1. Engineering Fundamentals
   • Example

Engineering Fundamentals

We will consider concepts and equipment primarily involved in handling and placing bulk material, namely earth.

Broad factors or characteristics to be considered:

~ equipment to be used

~ material to be worked

~ job to be accomplished

Equipment power units:

~ diesel (most common)

~ gas

~ gas turbine

Diesel engine characteristics:

~ less servicing, long life, less fuel used, fuel costs

~ less, lower fire hazard, less insurance premium, etc.

Equipment power unit mounting:

~ stationary

• permanently installed

· batch plant (pg. 572, textbook)

· quarry crusher (pg. 530, textbook)

· electric power generator

• transportable to new sites

· tower crane (pg. 173, textbook)

· air compressor (pg. 332, textbook)

~ mobile

• self-propelled

· separate propelling & performing engines

· integrated - same engine propels & performs

· crawler mounted (pg. 114, class notes)

· truck mounted (pg. 114, class notes)

~ towed

~ prime movers

• tractors - track, wheel (pg. 97, 359, 361, textbook)

• graders

• trucks (pg. 238, textbook)

• excavators - back hoes & front shovels (pg. 206, 213, 214, textbook)

Equipment running gear:

~ Tracks (pg. 97 & 359, textbook)

• traction maximized

• flotation - low ground pressure

• low speed & low mobility

~ Wheels (pg. 97, 189 textbook)

• rubber tires

· pneumatic

· solid

• ground pressure

· tire size

· inflation

• good speed & mobility

• steel tires

Rolling resistance considerations:

~ varies with road surface type & condition.

~ rubber tire’s resistance also varies w/size, pressure & tread.

~soft earth increases a vehicle’s rolling resistance due to earth depressing.

~tire or track characteristics are best decided after road conditions are known.

~ haul road conditions change:

• too dry, too wet, need maintenance & drainage

Rolling resistance for representative haul road conditions and different wheels or tracks are available (Table 4-2), pg. 73

Rolling resistance calculations (flat surface) (pg. 75, textbook):

~ R = rolling resistance (lb/ton) - Table 4-2, pg. 73 of your textbook

~ P = tension in tow cable (lb) - dynamometer test for total rolling resistance

~ W = gross weight of vehicle (tons)

~ R = P/W (for level haul road) - lbs. tractive pull per ton vehicle wt. moved

If other than for a level haul road, R must be corrected for the road’s grade.

Grade resistance considerations:

~ varies with slope or grade of road

~ for down hill, grade resistance subtracts from rolling resistance (RR - GR)

~ for up hill, grade resistance adds to rolling resistance (RR + GR)

~ rule of thumb: (accurate up to 10% slope)

• grade resistance force = 20 lb/ton/% grade (20 lbs. per gross ton of wt. for each 1% grade)

~ see Table 4-3 (pg. 75 of your textbook)

~ weight in tons is for the tractor plus any towed load

Total resistance (TR) (note: we will neglect wind resistance & acceleration):

~ TR = RR + GR (for up hill)

~ TR = RR - GR (for down hill)

Grade resistance derivation: (also in textbook, pg 75)

Component of force to overcome to push [W] up the slope is W × sine "A".

For small "A" (A < 10°) sine A _ tangent A which = "A" expressed in radians.

For 1% slope, tan A =1/100

F/W = sin A, F = W sin A = W tan A ;

sin A = tan A

F = W × tan A = 2,000 lb/ton × 1/100

Traction is the ability of drive wheels or tracks to transmit rimpull force to haul road surface.

Rubber tires on dry rough concrete have excellent traction, on wet sand have fair traction, and on ice have very poor traction.

A measure of this relative traction or slippage for varying surfaces is the coefficient of traction

Even though engine power is available, it will be nullified if sufficient traction cannot be developed.

Example in text: Rubber-tired tractor (see pg 78)

~ wt on drivers = 18,000 lb

~ max. rimpull = 9,000 lb (low gear)

~ on wet sand C_t = 0.30

•T_max = W × C_t = 18,000 × 0.30 = 5,400 lb

•here we can only use up to 5,400 lb of the 9,000 lb of avilable rimpull before experiencing loss of traction by slippage.

~ if C_t were 0.60 for a dry clay loam haul road:

•T_max = W × C_t = 18,000 × 0.60 = 10,800 lb

•here the total available rimpull in low gear would be useful if needed.

Drawbar pull (engine pulling effort)

~ crawler tractor’s pull in pounds at its drawbar (pg. 77, textbook)

~ drawbar pull is engine’s pulling effort minus RR & GR

~ drawbar pull varies with gear speed

(see table, pgs. 73, 77, textbook)

• highest in first gear

• lowest in high gear

Rimpull

~rubber tired vehicle’s tractive force (driving wheels on road surface) (pg. 85, textbook)

~ w/o slippage, rimpull depends on engine power & gear ratio

~ w/ slippage, effective rimpull is wt. on driver’s times C_t

~ available rimpull formula: (see table, pg. 85, textbook)

~load pulling capability is rimpull minus RR & GR overcoming forces (see example, pg. 85, textbook)

Temperature & altitude have effects on engine horsepower.

Engines are rated at standard conditions:

~ 60°F; 29.92 inches hg (sea level pressure)

~ other temperatures & altitudes may be used

Engine hp decreases as temperature increases. HP T

Engine hp decreases as altitude increases. HP Alt.

(See Tables 4-5 & 4-6, pgs. 82 & 83 in textbook)

where: HP_avail =horsepower available

P_actual =altitude at which the machine will be operated in inches Hg (inches of mercury) barometric pressure (see Table 4-6)

P_std =standard condition altitude, usually sea level, 29.9 in. Hg

T_actual =Rankine temperature at which the machine will be operated

T_std =standard condition temperature in Rankine units, usually 60°F, which equals 520°R

Factors determining actual fly wheel hp output

~ standard engine rated hp supplied

~ altitude correction

~ temperature correction

~ accessory load: fans, generators, compressors, A/C, etc.

~ load duration - without losing speed (normal accessories)

• peak hp up to 5 minutes (W/1 5% engine hp)

• continuous hp 8-24 hrs/day (_ 80% engine hp)

• intermittent hp for < 1 hr (_ 90% engine hp)

From Table 4-6 observe rule of thumb (pg. 81, textbook)

~ 3% engine hp output/1000 ft altitude (loss due to altitude)

~ 1% engine hp output/10°F above 60°F (loss due to temperature)

~ 1% engine hp output/10°F below 60°F (loss due to temperature)

Example

Example: A gasoline engine was tested under the given conditions and was found to develop the indicated horsepower. It is desired to convert the results to bhp (brake horsepower) for standard conditions.

Observed hp, 86.43

Observed pressure, 29.52 in. Hg

Observed temperature, 42° F

Solution:

Example: Rubber tired tractor scraper hauling fill

• GVW loaded = 250,000 lb

• load weight = 125,000 lb

• wt. distribution: 50% tractor, 50% scraper

~ Three haul road segments

~ Rubber tires mean work with rimpull. Assume sufficient hp.

Haul, 250,000 lbs:

• TEG% = RR(lb/ton)/20 + GR%

• useable rimpull = W_drivers × C_t = .5 × 250,000 × C_t

• required rimpull = W_tons × 20 lb/ton/% × TEG%

Return, 125,000 lbs

• useable rimpull = W_drivers × C_t = 62,500 C_t

• required rimpull = W_tons × 20 × TEG

~For NG results we have a problem. We could use pusher, pave road, reduce the grade, change to track type prime mover, etc.

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