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[dirkske]

Could a gimbal motor be used to power a small paddle tug, direct drive?  Say, non-scale-just-for-fun 600mm length?
Stupid idea, not possible?

[chas]

Possibly not. As I understand them, gimbal motors aren't many to have a significant load.

[dirkske]

I read that too. Sadly I have yet to find any reference to actual tests on what load is acceptable. I had hoped someone on this forum had some practical experience with them in a similar application.

[malcolmfrary]

A description of one from an ebay seller-





Specially designed for brushless gimbal, suit for gimbals for card cameras like Gopro or SJ4000 within 100-200g in weight.
2208 39g 3mm shaft Gimbal Brushless Motor 90KV for 100-200g GoPrO SJ4000
Item name:G-SUN 2208
Weight:39g
Shaft Diameter:3mm
Motor diameter:28mm
Length:26mm
Load weight:100-200g cameras like SJ4000
Connector type:Futaba/JR compatible
Camera range:100-200g (SJ4000)


There might be enough information there for somebody who understands the figures to give an opinion.  If usable, the main advantage, rather than direct drive, might be in making the gearing/pulley ratios simpler to arrange.

[dirkske]

Malcolm,I'd guess that one to be a tad on rhe small size. Was thinking about twice that diameter.
I know they are not build for power. Yet a slow paddler surely cannot require much power. Or does it? Newby here.

[malcolmfrary]

It doesn't really matter what the final propulsion is - paddle, prop or jet, to move a boat of a given size at a given speed requires enough power.  That is power output from the motor, not electrical power in.  You still need to move the boats displaced weight of water by its length to move the boat forward by that length, plus slippage.  There is nothing in that specification to indicate what power the motor is capable of handling, is there a specification or a link to the motor you were thinking of?

[dirkske]

Here is a link to a motor:

https://hobbyking.com/en_us/turnigy-hd-5208-brushless-gimbal-motor-bldc.html

Not much power info eighter, for gimbals the usual statement is what camera weight they can handle (this one up to 1.8kg). But thats misleading as power reference since those cameras are balanced. The motors must overcome the camera inertia while swiveling them, not lift them.

[andrewh]

Hi, dirkske

sorry - copying has led to some formatting thing I hhave not been able to remove (yet)
I think we have enough data from malcolmfrary and yourself to make some headway

2208 39g 3mm shaft Gimbal Brushless Motor 90KV for 100-200g GoPrO SJ4000

forgetting the go-pro bit - which I don't profess to understand or use - and we have a full description of the motor

its got 22mm diameter (normally stator OD in mm) and is 08mm thick core with a very low kV of 90

the kV tells us that it spins at 90rpm for every volt applied, so say a 6V battery would spin it at 90 x 6 rpm =540 rpm

that sounds prettty good for a paddler or small tug. 

Clearly the motor is designed for low speed, with high torque at low revs - hence it has a very fine wire winding and multiple poles, so we are not going to see huge power (or current)

Worth a try


The other motor is similar, only much more so!
its 52mm OD and the same 8mm thickness with a kV of only 31, so 6 V will spin it at approx 180rpm


It says nothing about power or current, but it is wound with lots of turns of 0.25 mm dia wire


Incidentally, the working of a gimbal is such (I believe) that these motors do not rotate - they are energised by gyro signals to make tiny changes in angle, but powerfully.


I would not hesitate to use them for very low speed high torque duties, especially if they were cheap
andrew


 

[Allnightin]

Instead of a gimbal motor, how about a continuous rotation servo?

This would offer speed control and reduction gearing in the same package at fairly low cost?

[malcolmfrary]

Probably too much speed reduction unless you find a good way to get drive off one of the intermediate gears.

[Allnightin]

There is a 180 rpm version of the Parallax/S148 servo which might be worth trying?

[dirkske]

I had thought about continuous rotation servo's.  Had two initial thoughts that led me looking into alternatives:
- gear noise
- gear stripping and/or paddle damage when something gets caught in the paddle (airplane esc's shut down temporarily when propeller blocks, hoping the marine/car esc's do this as well)

It's my uncertainty of the max current handling of these gimball motors that's making me uncomfortable.   
There's 0.27mm copper in the one I linked to; I'm completely in the dark as to what current it could handle. 

