Servos

[Tim Mackey]

Nothing too complex, and its very generalized, but there is a small chart summarizing things for beginners when they are faced with the bewildering choice of servos available these days over HERE.

This prompted a bit of chat about various aspects, which is absolutely fine and welcome - so here's the place for that.

Edited By Timbo - Administrator on 27/02/2010 09:09:39

[John Privett]

Just to be boringly pedantic...

 

The unit of torque is frequently mis-stated as kg/cm.  That is wrong,  it is kg.cm.

 

I wouldn't normally bother to mention it as it is such a frequent mistake,  but since the chart will be seen as having the authority of the forum and RCM&E behind it, it ought to be correct.  Otherwise it just perpetuates the error.  "Well, RCM&E says it kg/cm, so that must be right..."

[Former Member]

[This posting has been removed]

[John Privett]

The unit of torque is Force times distance. 

 

In your example;

 

If a servo can pull 1 kg at 1 cm from the center

 

then the torque is 1 *1 = 1 kg.cm.  (force multiplied by distance)

 

then at 2 cm it can only pull 1/2 Kg

 

then the torque is the same at 2 *0.5 = 1 kg.cm. (again, force multiplied by distance)

 

Further references:

http://en.wikipedia.org/wiki/Torque
Torque has dimensions of force times distance. Official SI literature suggests using the unit newton meter (N·m)

http://www.gordonengland.co.uk/conversion/torque.htm
The product of force and lever-arm distance, which tends to produce rotation. One newton metre is equal to the torque resulting from a force of one newton applied perpendicularly to a moment arm which is one metre long. The SI derived unit commonly use for torque is the newton metre.

http://www.boltscience.com/pages/convert.htm
Torque is a measure of how much twisting is applied to a fastener. The units used to measure torque are in the form of force times length.
The metric (SI) unit of torque is the newton metre (Nm).

 

 

[Myron Beaumont]

Isn't the dot following Kg partially misleading . Correct me if I'm wrong but in the oldays ('ere we go I hear you say) when torque was in foot pounds ,it  was generally written as follows      ft - lb   so perhaps nowadays (metriculated ?) it might be  kg-cm .Expressions of torque , both of them .Don't mention Newtons - never did understand them -then again Newton is 'er indoors' surname '

Otherwise ,a handy little chart Timbo

[Terry Whiting]

John,

 

I think you were being pedantic.

 

This is a thread for beginners. I thought Timbo's chart was and is a damn good guide

for any newbe to this hobby.

I'm sure when you entered this hobby you were not concerned about the forces of torque

when using your bog standard servos which at the time I'm sure were the only servos available. 

[Tim Mackey]

Ive had a google around and got more confused - as you do

It seems the Yankies use ft.lb or kg.lb ( the dot ) and we tend to use either the slash which is really "divide"  or a dash.

 

Heres a couple of other bits of stuff I found too.

 

Torque is the measurement of force given over a distance. For example, imagine a lever (like a crowbar) where you apply a force along its length (like to pry open something). This lever's torque is that force multipled by the length away from the pry point. For servos, think about the servo rotating a servo arm, the torque is the length of that arm multiplied by the force. In the USA, most servo torque is measured in oz-in (force in ounces times inches, or ounce-inch). Servo torque is measured by using force or weight at a radius from center of the output shaft to see when the servo stalls as it tries to lift the weight horizontally.

To convert oz-in to kilogram-centimeters (kg-cm) just divide by 13.9

 

The torque does not change as you get farther from the center.
A servo with 20 oz/in of torque has 20 oz. of force 1" from the center. The same servo has 40 oz. of force 1/2" from the center, and 10 oz. of force 2" from the center.

force * distance from center = torque

 

I think I can live with the dot - its not the aim of the chart to educate folk on the absolute correct technical terms etc, and I guess the people who will read it will understand it for what it is.

Thanks all for your input...

Edited By Timbo - Administrator on 26/02/2010 09:07:38

[Tim Mackey]

Cross posted there with Terry

No need to worry or argue guys, its just a little chart, but John was only trying to help

[John Privett]

Posted by Timbo - Administrator on 26/02/2010 09:04:25:

I think I can live with the dot - its not the aim of the chart to educate folk on the absolute correct technical terms etc, and I guess the people who will read it will understand it for what it is.

