# Homework Help: How to calculate torque on a shaft [solar panel]?

1. May 16, 2015

### JetLife

< Mentor Note -- thread moved to HH from the technical engineering forums, so no HH Template is shown >

I'm busy designing a solar panel mount for a 300W solar panel, roughly 2m x 1m in size and 30kg in mass. I would like to know how to calculate the torque on the shaft (situated in the centre of the panel) when experiencing a wind force of 300 km/h.

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2. May 16, 2015

### BiGyElLoWhAt

Integrate $r* d\vec{r}\times \vec{F} d\theta$ where F is the force of the wind on the panel (you can probably google this, or model it using the quadratic drag equation with v=300km/h), w is the width and dr is the radial distance from the shaft. This will probably be zero if you assume a constant force (as long as v=300, the force will be constant). The best thing you could do is only calculate the torque for one side of the panel, that will give you the upper bound for the torque from fluctuations and the panel shaking around. This is actually more along the lines of something to the effect of a meter-torque, which makes me wonder, but I'm pretty sure that's what you want, the sum of all the torques, which on an infinitesimal scale will result in integrating the Force down r to get $T(r)$ and then around theta to sum torques.

Last edited: May 16, 2015
3. May 16, 2015

### JetLife

Thanks. Having done that, I get a torque of 1450,158 [N.m]. Surely it can't be that much? Would the motor for that torque not need to be huge?

4. May 16, 2015

### SteamKing

Staff Emeritus
You're not the first guy posting at PF this week about designing some sort of solar-gizmo:

But that was because he made a mistake in figuring the density of air.

Now, a wind speed of 300 kph is not quite as breezy as that poor fellow's, but it's not something to sneeze at either.

Your torque values are so high because you are trying to mount your solar panel in a Category 5 hurricane-force wind or a category F3 tornado:

http://en.wikipedia.org/wiki/Saffir–Simpson_hurricane_wind_scale

http://en.wikipedia.org/wiki/Fujita_scale

You'll be lucky if the whole shebang isn't torn off its mounts and blown away entirely.

Last edited: May 16, 2015
5. May 16, 2015

### BiGyElLoWhAt

Yea, it would be pretty huge. 300 kmph winds only hitting one side. Of course, it will probably never hit that, because realistically there will be cancellation from the wind pushing on the other side, but the number you calculated is the absolute maximum torque that could ever be exerted, which is 300 kmph winds on half of the solar panel (on one side of the pivot point) and 0 kmph on the other.

Stupid units... I wrote mph instead of kmph on the last couple speeds.

6. May 16, 2015

### JetLife

I will check it out. The 300km/h wind is due to South African standards [SANS 10160-3: Structural Design for Wind]

7. May 16, 2015

### BiGyElLoWhAt

Dang, that's intense man.

8. May 16, 2015

### BiGyElLoWhAt

Another thing you might try:

Find out how much winds tend to vary/meter. So say winds over the span of 1 meter reasonably vary 10kmph (I don't know, that's a made up number) at max, then you can say ok, now I know what's the minimum amount of cancellation I can expect. With that information, calculating the same thing again, but with F being a function of r (F is a function of v and v is a function of r), this will give you a probably more realistic calculation, and it will be tremendously lower. The only problem is that I don't know if that information is readily available.

**Sorry, I got negligible sleep last night and am making grammar, units, etc. mistakes all over the place.... =/

9. May 16, 2015

### JetLife

Ultimately what this boils down to is the selection of a suitable motor; however the torque provided by low speed, high torque motors I've found on the net aren't sufficient.

I have a suspicion that other post about the solar panel may be one of my peers. He/She seems to have gotten stuck at the same point.

10. May 16, 2015

### BiGyElLoWhAt

I've always wanted to run into somebody I know on here.

But yea, the most realistic approximation will be using what I said in post 8. My original post is the absolute maximum for unrealistic conditions.

11. May 16, 2015

### OmCheeto

Reminds me of the gentleman who responded to a request for daily driving data, to and from work, from Carnegie Mellon University, for an EV study:

That's one hell of a commute. No wonder he was speeding.

ps. Ok to delete.

