I see that some retailers are selling lawn mowers with their engines
labeled in foot-pounds of torque instead of horsepower. One had an
impressive figure of 6.5 ft-lb. If this thing runs at the max of 3600
RPM it works out to a hair under 4.5 HP. The uninformed buyer won't
know the difference and will get a machine with considerably less
power than he thinks it has.
I wonder if Sears started this. It ain't dishonest, but it's counting
on mass ignorance to sell stuff and I bet it works.
On Sat, 2 May 2009 19:24:06 -0700 (PDT), Dan_Thomas firstname.lastname@example.org
The engine doesn't necessarily (or even probably) develop max torque
at 3600 RPM. The torque cited may be developed at some speed well
above or more likely below 3600 RPM that doesn't happen while mowing.
An engine that delivers 5HP at rated speed (3600 RPM) would be
developing 7.3 lbf-ft of torque at that speed.
This Briggs 250 cc engine is rated at 7.5 HP, 11.5 lbf-ft of torque,
speed not cited for either spec but not to exceed 3600 RPM. 7.5 HP
at 3600 RPM is 10.9 lbf-ft of torque so ya don't get the rated torque
at 3600 RPM.
Did I get me sums wrong here? Same as 9.89 newton-meters?
lbf is pounds force, the force of one lb of mass accelerated by
gravity. Torque is (or was once) expressed in pound-feet rather than
foot-pounds to distinguish it from work and energy in ft-pounds. The
pounds are units of force in both cases. It used to be that lb was a
unit of force and the slug was the English unit of mass but I think
that changed and now the pound is regarded as a unit of mass.
Lbf is the convention used by MathCAD. It makes sense to me, and I use
it because I rely on MathCAD to keep my units straight for me. YMMV.
It's a push mower. It doesn't take much figuring to realize that push
mower engines haven't gotten bigger (other than the fancy plastic
housings), burn about the same amount of gas, and still bog down about
the same in wet grass with the same size blade. Only the marketing
numbers seem to have changed (well, that and the "safety" features).
I keep waiting for them to push deck measurements in mm rather than
If they really wanted a proper mower rating, then they would rate them
by inches of cut per area mowed per hour, at a certain reference grass
density, type, and moisture content. That they don't pretty much says
I saw all of this twenty years ago when I used to work retail, with
motor amperage ratings for vacuum cleaners. One mfr. changed over to
"cleaning effectiveness amps" and made their competitor hopping mad
that their oversized inefficient motor no longer had the biggest
marketting number. I had fun telling the competitor's sales rep to
stuff it when they cornered me to complain. I happenned to know from
talking to our return guy their product didn't work as well--when they
worked at all--big motor or no. Now it's all cyclone this, and
tornado power that, and completely meaningless, so you have to resort
to seeing how much of a handful of sand each will pick up on your
own. I was horrible when it came to upselling the units, too, I'd
tell people straight up the only thing they got for another $30 on the
next bigger model was the $2 headlight.
To tie it all back to metalworking, it's like someone else commented a
while back, about a twist drill company losing sight of the fact that
in the end, the customer doesn't want to buy a drill bit--he wants a
Give it all the names and numbers you want, but does it cut grass?
Does it pick up dirt? Is there a hole the size I need where I want
I guess in some ways it's also about customers knowing just enough of
the science to be fooled by the marketing, rather than taking the time
to really learn the difference. Trusting a marketeer for the accuracy
of his science sounds a little like asking the fox to count the
chickens for you.
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