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ISC, ISCe, QSC8.3, ISL, ISLe3, ISLe4 and QSL9 Engines – 016-010   Engine Mounts

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General Information

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Some vibration exists in all piston type engines, due to the pulsating power inputs and reciprocating components. Some of these vibrations are internal to the engine and are compensated, or balanced, by opposing forces within the engine structure. These are generally
not of interest to vibration isolation designs. The vibrations that are offset or balanced internally will cause shaking moments and forces that
must be reacted to by the engine mounts. If these moments and forces are
not adequately reduced by the engine mounting and isolation systems, they can cause customer dissatisfaction and/or damage, due to component fatigue.

The effectiveness of an engine mounting system in isolating the vehicle structure from engine vibration depends on the relationship between the frequency of the vibration coming from the engine and the natural frequency of the engine mounting system. The mounting system effectiveness is commonly measured with the term “transmissibility”. Transmissibility is the amount of engine vibration which is transmitted through the mounting system to the vehicle structure.

Transmissibility values greater than one indicate the engine mounting system is actually transmitting more vibration into the vehicle structure than is coming from the engine. This is possible if the natural frequency of the mounting system is close to the frequency of the engine vibration. This can result in the mounting system operating at or near resonance, with a resulting magnification of the input vibration. This is obviously an undesirable situation.

Transmissibility values of less than one indicate the mounting system is transmitting
only a fraction of the vibration input from the engine, thus isolating the vehicle from engine vibration. Good engine mounts will reduce the amount of engine vibration transmitted to the chassis frame by at least 50 percent at idle.

Stiffness (durometer) and size of the isolator, along with the weight of the engine or component applied, are the determining factors when designing a mounting system. An isolator that is correct for one engine may
not be right for another. Likewise, because of weight differential, a particular isolator designed for the rear of an engine probably will
not be ideal for the front. Hard engine mounts will give little or no isolation, and can actually magnify the vibration transmitted to the chassis.

The following are illustrations of typical FRONT engine mounts.

  1. Bolt
  2. Supported member
  3. Snubbing washer (or flat bracket surface of equal diameter
  4. Rebound tail
  5. Bonded metal center
  6. Locknut
  7. Supporting member
  8. Rubber mount.

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The following are illustrations of typical REAR engine mounts.

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Remove

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 WARNING 

The engine lifting equipment must be designed to lift the engine and transmission as an assembly without causing personal injury.
 WARNING 

This component or assembly weighs greater than 23 kg [50 lb]. To prevent serious personal injury, be sure to have assistance or use appropriate lifting equipment to lift this component or assembly.

Use a hoist or lifting fixture to support the engine.

NOTE: When removing the engine mount fasteners, note the location of any shims or spacers used.

Remove the capscrews from the engine mounts.

NOTE: Certain applications will require loosening of the rear engine mount fasteners to allow removal of the front engine support bracket.

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Inspect for Reuse

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 CAUTION 

Damaged engine mounts and brackets can cause engine misalignment. Drivetrain component damage can result in excessive vibration complaints.

Inspect all rubber-cushioned mounts for cracks and other damage. Look for interference or contact between metal components.

Inspect all mounting brackets for cracks and damaged bolt holes.

Inspect the mounting capscrew to make sure it is
not too long, which will
not provide enough preload on the mount.

Replace any damaged parts as necessary.

NOTE: Damaged engine mounts, brackets, and mounting hardware can cause the engine to move out of alignment and damage the driveline components in the equipment. This can result in vibration complaints.

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Inspect the capscrew for the following:

  • Damaged threads
  • Rust or corrosion-caused pitting
  • Nicked, bent, stretched, or galled.

The capscrew
must be replaced if it has any of the listed damages.

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For barrel mounts, inspect for signs of contact between the side brackets and the front engine support bracket.

Contact between the engine mount and side brackets can cause vibration complaints. If contact is found, replace the front engine support bracket.

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Install

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NOTE: Make sure to install any shims or spacers in the same location as removed.

Align the engine in the chassis.

Install the engine mount fasteners and tighten. Refer to the OEM for torque specifications.

Remove the lifting fixture or hoist from the engine lifting brackets.

Connect all engine and chassis mounted accessories that were removed.

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Last Modified:  13-Nov-2008
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