- - AGRICULTURE CORE CURRICULUM - - (CLF2000) Advanced Core Cluster: AGRICULTURE MECHANICS (CLF2900) Unit Title: WORK AND POWER ____________________________________________________________________________ (CLF2912) Topic: USES OF WORK AND Time Year(s) POWER 3 Hours 1 / 2 / 3 / 4 ____________________________________________________________________________ Topic Objectives: Upon completion of this lesson the student will be able to: Learning Outcome #: (S-2) - Demonstrate knowledge of energy, force, pressure, work, and power, using applications of levers, gears, pulleys, and shafts. (T-3) - Demonstrate knowledge of the transmission of power, using chains, belts, gears, and shafts. Special Materials and Equipment: six simple machines: 1. lever 2. inclined plane 3. wedge 4. screw 5. pulley 6. wheel and axle References: Jacobs, C. O., & Harrell, W. R. (1983). AGRICULTURAL POWER AND MACHINERY. New York: McGraw-Hill. Smith, H. P., & Lambert, H. W. (1976). FARM MACHINERY AND EQUIPMENT. (6th ed.) New York: McGraw-Hill. Evaluation: Quiz by instructor and evaluation in discussion and lab work. TOPIC PRESENTATION: USES OF WORK AND POWER A. Simple machines are used to compound force and increase the efficiency of work. There are six types of simple machines: 1. Levers are a simple and useful device to multiply force, extend a machine's ability to reach or extend, and also to reverse the direction of applied force. The formula for calculation of leverage problems is: Force X Force Arm Length = Weight X Weight Arm Length a. Levers are divided into three classes: 1) Class I - The pivot or fulcrum falls between the point where the force is applied and the weight or resistance to be moved. Force Weight | __ Example: Pry Bar _\|/__________________|__|_ /^\ Fulcrum 2) Class II - The weight falls between the fulcrum and the applied force. Weight __ Example: Wheelbarrow ______________|__|________ /|\ /^\ | Fulcrum Force 3) Class III - The force falls between the weight and the fulcrum. Weight __ _|__|______________________ Example: Tractor /|\ /^\ (front loader) | Fulcrum Force 2. Inclined Plane - The slope of the incline multiplies the force as a ratio of the height lifted compared to the distance moved up the slope. Inclined planes are used for ramps, slides, and adjusting mechanisms. The formula for computation of inclined plane problems is: Force X Distance Moved = Weight X Distance Lifted | | | | | | <--- height lifted | | |....................| |<---dist. moved----> 3. Wedge - The wedge allows applied force to be split into two equal force vectors which are directed at 90 degrees to the slope of the wedge. The total force is equal to the addition of the two slope vectors. | | | | | | |<-------Force Applied | | | | | 4. Screw - The screw is also an inclined plane, but one that wraps around a rod. One complete turn of the screw moves the head of the screw just the short distance between the two threads. This distance is called pitch. The formula for the computation of screw problems is the same as the formula for inclined plane problems. 5. Pulley - A pulley is a continuous Class I lever. It can rotate to allow the leverage to continue to be applied without having to be reversed. A combination of pulleys is called a block and tackle. It can lift heavy weights in direct proportion to the number of supporting ropes used. Pulleys are also used to reverse the direction of the applied force. If the number of supporting ropes is three, the calculation of pulley problems is accomplished by the following formula: Force X 3 Ropes = Weight X Distance Weight Moves 6. Wheel and Axle - When two pulleys or circles of vastly different diameters are joined together to form a wheel and axle, the result is a multiplied force without cumbersome ropes or cables (as in a block and tackle). An example is the classic water lifting device used in a shallow well, the hand crank and bucket lift. The formula used to calculate problems is as follows: Force X Radius of Wheel = Weight X Radius of Axle B. Power transmission involves many different types of systems, some simple and others complex. In order to provide an efficient method of the transmission of power from source to use, many new materials have been developed and used. Although gears, pulleys, belts, and shafts are still used, pneumatic and hydraulic systems have simplified and streamlined the process of power transmission. Improved bearings and lubricants have also reduced friction and increased efficiency in power transmission systems. C. Power Transmission Systems 1. Shaft - The shaft supported by some type of bearing is used to convey power in direct drive systems where the power source is in line with the point of use. Sometimes disconnect couplings, universal joints, or torque absorbing units are used in