Hourly Productivity of Trucks
A function of :
a. Load carried, thus truck size
b. Number of cycles, thus truck cycle time (CT)
c. Job efficiency, thus how efficient we are
PR[t/h] = LOAD[t] x # CYCLES[1/hr] x J.E.F.
# CYCLES = 60 [min/h] / CT [min]
Equipment Cycle Time
1. The sum of :
a. All variable times
b. All fixed times
c. Spot and wait time
2. This time has to be derated to reflect various “inefficiencies” of the job
a. Does your operator need a pee….?
b. Is he willing to give the job his best?
c. Do you (an ENGINEER) do all right?
Cycle Time “Derating”
1. “Rough” approach: to derive number of cycles include job efficiency factor:
a. Either deduct time (e.g. 50 min. hours)
b. Or multiply by a factor (e.g. 0.85)
…. but NOT BOTH!
2. “Educated” approach:
a. Quantify the delays / interferences
b. Thorough understanding of the job required
Job Efficiency Factor (After Dresser)
Favorable Job Conditions
1. Material being excavated & hauled:
- Topsoil
- Loam/clay mixture (low moisture content)
- Compacted coal
- “Tight” earth (no rock)
2. Loading Area (“Cut” or “Borrow Pit”):
- Unrestricted in length or width
- Dry & smooth (or maintained by dozer /grader)
- (For Scrapers): Units load downhill
3. Total Rolling Resistance is under 4%.
4. Constant supervision at both loading & dumping areas.
Unfavorable Job Conditions
1. Material being excavated & hauled:
- Heavy (dense) or wet clay
- Loose dry-blown sand with no binder
- Coarse gravel (no fines)
- Caliche or unripped shale
- Frequent boulders or rock outcroppings
2. Loading Area (“Cut” or “Borrow Pit”):
- Restricted in length or width
- Wet, slippery and/or soft (not maintained)
- (For Scrapers): Units load uphill or on a side slope
3. Total Rolling Resistance is over 7%.
4. No supervision in loading or dumping areas
How to Quantify JEF? A CAT Example ….
Consider the two major factors
1. Operator efficiency
a. A function of cycle length
b. Most of the loss occurs in loading/dumping area
2. Fleet match
a. Number of haulers vs. Loaders?
b. Variability of cycle times?
“Operator” Efficiency
Note: do not discount your performance twice!!
Loader-Hauler Interaction
Production is lower than the sum of production of individual units:
1. Mismatch of hauling and loading equipment
2. Bunching of units
3. Randomness of availability
Consider This Situation :
994 loader (cycle time 1 min) loads 793 trucks (cycle time 20 min) in six passes
Fleet “Undertrucked”: production limited by haulers
Fleet “Overtrucked”: production limited by loaders
Mismatch
Bunching
Efficiency is percent of maximum fleet production
Coefficient of variation = Square of standard deviation
Fleet Efficiency (Loaders)
So, how would this relation look for trucks ?
How Many Trucks ?
Overtrucking
1. Good shovel productivity & utilization
2. Normally higher unit cost as trucks are more expensive
Undertrucking
1. Used most often
2. Usually lower capital and operating cost
3. Good utilization of truck fleet
More flexibility in truck assignments ?
Optimum Number of Trucks
Production
Required production level may be the main determinant of the choice
What is wrong here? No other shovel to send the trucks to ?