Thanks all for your input...

Edited By Timbo - Administrator on 26/02/2010 09:07:38

 

Absolutely right - the chart (and a good chart it is too) doesn't aim to educate about the units used.  But since the units are mentioned then they really ought to be correct.  Anything else just makes the source look a little dubious and helps spread the confusion. 

 

Any site that quotes torque as force/distance is wrong, and always makes me wonder, if they've got that wrong,  what else are they incorrectly stating as a fact.

[Ultymate]

Please don't forget Newton x . / +  metres please guys Lighten up and fly straight  

[Tim Mackey]

I know Andy Ellison did an article about servo selection and the effect of output arm length etc on forces and so on, but I am blowed if I can remember when or where it is.

David A / Andy - any ideas ?

[A.A. Barry]

Guys, don't take this the wrong way......in 1969 Aus changed every measurement ,wieghts,sizes,fluid amounts,.....every thing is by 10 eg...

1000g weight is 1kg = 2.2lb  the K factor is a cosign of 000, i think

It took a long time for us who grew up with inchs feet yards miles lbs tons, pennys shillings pounds to get to understand it all

In  US they have monies in x 10 but stick to the lbs, pints very odd

 I seem to believe the UK has a monies in 10 but nothing else

The metric  ( x 10) system, once learnt is so much easier, the kids here have no problems div. sub. x. weights and measures 'cause it's all in x 10

Absolutly nothing to do with the chart, but it is a valid point

a.A.Barry

 

OH I forgot NZ has the same. correct me if I am wrong Kiwi

 

Edited By A.A. Barry on 26/02/2010 10:06:52

Edited By A.A. Barry on 26/02/2010 10:10:32

[David Ashby - Moderator]

Steve Dorling wrote an article that's here on the site while I seem to remember Andy's articles on 3D flying and set-up also mentioning something along those lines. 

Edited By David Ashby - RCME Administrator on 26/02/2010 10:08:04

Edited By David Ashby - RCME Administrator on 26/02/2010 10:08:56

[Biggles' Elder Brother - Moderator]

I just happened to read Andy's article a few days ago, but like you Timbo, frustratingly, I can't find it again. If my memory serves me right he also uses the Kg/cm notation.

 

To be honest I don't think John is being pedantic. I mean no critism of Timbo - great, useful post. And I know that a lot of people - including in the trade - use the notation Kg/cm. But it is "wrong" I'm afraid, and it does matter because using this wrong notation obscures an important fact about troque - i.e the torque is fixed (as Timbo sates) so the multiplication of force times distance is always going to give a fixed answer which is property of the servo. So if you double the distance, you half the force. And that's important to understand, if you go for more linear movement you get it at the expense of less actuating force. Only using the correct units makes this obvious.

 

Now if you want real pedantic - then it should be in Nm (Newton-metres) not Kg-cm. The Kg is a unit of mass - not force!

 

But anyway - Note to beginners: This is a really useful chart and all this other stuff is just friendly, maybe interesting?, chat by by a load of boring old you-know-whats around the subject!

 

BEB

[Tim Mackey]

Thats it thanks David

Dont worry your memory will fade as badly as mine with the advancing years

 

Edited By Timbo - Administrator on 26/02/2010 10:29:15

[Tim Mackey]

To take an extract form Andy's piece, you will see that he does indeed use the /

As does Steve ( must be something on the slope thats causes it )

 

"Servo torque is usually rated in 'kilograms per centimetre' (kg/cm), e.g. a 3kg/cm rated servo will provide 3kg of force to the control surface from a point 1 centimetre out along the servo arm. Attach your pushrod at 2cm from the centre of the hub and the servo can only apply 1.5kg of torque. Put an overly long, 3D-type arm on your servo at 4cm from the hub and you'll only get 0.75kg of torque"

Edited By Timbo - Administrator on 26/02/2010 10:30:07

[Erfolg]

I am surprised at the wide differences of opinion.

