1. CONSTRUCTION EQUIPMENT MANAGEMENT
BFP 40203
CONSTRUCTION EQUIPMENT MANAGEMENT

2. LOADERS AND OTHER EXCAVATORS (2 HOURS LECTURE)
CHAPTER 3.0
LOADERS AND OTHER EXCAVATORS (2 HOURS LECTURE)
3.1 Determining the number of haul units required
3.2 Queuing theory method
3.3 Haul unit operation
Test 1
Project

3. The Haul Cycle
Total haul unit cycle time is found by summing the time required for each of the following components of the haul cycle:
Load—at the excavator/loader
Haul—from the loader to the unloading site
Dump—at the unloading site, including maneuvering
Return—travel back to the loader
Spot—move into loading position at the loader

4. The total haul cycle time
calculated as the sum of fixed time and variable time.
Fixed time
is the sum of load, dump, and spot times.
These times are referred to as fixed since they do not depend on the haul distance or travel speed of the haul unit.
Fixed times can usually be closely estimated for a particular operation.
Travel time
are added together and referred to as variable time.
Travel time can be found by using travel time curves or by dividing the travel distance by the average speed of the haul unit.

5. Loading Time The time required to load a haul unit
Load time = Number of buckets X Excavator cycle time

6. loading rate at 100% loader efficiency is used because a loader typically operates at or near 100% efficiency when actually engaged in loading
Either bank measure or loose measure

7. Calculating the Number of Haul Units Required

8. EXAMPLE 3-1 Shovel production
job efficiency of 75% is 229 BCY/h (175 BCM/h)
truck transit time is 0.5 hr
determine how many trucks having a capacity of 16.5 BCY (12.6 BCM) would be required to fully service the shovel.

9. SOLUTION Shovel production @ 75% = 229/0.75 = 305 BCY/h (233 BCM/h)
Load time: BCY hr ?
16.5/305 = h (12.6/233 = h)
Transit time = 0.5 hr
= 11 trucks.

10. EXAMPLE 3.2
If a 3 LCY (2.3 LCM) hydraulic shovel has a dipper cycle time of 22 sec
how many trucks would be required to service this shovel?
tough clay with a bucket fill factor of 0.80
load factor of 0.77.
trucks will carry 16.5 BCY (12.6 BCM) of soil.
truck transit time is 0.5 hr.

11. SOLUTION Bucket load = 3 X 0.77 X 0.80 = 1.85 BCY (1.42 BCM)
Number of dippers to fill haul unit
= 16.5/1.85 = 8.9
= (12.6/1.42 = 8.9)
Use 9 dippers.

12. Load Time = (9 X 22) / (60 X 60)
= hr
Number of units required
= ( ) / = 10.1
Use 11 trucks.

13. Effect of Reduced Haul Units
If the rational method of determining the number of haul units required is used,
no increase in production occurs if more than the required number of haul units is provided.
if less than the required number is available, the expected production is reduced in proportion to the shortage.

15. EXAMPLE 3
In the situation of Example 3-1, only 8 trucks are available. What is the expected production of this system?
SOLUTION
Expected production
= (8 /10.3) X 229
= 178 BCY/h (136 BCM/h)

16. QUEUING THEORY

17. Application of Queuing Theory
The following terms and symbols will be used in applying queuing theory to the loading and hauling problem:
n = number of haul units in the fleet
a = mean arrival rate of a particular haul unit (arrivals/h)
l = mean loading rate of the excavator (units/h)
r = ratio of arrival rate to loading rate
Po = probability that no haul unit is available at the loader
Pt = probability that one or more haul units are available at the loader

18. Since there must be either a haul unit at the loader or no haul unit at the loader, the sum of Po and Pt must equal one.
Hence,
Pt = 1 - Po

19. In order to find Pt and Po, it will be necessary to calculate the ratio, r defined above.
A simple equation for computing r can be developed as follows:
a = arrival rate = 1/transit time
l = loading rate = 1/loading time Or
l = loader production/truck capacity
r = loading time/transit time
r = truck capacity/(loader production x transit time)

20. The value Po (probability of having no haul unit available for loading at any particular instant)
the value of Pt will depend on the number of haul units in the fleet as well as the ratio r.
For greater accuracy, the value of Po may be calculated from the following equation:
Po =

21. EXAMPLE A shovel has loading rate of 305 BCY/h (233 BCM/h),
truck capacity is 16.5 BCY (12.6 BCM)
truck travel time less loading time is 0.54 hr. for fleet of 5 trucks
what is the probability that there will be a truck available for loading at any particular instant?

22. Solution
r = truck capacity/(loader production x transit time)

23. Po =
n = 5, r = 0.1

24. Pt = probability that one or more haul units are available at the loader

25. Table 3.0 Probability Of Haul Unit Being Available (Pt)

27. Optimum Number of Haul Units
The expected production of an excavator/haul system using queueing theory is determined
by multiplying the normal production of the excavator by the probability of having a haul unit available at any instant.
Expected production = Normal excavator production x Pt
the expected production of the system would be
only 43.6% of the normal production.

28. (i.e., lowest unit cost of production).
When queuing theory is used, the optimum number of haul units for a particular operation is selected as the combination of excavator and haul units which yields the best cost performance
(i.e., lowest unit cost of production).
Cost performance over a range of haul unit numbers is determined and the optimum number of units selected.
An approximate value of the optimum n may be found by taking the reciprocal of r.
n = 1/r
A range of n values about this value should then be investigated.

