate enough power (in this case rimpull) to overcome this resistance.
Grade resistance is the force-opposing movement of a vehicle up a frictionless slope (does not include rolling resistance). The effort required to move a vehicle up a sloping surface increases approximately in proportion to the slope of the surface. The effort required to move a vehicle down a sloping surface decreases approximately in proportion to the slope of the surface. For slopes less than 10%, the effect of grade increases for a plus slope and decreases for a minus slope. The required tractive effort increases or decreases 20 lbs per gross ton of weight for each 1% of grade.