Re: SPEED SENSER QUESTION?

max dodge wrote
that is the point that i disagree with. a horsepower number does not
describe an engines ability the same at different rpms. 100
horsepower at 1000 rpm is 10 times stronger than 100 horsepower at
10000 rpm. 100 lbs/ft of torque has the same ability no matter what
rpm it occurs at.

um... no. 100ft lbs at 1000rpm is 19hp. 100ft lbs at 6000rpm is
114hp.
if it takes 100ftlbs to turn a shaft one shaft will turn 6 times as
fast meaning more work is done. looks like different abilities to me.


100 hp @ 1000 rpm = 525.2 lbs/ft torque
100 hp @ 10000 rpm = 52.52 lbs/ft torque

whats this for

whats this for its to show that a horsepower number alone does not
accurately describe the ability an engine has to do work.

you said if it takes 100ftlbs to turn a shaft one shaft will turn 6 times
as fast meaning more work is done.
well use that same shaft again comparing the two 100 horsepower engines.

the engine that makes 100 hp @ 1000 rpm has 525 lbs/ft of torque. thats
plenty of torque to do the job. in fact it can run 5 shafts each requiring
100 lbs/ft of torque all at the same time. the engine that produces 100 hp @
10000 rpm makes less than 53 lbs/ft of torque. if it takes 100 lbs/ft of
torque to turn the shaft it will not be able to turn the shaft at all. no
work is being done.

ill give you another example. say im out shopping for a new push mower. i
get frequent rain and one corner of my back lawn doesnt drain very well.
the lawn is always wet there and the grass grows very fast. my old mower cut
fine everywhere in my lawn including the high wet grass. it had a good 5
horsepower engine when new but now its old and uses too much oil. i leave a
trail of smoke behind every pass. i assume any 5 horsepower push mower with
the same size blade will work the exact same way so thats what i go
shopping for. i want a brand new mower just like my old one but the old
model is discontinued. i find the same brand mower with the same 21 blade
a 5hp engine and rear mounted collection bag. same thing as the old one
right well no. it looked the same and had all the same advertised
specifications but it wasnt the same thing. i didnt realize that my old
mower was rated 5 horsepower @ 500 rpm and the new one is 5 horsepower @
1000 rpm. the grass doesnt really care if it gets whacked by a blade
spinning around at 500 rpm or 1000 rpm so the end result should be the
same. it should make tall grass short.
i try out my new mower. i get the front lawn done and it looks very nice.
that 1000 rpm mower cuts the lawn very cleanly and deposits much more of
the clippings into the bag than my old mower ever did. im very happy with
my new mower even though i still dont know about the higher blade speed. i
assume the old engine was getting weak and this new one is more powerful.
now i go around back. i get three feet into the wet high grass and the mower
stalls. i have to drag it back out of the wet area to get it started again.
i push it another three feet into the wet grass and it stalls again. it
takes me hours longer with this new mower to cut the wet area than it did
with my old mower. 5 horsepower isnt always exactly the same as 5
horsepower. the old mower had twice the torque that my new mower has. the
new mower has twice the blade speed that the old mower had.
which one gets more work done if you ask the guy that has to push the mower
through the tall wet grass its the old one.

im not saying that horsepower ratings are meaningless. im only saying that
horsepower ratings are less meaningful without an rpm to go along with them.
--
ken


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