conjunction with the power shaft. a. Universal Joint - This drive unit allows the shaft to be slightly out of line with the end use and still convey power without binding up or jamming. Tractor P.T.O. shafts make use of two universal joints to align and also to allow for turns with pull type equipment. Universal joints should receive good lubrication, maintenance of clean sliding splines, and prevention of damage from bending or impact. b. Torque Coupling - Several types of torque couplings are available which absorb shock loads and provide a point for disassembly of the shaft. Usually they are comprised of a metal adaptor which fits on one shaft, a rubber absorbing ring in the middle, and a second metal adaptor to mate with the ring and first adaptor. They are available in different sizes for large and small horsepower applications and are a low maintenance item. c. Chain Coupler - This consists of two chain sprockets and shaft collars which are joined by a short length of roller chain with a master link for easy disconnection. This low maintenance shaft coupling does not absorb any torque but is very strong and it will accept a slight amount of misalignment on a direct drive power transmission system. 2. Chain and Sprocket - Whenever the power must be a 100% positive drive system and the point of use is in a parallel axis to the power source, a chain and sprocket system can be used. These systems can use either lubricated or non-lubricated chain and they have both slow and high speed applications. The two common types of chain used are hook link and roller. The sprockets are designed to mate with the different types of chain. a. Hook link chain is a slow speed chain using a separating link-type chain made of malleable iron or pressed steel. They are low cost and can be run dry and in very dirty conditions. Common examples are planters and manure spreader paddle chains for the spreader floor. Hook link chain is normally operated with the hook part of the chain leading in the direction of movement of the chain. This type of chain can be hammered together or hammered apart at any link by using a backing block. b. Roller chain is high quality, high speed chain which can be operated in oil or semi-dry. Some oil is necessary to lubricate the interior parts of the chain. Roller chain is made of high quality, precision ground steel and consists of alternating roller links and interconnecting pin links. The roller has a bushing pressed into the roller link plate so the movement is on the roller, the bushing, and the pin which is also allowed to rotate freely. 1) Chain size is designated by the measured distance between centers of the links and is called pitch. Double pitch chain is used on light duty applications and at slower speeds while multiple-strand chain is designed to carry increased power loads. 2) Sprocket size must be the same pitch as the chain but the number of teeth, type of hub, and hub bore will vary with the application. 3) Common sizes of chain are 35, 40, 50, 60, and 80. These correspond to 3/8," 1/2," 5/8," 3/4," and 1." c. Chain Tensioning Adjustment - Correct adjustment of the chain decreases wear and maintains alignment of the chain with the sprockets. This eliminates chain slap and the possibility of a chain "jumping time" or becoming out of phase with the rest of the machine. Correct adjustment depends upon 1) Distance between sprockets 2) Size of load 3) Type of application 4) Type of chain 5) Type of support bearings Normally the chain is adjusted to eliminate all slack but not tight enough to cause binding or undue stress on the drive system. The adjustment block or sprocket is usually on the non-powered side of the drive chain and can be either a spring-loaded constant-tension type or adjusted using a slotted hole sliding bolt system. 3. Belt Drives - A belt drive system uses various sizes and numbers of belts and is popular because the belts provide some slip or torque release and operate without lubrication. Recently the application of the toothed belt has allowed the quiet operation of a belt to be applied to drive systems which need to be timed. An excellent example of this is the timing belt used on gasoline engines to power the valve system. No slippage is allowed here while with normal flat or V-type belt drives up to 5 percent slippage is common. Belts run on sheaves to connect the shaft or engine with the belt. Pitch refers to the neutral axis diameter size of the belt as it moves over the groove in the sheave. (Neutral axis means the point on the cross section of the belt where there is neither compression nor tension as the belt runs over the sheave.) a. Types of Belts 1) Flat Belt - A drive system which uses a thin, flat belt and relies on the tension of the belt to give traction at the sheave. The belt stays centered on the sheave because the center of the sheave is crowned higher than the sides so the belt climbs to the highest point on the sheave. These belts are still very useful in the very dusty conditions under which many agricultural machines operate. 