Re: Mechanical Reliability
 

a tie rod, you know, it ties the piston to the crankshaft :^)

good catch - I meant the connecting rod bearings (this is why Im not a M.E.)

Re: Mechanical Reliability
 

its equally distressing when your robot spews all of its magical noise all at once

because mechanical parts are pumped full of noise in a machine shop. If the noise leaks out all at once, it stops working

sometimes you can force a little noise back into mechanical parts by wacking them, but usually it has to go back to the machine shop, to be refilled with all the noise it can hold.

Re: Mechanical Reliability
 

LOL, I was trying to figure that out for the longest time. I'm studying to become and ME and I'm like "there is no way 10 tons of force is exerted on the tie rods." For the connecting rods (and bearings) it makes perfect sense because the pistons have a respectable amount of mass to them and at the top and bottom of each stroke, it goes through an extremely massive acceleration because it is a very large change of velocity that happens in the blink of an eye and because F=ma the force will be very large.

Re: Mechanical Reliability
 

its not just the force that is being transferred to the driveshaft, its the angular acceleration as well, because it is spinning at 4 or 5,000 RPM in a 6 or 8 inch diameter circular path, with all the weight of the connecting rod and piston attached.

you tie rods should not see too much force, unless your name is Joey Chitwood

but even there, when you turn your wheels on dry pavement, with the car stationary, Ill bet there is a lot

Re: Mechanical Reliability
 

actually the force in that part of the engine is still not that large. The moving parts of an egine weigh almost nothing because that engine will turn faster if it is lighter so the rotation of the engine alone isn't going to cause the largest force in the engine. I mean if they actually weighed a lot then the engine would need a fly wheel in the sense of keeping the engine turning (the starter would have to turn the engine in a different method).

The largest force in an engine will come from the expanding gas in the piston causing the engine to turn. In my fathers car the engine is a 4 piston engine meaning that the torque put out is constant and only one piston is firing at a time. the engine has 205 ft lbs. of tourqe so that is the amount each piston puts out while firering. The distance traveled by each piston is 3.042 inches. divide that by 2 to get the radius. So the piston is pushing on a 1.701 inch lever. then divide 12 by that and mutiply by 205 to get the number of pounds on the be 1446.208 pounds.

Also just think that if the force exerted by the pistons of the bearings was 10 tons then engine would take a very long time to reach red line and most engines only take seconds.

Alex

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.