Torque versus power

On Mon, 11 Aug 2003 01:23:16 -0400, "Ziguy" posted:

You are. High power is only obtained from high torque at high frequency (rpm). High torque at one rev per day is very low power. At 5000 rpm, it is much higher power. So power is the amount of work done (torque times angle of rotation) per unit time.

HTH from an amateur :)

both - different definitions of "power".

using the defintions from the world of physics, power is the rate of doing work

- force times distance per unit time or torque times revolutions per unit time

using the definitions from the layman's world of auto racing, power is the ability to accelerate rapidly, which requires torque delivered to the drive axles - torque being turned into the force at the tire-road used to accelerate the vehicle mass.

if your car engine has a lot of rpm and moderate torque (ferrari comes to mind), and the numbers work out to 500 hp with 100 ft-lbs of torque, the guys at the dragstrip will tell you your car does not have any power - they know what they mean by power

if you tell an engineer the same thing, he will say 500 hp in a car is a lot - he knows what power.by its technical definition

Now, if you ask an automotive engineer at the dragstrip whether the same car has a lot of power, being experienced in the field he will likely either say no or ask you "what do you mean?" If he finds out you are a layman, he will likley say "not much power". If he finds out that you are an engineer, he will likely say "got power, but not much torque for a dragstrip"

Now I've got a question.

How does the universe treat angles? I mean, they say that the heisenburg uncertainty principle also applies to anglular momentum, but then what is planck's constant for angles? It HAS TO BE in angular units because angular momentum times angular position is NOT THE SAME UNITS as regular momentum times regular position, just as angular enegy times time is NOT THE SAME UNITS as regular energy times time.

So, in which units does the universe prefer? I would think it would prefer units 'normalized' so that 2 pi radians = 1 cycle = 1, so each cycle carries no units. However, with h-bar being h/2pi being the angular momentum of a spin 1 particles and angular momentum being quantized as half that in, say, electrons, how exactly does that sit with this idea?

Wouldn't one think it would be something like h / 1 cycle = the angular momentum, where the angular velocity term was simply the number of cycles per second? Or, even if not, then maybe h / 2pi radians = the angular momentum if the angular velocity part was the number of 2 pi radians per second.

It... just bothers me, that's all. I mean, what's so special about radians? Isn't the universe indifferent to the types of units used?

(...Starblade Riven Darksquall...)

The two terms are directly and relevantly coupled.

For an internal combustion engine, torque is always defined at a particular rpm. Once you work out for yourself whether or not that actually gives one a different, more specific, way of describing "power" you'll have your answer. I've actually given you the only additional fact you need to know in order to solve this homework answer.

In general, Power = Torque * Angular_speed. In the units commonly used in automobiles, Horsepower = (Torque in foot-pounds) * RPM * 0.01142

Paul Cardinale

Hi,

No they're not. Torque is the rotational equivalent of force. Power = Torque * Angular velocity (in radians/sec)

Cheers,

Zigoteau.

You are right. Power and torque are different quantities. Power is torque times speed.

now how can you say that one definition of power is more accurate than another? Both were made by humans, both are used in a specific context by specific groups, and both are equally able to be misused. Many of the group posting here hold with one concept. A great many more hold to another. The context defines the answer, not a textbook.

In the real world, the dictionary always trumps a professors chalk - it has more breadth and thus more capacity for truth in application. I have not checked a dictionary for this definition, but I would bet it does not define power exclusively as work per unit time, if that.

Now, I am fully aware of the physical defintion of power used in the sciences - as a PE degreed in ME and EE with over 30 years experience from design to field, component to system, a chief engineer, a VP engineering, and having taught the concept as what some fools called 'an authority" , and having listened ad nauseum on ANSI committees to inexperienced newbie engineers and non-technical appointee managers, that is just one of the many diverting concepts young engineers find intriguing. The answer, however, goes way beyond the pablum fed in the first year at a univeristy - it goes to what senior engineers are supposed to do - communicate outside the holding pen.

Like any question posed to a technical person outside of a paper for peer review, the question goes not to the technical defintion used in physics, it goes to the context of the vocabulary.

