AGRICULTURAL

TRACTOR
by

Engr. Alexis T. Belonio

Department of Agricultural Engineering and
Environmental Management
College of Agriculture
Central Philippine University
Iloilo City
Atbelonio@yahoo.com
Definition:
A machine used primarily for traction or
pulling an implement.
A versatile machine and a mobile power
source for wide variety of agricultural task.
A machine that can deliver both stationary
and mobile jobs.
A machine that can pull or tow implement
through traction from the drive wheel and or
pull implement through the draft developed
from its drawbar hitch or power implement
through it PTO drive.
Functions of
Tractors
Pulling machines behind
it
Pushing machine in front
of it
Operating machine
mounted on it
Transmitting power to
other machines through
the PTO drive
Raising, lowering, and
controlling machines
and attachment by its
hydraulic power
Driving machines
through belt power
Transmitting power
through flexible shaft
Furnishing power for
fast highway
transportation and
handling
Advantages of Tractors
Can work continuously at heavy load
Not affected by bad weather
Adapted to draft, belt, and PTO work
Has considerable range of working speed
Little attendance required when not in use
Quickly available when needed in an
emergency
Requires small storage space
 Disadvantages of
Tractors
Limited overload capacity
Requires cash expenditures for fuel and
lubricants
Requires some mechanical skill for
successful operation
Inflexibility of size of power unit for
economical power production under
some condition.
 Classifications of
Tractors
Use
z Farm or Agricultural
z Forestry
z Earth working
z Industrial
z Military
Farm Tractor
z Standard
z General Purpose, All
purpose, Utility, Row Crop
z Special
z Orchard, vineyard, Lawn
Running Gear
z Wheel (Two wheel,Tricycle,
Four Wheel)
z Crawler, Track Layer
z Semi Crawler, Half Track)
Type
z Walking, Garden
z Riding
z Tool Carrier
z Tandem
z Two Way
z Compact, mini
z Large, Extra Large, super
large
z Frameless, Unit construction
z Frame, chassis construction
Drive
z Rear, wheel drive
z Four wheel drive (unequal
wheel, four even large
wheel)
Steering
z Front wheel
z Rear wheel
z Articulated frame
z Four wheel
z Crab
z Front axle
z Skid Steering
Engine
z Diesel
z Gasoline
 Brief Description of
Agricultural Tractors
Compact Tractor – A four-wheel tractor of
less than 20 hp. They are used for
landscaping and gardening rather than
massive agricultural task. They are
equipped with single 12-in moldboard plow
and capable to plow 1 hectare for 11 to 14
hours.
Farm Tractor – A four-wheel tractor with no
less than 20 brake horsepower. They are
used for massive agricultural tasks such as
plowing, harrowing, seeding and planting,
and other operations.
Special Type Tractors
z Tract-type Tractors – They consist mainly of two heavy
duty endless link device know as tract for traction
purposes. They are used for heavy duty operation such
as earth moving and other industrial jobs requiring
higher power.
z All Purpose or Row Crop Tractors – They are
designed to handle field and belt jobs including the
planting and inter-tillage activities.
z Orchard Tractor – they are small and medium size.
They are designed to be operated with advantages
under tress. They are built with lower lower and few
projecting parts and has fenders for protection purposes
z Industrial Tractors – The are machines of any size
designed for various industrial operations and heavy
hauling. They have hoisting, excavating, power-loading
attachments.
Construction and main
system of Tractor
Engine
Cylinder, cylinder
head
Piston, piston ring,
piston pin
Connecting rod
Crank shaft, crank
gear, timing gear, fly
wheel
Valve system
Electrical system
Cooling system
 DRIVER
SEAT
HOOD
STIRRING
WHEEL
ROLL ON ENGINE
PROTECTION
DEVICE

REAR CHASSIS
WHEEL
FRONT
WHEEL
 LIGHT FUEL THROTTLE
CONTROL
LEVER

DIFFERENTIAL
BREAK
CLUTCH

HYDRAULIC CONTROL
GEAR CONTROL LEVER
LEVER
 THREE POINT
HITCH SYSTEM

POWER
REAR TAKE OFF
SINGLE HITCH
WHEEL DRIVE
SYSTEM
 Power Train of Wheel Drive
Tractor
0.87 – 0.98

PTO Shaft

Engine Clutch Transmission Differential 0.75 -0.81

1.00 0.96-0.98
Final Drive

Wheel Axle
0.82 - 0.87
Turbo Charger of Tractor Engine – This
increases the output of the tractor engine
by about 20%.
Power Transmission – It is used to transmit
the engine power to the wheel axle and PTO
shaft of the tractor. It is also used to change
the torque and speed of the engine tractor
into high torque and low speed that is
required by the wheel axle and the PTO shaft.
Clutch – It is a device used to connect and
disconnect the tractor engine from the power
train so that the gear can be shifted or the
tractor can be stop without stopping the
engine.
Differential – It is a special arrangement of
gears so that it will permit one driving
member to rotate slower or faster than the
other and permit operation of driving wheels
at different speed.
Final Drive – Is a gear reduction located in
the power train between the differential and
the driving wheels used to transmit power of
the engine at low speed and large torque to
the rear axle and wheels of the tractor.
 Cab Muffler

