LO 9-7 Identify and describe the purpose, types, construction, and operation of the engine geartrain
1. Geartrains
LO 9-07: Identify and describe the purpose, types, construction, and operation of the engine geartrain. |
Geartrains
Because the crankshaft rotates twice as fast as the camshaft, the drive gear on the crankshaft is exactly one-half the size as the gear diameter on the camshaft. Synchronization of camshaft-to-crankshaft rotation requires the gears to be timed. Timing involves installing the gears together using gear markings designated as timing marks. To correctly time the gears together, one gear usually has a mark on two adjacent teeth, and the other gear has a mark on only one tooth Figure 53. This same arrangement is used whether the valve train uses a gear, chain, or belt drive. A keyway or shaft shape locates the gear in the correct position on each shaft. An idler gear is placed between two other gears to transfer motion from one gear to the other without changing their direction of rotation. To obtain a 2:1 gear ratio between the crankshaft and the camshaft, one or more idler gears can be placed between the crankshaft gear and the camshaft gear. Torsional vibration dampers or scissor gears may be incorporated into the geartrain mechanism Figure 54.
FIGURE 09-53: Marks on these two gears are used to synchronize the position of the camshaft in relation to the crankshaft. All timing marks on the gears of geartrain must be correctly aligned whenever the gears are installed. |
FIGURE 09-54: Scissor gear removal from a Cummins ISX Signature engine. |
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The geartrain will typically drive accessory components, such as air compressors, power steering and water pumps, fuel injection pumps, engine fans, and air-conditioning camshafts Figure 55. Helical- or spur-cut gears used in geartrains are commonly used on HD applications because they are the strongest and most durable method to transmit torque to the engine accessory drives and camshaft. Gears are, however, noisier than a chain or belt drive system due to gear backlash. Gear backlash is the clearance established between gear teeth for lubricating oil. Measuring gear backlash is a service procedure followed whenever geartrain noises are heard. On heavier engines, an idler gear with a hunting tooth gear ratio is used to add tension to a geartrain to establish correct backlash for multiple gears. If it is adjustable, a specific idler gear with elongated bolt holes is loosened and turned slightly with a purpose-made tool or pry bar to change the gear teeth backlash. The gear is retightened and the backlash is measured Figure 56. Timing chains and sprockets are also used to turn the camshaft Figure 57. Roller-link chains are used in diesels with long distances connecting the camshafts to the crankshaft drive gear. A chain tensioner with a piston pressurized by engine oil is used to remove any slack in the chain. Chain sprockets also have timing marks. In light-duty (LD) and MD diesels, belt drive geartrains are used with sprockets on the crankshaft and the camshaft linked by a single reinforced elastomer belt Figure 58. The belt has cog-shaped teeth that mesh with the teeth on the sprockets. Like a chain drive system, belt drive valve trains use a belt tensioner to maintain a tight belt. Without the tensioner, a natural stretching of the belt or chain will change the length of the belt or chain, which would prevent accurate cam-to-crank synchronization during engine operation. Tensioners not only enable more precise valve timing but also compensate for component wear and stretch. Engines with a belt drive valve train usually use a spring-loaded tensioner and idler wheel. |
FIGURE 09-55: The geartrain of a Detroit Series 60 with a single overhead camshaft. Note the odd hunting-type gear ratio of the adjustable idler gear used to adjust backlash. |
FIGURE 09-56: Gear backlash is commonly measured by rocking the geartrain back and forth and measuring the movement with a dial indicator. |
FIGURE 09-57: A timing gear and chain for a Mercedes-Benz OM642 engine. The gear is tied to the chain to prevent the loss of cam-to-crank synchronization when only the cylinder head is removed. |
FIGURE 09-58: The 2.8 L Duramax I-4 diesel engine uses a rubber timing belt to drive two overhead camshafts. |
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