Torque calculation

I would think trying to 'calculate' it is a complete waste of time..

Measure it, or look up suggested solutions from people with experience or a rule of thumb or look-up table...

But calculate? Nah.

A mass of 8.5kg on a perfect bearing with no wind resistance would take very tittle tourque.... It's the real life effects which are of most importance....wind resistance being pretty significant.

I'm happy to get shouted down on this...but my money is on the guy who'se been there, done that and says you need 1/2 a hp per 10' of mast...

How's that for a comple guess?....any offers. Tonights star prize will be a bad limerick.

I think Del's got it.

Yeh, but the ointment does help!..

I agree - it's meaningless to try to calculate the torque requirement. Assuming real engineering - there's some kind of bearing the antenna is sitting on - which is probably lubricated by grease. I bet that grease's viscosity changes with temperature, and with the amount of dirt it accumulates.

I suggest measuring is the best way to go. Attach a bar radially; to that bar attach a sensitive (and low static-friction) force gauge, preferably one with a gizmo that allows it to remember the maximum force measured.

Measure the distance from the center of the antenna's mast to the point the gauge attaches to the bar.

Pull very gently on the free end of the gauge, with slowly increasing force. At some point the antenna will start to move.

Multiply the maximum force measured by the distance you measured - that is the minimum torque your motor must be able to deliver (for an ambient temperature the same as the temperature at the time you do the measurement). But note - depending on the gear ratio between the motor's shaft and the antenna's mast, the motor will need to be that much less strong. For example, say you attached the gauge at 1m and it registered 0.1N, a minimum torque at the antenna mast of 0.1Nm is required; if the gear ratio is 20:1, however, the motor only needs to deliver 0.005Nm at its own shaft.

If you can, I recommend making up a gallon of crushed ice and encase the bearing housing in ice for an hour or so before making your measurement - that will give you a better reading of its torque requirement when cold.

Note: as the previous post said, you may need to take into account wind acting on the antenna. You may also need to take into account the mast freezing to the base (in which case you may have to have an overload cutout on the motor controller).

Assuming that the antenna is (more or less) symmetric about the mast, then wind effects will mostly have the effect of increasing the friction between the mast and its bearings, plus some direct torque as the wind impacts any asymmetry of the antenna. To measure the effect of the added bearing friction, have a friend push the mast (radially, so he/she doesn't add any torque) while you are doing your measurement.

When you have a motor torque estimate you are happy with, I recommend you double it to account for things you haven't anticipated.

Note about the design of the measurement: the stepper motor may have to overcome static friction, and it will have to impart acceleration to the antenna. By applying a low and increasing amount of torque to the antenna, with it stationary at the start, allows you to directly measure the maximum torque it requires to (a) overcome the static friction and (b) accelerate the antenna (while overcoming the dynamic friction of the system).

HTH,

Odessey2001

Is it going to b a direct drive? Chain and sprocket? how fast u gota turn it?Steppers like to be ramped up to get going. What about drive ratio? U know there's a question or 2 needs addressed before you get around to the motor part of the deal. The "steps" may turn out to be the easiest part.

Good luck

Del's pretty much right on the mark.

There are too many questions, such as, does the mast only turn or the entire platform? what supports the mast? are there any guidwires? How big is the platform if this is what you are turning. What is the platform rotating on? ETC.

Then again gear ratio's also change the requirements. Search out an existing similar configuration and question the facility or engineer.

I could have a 8.5 kg antenna that was 12mm in diameter and 1 meter long rotating about it's center axis or a multiple arm beam antenna that was 3 meters long with 2 meter long arms and mounted off the center of it's axis.

I would only rotate it when there was no wind or when the wind was 100kph.

If you want assistance with a problem post a question with all the parameters listed.

Otherwise you are just wasting your time and everyone else's.

Measuring is a pretty good idea...but you'd still need to allow a v good safety magin for windage....factor of 5? 10?

The way I look at it ..you are unlikely to regret putting on a motor which is too powerfull..whereas...