COVER GP-CAB 3727796 - Caterpillar

Linear Vibration
Most Caterpillar engines are naturally balanced by design. Those engines that are not naturally balanced use balance gears or balance shafts to keep vibration to an acceptable level.
All twelve and sixteen cylinder vee engines, eight cylinder 90° vee engines and in-line six cylinder engines are naturally balanced. Because of their cylinder arrangement and crankshaft design, the D346, D379, 3204, 3304, 3408 and 3508 Engines are not naturally balanced.
Balance Gears And Shafts
To control linear vibration in engines that are not naturally balanced, balance gears and balance shafts are used. The gears and shafts are timed to the crankshaft rotation. They produce a counter force to the unbalanced forces produced by the engine during rotation. This action results in a balanced engine.
The balance gears or shafts are driven by the front or rear gear trains. The correct alignment of the timing marks on the gears and shafts is very important during assembly. For example, if the balance gears or shafts are installed with the timing marks 180° out of phase, the unbalance forces will double. This will cause a large increase in the engine vibration. If the vibration is severe enough, it can cause parts to break loose and can cause damage to the engine support structure.
During engine overhaul, it is very important to make sure the balance gears, shafts and bearings are inspected for damage. Damaged parts must not be used again. Special Instruction, Form SEHS7914, "Troubleshooting Engine Vibration In Vehicular Equipment" can be used to help troubleshoot linear vibration problems.
Torsional Vibration
Torsional or twisting vibration is caused by the power impulses produced along the length of the crankshaft during rotation. See Illustration 1. Every crankshaft has a natural frequency of vibration. This frequency will remain constant if equipment is not added or removed from the engine. As rotation of the crankshaft starts, torque is applied to each successive crankshaft throw by the power impulses. If the frequency of the vibrations produced by the power impulses equals the natural frequency of vibration, a condition called "resonance" will be the result. This condition will greatly increase the amplitude (size) of the torsional vibrations. If not controlled, the torsional vibration will cause a failure of the crankshaft. To keep torsional vibrations at an acceptable level, rubber and viscous dampers are used.
Illustration 1. Twisting vibration that results from the power impulses.Rubber Dampers
Rubber dampers are usually found on smaller or older engines. This type of damper has a rubber ring that is under compression between the outer damper weight and the inner damper hub. The assembly is then fastened to the crankshaft as shown in Illustration 2. The rubber damper is tuned by design for a specific vibration frequency. During the normal load speed range of the engine, the torsional vibrations are dampened by the elasticity of the rubber and the inertia of the weight. Because rubber dampers are tuned to a specific frequency, they are limited to the system for which they were designed. The addition of a marine gear, power takeoff