motor torque - essentials 3

[cervantes]

hi all,

this is really a fundamental question
i need someone who can explain it in "mechanics for dummies" way

imagine I am not a technician, I completely do not know anything about mechanics....

on motor datasheets torque is specified - is it a torque directly on the motor axis?... is it increasing with pulley diameter if pulley is attached to the motor?...

see attachment:
if the motor has torque=1Nm and the load on the belt is 200N is it powerful enoough to move it? (does pulley size matter here... should I multiply torque x radius of pulley, then I will have bigger torque)?

thanks for understanding

 

[desertfox]

Hi Cervantes

The torque on the motor is the rated torque the motor will produce (torque produced by the motor electrical coils and transferred to the motor output shaft) at a given speed or speed range.
So in mechanical terms if you put a pulley on the motor that was 2 metres in diameter and exert 1N force at the pulley outer radius in the opposite direction to that the motor rotates, it will stop the motor.

We need more information to properly solve the belt pulley problem:-

type of electric motor =???

motor speed in RPM = ????

motor power=????

The will govern what you actually achieve in practice however the motor should be selected on the basis of the load it as to work against so:-

load = 20kg

velocity of load = ???

 

[desertfox]

Hi

Found this link, it explains basic torque on page 6 or 7

http://ecatalog.weg.net/files/wegne...f-electric-motors-50039409-manual-english.pdf

 

[cervantes]

thanks
so it looks like the drum diameter is not included in calculations so it doesn't matter what's its diameter?

 

[cervantes]

sorry. pls delete my last post

 

[rb1957]

this 2nd pic is very different to your first one.

the way i read your first pic, the 2nd motor needs 2x the torque of the first. It can work with the same power rating, only it'll run at 1/2 the speed. clear as mud ?

"so it looks like the drum diameter is not included in calculations so it doesn't matter what's its diameter?" ... i don't know what picture you're looking at, but the radius is clearly mentioned in the calculation ... the diameter of the load (20N) is 0.2m, so the applied torque is 20*(0.2/2) = 2Nm (and they clearly show different radii need different forces to create the same torque.

Quando Omni Flunkus Moritati

 

[rb1957]

likely a dumb question, but why is the unit of power in imperial units written "cv"? ... is it a translation of "hp"?

Quando Omni Flunkus Moritati

 

[TheTick]

You need to back away from motors and just get an understanding of torque. There is no "for dummies" explanation that will work if you do not. You also need to understand the relationship between torque and force.

Torque does not change with distance from axis. The force required to resist torque decreases the further you get from the axis. Think about using a long wrench versus a short wrench. Longer wrench = same torque but less force.

 

[3DDave]

About cv, From Wikipedia Horsepower:

"Metric horsepower (PS, cv, hk, pk, ks, ch)[edit]The various units used to indicate this definition (PS, cv, hk, pk, ks and ch) all translate to horse power in English, so it is common to see these values referred to as horsepower or hp in the press releases or media coverage of the German, French, Italian, and Japanese automobile companies. British manufacturers often intermix metric horsepower and mechanical horsepower depending on the origin of the engine in question. Sometimes the metric horsepower rating of an engine is conservative enough so that the same figure can be used for both 80/1269/EEC with metric hp and SAE J1349 with imperial hp.

DIN 66036 defines one metric horsepower as the power to raise a mass of 75 kilograms against the earth's gravitational force over a distance of one metre in one second;[13] this is equivalent to 735.49875 W or 98.6% of an imperial mechanical horsepower."

 

[LittleInch]

As a real starter here

Torque is a measure of how much force something is twisting. This is normally applied to a shaft or circular object rotating around a central point e.g. a bicycle wheel.

Torque is measured as force time distance, where the distance is measured from the axis in a radial manner.

Hence 10 Nm on a shaft can produce a force or 10N at a distance of 1m from the axis - e.g. the force on a bicycle brake block whilst someone is still pedalling.

alternatively if you apply a force of 10N on a lever 1m long, it will produce a torque of 10Nm at the end of the lever, like a long spanner

10Nm will also produce a force of 5N at a distance of 2m and similarly a torque of 10Nm if you apply a force 5N on a 2m long lever and so on and so on.

For your motor the torque is a result of the size of the motor. The torque figure you are probably interested in is the amount of torque it produces when it is rotating. This stays the same regardless of whether your pulley or wheel or whatever that is fixed to shaft is 10cm in radius or 1m in radius. What changes is the force it can apply at the outer radius. Basically the bigger the wheel the lower the force. Think of the bicycle with a fixed gear ratio. The motor (you) can exert the same torque on the shaft of the main gear. However if you put a 16" wheel on, you can go up steeper hills than if you put a 24" wheel on. Same torque, but less force to haul you and the bike up the hill.

does that help?

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[rb1957]

cv = metric horse power ? ... what happened to Nm ?

Quando Omni Flunkus Moritati

 

[LittleInch]

cv is the French version - cheveaux or "horse". Most metric powers will be kW, not hp.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[cvg]

so metric doesn't really simplify things after all...

and we need simplistic definitions for torque, work and power

a newton-meter is not a measure of power or force, it is a moment (aka torque), ie force times moment arm

work equals force integrated over distance

and power is the work (or energy expended) integrated over time

 

[kjoiner]

If you take the analogy of a wrench and look at it from the nut's point of view, you will have an idea of what the motor must do to overcome a load. Take a socket wrench and hold onto the socket with one hand and turn the wrench with the other. Do this flat on a table to negate gravity. The further your "turning hand" is out on the wrench handle, the harder it is to hold the socket still.

If you have 1Nm of torque on the motor, that means it can generate 1N of force on an arm 1 meter long. You have 200N of force so you need a much shorter arm. You need an arm length of 1/200 meters long or .005 meters. That's only a 5mm radius or a pulley diameter of 10mm.

There are other things to consider with an electric motor. It will draw more current the harder it's worked - up to a limit. That could be the power supply or the motor itself. Also, there is overhung load on the shaft. When you convert torque to a linear force, the shaft sees a bending load. You may be able to use a pulley small enough to generate the force, but you could also end up bending the motor shaft or damaging the bearings.


Kyle

 

[ades]

Torque es forcé x distance, if you put a bigger pulley, the toque is the same, but the forcé at the circunference of the pulley will change proportionally.

 

[electricpete]

cv literally tranlsated means horse vapors which somehow I still find funny.

http://www.eng-tips.com/viewthread.cfm?qid=278021

=====================================
(2B)+(2B)' ?

 

[LittleInch]

Err, no it means steam horses. Vapeur translates as steam, not vapours.

Still fairly funny but in a different way.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[electricpete]

multiple ways to translate...

http://translation.babylon.com/french/vapeur/

I choose this one...

http://www.seinology.com/scripts/script-121.shtml

=====================================
(2B)+(2B)' ?

 

[LittleInch]

Maybe, but which one makes more sense. Steam horses still sounds like a great way of talking about power.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[yoshimitsuspeed]

It's been explained pretty well in here but I thought I'd post up these links. I was stoked when I found them and I feel like they describe the concepts very well.
Proper usage of torque and HP terminology are huge pet peeves of mine and in the automotive world it's pretty universally misunderstood and misused from individuals posting on line all the way up to manufacturers commercials and selling points.

One that I still have trouble trying to remember do do is properly use pound foot and foot pound in the proper situations.

http://www.epi-eng.com/mechanical_engineering_basics/energy_and_work.htm

http://www.epi-eng.com/mechanical_engineering_basics/power_and_torque.htm