some wear questions
#1
12-03-2003, 03:35 PM
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some wear questions
I doing my senior project over the bottom end construction on a 351M. I have a few wear questions. what would cause crankshaft out of round...taper? same with rod bearings. Also how can I tell if my crank is forged? My connecting rods? my crank and rods are out of a 1977 pickup.
#2
12-03-2003, 03:52 PM
Lead Driver
#3
12-09-2003, 02:47 PM
Not an engineer but will get you started on the wear question until bigger brains take over.
Rod wear: in a perfect world there would always be oil between the bearing shell and journal and we would never see wear. The world is not perfect so the shell does contact (start-up for sure!) The joint is not loaded evenly: highest load when the power making downstroke occurs. Also, examine the radial distribution of load on the joint. Notice the force is spread over a planar projection of the arc and is distributed on a point basis that follows a sine (or cosine or whatever) function. ie more force (trending towards infinitely more force) is transmitted per unit area at the apex (top of shell) of the arc than at the line tangent ("edge" of shell). (This is first year college physics statics and dynamics so go see your physics teacher after school... it is fascinating and very practical knowledge.)
Add dirt, contaminants, fuel dilution and you get what we see when we tear apart an old enginge. Lots of wear at the point of max load and little or no wear at the points of (near) zero load.
Main Journals: pretty much the same thing except now we have many more forces acting from different directions which tend to spread the wear out more evenly. You can also throw harmonic stresses into the equation.
Taper: can have two sources I can think of now. a. The applied load is applied unevenly (think center of rod bearing not lined up with center of piston) resulting is a bending load. or b. parts (I'm thinking crank here, mostly) bending under load (whick is really the same as a. when you think about it.) Harmonics again... the crank is wiggling like a snake, more so at certain RPM's (hence the name).... these vibrations are dampened (not eliminated) by the balancer.)
Hope this gets you started.... anyone else feel free to correct or expound on this.
Rod wear: in a perfect world there would always be oil between the bearing shell and journal and we would never see wear. The world is not perfect so the shell does contact (start-up for sure!) The joint is not loaded evenly: highest load when the power making downstroke occurs. Also, examine the radial distribution of load on the joint. Notice the force is spread over a planar projection of the arc and is distributed on a point basis that follows a sine (or cosine or whatever) function. ie more force (trending towards infinitely more force) is transmitted per unit area at the apex (top of shell) of the arc than at the line tangent ("edge" of shell). (This is first year college physics statics and dynamics so go see your physics teacher after school... it is fascinating and very practical knowledge.)
Add dirt, contaminants, fuel dilution and you get what we see when we tear apart an old enginge. Lots of wear at the point of max load and little or no wear at the points of (near) zero load.
Main Journals: pretty much the same thing except now we have many more forces acting from different directions which tend to spread the wear out more evenly. You can also throw harmonic stresses into the equation.
Taper: can have two sources I can think of now. a. The applied load is applied unevenly (think center of rod bearing not lined up with center of piston) resulting is a bending load. or b. parts (I'm thinking crank here, mostly) bending under load (whick is really the same as a. when you think about it.) Harmonics again... the crank is wiggling like a snake, more so at certain RPM's (hence the name).... these vibrations are dampened (not eliminated) by the balancer.)
Hope this gets you started.... anyone else feel free to correct or expound on this.
#4
12-09-2003, 02:56 PM
NOT SO FAST THERE!
Look at any used crank and you will see that it is the bottom of the crankpins that wear first. That is because the biggest force on the rod bearing is turning the piston around at the top end of the exhaust stroke. The pressure from the power stroke is secondary. I recently threw out a 350 olds crank that was cut about 1/8" on the bottom of the crankpin. The top of the crankpin was untouched. Detonation is another story and will cause wear from the oil film being ruptured at or before TDC.
R.
Look at any used crank and you will see that it is the bottom of the crankpins that wear first. That is because the biggest force on the rod bearing is turning the piston around at the top end of the exhaust stroke. The pressure from the power stroke is secondary. I recently threw out a 350 olds crank that was cut about 1/8" on the bottom of the crankpin. The top of the crankpin was untouched. Detonation is another story and will cause wear from the oil film being ruptured at or before TDC.
R.
#5
12-09-2003, 07:12 PM
Why is it harder to turn the piston around at the top than the bottom?
I have not payed a lot of attention to crank wear pattern, I send it to the shop and let them fix it. From what I remember (and that's not a lot) the bearings out of my 400's wear more on top than bottom. I bet some like Bubba has some great pictures for Steman's report. (Which he better share with us.)
I have not payed a lot of attention to crank wear pattern, I send it to the shop and let them fix it. From what I remember (and that's not a lot) the bearings out of my 400's wear more on top than bottom. I bet some like Bubba has some great pictures for Steman's report. (Which he better share with us.)