MACHINE POWER REQUIREMENT

Introduction• To match power units to the size and type of

machines so all field operations can be carried out on time with a minimum of cost

• If tractor is oversized for implements, the costs will be excessive for the work done

• If the implements selected are too large for the tractor, the quality or quantity of work may be lessened or the tractor will be overloaded usually causing expensive breakdowns

Factors to consider when selecting a power unit

1. Engine type2. Power ratings3. Soil resistance to machines4. Tractor size5. Matching implements6. Sizing for critical work

Engine type• The combustion process in the cylinders

converts the energy contained in fuel to a rotating power source

• This rotating power source can be further converted into 3 forms:– Drawbar Pull– PTO output– Hydraulic System Output

Power Ratings

• Power is a measure of the rate at which work is being done

• The English power unit is defined as 550 foot-pounds of work per second

• The metric power unit is measured in kilowatts (kW)

1 kW = 1.34 horsepower

If a load require a force of 20 pounds to move it vertically a distance of 3 feet, the amount of work done is :

Work = Force x Distance= 20 lbs x 3 feet= 60 ft. lb

The amount of work done is 60 ft.lb with no reference to time.

If a 1000 lb force is moved 33 feet in one minute, the rate of doing work is one horsepower, because one horsepower equals 33,000 ft. lb. per minute

The equivalent rate of work in one second to equal 1 horsepower is :

1 HP = 33,000 ft.lb = 550 ft. lb per second60 seconds

When working with field machinery, we usually think of miles per hour and pounds of draft. For these conditions the formula for horsepower is :

HP = Force,lb x Speed, mph375

Metric Equivalent

Metric unit for power is kilowatt (kW)

Force is measured in newtons or kilonewtons

1 HP = 0.746 kW1 kW = 1.34 HP1 N = 0.225 lb1 kN = 224.8 lb force

Formula for kilowatt is:

KW = Force (kilonewtons) x Speed (km/hr)3.6

Eg: If the draft of a trailing implement, such as a disk harrow, is measured at 11.1 kilonewtons and is pulled at a speed of 8 km/hr, what is the drawbar kilowatt ?

Drawbar, kW = 11.1 kN x 8 km/hr =24.7 kW3.6

The formula can also be used to determine speed.

Eg: A tractor is pulling a plow with a total draft of 22.2 kilonewtons. How fast can the plow be pulled if the tractor has 50 drawbar kilowatts ?

Speed = 50 kW x 3.6 = 8.1 km/hr22.2 kN

To determine draft :

Eg: Given 65 kW tractor, speed 8 km/hr., field cultivator draft is 4 kN per meter of width when used in a given field. What width of cultivator could be pulled ?

Draft = Power,kW x 3.6 = 65 kW x 3.6 = 29.25 kNSpeed, km/hr 8 km/hr

Width, meters = Total Draft = 29.25 = 7.3 metersDraft per meter 4

Eg: If the draft of a trailing implement like a disk harrow is measured at 2,500 pounds and is pulled at a speed of 5 mph, what is the drawbar horsepower ?

Drawbar horsepower = Force, lbs x speed,mph375

= 2500 x 5375

= 33.3

• This formula can be used to determine how fast an implement could be pulled with a given size of tractor

Speed = Drawbar horsepower x 375Draft, lbs

• This formula can also be used to determine how large an implement can be pulled but an extra step or two is involved.

• Size of the implement have to be related to the amount of soil resistance

Determining tractor size needed

• There are various kind of power– Brake– PTO– Drawbar

• Tractor power is measured in horsepower (USA) or in kilowatts (kW)- metric equivalents

Brake Horsepower

is the maximum power the engine can develop without alterations

the engine can be hooked to a dynamometer to determine how much brake horsepower can be developed

useful in sizing stationary engines for operating irrigation pumps, grinders and other large equipments

the same engines used for large tractors are often used as stationary engines

Power Take-Off Horsepower (PTO hp)

• the power measured at the PTO shaft

Drawbar Horsepower *• is the measure of the pulling power of the engine by way of tracks,

wheels or tires at a uniform speed

• drawbar horsepower varies depending widely on several factors -soil surface and type of hitch

• drawbar horsepower is the function of drawbar pull and speed of the various kind of horsepower, maximum PTO hp is the most commonly used in designating the size of a tractor

• large tractors that do not have PTO shaft, they may be given a brake hp (flywheel) or maximum drawbar hp rating

Matching tractors and implements

When matching a tractor and implement, 3 important factors must be considered :-

1. The tractor must not be overloaded or early failure of components will occur

2. The implement must be pulled at the proper speed or optimum performance cannot be obtained

3. The soil conditions and their effects on machine performance must be considered

With a given tractor, there is a set of amount of power available. This available power is used for:-I. moving the tractor over the groundII. pulling the implement over the groundIII. powering the implement for useful work

The softer or looser the soil conditions are, the greater amount of power that will be consumed because of greater rolling resistance

This reduces the available usable drawbar powerSOIL CONDITIONS---POWER

Condition Usable Drawbar Power Ratio of MaximumOf Soil As a % of Maximum PTO Power To Usable

PTO Power Drawbar Power_________ __________________ ___________________

Firm 67 Percent 1.5Tilled 56 Percent 1.8Soft or Sandy 48 Percent 2.1___________________________________________________

Eg: A 5-bottom, 40 cm plow operates in medium to heavy soil with surface conditions considered as firm. Given 32.5 drawbar kW per meter of width for gumbo and 27.4 kW per meter for clay. We will use 30 drawbar kW per meter as the average.

• Plow width = 5 x 40 = 200 = 2 meters100

• 2 meters x 30 kW per meter = 60 drawbar kW needed

if tractor size is known, determine how large an implement can be pulled.

Tractor size – 110 PTO kWSpeed - 8 km/hSoil condition – firmDraft – 5.83 kN per meter of widthUsable drawbar kW – 67 % of maximum

PTO kW = .67 x 110 = 73.7 kN

Width = 73.7 kW x 3.6 = 5.7 meters8 x 5.83

Position Control System

Hydraulic control of an attached implement eg. spreader or broadcaster whereby the operator will preselect and position the implement as determined by the position of hand control lever

The position of the hand lever and the hydraulic cylinder are always the same

Pressure is controlled by the relief valve and the hydraulic cylinder will automatically move the implement to its predetermined position and maintain it there

Draftis the horizontal component of pull, parallel to the line of motion

Poweris the rate of doing work

1hp = 550 ft.lb/sec.1 kW = 1.34 hp (metric unit)

Drawbar Power (dbhp)

is the measure of pulling power of the engine by way of tracks, wheels or tires at a uniform speed

1 dbhp = 4500 kg. m / minute

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