Transmitting Torque: Meaning & Why Not Power?

[ZdMh]

When we talk about for ex. a driver rotates a pump, we say "a torque is transmitted to the pump". What does exactly this mean "Transmitting torque"? and why we said Transmitting torque and not transmitting power for ex.?

 

[A.T.]

why we said Transmitting torque and not transmitting power for ex.?

power = torque * angular_velocity

 

[anuttarasammyak]

I assume they would like to mention that some power of rotation is transmitted to another power of rotation with combined rolls or gears in mind.

 

[jbriggs444]

Summary:: Why we say "Transmitting torque"? not something else?

When we talk about for ex. a driver rotates a pump, we say "a torque is transmitted to the pump". What does exactly this mean "Transmitting torque"? and why we said Transmitting torque and not transmitting power for ex.?

Six of one, half dozen of the other. Depends if you are thinking about a socket wrench or a drive shaft.

 

[ZdMh]

Six of one, half dozen of the other. Depends if you are thinking about a socket wrench or a drive shaft.

I'm thinking of a drive shaft in this case. But I didn't get exactly what you meant...

 

[Lnewqban]

Welcome, ZdMh
I don't know the exact answer to your question.
The use of that word is more common for mechanisms that have rotating parts, such as gears, shafts, pulleys, flywheels and sprockets.
In most cases, torque is transferred between rotating parts via shafts to which they are solidly linked by keys or splines.

Perhaps it is associated to vehicle's transmission, which includes gear box, shafts, torque converter and differential and which function is to transfer torque from the engine to the rear wheels.
When we combine the concepts of transmitting torque and the rotational speed of parts, we arrive to the concept of transmission of power.

For other mechanisms, such as linkages, linear actuators, hydraulic cylinders, etc., the word is also used for transferring of forces from one part to another, but it is less common.

Then, we have the generation and transmission of electric power through interconnected nets.
If produced in a power plant, we again have the case of transmitting torque and the rotational speed of steam turbines and generators.

 

[vanhees71]

Maybe it's because English is not my mother tongue, but I have problems to understand what "transmitting torque" should mean. I also couldn't make sense when you'd say a "force is transmitted to a body". A torque or force is applied and what's transmitted is angular momentum or momentum, as described by the equations
$$\dot{\vec{p}}=\vec{F}, \quad \dot{\vec{J}}=\vec{\tau}.$$

 

[Leo Liu]

Do you mean the word "exert"?

 

[jbriggs444]

Maybe it's because English is not my mother tongue, but I have problems to understand what "transmitting torque" should mean. I also couldn't make sense when you'd say a "force is transmitted to a body". A torque or force is applied and what's transmitted is angular momentum or momentum, as described by the equations
$$\dot{\vec{p}}=\vec{F}, \quad \dot{\vec{J}}=\vec{\tau}.$$

If the body (e.g. a drive shaft) is rotating at a uniform rate and if one is applying a torque at one end then one can harvest an identical torque at the other end. One might reasonably say that the torque has been "transmitted".

The same could be said about power.

 

[ZdMh]

Thanks guys for your time to explain the idea, I truly appreciate!

Do you mean the word "exert"?

I didn't get what you mean by that?

 

[vanhees71]

If the body (e.g. a drive shaft) is rotating at a uniform rate and if one is applying a torque at one end then one can harvest an identical torque at the other end. One might reasonably say that the torque has been "transmitted".

The same could be said about power.

Well, in German I would not say "eine Kraft wurde übertragen" (which I'd translate to "a force is transmitted"), I'd say "Impuls wurde übertragen" ("momentum is transmitted"). "Eine Kraft wird angewandt/ausgeübt" ("a force is applied/exerted".

In general I think you cannot transmit "entities of intensity" ("Intensitätsgrößen") but only "entities of quantity" ("Quantitätsgrößen"). See the very instructive preface to Sommerfeld's Lectures on Theoretical Physics vol. III.