2) V-belt - This is the most commonly used belt drive system because it is very efficient, works in dusty conditions, provides elastic power absorption for shock loads, and is inexpensive to purchase and maintain. It derives its superior tractionability by wedging itself into the "V" shape of the sheave as the load increases. V-belts have a number printed on the back which together with the manufacturer's name can be cross-referenced to determine the belt size. Belts are purchased by length, cross-sectional width, and the "V" depth of the belt. Formula to determine the length of a belt: 2 (D - d) L = 2C + 1.57 (D + d) + --------- 4C Where: L = effective length of belt, in.(mm) C = distance between centers of sheaves, in. D = effective outside diameter of large sheave, in. d = effective outside diameter of small sheave, in. 3. Toothed Belt - Also called a positive drive belt, it has the strength of the flat belt with the added advantage of zero slippage. Many timing belts now use the positive type drive belt system to provide a light weight, maintenance free drive for timed power transmission applications. The term pitch refers to the center-to- center distance between the neoprene rubber teeth. The five standard pitches are 1/5," 3/8," 1/2," 7/8," 1 1/4." Within each pitch size there are also several different widths available. 4. Gear Drive - Although the most costly, the gear drive system of power transmission is the longest wearing and most trouble free. Gears are run in pairs and can take power around corners as well as change shaft speeds up and down. Some gear sets operate exposed dry or with limited lubrication, but the best application of this drive system is to run in a gear oil bath. a. Types of Gears 1) Spur - Spur gears are straight cut gears which must run their shafts parallel. They can be either internal or external gears and are relatively low cost to produce. 2) Bevel Cut - Beveling allows straight cut gears to run at angles up to 90 degrees so they can be used to go around corners. 3) Worm - This gear set has a shaft with teeth cut around it with screw-like threads meshing with a helical cut gear forming the sector unit. Worm gears are used in steering gears on tractors. 4) Helical - These can be either spur-type or bevel-type gears but without straight teeth. The teeth are slanted so more tooth area stays in contact longer to provide superior wear life, better power transmission, and quieter operation. 5) Hypoid - Spiral bevel or hypoid gears have curved teeth which mesh very quietly and provide excellent power flow by staying in contact longer than spur, helical, or bevel cut gears. They are used extensively in automotive and machinery differentials or on rear end powered axles. b. Planetary Gear System - This system is usually a spur gear system and is composed of a center sun gear, two or more planet gears going around the sun gear, and an outer ring gear meshing with the the planet gears on the outside. The planet gears are kept separated by a planet carrier unit. __________________________________________________________ ACTIVITY: 1. Draw different belt, chain, and gear systems and then label and identify the parts. 2. Determine the direction of rotation of the shafts in a complex chain or belt driven system such as a combine or harvester. __________________________________________________________ ============================================================================ ***INSTRUCTORS PLEASE NOTE*** The detail of this topic presentation goes beyond the scope necessary to meet the requirements of the Core Cluster in this area. It will take longer to teach than indicated above if covered entirely. It is included for local enrichment as appropriate to the class. ============================================================================ A. Any discussion of power transmission in machines would not be complete without the inclusion of fluid power systems. In the past 40 years hydraulic systems have evolved into an integral part of agricultural machinery power transmission. Hydraulic assist is commonly used for power steering, implement hitching, braking, hydrostatic transmissions, among other things. Fluid power provides the flexibility of remote system drive in a low-maintenance, safe, and inexpensive way with easy, complete control at the operator's fingertips. B. Principles of Hydraulics 1. Fluids will not compress. 2. Fluids have no shape of their own. 3. Fluids transmit applied pressure equally in all directions. 