Power lifting - Power shot - Personal power- Star power - Towing power - Firepower -

all are work per unit time? no, they are not - sport, energy density, social, economic, weight, and destructive force - but not work/time

power = force times distance per unit time? power = torque times revolutions per unit time? power = flow times volume per unit time? power = field stength times energy per time?

all of these funndamentals are valid, and yet most are incomplete and thus invalid - (think phase shifts from vibration, waves, holes and zeros, and transit interference)

but when two engineers at the proving ground say a vehicle has a lot of power, they aren't talking about work per unit time. Hell, when a chemical engineer and a mechanical engineer are looking at a 357 magnum and they both say it has a lot of power, they aren't even talking about the same thing, and I'd bet is that work per unit time is not the concept in their heads. . And if a civil engineer is in the room, he isn't talking about anything close to the other two - he means it kicks a lot and has a big bang and makes a large hole in concrete - but maybe so do the chemE and the ME - its in the context. Are they wrong? Or are the ones who hold pedantically to a definition outside the context of their conversation wrong? depends on whhether you sit in an office on the top floor or a cube down below.

engineers of all people need to understand that the context defines the solution - it's what we do. Define the problem's context first.

need to look outside the holding pen, people.

take care........

Given a constant force, F, at a given radius, R, the radius of a tire, the torque is N = R*F, and the power delivered is P = (2*pi*R)*F*f, where f is the rotation rate of the tire in revolutions per second (rps). The ratio of power, P, to torque, N, is then

P / N = 2*pi*f = w or Power= w * Torque

Where w is the rotation rate of the tire in radians per second when Power is given in Watts and torque is given in Joules or in Newton-meters. It can also be done in archaic form with power in horse power and torque in foot-lbs with the appropriate conversion factor. [Old Man]

You work up a sweat, but that is your muscles beng heat

Uncle Al has not thought this one out carefully. Muscles are *not* heat engines.

You are. Torque is the ability to do a task. Power is the speed at which that task can be done. Gears alter torque. With the right gearing, a low power engine will lift a heavy weight slowly. A high power engine will lift the same weight, but faster.

John

Torque can be stated as effort, whereas power is the rate of exerting that effort.

As far as internal combustion engines (spark ignition), don't get too carried away with the actual figures. You see, the characteriscs of an engine vary considerably depending on the amount of throttle opening used. The impressive figures quoted by manufacturers are wide open throttle figures. In normal use, car owners rarely if ever drive their cars with wide open throttle.

About the only thing you can discern from the power and torque characteriscs quoted by manufacturers is the drivability. The greater the separation between the speed at peak power and the speed at peak torque, the better the drivability. The closer these speeds are, the more "peaky" the engine is. A peaky engine needs to be "rowed" along with a manual gearbox to keep the engine operating within the narrow "power band". An engine with a broad power band does not need as many gear changes or as many gear ratios to keep it drivable.

On Tue, 12 Aug 2003 07:22:50 -0700, Russell Blackadar posted:

Well "engines producing only heat" then :)

You aren't reading carefully. He said (and correctly so) that Torque is Force. Force is not Power.

And on another point. A dictionary is not the defining item you think. A dictionary simply shows the definition of the word in current, lay usage. Since we are talking about phsyics and engineering, you need to consult an engineering dictionary for how Torque is used by engineers, designers and physicists.

I read quite well - and you need to check a basic physics book - torque is not force - torque is force cross length times phase. For you lay persons trolling, Torque requires force to be multiplied by another quantity, length, in order for torque to exist. It is not force.

that is exactly the point. The technical answers to the original question defined only one half of the conversation in question, the physics-based half. They missed the industry context half.

First, "we" did not ask the question - a lay person asked the question, and he asked it about terms used in the conversation and in the use ( context ) he described.

second, "we" are not talking about physics and engineering - unless you mean all persons talking about physics context, to have a self-fulfilling position. This half of "we" is talking about communication of an industry-specific term.

you need to consult an

Read the credentials, troll - I personally designed and built test equipment for the auto industry - GM test track simulator fourier analyzer, reflected impedance machines for Toyota engines, rotary fatigue testers, axle testers, tire impact testers, electric and internal combustion driven gear systems - from epitrichoidal to overhung worm, and hundreds of other machines and systems using advanced mechanical concepts - and I directed and led teams for design thru installation of a fair number more. And led a couple seminars at MIT for the ME profs and grads. Been at it since 1974. I do have some knowledge of the freshman-level view of force and torque.

Dear Hobdbcgv:

Lest we start a units war, I will point out to all that torque has units of force times distance... which looks like work. It is NOT work, since there is no inherent motion in (or counter to) the direction of the force. Where he says "cross length" be refers to the cross product of the force vector and the length vector. No "work" survives this operation, since it does NOT describe a motion, only a geometry.

David A. Smith