victorzamora

Hey there! Quick question, when the servo manufacturers post a certain torque for servos....is that holding torque or dynamic torque? For example, the 8611a is reported to have up to 320oz-in of torque...is that holding or dynamic? The reason I'm asking is because I found these servos that are 8.2kg-cm dynamic torque and 16.4kg-cm holding torque per spec's. I don't know how to compare these to (for example) the 8611a. FWIW, 8.2 and 16.4 are: 113.9 oz-in and 227.8oz-in, respectively. Which one of these numbers do I compare to Horizon's claimed 320oz-in.

Now please don't start the claimed vs actual stats argument...I know, no one is honest and companies suck unless it's the company you support. Anywho, thanks for the help!

Flying Geezer

My Feedback: (14)

I don't know whether the holding torque and the moving torque are the same, but I doubt it. When you talk about 320 in. oz. of torque, I believe that means that the servo can lift 320 oz. one inch in one second.

I may be wrong but I think the holding torque would be less. On less powerful servos, I find it a lot easier to move them a little with my hand at rest. On the other hand it's really hard to hold them back when they are moving.

Also, the specs you give for holding torque is in. oz in centemeters, (metric), and the working torque is in inches. You'll have to convert the cm to inches, or the inches to cms to compare.

This is not an answer, but I hope it helps you research. There are metric to inches converters online, just google it.

I would suggest that you convert the inches to centemeters, that would make the math easier for me because the centemeters are smaller.

Campgems

Good question. I have done a bit of CNC machine setup over the last few years and the stepper motors are rated in in/oz Holding torque. I've never notice the difference on servos. Perhaps Bax can answer that question.

Don

A.T.

Servo - FAQ Torque Measurement
much more info under sub sections
"Servo - Alterations, Calculators, Databases, Leads, Repairs, Convert to an ESC or winch & FAQ" at
Alan's Hobby, Model & RC FAQ Web Links
Regards
Alan T.

Howard

My Feedback: (55)

There is no difference between holding torque and dynamic torque. Torque is simply the component of force that is perpendicular (tangent to) the radius multiplied by the length of the radius at that point. Therefore the units are always in distance x force with common measurements being ft-lb, in-oz, newton-meter, kg-cm, gm-cm, but, in theory it can be any set of units as long as it is the product as stated in the first sentence.

In this case I believe the issue being asked is whether the servo torque rating is measured when it is moving or when it is standing still which can be different because of the design (and usually cost) of the internals of the servo. For example, an analog servo has to be moved in order to develop rated torque to push back whereas a digital servo is able to push back without having to move (almost no movement). I don't know if there is a formal standard for the process of measuring servo torque - my guess is that each mfg is honest about their torque values but, tests them in a fashion to put their best foot forward.

If you want to make fair comparisons I would think it would be best to only compare analog to analog and digital to digital - for similar torque ratings. I now use only digital servos (not the expensive ones) for my flight control surfaces, not because of speed but because they resist the onset of flutter by not moving.

One last comment - it is easy to convert the different units of torque to the one you want by simply knowing the relationship of how many centimeter,meters are in a foot/inch and how many grams are in a lb/oz. Jusst muliply/divide it out.

Howard

victorzamora

Well, I already did the conversion....it's in my first post (114 and 228 oz-in). I know that the definition of torque is the same whether or not the servo is moving. Also, I know the difference between digital and analog servos (kind of). Also, the stats given for the other servo (not the 8611a) show that its dynamic torque is about half its holding torque. That means that it can hold twice as much weight as it can move.

I don't really know how to phrase my question. I guess I'm asking if anyone knows if the advertised torque of most servos is dynamic or holding. I mean, do I compare the 8611a's 320oz-in of torque to the Hyperion's 114oz-in OR its 228oz-in. I might have to ask Bax unless one of you know.

A.T.

Perhaps what you seek is a list of specifications compiled by a neutral party e.g.:
Servo - Database + Download Winzip Program "Choisir ces servos"
As mentioned in post 4 there is much more info under sub section
"Servo - Alterations, Calculators, Databases, Leads, Repairs, Convert to an ESC or winch & FAQ" amongst others at
Alan's Hobby, Model & RC FAQ Web Links
Regards
Alan T.

XJet

In my testing I've found that holding torque is generally higher than stall torque.

The holding torque is defined as the amount of force at which the servo will still hold the commanded position against a load.

So if you hang a weight from the end of a 1 inch horn which is parallel to the ground, the holding torque is the maximum amount of weight you can add before the arm starts to rotate away from the horizontal.

Stall torque is different (and usually less). If you imagine you again hang weight from that 1 inch servo arm, the stall torque is the amount of weight it takes to stop the servo from moving when commanded to rotate against the load.

Why is holding torque usually greater than stall-torque?

Well there are probably a couple of reasons.

First, there's inertia. Ever tried pushing a car? It's a lot harder to get it moving than it is to keep it rolling. Overcoming that initial resistance requires more effort than maintaining the movement. The same is probably true with servos. Because of the gear ratios, the mere inertia of the motor's armature requires quite a bit of force to overcome. Try turning a regular servo arm (with the servo unpluged) and you'll see that it does require more force to start it turning than to keep it turning.

Secondly, as the amount of force applied to the servo's mechanics is increased, so does the distortion of the gears and deflection of the shafts. When the gears are distorted and the shafts deflected, they become harder to turn because the gear-mesh is imperfect and the friction created by the side-loading rises significantly. This means that the higher the load on the servo, the less of that force goes into trying to fight the motor torque. If it requires a force of x to stall the servo, it requires a force of more than x to make it rotate further because as you increase the force, some of it is absorbed by the increasing friction in the mechanics.

Of course the servo industry is a highly competitive one so most manufacturers tend to "fudge" the numbers. When Hitec claimed huge torque figures for the HS5955TG for example, they forgot to tell you that this was only for a few seconds and after a short period, the torque would drop dramatically as the servo got warm. Some other manufacturers just outright lie about the torque (and speed) of their products so as to compete with others doing likewise.

In short, don't accept the claims of any servo-maker as gospel.

________
When I'm not here I'm at [link=http://rcmodelreviews.com]RCModelReviews[/link]

txtkiller

All servos have a starting power rating. It will always be lower than holding power. Look for a servo with a high start rating Not a holding power rating. If a servo has a high holding power and a low starting it can't push pass the holding power. I wouldn't use it. It's all about how much you can move not hold.