1. Tractors
   • Examples

Tractors

Primary purpose is as a prime mover, to pull or to push or to mount a major construction accessory.

"On-highway" tractors are known as trucks

~ single unit:cab, engine & load container (pg, 97, textbook)

~ combination unit:cab & engine but detachable load container (pg 77, textbook)

"Off-highway" tractors

~ crawler - powerful & slow with excellent traction (pg 77)

~ wheel - higher speed _ 30 mph, less powerful and greater slippage

Many specialized uses

~ w/blade - bulldozer (pg 97)

~ w/plow - snow removal

~ w/ripper - tear up pavement or rock (pg 133)

~ w/boom - lay pipe in trench (pg 214)

~ w/bucket wheel - trencher (pgs 215, 217)

Crawler tractors rated by

~ weight - controls maximum tractive force

~ power - controls available rimpull

~ size - relates primarily to weight

Tractor drives (crawler or wheel)

~ direct drive (manual)

~ torque converter drives with power shift

Horsepower drawbar pull

~rated - continuous operation

~ maximum - short period peak effort

Wheel tractor drawbar pull is the smaller of:

The product of speed × rimpull is approximately constant as a tractor is shifted through gears at rated power.

Wheel tractor develops pounds rimpull.

Gradability is the maximum % slope up which a tractor may move at a uniform speed.

Gradability varies with:

Review Example 4-2, pg. 76, textbook, Effective grade for wheel-type hauling units

(Nebraska test 110 lb/ton for crawler, 85% safety factor applied to drawbar pull) (pg. 84)

• In the performance of crawler tractors, based on the Nebraska tests to determine the maximum drawbar pull at each of the available speeds, the haul road is calculated to have a rolling resistance of 110 lb per ton.

Examples

Example:Determine the gradability of a crawler tractor pulling a high-pressure rubber tired self-loading scraper and its load. The following information is given:

Tractor horsepower: 180

Weight of tractor: 40,500 lb or 20.25 tons

Drawbar pull in 1st gear: 33,714 lb

Maximum available drawbar pull:

Weight of loaded scraper: 78,960 lb or 39.48 tons

Haul road condition: rutted, uneven earth

Rolling resistance for tractor: 160 lb/ton

Excess rolling resistance for tractor:

Rolling resistance for scraper: 210 lb/ton

Solution:

Gradability:

Rolling resistance of tractor =

Rolling resistance of scraper =

Combined rolling resistance =

Drawbar pull available to overcome grade:

Max. available drawbar pull =

Required for rolling resistance =

Pull available for grade =

Combined weight of tractor and loaded scraper:

Tractor =

Scraper =

Total =

Pull required per ton per 1% grade =

Pull required per 1% grade of the total load:

Max. possible grade =

For the tractor alone, the max. possible grade will be:

Max. available drawbar pull =

Pull required for rolling resistance =

Pull available for grade =

Pull required per 1% grade =

Max. possible grade =

=

Operations within typical basic cycle (pg 213, textbook)

~ fixed elements:

~ variable elements (pg 97):

•haul time (may include loading)

•return time (may include dumping)

•wait time

Pull varies with speed and the gear employed:

~ crawler -pounds drawbar pull (see Example 4-6, pg. 84, textbook)

~ wheel -pounds rimpull, wheel on road (see example, pg. 85, textbook)

~consult performance data tables or curves

~ select highest gear providing the required pull

Pull characteristics:

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