Transcript EQUIPMENT COST - Civil Engineering Society @ Legenda

Chapter 6
Machine Equipment
Power Requirements
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
POWER
“Why does a machine only travel
at 10 mph when its top speed is
30 mph?”
This is a critical question because:
• Speed affects cycle time
• Cycle time drives production
• Production determines cost
POWER
Speed affects cycle time
Production
determines
cost
Cycle time drives
production
PAYLOAD
The payload capacity of
construction excavation and
hauling equipment can be
expressed either
Volumetrically
or
gravimetrically.
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
Rolling resistance is caused by
• Internal friction • Tire flexing
• Tire penetrating the surface
ROLLING
RESISTANCE
If tire penetration is known
Rolling resistance (lb) =
(40  [30  TP])  GVW
• TP = tire penetration, inches (may
be different for haul and return)
• GVW = gross vehicle weight, tons
ROLLING
RESISTANCE
If tire penetration is
known
Rolling Resistance (lb/ton) can
be estimated from the
information in
Text Table 6.1
HAUL ROAD
CONDITION
If haul roads are well maintained
rolling resistance is less and
production improves. Good haul
roads require graders and
water trucks,
so there is a
cost.
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 ft
100 ft
5 ft
 100  5%
100 ft
GRADE RESISTANCE
Fig. 6.4 page 146
GRADE RESISTANCE
You need to review the
derivation of equation 6.8.
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 x -4% grade
GA = -960 lb
POWER REQUIRED
A machine must overcome the
forces of rolling and grade
resistance to propel itself.
These can be expressed as:
• lb/ton
• % effective grade
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.
PRACTICAL EXERCISE
This
scraper has
a:
EVW of
96,880 lb
Rated load of 75,000
lb
PE Step 1
Calculate the operating
weight in tons for the
haul.
96,880 lb  75,000 lb
Haul weight = 2,000lb / tn
Haul weight = 85.94 tn
PE Step 2
Calculate the
operating weight in
tons for return.
Return weight =
96,880 lb
2,000lb / tn
Return weight = 37.5 tn
PE Step 3
Calculate the rolling resistance.
•Earth haul road poorly maintained
Table 6.1
Use an average value; 120 lb/tn
Convert to equivalent grade (eq. 6.9)
120 lb / tn
 6%
20 lb / tn
PE Step 4
Calculate the grade resistance.
• Grade from cut to fill is 2%.
Haul grade
(GR) = 2%
Return grade (GA) = -2%
Haul
Return
PE Step 4
Calculate the grade resistance.
Haul grade (GR)
= 2%
Return grade (GA) = -2%
Equation 6.8
Haul GR
= 2%  40 lb/tn
Return GA = -2%  -40 lb/tn
PE Step 5
Calculate Haul
Total Resistance
TRhaul = 6% + 2%  8%
TRhaul = 120 lb/tn + 40
lb/tn
= 160 lb/tn
PE Step 5
Calculate Return
Total Resistance
TRreturn = 6% - 2%  4%
TRreturn = 120 lb/tn - 40
lb/tn
= 80 lb/tn
PE Step 6
Calculate Haul
Total Resistance in lb.
TRhaul = 8%
GVW
TRhaul
= 160 lb/tn x
85.94 tn
= 13,750 lb
PE Step 6
Calculate Return
Total Resistance lb.
TRreturn = 4%
EVW
TRreturn = 80 lb/tn x 37.5
tn
= 3,000 lb
POWER AVAILABLE
Engine horsepower and operating
gear are the primary factors that
determine the power available at
the drive wheels (drawbar) of a
machine.
POWER AVAILABLE
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.”
POWER AVAILABLE
Performance charts are provided
for machines enabling us to
estimate machine speed.
Text page 159-160
The charts relate rimpull
(drawbar pull), GVW, speed
and total resistance (%).
Page 230
Haul
Empty
(EVW)
Loaded
(GVW)
Haul
Haul
Haul
Speed  9 mph
Return
Speed  31 mph
POWER AVAILABLE
What if the total resistance
is negative?
See Text page 163 and 231
Retarding Performance chart
The effective grade numbers
are negative numbers.
POWER USABLE
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.
POWER USABLE
Consider the scraper in
the previous example.
What is the weight on the
drivers during the haul?
POWER USABLE
Total weight is
96,880 lb  75,000 lb = 171,880 lb
Table 8.1 p. 231 Weight distribution
loaded:
Drive axle 53%
171,880 lb  0.53 = 91,096 lb
Also see Fig. 6.8
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
= 0.15
91,096 lb
POWER USABLE
The haul road is a 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 6.4, page 156
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.
See Table 6.5 page 157