4. Fluids provide a great increase in work force, sometimes called "the hydraulic lever." C. Pascal's Law - "Pressure applied to an enclosed fluid is transmitted equally and undiminished in all directions to every part of the fluid and of its restraining surfaces." D. Hydraulic System Advantages 1. Simplicity - Fewer moving parts are needed than in mechanical systems. 2. Compactness - Very small units are able to create large force. 3. Flexibility - Power can be utilized in any location without concern for the angle or placement of the engine location. 4. Increased Force - Very small forces can be used to create large leverage advantages. 5. Safety - There are fewer moving parts with easy overload protection and much more positive control of operation. E. Hydraulic System Parts 1. Pump - The pump is the primary unit of the system. The pump produces flow in gallons per minute (GPM) which is directed to all parts of the system. There are three basic types of pumps: a. Gear Pump - The meshing of two gears in an enclosed housing provides a simple, low cost pump. As each tooth meshes, the oil is squeezed out and forced out of the pump and into the hydraulic lines. Gear pumps can be external gear or internal gear and can build pressures up to 1000 - 2000 psi. b. Vane Pump - This pump has a center rotor which drives sliding vanes around an outer housing. This paddling action moves the oil into the hydraulic lines and follows this displaced oil with more oil. All oil hydraulic pumps are positive displacement type pumps. This means that they force out an exact amount of oil each revolution or cycle. Vane pumps are used to generate pressures of 1500 - 2500 psi. c. Piston Pump - A piston pump is an expensive hydraulic pump but also one which can build very high pressure (5000 psi). It is a pump which can develop either fixed or variable displacement flow, gallons per minute (GPM) and is either a radial or a axial design. The working parts of the pump consist of several pistons operating in cylinders. 2. Valve - System control of the oil pressure and flow is accomplished by using valves designed to provide safety and set the operational characteristics for the system. There are three basic types of valves: a. Pressure control valves are used to limit system pressure, set reduced pressures, and unload a pump. Such valves include: 1) Relief valves 2) Pressure reducing valves 3) Pressure sequence valves 4) Unloading valves b. Directional control valves control the circuit selection for oil flow within a hydraulic system. They include the following: 1) Check valves 2) Spool valves 3) Rotary valves c. Volume Control valves regulate the GPM going into a circuit by throttling or diverting the oil. Example of volume control valves are: 1) Compensated flow control valves 2) Non-compensated flow control valve 3) Flow divider valves 3. Fluid Actuator - The point at which the hydraulic fluid is used to perform work can be either linear or rotary type motion. The system parts used to accomplish this are: a. Cylinder - either single-acting (powered motion one direction), or double-acting (power both directions) b. Ram - single-acting, heavy construction rod serves as piston to develop force c. Vane or Rotary Cylinder - produces twisting or rotary action to move a shaft less than 360 degrees of rotation d. Motor - revolves a shaft, sprocket, belt pulley, or gear 360 degrees either clockwise or counterclockwise to produce torque 4. Reservoir - This provides oil storage, cooling, air removal, and cleaning. It is a very important part of the hydraulic system design. The reservoir is usually sized to be 4 times the capacity of the pump (8 GPM pump = 32 gal. reservoir) but this can vary with the system design. 5. Accessories - Several devices provide increased utility of the hydraulic system and prolong its life. a. Oil Filter - The filtration of dirt and water out of the hydraulic system is the most important single factor necessary to increase system life. The filter can be used on either the inlet or the outlet side of the pump. It should be able to filter out particles as small as 5 microns. b. Oil Coolers - Additional cooling on the high GPM and high use circuits in a hydraulic system can save repair dollars and extend the system life. The reservoir can also be designed to allow for additional cooling capacity. c. Accumulators - These store hydraulic energy, absorb shock loads, and provide a means of adding additional flow to a hydraulic circuit at specific times in its cycle. __________________________________________________________ ACTIVITY: 1. Identify hydraulic system parts and describe the relationship between the parts. 2. Disassemble simple hydraulic components and draw the parts. __________________________________________________________ 7/23/91 YNJ/JR/sg #%&C