29. n = number of haul units in the fleet
a = mean arrival rate of a particular haul unit (arrivals/h)
l = mean loading rate of the excavator (units/h)
r = ratio of arrival rate to loading rate
Po = probability that no haul unit is available at the loader
Pt = probability that one or more haul units are available at the loader

30. What is the expected production and unit cost of excavation and haul?
Example
A shovel has a loading rate of 305 BCY/h (233 BCM/h)
job efficiency of 75%
cost $83.00/h
truck capacity of 16.5 BCY (12.6 BCM)
cost $31.00/h
travel time less loading time of 0.54 h
What is the optimum number of haul units to use for this operation?
What is the expected production and unit cost of excavation and haul?

31. Solution

33. Production = normal production x Pt
r = 0.1; n = 8; Pt = 0.662

34. Optimum solution
= 9 $2.174/BCY ($2.846/BCM)
Expected production
= 167 BCY/h (127 BCM/h)

35. JOB MANAGEMENT Sizing Haul Units Spotting Haul Units Standby Units
Hauling Operations

36. Sizing Haul Units
A difficult factor to measure in estimating excavator performance is the effect of the size of the target which the haul unit presents to the excavator operator.
It has been found that the use of too small a haul unit will both increase the excavator cycle time and lead to excessive spillage during loading.
Job studies have shown that these factors often result in production losses of 10% to 20%.
As a rule of thumb, it is suggested that, haul units have a minimum capacity of 4 times the excavator bucket capacity.
Draglines require even larger target areas.
For dragline operations, haul unit sizes of 5 to 10 times bucket capacity are recommended.

37. desirable to have haul units that hold an integer number of bucket loads.
The use of a partially filled bucket to top off a load is always inefficient and is especially costly for haul units that hold a low number of bucket loads (4 or less).
Spillage during loading must also be minimized.

38. Spotting Haul Units
It has been found that careless spotting of haul units at the excavator is one of the most common causes of inefficiency in excavator operations.
The location of the loading posi­tion of the haul unit should be carefully planned to minimize excavator cycle time.
Reducing the swing of a shovel by 30° will increase production approximately 15%.
The use of a 180° swing for loading instead of a 90° swing will reduce shovel production by almost one-third.

39. Spotting time for back-in loading may be reduced by using spotting logs or bumpers to help the haul unit operator position his vehicle for loading.
For shovel loading, haul units should be spotted as close to the bank as possible within the radius of the dipper as it leaves the bank.
To further reduce the loss of production during spotting, it may be advisable to use two spotting positions, one on each side of the excavator/loader.
In this case, one haul unit may be spotted while the other unit is being loaded.
Continuous in-line spotting is also efficient whenever job layout permits and a minimum swing angle can be obtained.
Having a supervisor direct spotting may also increase production and be economically feasible.

40. Standby Units
Since fixed costs for operation of a haul unit are usually small in comparison with those of the excavator, standby haul units are frequently provided to ensure that the produc­tive capacity of the excavator is fully utilized.
Standby units are used to replace haul units that break down or are unable to perform in synchronization with other haul units.
It is suggested that standby units be provided on a ratio of 1:5 for average multiple-excavator operations.
This ratio may be reduced for large haul fleets.

41. For single-excavator operations with a small haul fleet, the ratio should be increased to about 1:4.
If rented equipment is available on short notice, it may be possible to reduce the number of standby units even further.
In any case, the exact determination of the number of standby units to be provided should be based on the construction organization's past experience, manufacturer's data on haul unit breakdown, and an analysis of the opera­tional situation.

42. Hauling Operations
To obtain maximum efficiency in excavation and hauling operations, the operation of the excavator and haul fleet must be carefully synchronized.
Ideally, the haul units would be separated by a fixed distance, travels at exactly the same speed, and arrive at the loader precisely when required for loading.
In such an idealized situation the excavator and haul units would all be kept fully utilized.
In actual practice, of course, such perfection cannot be obtained.
Nevertheless, the objective in managing such an operation is to approach the ideal situation as closely as possible.

43. Techniques that will assist managers in obtaining maximum hauling efficiency
Whenever possible, stagger the starting and stopping time for haul units in order to avoid the bunching up of units at the beginning or the end of a shift.
Load haul units as close to the rated load as possible. Do not overload because excessive maintenance and breakdowns will result. Poor haul road conditions may require units to operate at less than rated load. It will usually prove eco­nomical to improve the condition of such haul roads.
Keep dump bodies clean and in good condition in order to facilitate dumping.

44. Operate haul units at the highest legal safe speed and maintain the desired interval between units. Sluggish units should be replaced by standby units, if standby units are unavailable, load the sluggish unit lightly so that it can maintain the required speed. Do not allow speeding. Speeding is not only unsafe, but it also results in excessive equipment wear and upsets the uniform spacing of haul units.
Provide separated haul and return lanes whenever possible. This decreases the chance of accidents and allows higher safe vehicle speeds.

45. Develop an efficient traffic pattern for operation of haul units in loading and dumping areas. Minimize backing and interference between units.
Have haul units assist in spreading of fill by spread dumping (dumping while moving forward).
Whenever conditions at dump sites are not uniform, alternate haul units between slow and fast dumping sites in order to maintain more uniform intervals between haul units.
Use time studies and job observation to determine the factors that limit produc­tion. Methods may then be devised to improve the situation. Use actual job data to refine estimates based on average or theoretical data.