1984/01/18 Caterpillar

Caterpillar-Built Engines

This article explains the different types of vibration. It also tells how balance gears, balance shafts and dampers are used to control vibration and it gives specific service recommendations for balance gears, balance shafts and vibration dampers.

Two types of vibration, linear and torsional, can be found in engines. Linear vibration is movement of the engine structure that can be seen, felt and heard. Torsional vibration is twisting vibration, it usually can not be felt or seen but can sometimes be heard as gear teeth noise.

For more detailed information on engine vibrations, see Service Training Meeting Guide 403, Form SESV1403, "Introduction To Vibration." Also see the other publications that are shown at the end of this article.

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 or a generator, which is not a part of the design package for that specific engine, will change the natural frequency of crankshaft vibration. This will make the damper ineffective. Before a change is made to a system with a rubber damper, a torsional vibration analysis should be made. For information on obtaining a vibration analysis, refer to the Engine Price List and the Application and Installation Guides (shown in Chart B.)

Illustration 2. Cross section of rubber damper.

Viscous Dampers

Viscous dampers are more effective over a wide range of engine speeds than rubber dampers. Viscous dampers have a sealed housing that contains a heavy weight. The weight is suspended in a highly viscous silicone fluid. See Illustration 3. As the crankshaft turns, the silicone fluid turns with the housing. The heavy metal ring is driven by the fluid but, because of inertia, it has a resistance to any sudden change in the direction or speed of rotation. The silicone fluid drags or pushes the heavy ring as a result of the vibrations generated by the engine. The vibrations are dampened at most speeds and absorbed into the viscous fluid as heat.

See Chart A for a summary of damper usage on Caterpillar engines.

Illustration 3. Cross section of viscous damper.

Chart A--Engine Summary And Damper Usage

Service Of Dampers

It is our recommendation to inspect rubber dampers at 3000 hour intervals on the 3208 Engine and 5000 hour intervals on all other engines. It is also our recommendation to replace rubber dampers for any of the reasons that follow. For Steps 1 through 3, the inspection must be made during overhaul.

1. The engine has had a failure because of a broken crankshaft.

2. The crankshaft front bearing is badly worn.

3. There is a large amount of gear train wear that is not caused by a lack of oil.

4. Inspection shows slippage (movement) between the inner and outer member of the damper as shown in Illustrations 4 and 5.

5. Inspection shows rubber deterioration because of cracking, spalling, splitting, etc. See Illustrations 6 and 7.

If none of the above conditions is found, rubber dampers can be used again at overhaul.

Illustration 4. Slippage between inner and outer member. DO NOT USE AGAIN.

It is our recommendation to inspect viscous dampers at 20,000 hour intervals. It is also our recommendation that viscous dampers be replaced for any of the reasons that follow. For Steps 1 through 3, the inspection must be made during overhaul.

1. The engine has had a failure because of a broken crankshaft.

2. The crankshaft front bearing is badly worn.

3. There is a large amount of gear train wear that is not caused by a lack of oil.

Illustration 5. Slippage between inner and outer member. Reference marks are not in correct alignment. Misalignment of the reference marks can be 1.5 mm (.06") due to assembly. If there is evidence of movement. DO NOT USE AGAIN.

Illustration 6. Rubber deterioration. DO NOT USE AGAIN.

Illustration 7. Rubber deterioration. DO NOT USE AGAIN.

4. Inspection shows fluid leakage or a dented (damaged) housing. Either condition can cause the metal ring to make contact with the housing and cause damper operation that is not correct.

If none of the above conditions is found, viscous dampers can be used again at overhaul. Be careful not to damage the damper during removal and installation.

For either rubber or viscous dampers, never exceed engine application guidelines without doing a vibration analysis to determine if the damper is adequate. Refer to the Application and Installation Guides shown in Chart B for the technical data required to perform a torsional vibration analysis. Chart B also shows reference literature that discusses vibration, correct installation methods, troubleshooting, etc.

Caterpillar engines will perform properly when used in an application for which they are designed. The torsional compatibility of the installation is the responsibility of the system designed. Caterpillar will provide the customer, upon request and without charge, the necessary information to do a theoretical torsional analysis. If the customer desires, Caterpillar will perform a torsional analysis for a nominal charge. See the Engine Price List for cost information.