Engine
Final Drive

Transmission Box
Differential
Front Wheel Ballast
Rear Wheel
 Tractor Engine Indicated
and Brake Horsepower
Formula
IHP = [P L A N n ] / [ 33,000 x 2 ]
BHP = IHP – FHP
= IHP x ξe

Where:
IHP - indicated horsepower, hp P - mean effective pressure, psi
BHP - brake horsepower, hp L - piston stroke, ft
FHP - friction horsepower, hp A - area of the piston, in2
ξe - engine efficiency, decimal N - crankshaft speed, rpm
n - number of cylinders
Tractor Wheel and PTO
Power Formula
Pw = ξm x Pe
Ppto = ξm x Pe

Where:
Pe - engine power, kw
Pw - wheel axle power, kw
Ppto - PTO power, kw
ξm - mechanical efficiency, 0.75 to 79
 Example
What is the wheel axle power of a four
wheel tractor whose engine power is equal
to 150 hp? The mechanical efficiency is
85%.
Given: Engine power – 150 hp
Mechanical efficiency – 85%
Required: Wheel axle power
Solution:
Pw = ξm x Pe
= 0.85 x 150 hp
= 127.5 hp
 Wheel Axle Torque
Formula
T = 1000 Pw / 2 π n
= F Wr

Where: T - wheel axle torque, N-m

Pw - wheel axle power, kW
n - speed of wheel axle, rpm
F - wheel force, kg
Rw - radius of wheel, m
 Example
The computed wheel axle power of the tractor is 127.5 hp. What
is the wheel axle torque of the tractor if its wheel rotates at 40
rpm?

Given: Pw- 127.5 hp, n- 40 rpm

Required: wheel axle torque

Solution:
T = 1000 N / 2 π n
= [1000 (127.5 hp x 0.746 kW/hp)]/ [2 π (40 rpm)]
= 378.45 N-m
 Example
If the tractor above has a wheel diameter of 2 meters, what is
the force available at the wheel of the tractor?
Given: wheel diameter – 2 m, wheel torque – 278.45 N-m

Required: force at the wheel

Solution:
T = F Wr
F = T / Wr
= 378.45 / 1m
= 378.4 N
 Reduction Ratio of
Transmission of Wheel
Tractor

Gear Position Reduction Ratio

1st 4.48
2nd 2.49
3rd 1.90
4th 1.45
Tractor Speed Formula
Vt = 0.033 x R x Ne / i
Where:
Vt - tractor speed, km/hr
R - diameter of wheel, m
Ne - engine speed, rpm
i - reduction ratio
 Example
Compute the traveling speed of a tractor at 2nd gear with engine speed set
at 2400 rpm. The diameter of wheel of the tractor is 2 meters.

Given: Gear position - 2nd , Engine speed - 2400

rpm, Wheel diameter- 2 m
Required: Tractor speed

Solution:
Vt = 0.033 x 2400 rpm x 2 m / 2.49
= 63.6 kph
Inflation Pressure of
Tractor Tire
Tire Inflation Inflation
Pressure Pressure
KPa Kgf/cm2
Front Tire 2 150 1.5
ply
Front Tire 4 150-200 1.5-2.0
ply
Rear Tire 80-130 0.8-1.3
 Tractor Wheel Slip and
Tractive Efficiency Formula
% Slip = 100 [ (R – r ) / R ]
ξt = Pd / Pw
Where:
R - total drive wheel revolution count to traverse the
drawbar runway under load
r - Total drive wheel revolution count to traverse the
drawbar test runway under no load
Pd - drawbar power, kW
Pw - wheel axle power, kW
 Example
Wheel slip test of a tractor had shown that a
wheel rotates by two and a half revolutions
at a distance of 20 meters. If the tractor
wheel has a diameter of 2 meters, what is the
percentage slip pf the tractor wheel?
Given: Wheel diameter – 2 meters, wheel rotation – 2-1/2 rev,
distance travel – 20 meters
Required: Percentage wheel slip
Solution:
Dl = π D Nr = π x 2 m x 2.5 rev = 15.7 m
Dn = 20 m
S = [( 20 m – 15.7 m) / 15.7 m] x 100
= 27.89%
 Minimum Slip for
Maximum Tractive
Efficiency
Condition Slip Range (%)
Concrete 4–8
Firm soil 8 – 10
Tilled soil 11 – 13
Soft Soil and Sand 14 – 116
Power Take-
Off Shaft
PTO is used for
driving field machines
such as rotary plow,
hay baler, corn
picker, self-unloading
forage wagon,
irrigation pump, etc.
 PTO Standards
PTO Shaft Speed Spline Shaft Number of
(Rpm) Diameter (mm) Splines
540 35 6