The 'Eppleton Hall' paddle tug apparently had 370kW power on-board. Using a ballpark-rule-of-thumb conversion I found on the net:
Full scale power * cubed scale * 150% (scale-down efficiency loss) = 370000 / (50)^3 * 1.5 = 4,5W 
That can't be right can it?   If it where right; these gimball motors could probably handle 1A. I'd be using two of them, thus 10W total if I where to run them on 5V.

I like Malcolm's reasoning, just have to wrap might head around how to do the math.
 

[malcolmfrary]

Your power scaling maths is fine.  You have to remember that full size power plants are rated by their output, model motors are rated by their input without regard to how efficient any motor might be.  After that, you can start to ponder about the relative efficiency of model propellers, or in this case, paddles.  If one motor per paddle is used, current needed from a 2S supply is about 1/3 Amp per motor, possibly what they are designed for.  Of course, that very much depends on the load being driven.
With the available information, since you are planning to move into unknown territory, the only way is to experiment.  The worst thing that is likely to happen is that you end up with a nice motor and need to think of another use for it.

[dirkske]

As suggested, I'll place an order and see where I get. As Malcom said, if unsucessfull I can always use them to have option of sailing FPV without getting seasick :)

I'll cobble up a representative quick and dirty foam slab hull for testing with some 3d printed parts. If it works out, might be fun platform for paddle entry and/or kids. Quick & easy to build, maybe even cheap providing 3d printer access.  I'll post feedback, likely somewhere January 2017.

[Colin Duerden]

Hi Malcolm I noticed that you have been discussing motors for a paddle tug. Its Colin Duerden here for a question. I am asking what would the ideal motor be for my Director Paddle Tug. It is fitted with motors rated at 40 RPM. It works in the bath but travels very slowly. I dont think this rating is high enough so with that in mind I have ordered more motors one set at 60 RPM each and one set at 200 RPM each. I wondered if you could advise me which would be the most suitable motors to use in your opinion? With Kind Regards and a Happy New Year to you also Colin Duerden

[dirkske]

It's taking a bit longer as expected, but.... proof of concept says it is a GO

https://youtu.be/3nFb9EhR_sM

- Torque more then adequate (even 1:1 for the Tipstaff I'm building)
- Speed more then slow enough
- Topspeed can be adjusted via different reduction ratio and or 3s versus 2s Lipo (did not do measurements, nor did I check if motor is OK for 3s or up)

I'll likely redesign the printed parts to using:
- HTD belts
- Belt & pully to be swappable without having to remove axles

Caveats known so far:
- Customs make the controller a bit pricey (ordered 2 for tank-style operation)
- Too much fun

[malcolmfrary]

If that is the fastest that it rotates, it just won't move the boat quick enough unless it has tremendous torque and can be geared UP enormously.  A scale model will need its paddles to rotate faster than the real thing - if it is (say) a 1:25 scale model, the wheels must turn at least 5 times as fast (multiply by square root of scale).  With most models this works at between 100-200 RPM.
Colin is approaching the same problem of using unusual motors - if they do not have the basic power to turn the loaded wheel at the right rate, it is simply not going to work.

[dirkske]

I geareded it down immensely using the big pulley; not needed. Motor on 2s has a max theoretical RPM of 303 at nominal voltage. I could add another cell if the controller would be limiting RPM too much on 2s.  Both motor & controller can handle the extra voltage & current.

I'll likely keep the belt system though; even without gearing. This to compensate for minor misalignment/axle wobble.

The biggest gamble was on torque using the motor without reduction; this however seems to be OK for the small Tipstaff. Gut feeling; no actual in-water-tests yet.  Hull waterproof but no wheel yet....

I'll keep you all posted.

[dirkske]

..... I could add another cell if the controller would be limiting RPM too much on 2s.....

Adding voltage does not matter relating RPM, current does not change much either.  Appears the controller is limiting current & RPM.
Max RPM measured at 230 on both 2s & 3s LiPo cells. 

Following malcomfrary's advice; I won't be gearing up nor down. 
I could have bought bigger diameter motor's but didn't; so let's hope the torque is adequate in real life.  It will be a while before I can come back with actual in-the-water results.  Need to build the wheels first.