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.