12. May 16, 2015

### Matthew Titus

oh rub it in SteamKing

13. May 16, 2015

### JetLife

Haha thanks for the help guys. Hopefully I come up with a solution that makes some sort of sense.

14. May 16, 2015

### JetLife

Aah Titus, so that was your post? You manage at all?

15. May 16, 2015

### Matthew Titus

still working on it will inbox you if I figure out something.

16. May 16, 2015

### JetLife

Shot boet

17. May 16, 2015

### SteamKing

Staff Emeritus
For what? An air burst from an atomic bomb?

I've looked at this standard. There are a lot of factors going into the determination of the design wind speed. Are you sure you haven't made a mistake somewhere? I find it hard to believe that structures are regularly designed for 300 kph wind loadings in SA which don't look like bunkers and which don't cost a fortune to build.

18. May 16, 2015

### JetLife

Haha I questioned this myself when I first took a look at the standards. Then contacted a peer that is busy designing a parabolic reflector, he too used a wind speed of ± 300km/h

19. May 16, 2015

### Staff: Mentor

So I have two suggestions about the wind issue. First, verify with your instructor that it really is a requirement that must be met by your design.

Second, if it is, I would suggest a shaft locking mechanism, to be used when the wind speeds exceed some reasonable amount (like 50km/hr). It is not practical for things like solar panels to be expected to continue tracking in very high winds. They would typically be set in an average position and locked for the duration of the wind storm. you might also check with the instructor to verify that this is a reasonable alternative to over-designing the tracking motor and mechanism.

20. May 16, 2015

### SteamKing

Staff Emeritus
Amplifying on what berkeman said in his post, when there are high winds around, there is typically little sunlight available to shine on your solar panel, 'cuz the rain, clouds, and flying debris block it out.

In addition to the large torque, what about the structure holding up the solar panel itself? It looks rather lightly constructed, judging from the attached sketch. Is this frame even strong enough to support loads generated by hurricane-force winds?

21. May 16, 2015

### JetLife

I'll speak to my lecturer about the wind speed but we were required to adhere to the standards. If you have a look at the "Twerly Street Light" [http://groundup.org.za/article/renewable-energy-streetlights-could-change-lives_1874] [Broken], which was designed in my city, it was also designed for 300 km/h wind speeds.

All the structural supports are 100%. Largest stress I got in the frame was 140MPa. Just note that the sketch is just a rough concept. My CAD files are obviously of much greater detail. Also, in my city, which is where this mount is being designed for, we get gale force winds when its sunny out, its dubbed "The Windy City" for a reason.

Additionally, the weather, especially the nature of the wind, can change drastically fairly quickly, making it quite a mission to climb up on the roof to lock the shaft every time the wind reaches an excess speed. But, I'll have a look into it, maybe there's some automated way to do it, although it is way beyond my level of experience.

Last edited by a moderator: May 7, 2017
22. May 16, 2015

### SteamKing

Staff Emeritus
For some reason, this link no longer works (at least for me).

I don't think locking down the solar panel is a good idea. If anything, you want the panel to be free to weather vane in the wind, so that the wind pressure does not build up unduly. High winds, especially when other structures are present nearby, can change in velocity and direction quite rapidly.

If it's not feasible to permit the solar panel to weather vane, when high winds approach, then you want to enclose the apparatus inside a fairing or shelter of some sort, if nothing else to prevent it from being damaged or destroyed by any flying debris picked up by the winds.

Last edited by a moderator: May 7, 2017
23. May 16, 2015

### JetLife

24. May 17, 2015

### JetLife

Next Problem, since the rotational speed for tracking the sun is extremely low, average is 0.000694 rpm, and using a motor of 110 W, the torque experienced on my shaft is impossibly high. Note, this has nothing to do with the wind load, just power delivered to the shaft by the motor and gears. Any suggestions?

25. May 17, 2015

### BiGyElLoWhAt

It doesnt output 110w constantly, thats just the max. The actual output will be determined from the variables in your power equation.