Torque BS Filter - explained

[Mark Gagner]

Stephen Dubovsky wrote:
> 
> ...
> >:By definition:
> >:Horsepower = (rpm * Torque) / 5252
> >:
> >:Thus, at 5252 RPM, HP = 5252*Torque/5252 = Torque
> >
> >
> >Please don't stop now., the bulbs are going off....  What are the units
> >used to get the 5252 number?
> >
> ...
> 
>   Ok, Ill try to get all of this right;)
> 
>   By definition Force = mass * acceleration (F=m*a) (Ill work in the metric
> system first and give the conversions later - soory, Im a 'new' math kind
> of person) so Newtons (N) = kg * a which will have units of kg*(m/sec^2)=N.
> 
>   Torque is force times a distance (NOT a distance traveled, but how far
> away the force is applied.  i.e. 1 lb-ft is 1 lb of force applied on a
> lever/bar 1 ft away).  So T (in Newton meters (Nm)) = N * m which will
> carry the units of kg*m^2/sec^2 (which is exactly 1 Nm - isn't the SI
> system great;).
> 
>   Rpm is a is an angular rate of change (has units of 1/sec - actual
> constant is 1/(60*sec)).
> 
>   Work (Joules) is Force times a distance traveled (apply 1 N of force to
> move something 1 m = 1 joule or work).
> 
>   Now Power is the rate at which you do work.  Watt = joule/sec = N*m/sec =
> kg*m^2/sec^3
> 
>   So if you have a motor rotating at 'x' rpm w/ 'y' Nm of torque.  You have
> 'y' N of force (at 1m) traveling 2*pi*1m (the circumference of a 1m radius
> circle) every 1/(60*sec).
> x*rpm * y*Nm/m * 2*pi*m =
> [x * 1/(60*sec)] * [y * kg*m/sec^2] * [2*pi*m] =
> x*y * (2*pi/60) * kg*m^2/sec^3 =
> x*y * (2*pi/60) * watt
> 
> i.e. 1000rpm and 500Nm -> 1000*500*(2*pi/60) = 52360 watts of power
> 
> So for you non SI guys:
> 1 lb*ft = 1.356 Nm
> 1000 * watt (1 kW) = 1.341 hp
> 1 hp = 745.7 watts
> 
> And:
> So, 'x' rpm and 'y' lb-ft yield:
> x * y*1.356 * 2*pi/60 * (1/745.7) =
> x * y * 1.9042*10^-4 = hp
> 
> The above example becomes 1000rpm and 500 Nm = 369 lb*ft
> 1000*369*1.904*10^-4 = 70.2 hp (which is the same as 52360Watts/745.7)
> 
> The reciprocal of the constant yields the magical 5252 rpm number.
> 
> Nuf' physics for everybody?  I hope I got everything right.  Please flame
> me if I left something out, or got it wrong;)
> SMD
> 
> ---
> Stephen Dubovsky
> dubovsky at vt.edu
> 95 Yamaha FZR600 - to go fast
> 83 Porsche 911SC - to go a little faster
> 84 Jeep Cherokee - to haul things that go fast

The only thing left out is that horsepower (like most units in the
English
system of measurement) comes from a wierd, historical event. According
to
what I read, James Watt (the inventor of the steam engine) wanted a way
to
compare his steam engines against the work produced by the standard
"engine"
of his day... the horse.

He had draft horses lift weights and measured the time they required.
>From this
he derived their rate of work and thus the horsepower was born. :)

And yes the SI system is great!

Mark