You slipped
a decimal somewhere.
The speed
at the tip distance divided by time. At 3600 RPM and 24" dia (to make it
even, 2' dia and 1' radius) you get 3.14159 * 1' or
3.14159'
per revolution *2 for the circumference. 3600 revolutions per minute is
21,600 revolutions per hour * 2* 3.14199' per revolution is
1357166'
per hour. There are 5,280' per mile, so 135,7166/5280= 257 miles per
hour. If you take a different diameter or RPM you will get a
proportionally different speed.
With 29"
and 7000 RPM I get 29" / (12"/ft) * 3.14159 *7000 (RPM) *60
(Minutes/hour) /5280 (feet/mile)= 603 MPH .... REAL FAST, the speed
of sound is
about 720 MPH at sea level and 70 deg. F!
BTW, as an
ex-pilot, tip speeds on a 72-84" dia prop (small plane) start going supersonic
at about 2800 RPM depending on altitude (the
speed of
sound drops with altitude). Nobody wants a mower turning THAT
fast!
Larry
Elie
I tossed in 29" diameter and found
that it took over 7000 rpm to make
the recommended speed; what blades can be run at
that speed and
last? Or won't they fatigue? I've
seen car water pump fan blades
that fatigued (developed cracks, some came apart
destructively to
their surroundings) when run in excess of 7000
(engine rpm, don't
know what the blade rpm was, anyone have a
vehicle with the older
style water pump mounted fans that could measure
the pulley ratio?).
----- Original Message -----
Sent: Wednesday, August 27, 2003 9:30
AM
Subject: RE: (ET) Mower blade
speed
The speed
is right when the batteries are doing well and the voltage hasn't been
drooped by excessive current.
Yes,
re-winding will add speed, and take a ton more current to power. For
electro-magnets, the governing factor is the
FIELD. The field is proportional to the current times
the number of turns of winding. Reduce the number of turns and
the
current has to go up by the same amount to preserve the field. The
power is just the product of the current times
the
voltage. Since would still be powering with the same voltage, and the
current would be higher, the power would be
higher.
Double
blades do up the current a bit. Whether it lets you cut faster is
questionable; you are now cutting at two heights and
mulching
as well. There isn't any free lunch.
Larry
Elie
Does anyone know what the minimum blade tip
speed should be before the blade tears the grass versus cutting it
clean?
I found the following links that calculate
this speed. It looks like our Elec-Trak blades max out at 155
mph.
Should a person rewire the motor for 24 volts
and run it at 36 volts to raise the rpm? Maybe run 2 blades
per motor in a +?
Any thoughts?
_______________________________________________
Elec-trak mailing
list
Elec-trak cosmos phy tufts edu
https://cosmos.phy.tufts.edu/mailman/listinfo/elec-trak