 

I am sure that many engineers and scientist types will have been taught the same method as I have (Myron Biggles etc). It is simple.

 

At the side of the relationship you write the units being used. You then proceed to strike out those which are common to the numerator and denominator

 

so cm * m/cm becomes m, simples.

 

As has been stated T=Fd which is kg * m. It has been pointed out that this is wrong, I know, but this is common usage by modellers ignoring g = 9.81 on Earth. If you take the engineering approach it can become much more complicated where the torque is rotational, bringing in angle and vectors 

 

It can be made to seem  worse as Nm now has become a Joule. and you can play around with the units as a Newton is the work done in moving through 1m. per sec. Now we keep correcting each other as debating the number of angels can stand on a pin head.

 

We can all see it is a relative value as in the table, I believe that in engineering terms it is Nm,  in modelling terms it is Kg*cm as the accepted description 

Edited By Erfolg on 26/02/2010 12:09:12

[Biggles' Elder Brother - Moderator]

Actually Erfolg you bring up an interesting point. One of the questions we often challenge engineering undergrads with is

 

"The units of torque are Nm, while the units of work done are also Nm - how can two different (unrelated) things have the same units?"

 

The answer of course is exactly as you stated - torque is a vector quantity in that it has magnitude and direction and both must be stated to completely define the torque. Whilst work done - is a scalar and has only maginitude - no direction.

 

BEB

[Myron Beaumont]

BEB 

Work does have a direction -------Walk away from it  

[Erfolg]

Biggles

 

In my mind modeling has many similarities with engineering. Much is art, understanding of materials and the application of science. Most modellers I suspect preffer the art and the craft side of materials.

 

The table is useful in that it allows us to see the relative attributes of the various types/class of servo. It is structured  to allow comparisons in a world which is occupied by modellersin a way that is relevant tor modellers.

 

How necessary is it useful to know what a Pascal is? Is it wrong  to write N*m-^2 or kg*m--^1 * s-^2, in my mind it is what helps engineers manipulate data and not to be treated as absolute right or wrong in presentation, other than that some units are preferred, often for convenience and allows us to have discussions that go on for hours. Most modellers could not care less.

 

I suspect that like myself you get your satisfaction from using your engineering/scientific knowledge in rationalising what will or has happened. Most find this approach boring and toatally irrelevant, they are right of course, that is, for them.

 

I like the table, and wished we had others. Although we have a Lipo chart, Timbo could be put to work on others.

 

I have to congratulate Timbo for the effort, putting his head above the parapet and now what about the other tables, I am waiting in anticipation. 

 

Erfolg

[Biggles' Elder Brother - Moderator]

Totally agree Erfolg. The table is great - just having a bit of fun "chewing the fat".

 

Its true of course on Units - each can use whatever he/she wants - and for 99.9% of cases one set is as good as another. The problem is, if each person/manufacturer uses different units it becomes very difficult to make comparisons. Also muddling Kg's and Newtons for example is a big problem and is one of the reasons UK science went away from imperial - to avoid the pounds/ poundels/pound-force confusion - but it seem we have it back only with Kg's and Newtons!!! Ah, well twas ever thus

 

BEB

[Former Member]

[This posting has been removed]

[Biggles' Elder Brother - Moderator]

Old Syllabus                     New Poly Syllabus

Physics                              Aerodynamics and Mechanics

Chemistry                         Adhedsive Technology

Woodwork                         Model Building

Geography                        Navigation

English                              Reading RCM&E

French                               Reading RCM&E 7 days later than everyone else!

Mathematics                    Calculating the number Lipo's needed

PE                                      Jogging around the strip carrying all your gear

 

BEB

 

[Tim Mackey]

Me thinks I am gonna have to move the OP and lock it off from silly stuff - maybe we will keep this for..... ahem..."discussion"

Done...Chart is now HERE

Edited By Timbo - Administrator on 27/02/2010 09:19:18

[Myron Beaumont]

BEB

You forgot  IT that I think is on the syllabus!------- Programming R/C equipment and use of laptop for all modelling requirements ----No games -just flight sims ?

All homework ( shed work) to be scrutinised by Professor Timbo  RCME-SMAE