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Thread: Bearing Overload?

  1. #1
    Associate Engineer
    Join Date
    Oct 2013

    Bearing Overload?

    I am trying to determine the rod bearing load when a starter motor incurs a 90 ft lb load. The engine is a Chevy 350. I have drawn the geometry in the picture below. I am figuring using a 10 tooth starter gear & a 153 tooth flywheel, so I calculate 15.3 x 90 ft lbs = 1377 ft lbs torque at the flywheel. So if I consider the distance traveled by the flywheel at 12 inches from it's center during a transition from 5 degrees BTC to TDC I get 1.04687 inches. During this same transition, the piston moves .00864. To calculate the effective leverage, I divide 1.04687 by .00864 to get 121.165, then I take 121.165 x 1377 lbs at 12 inches to get 166,844 lbs on the piston, and divide it by 2.704 square inches of rod bearing area to get 61,703 PSI on a rod bearing. Where did I go wrong? is the load really that high? A soft metal bearing could not endure that load could it?, or even the compressive strength of a rod for that matter.

    Last edited by Kelly Bramble; 10-24-2013 at 01:28 PM.

  2. #2
    Administrator Kelly Bramble's Avatar
    Join Date
    Feb 2011
    Bold Springs, GA
    Well, without examining the calculations very hard I think you are neglecting the fact that not all of the force is exerted on the bearing. The crank shaft will rotate and the mechanical energy is dissipated into motion, strain, heat energy and torque applied at the PTO of the engine.

  3. #3
    Lead Engineer
    Join Date
    Aug 2013
    Houston TX USA
    The maximum rod bearing load from the starter torque will be determined by the air pressure at TDC under cranking conditions and therefore by the compression ratio of the engine not the maximum torque of the starter motor.

    The only two conditons where the maximum torque of the starter might be applied to a rod bearing is if a head gasket leak or a cracked head causes one of the engine's cylinders to fill, or partially fill with water while the engine is shutdown. Another condition where a bearing overload would occur on older vehicles with manual transmissions is when the driver attempted to start the engine without depressing the clutch pedal and in this case the excessive loading is determined not by the maximum starter torque but by the vehicle's inertia and the gear selected unless the vehicles parking brake is still engaged.
    Note: This type of overload cannot happen on newer cars with manual transmissions because, as safety measure, the clutch pedal must now be depressed before the starter will operate.
    Last edited by JAlberts; 10-25-2013 at 10:31 AM.

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