Chp 6 Power 10th ed

Mobile Equipment Requirements

  • Textbook Title: Construction Planning, Equipment, and Methods 10th Edition by Peurifoy, Schexnayder, Schmitt, Shapira, Cohen

Importance of Power

  • Critical Question: Why does a machine only travel at 10 mph when its top speed is 30 mph?

  • Influences on Production:

    • Speed affects cycle time

    • Cycle time drives production

    • Production determines cost

Caterpillar Machinery

  • Relates to power and efficiency of equipment o CATERPILLAR systems involve analyzing speed, cycle time, production cost.

Payload Considerations

  • Equipment payload can be expressed:

    • Volumetrically

    • Gravimetrically

Rolling Resistance

  • Definition: Measure of the force (lb/ton) to overcome when a wheel travels over a surface.

    • Factors contributing to rolling resistance:

      • Friction of mechanisms

      • Tire flexing

      • Shear through the surface

      • Continuous uphill grade

  • Causes of Tire Rolling Resistance:

    • Internal friction

    • Tire penetrating surface

  • Equation for rolling resistance (lb):

    • RR = [40 + (30 X TP)] X GVW

      • where TP is tire penetration in inches and GVW is gross vehicle weight in tons.

Estimation of Rolling Resistance

  • Rolling Resistance estimations can be found in a table for different wheel types and surfaces.

Importance of Haul Road Condition

  • Well-maintained haul roads:

    • Reduce rolling resistance

    • Improve production

    • Requires graders and water trucks, incurring costs.

Grade Resistance

  • Definition: Haul roads are rarely level from point of loading to dump location.

  • Grade Calculation:

    • Grades in % slope ratio:

      • Vertical rise (or fall) / Horizontal distance

  • Example Calculation of Grade:

    • 5 ft fall over 100 ft horizontal: 5 ft/100 ft = 5% grade

  • Grade Resistance Equation (for small angles):

    • GR = 20 lb/ton × % grade

  • Example Calculation:

    • Truck with a GVW of 23 tons moving up a 4% grade: GR = 20 lb/ton x 23 tn x 4% = 1,840 lb

GRADE Assistance and Resistance

  • Gravity's Role: Assists machines downhill (Grade Assistance).

  • Example: Downhill calculation for a truck with 12 tons at 4%: GA = 20 lb/ton x 12 tn x -4% = -960 lb.

Total Resistance Calculation

  • Formulas:

    • Total Resistance (TR) = Rolling Resistance (RR) + Grade Resistance (GR)

    • TR = RR - GA (when applicable)

Exercises for Practical Applications

  1. Scraper exercise on a poor haul road:

    • Calculate total resistance in pounds and equivalent grade.

  2. Steps outlined for weight calculations and resistance assessment.

Power Available

  • Definition and Measurement: Power available determined by engine horsepower and operating gear.

    • 1 hp = 33,000 ft-lb per minute.

    • Performance charts estimate the speed of machines based on rimpull, GVW, and resistance.

Coefficient of Traction

  • Understanding: Ratio of maximum pull before slippage to total weight on drivers.

  • Example Calculation: For a scraper:

    • Weight on drivers = 60,000 lb

    • Rimpull requirement assessed against coefficient of traction for different conditions (dry/wet).

Altitude Limits

  • Performance effects due to reduced atmospheric density at higher altitudes, leading to reduced engine output.

  • Example case for equipment operation at altitudes exceeding 15,000 ft.

Summary of Key Equations

  • Rolling Resistance = [40 + (30 × TP)] × GVW

  • Grade Resistance = 20 lb/ton × % grade

  • Total Resistance = Rolling Resistance ± Grade Resistance