1. Basic Principles of Power
Harrell, Chapter 6

2. Goals Terminology Levers and their influence on torque
Relation between torque and power
Method by which an engine develops torque and power
Identify three terms used in ag power and machinery that describe power
Describe the two ways by which agricultural tractors are rated for power and how each can be measured
Interpret engine performance test data in accordance with power and torque

3. Force
Energy that is exerted by one source upon another with a tendency to change the action of the one being acted upon
There is no motion involved in the expression of force
Weight expresses the mass that must be overcome based on the actions of gravity (1 lb = kg)
Force required to hold mass in place against the force of gravity is expressed as a Newton (9.81 kilograms of force)
What is the force required to support 205 kg of weight?

4. Work Involves the action of force to move a body at rest
Involves both force and distance, which can be calculated
Work = distance X force (results in ft lbs of force in standard, or Joules in metric, which is a Newton meter)
If a football weighs 1 lb, how much work must be performed to throw it 25 yards?

5. Torque Term used to describe a rotating or twisting force
Requires the use of a lever
Identifies the amount of twisting or turning effort that is applied by using a lever to achieve rotation
Torque = force X length of lever (ft. lbs. or N*m)
Torque is influenced by the length of the lever
Cable or chain drum must also be considered when using hoists, so weight X drum radius length of crank lever

6. Engine Torque
The internal combustion engine: piston inside a closed cylinder, connecting rod, crankshaft
Combustion and expansion of gases creates force that pushes the piston
Length of the crankshaft throw is half the stroke
Calculate: Lb of Force X length of throw
To convert Lb Ft. to lb inches, multiply by 12

7. Power
The term Power is used to express the rate at which work is being performed (time)
Formula: work/time or Force X distance time
Figure 6-8

8. Horsepower
The term horsepower is a description of the amount of work that an average horse can produce in a specific period of time.
1 hp = 33,000 ft lb/min
So…..
Formula: HP = force X distance time x 33,000

9. Types of Power Drawbar Power Power takeoff (PTO) power Brake Power
Belt power (old tractors)

10. Drawbar Power
Drawbar power refers to the power that a tractor will produce when pulling a load attached to a drawbar.
Example, the size of tractor can be determined for pulling a six bottom plow if you know:
With of cut of each bottom in inches
Speed of travel in mph
Depth of plowing in inches
Classification of soil
See page 107

11. Drawbar Power Plow speed = 4.5 mph or 396 ft/min Plow depth = 8 in
Cut width = 14 in
6 bottoms (plowshares)
14 in/bottom X 6 bottoms X 120 lb pull/1 inch of cut = 10,080 lb
Force X distance time X 33,000 ft lb/min
10, 080 lb X 396 ft 1 min X 33,000 ft lb/min

12. PTO Horsepower
540 rpm
540/1000 rpm

13. Brake Power
Power which an engine or motor delivers when measured at the output shaft
Generally considered the same as PTO in a tractor
Drawbar power is generally 15% less than PTO or brake power

14. Belt Power
Old rating used with old tractors

15. Measuring Power Use a dynamometer
Places a load on a rotating output shaft in a controlled manner
Tachometer – measures the number of revolutions per minute which the crankshaft and friction wheel rotate
PTO dynamometer – commonly used in tractor shops

16. Interpretation of Data
Sales literature discussed by a salesman
Technician must also determine if a piece of equipment is meeting specs
Performance test data is usually represented as test curves on a graph
Temperature and altitude (elevation) will affect performance
Increased ambient temperature decreases performance
Increased elevation decreases performance

17. Summary Measuring and determining power is important
There are different ways to measure power in agricultural equipment
Loss of power indicates a problem in the equipment that should be repaired