 

[etotheipi]

Well, in German I would not say "eine Kraft wurde übertragen" (which I'd translate to "a force is transmitted"), I'd say "Impuls wurde übertragen" ("momentum is transmitted"). "Eine Kraft wird angewandt/ausgeübt" ("a force is applied/exerted".

In general I think you cannot transmit "entities of intensity" ("Intensitätsgrößen") but only "entities of quantity" ("Quantitätsgrößen"). See the very instructive preface to Sommerfeld's Lectures on Theoretical Physics vol. III.

I think you are right in the broader context of flux and transport phenomena, that the momentum is that which crosses (is transmitted across) the surface but, the force only describes the rate of flow.

Sometimes the word 'transmitted' is used for forces, but in a looser sense. For instance if you take 10 light springs, join them end-to-end vertically, and suspend a mass at the lower end, then the tension in the lower spring equals the tension in the top spring. We might say, colloquially, that the spring arrangement "transmits" a force equal to the weight of the suspended mass, to my hand.

 

[A.T.]

Well, in German I would not say "eine Kraft wurde übertragen"

I get a lot of hits for "Kraftübertragung", including scientific and engineering sources.

https://de.wikipedia.org/wiki/Kraftübertragung

 

[vanhees71]

Well, terminology in everyday language and unfortunately also slang among scientists is not always of the precision desirable for expressing science!

 

[A.T.]

Well, terminology in everyday language and unfortunately also slang among scientists is not always of the precision desirable for expressing science!

Differntiating between force/torque transmission and power transmission allows for more precision.

 

[etotheipi]

Differntiating between force/torque transmission and power transmission allows for more precision.

But for instance if some charge flows through a resistor, then we would say there is a current through a surface in the resistor, but it is the charge that is transmitted through that surface. Likewise we might speak of power through a surface within the resistor, but it is energy that is being transmitted.

I think it is still imprecise to speak of transmitting a torque, for instance, since all you are really saying is that there is a torque on one end that equals numerically another torque at the other end. Same for power.

 

[A.T.]

I think it is still imprecise to speak of transmitting a torque ...Same for power.

Doesn't change the fact that both is used, and doesn't help the OP to understand the usage.

 

[etotheipi]

Doesn't change the fact that both is used, and doesn't help the OP to understand the usage.

Okay, sorry. In any case it is instructive to differentiate between torque as a means of transmitting angular momentum to a system as opposed to its colloquial usage as a numerical equality of two different torques in an equilibrium case.

 

[wrobel]

transmission of the torque

 

[vanhees71]

Doesn't change the fact that both is used, and doesn't help the OP to understand the usage.

There's no better way than what we just have achieved! There are both expressions used and there's one that needs to be refined to get a full understanding.

 

[sophiecentaur]

Differentiating between force/torque transmission and power transmission allows for more precision.

Yes- its necessary in practice.
If we were discussing Electrical Power Distribution, we would mainly be interested in the Power and in the operating Voltage. Although it's the Power / Energy that we'd be paying for, the system Voltage is something that really needs to be known. Likewise, the torque must be known in any mechanical transmission system so that it is strong enough and cheap enough.

But I don't think it achieves a lot to get hung up on the use of words when in fact there is no confusion about what's meant. Seriously, is there a better word than 'transmit' to describe what happens when a force / torque is applied somewhere and turns up somewhere else? Trans and Mission imply sending across; what else would we want to say?

 

[Leo Liu]

I didn't get what you mean by that?

What I mean is that in the case mentioned, the particles making up the driver exert forces on the pump, and therefore "transmit" torques to the pump.

I guess you can also view this as a transfer of angular momentum.
$$\Delta \vec L=\int\limits _i ^f \vec \tau_{driver} dt$$
In the equation above, the torque of the driver changes the angular momentum
$$\vec \tau_{pump} = \lim_{\Delta t \to 0} \frac{\Delta \vec L}{\Delta t}$$
which subsequently creates a torque on the pump.