CE3220 Constructin Equipment (12).pdf
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Transcript of CE3220 Constructin Equipment (12).pdf
Machine Power
Chapter 10
Objectives
• An ability to calculate vehicle weight
• An ability to determine rolling resistance based on anticipated haul road conditions
• An ability to calculate grade resistance
• An ability to use performance charts to determine machine speed
Equipment Selection
Decision process for matching the best
possible machine to the project task
requires that the estimator take into
account both the:
– properties of the material to be
handled
– mechanical capabilities of the
machine
Material
Considerations
• Total quantity of material
• Size of the individual pieces
Payload
The amount of material that maybe
placed on or in a piece of equipment,
may be expressed:
– Volumetrically
• Struck
• Heaped
– Gravimetrically
Gravimetric Capacity
the machine
are designed
to handle.
Safe operational weight that the axles
or frame of
Payload
– bank cubic yards (bcy)
– loose cubic yards (lcy)
– compacted cubic yard (ccy)
Volume can be expressed in terms of:
Machine Performance
Why does a loaded machine travel
at only 12 mph when its top speed
is 33 mph?
– Required power
– Available power
– Usable power
Required Power
Required power is the power needed
to propel the machine, and two factors
establish this power requirement:
– rolling resistance
– grade resistance
These can be expressed as: lb/ton or
% effective grade
Rolling Resistance
Rolling resistance is a measure
of the force (lb/ton) that must be
overcome to rotate a wheel over
the surface on which it makes
contact.
Rolling Resistance
• Internal gear friction
• Tire flexing
Rolling resistance is caused by:
• Tire penetrating the surface
Rolling Resistance
Rolling Resistance (lb/ton) can
be estimated from the
information in Text Table 10.1
If haul roads are well
maintained rolling resistance is
less and production improves.
Grade Resistance
Grade Resistance
We seldom find a haul road which is level from point of load to point of dump.
Grade Resistance
Grades are measured in % slope:
the ratio between vertical rise (fall)
and horizontal distance in which the
rise/fall occurs.
Rise
Horizontal
Grade Resistance
Grade example: 5 ft fall in
100 ft horizontal travel.
5 ft100 ft
5%100ft 100
ft 5
Grade Resistance
• F = W sin
• N = W cos
• F = 20 lb/ton G%
Grade Resistance
You need to review the
derivation of equation 10.8.
What it tells us is that for small
angles (% grade) :
GR = 20 lb/tn % grade
Grade Resistance
Example: A truck with a 23 tn GVW is
moving up a 4% grade. What is the
force required to overcome grade
resistance?
GR = 20 lb/tn 23 tn 4% grade
GR = 1,840 lb
Grade Assistance
Gravity assists the machine when traveling down grade.
That force is referred to as grade assistance.
Grade Assistance
Example: Our truck has dumped its
load, the GVW is now 12 tn and on
the return it is moving down the 4%
grade. What is the force required to
overcome grade resistance?
GA = 20 lb/tn x 12 tn -4% grade
GA = -960 lb
Haul Routes
• The Constructor must study
the project's mass diagram to
determine the direction that
the material has to be moved.
Haul Routes
• Then the natural ground and
the final profiles depicted on
the plans must be checked to
determine the grades that the
equipment will encounter
during haul and return cycles.
Total Resistance
Total Resistance =
Rolling Resistance +
Grade Resistance
TR = RR + GR or
TR = RR - GA
Practical Exercise
A scraper is operating on an earth
haul road which is poorly
maintained. The grade from cut to
fill is 2%. Calculate the total
resistance in both pounds and
equivalent grade.
Operating wt. Empty 96,880 lb
Rated load 75,000 lb
Practical Exercise
Calculate the operating weight
in tons for the haul.
Haul weight =
Haul weight = 85.94 tn
96,880 lb 75,000 lb
2,000lb / tn
Practical Exercise
Calculate the operating weight
in tons for return.
Return weight =
Return weight = 37.5 tn
96,880 lb
2,000lb / tn
Practical Exercise
Calculate the rolling resistance.
• Earth haul road poorly maintained
Use an average value; 120 lb/tn
Convert to equivalent grade (eq. 10.9)
120 lb / tn
20 lb / tn6%
Table 10.1
Practical Exercise
Calculate the grade resistance.
• Grade from cut to fill is 2%.
Haul grade (GR) = 2%
Return grade (GA) = -2%
Haul
Return
Practical Exercise
Calculate the grade resistance.
Haul grade (GR) = 2%
Return grade (GA) = -2%
Equation 10.8
Haul GR = 2% 40 lb/tn
Return GA = -2% -40 lb/tn
Practical Exercise
Calculate the
Total Resistance (haul)
TRhaul = 6% + 2% 8%
TRhaul = 120 lb/tn + 40 lb/tn
= 160 lb/tn
Practical Exercise
Calculate the
Total Resistance return
TRreturn = 6% - 2% 4%
TRreturn = 120 lb/tn - 40 lb/tn
= 80 lb/tn
Practical Exercise
Calculate theTotal Resistance (haul)
TRhaul = 8%
TRhaul = 160 lb/tn x 85.94 tn
= 13,750 lb
GVW
Practical Exercise
Calculate the
Total Resistance return
TRreturn = 4%
TRreturn = 80 lb/tn x 37.5 tn
= 3,000 lb
EVW
Available Power
• There are two factors that
determine available power:
– Horsepower (machine specific
constant)
– Speed
• Available power will change as
machine speed is varied.
Available Power
Engine horsepower and operating
gear (speed) determine the power
available at the drive wheels of a
machine.
Available Power
Horsepower involves a rate of doing
work.
One hp = 33,000 ft-lb per minute
Therefore, must consider speed at
which the machine travels when
exerting a given amount of “pull.”
Rimpull
• Rimpull is a term that is used
to designate the tractive force
between the rubber tires of
driving wheels and the surface
on which they travel.
Drawbar Pull
• The available pull that a crawler
tractor can exert on a load that is
being towed is referred to as the
drawbar pullof the tractor
Available Power
Performance charts are provided for
machines enabling us to estimate
machine speed.
Text Fig. 10.9
The charts relate rimpull
(drawbar pull), GVW, speed and
total resistance (%).
HaulEmpty
Loaded
Haul
Haul
Haul
Speed 31 mph
Return
Available Power
What if the total resistance is
negative?
See Text Fig. 10.10
Retarding Performance chart
The effective grade numbers are
negative numbers.
Coefficient of Traction
The coefficient of traction is the
ratio between the maximum
amount of pull a machine exerts
before slippage and the total
weight on the drivers.
Usable force = coefficient of traction
weight on powered running gear
Power Usable
Consider the scraper in the
previous example.
What is the weight on the
drivers during the haul?
Power Usable
Total weight is
Weight distribution loaded:
Drive axle 53%
96,880 lb 75,000 lb = 171,880 lb
171,880 lb 0.53 = 91,096 lb
Power Usable
Considering the rimpull necessary
for the haul what is the minimum
coefficient of traction allowable?
Rimpull required 13,750 lb
13,750 lb
91,096 lb= 0.15
Power Usable
The haul road is:
wet clay loam.
Will coefficient of traction be sufficient?
Power Usable
The haul road is a wet clay loam.
Will coefficient of traction be
sufficient?
Table 10.4
Wet, clay loam - rubber tires
Coefficient of traction 0.40-0.50
Should be ok, 0.40 0.15
Altitude Limits Power
If equipment works at higher
altitudes, where the air is less dense,
the engine may produce at a reduced
power output.
Altitude Limits Power
Most machines with turbocharged
engines will operate at altitudes
above 2,500 before experiencing a
loss of power.