"Edwin Pawlowski" wrote in message


I have two cars that will turn 2,000 RPM at 70 mph. I drive a lot at that
speed every years. I had a rental car that I liked, but at the same
speed,
the engine was turning 2500 rpm. After driving it a while I decided I'd
not
buy one for myself. Why? I keep cars well over the 100,000 mile mark.
Given that I may drive it 100,000 miles at 70 mph, the crankshaft will
have
turned 43,103,000 times more than my present car. Lots of wear on them
bearings.

AAhhhhttt... Crank bearings are totally different than roller bearings.
They are more like split 2 piece bushings. The larger CS bearings will
distribute the wear and load over a larger surface. CS bearings do not spin
unless you have just blown the engine.. ;~) Good thinkig on the wear
factor of the higher RPM motor though.

So, if bearing "X" is 3" diameter it will have a ball travel of 9.42
inches
while a 2" bearing will travel 6.28 inches. At 4,000 rpm, the larger
bearing will cover 3142 feet while there smaller bearing will have gone
2094
feet. Given proper loading, I'd say the smaller would last longer purely
from a friction point. That would also mean proper operating temperatures
and lubrication.

AAhhhttt... Ball bearing balls and needle bearing needles don't circle the
bearing at a 1 to 1 ratio. The larger those balls are, the fewer times
thay will turn around the inner race. If the inner race is 1/2" in
diameter, and the outer race is 2" the balls are about 3/4" in diameter.
For each revolution of the inner race which is about 1.5" in circumference,
the ball that is about 2.3" in circumference will only turn .65 times for
each revolution of the inner race. For a larger bearing with the same size
1/2" inner race and an outer race that is 3" in diameter, the balls are
about 1.25" in diameter and 3.93" in circumference and those balls will
turn about .38 times for each revolution of the inner race.
So, regardless of the bearing size, the inner race that the arbor is
supported by will always be the same size and turn the same speed given the
arbor is the same size and turns the same speed. The larger the outside
race of the bearing the larger the balls will be and the slower thay will
turn.