1000 35 21
1000 45 20

As defined by: ASAE, SAE, DIN, BS, ISO, JIS

 Tractor Standard
Specification
Tractor Model
Transmission
Engine
z Bore and stroke
z No. of cylinders
z Displacement
z Speed Rpm
z Compression Ratio
Chassis
z Type
z Tire size (front and rear
z Weight Tested
PTO Performance
z Max power @ rated engine rpm (engine rpm; fuel
consumption, hp-hr/gal
z Max power @ standard PTO rpm (engine rpm,
fuel consumption, hp-hr/gal)
Drawbar Performance
z Max HP (short term) @ mph
z 100% load (2 hrs) Hp @ mph (fuel consumption,
hp-hr/gal
z 75% load (10 hrs) Hp @ mph (fuel consumption,
HP-hr/gal
z 50% load (2 hrs) Hp @ mph (fuel consumption,
hp-hr/gal)
z Max pull (lbs @ mph)
z % slip of drivers
z Lugging ability - % increase of pull at 80% rate
rpm
Sound level (at operator’s ear)
z 100 % load
z 75 % load
z 50 % load
z 50 % load at rated rpm
z By stander
Other specifications
z Hydraulic system
z Brakes
z Steering system
z Special equipment
z Accessories
z Optional equipment
z Overall weight and dimension
 Tractor Size
Tractor size can be determine based on the
following:
z Weight – The heavier the tractor the bigger is its
capacity.
z Horsepower – Higher tractor horsepower, the
bigger is its size and can be capable to do wider
operation.
z Implement Capacity – The more the tractor can
draw implement, the bigger is its size and the
faster it can finish the operation.
 Drawbar Pull

It is one of the most important factor in the

performance of the tractor.
Plowing and other operations are affected
by the drawbar performance of the tractor.
Drawbar pull is affected by the wheel slip
and the engine power. It is also affected
by the tractor weight and type, the type
and shape of the running gears or wheel
condition of the ground surface, inflation
pressure of tires and others.
 Running Resistance
It is the sum of all the resistance acting
on the tractor.
This includes the following:
z Rolling resistance from the soil surface
z Air resistance

z Acceleration resistance

z Slope resistance
 Rolling Resistance
Formula
R = Cr x W

Where: R - rolling resistance, kgf

Cr - coefficient of rolling resistance
W - weight of the tractor, kgf
 Example
A four-wheel tractor weighs 2.5 tons
and running on a deep mud soil
during rotary tilling operation. What
is the rolling resistance of the tractor
wheel?
Given: Tractor weight – 2.5 tons
Coefficient of rolling resistance – 0.25
Required: Rolling resistance of tractor wheel
Solution:
R = Cr x W
= 0.25 x 2,500 kg
= 0.625 kg
 Coefficient of Rolling
Resistance
Condition of Ground Surface Wheel Type Crawler Type

Asphalt Road 0.04 0.05

Dry Hard Ground 0.07 0.07

Hard Grassland 0.10 0.07

Mown Meadow 0.01 0.08

Soft Sandy Road 0.12 0.10

Field Just After Cultivation 0.20-0.30 0.10-0.12

Deep Mud 0.02-0.30 0.10-0.12

Fine Sand 0.30-0.40 0.10
Drawbar Power Formula
Pd = Pw – Pl
= Pw – (Ps + Pr)

Where: Pd - drawbar power, kW

Pw - wheel axle power, kW
Pl - power loss, kW
Ps - power lost due to slip, kW
Pr - power lost due to rolling resistance, kW
 Example
If the tractor above runs at a speed of 15 kph, what is the power loss and
the drawbar power that can be delivered by the tractor?
Given: Pw- 127.5 hp, Slip – 27.39%,
Required:
R –Power lossVand
625 kgf, – 15Drawbar
kph power
Solution:
Pl = Ps + Pr
= 127.5 hp (0.2739) + [625 kg ( 15 kph)
(1000 m/km)] / [( 76.2 kg-m/sec-hp) (3600 sec/hr)
= 69.09 hp
Pd = Pw – Pl = 127.5 hp – 69.09 hp
= 58.4 hp
 References
ASAE. ASAE Standards 1997. Standards
Engineering Practices Data. ASAE the
Society for Engineering in Agricultural, Food,
and Biological Systems. 2959 Niles Road, St.
Joseph, MI 49085-9659 USA. Pp. 407-428.
Tanaka, T. Farm Tractor.. Agricultural
Machinery management Course Handout
Sheet. Japanese International Cooperation
Agency. 45pp.
Thank you very much
and God bless!!!