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Unread 26-10-2005, 21:29
KenWittlief KenWittlief is offline
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Re: Mechanical Reliability

Alex, what is the angular acceleration of the connecting rod bearing journal, when the engine is spinning at 5,000 RPM, assuming your crankshaft is what? 1.5" radius (that sounds pretty small, I think an old VW crankshaft had a 3" radius)

and what is it at redline RPM?

another way to calculate it, the piston accelerates towards the crankshaft, then slows and accelerates the other way, at 5000 rpm, what is the force just moving the piston back and forth?

Edit: I did some quick calculations.

for an engine with a 4" piston throw (a 4" crankshaft diameter at the center of the bearing journals)

at 5,000 RPM, the piston and connecting rod are accelerating back and forth with A = 37,036 ft/S^2 (1157 g's)

and the angular (centripetal) acceleration of the connecting rod where its attached to the crankshaft is A= 45,654 ft/S^2 (1426 g's)

If your piston and connecting rod together weigh 1 pound, the force for the linear motion is about 1,100 lbs. The centripetal force depends on the weight of the connecting rod held by the bearing. If its a half pound then the force is 710 lbs - so we are close to one ton, and this is only with the engine spinning freely (not supplying any torque to the wheels).

the forces increase by the square of the velocity, so at 10,000 RPM (a high performace engine) you are looking at 4 tons of force on that one bearing, again, just to spin the engine parts themselves.

Last edited by KenWittlief : 26-10-2005 at 22:13.