Twin motor drive

[Peter Withers]

Having got my modified brake setup working OK in the 'ambulance' it has to be said that the UC does seem a bit down on power and has limited 'off-road' ability. To that end I have now decided to take the plunge and go for twin motors, one per track.

I got lucky in obtaining sets of gears that give me a 90 degree change in direction and about a 3:1 reduction. I bought a pair of Wickes 18V cordless drills that give me a 16:1 reduction and a nice quality motor in one simple package (the batteries, chargers etc are a bonus). I have started fabricating a gear assembly that will mount onto the existing bell housing that carries the drive shaft bearings in place of the differential axle. The major surgery to existing parts will be in chopping the existing drive shafts to length.

The photos show the motor and gearbox along with some of the bits and a partially assembled drive for one side. The motors will mount longitudinally and I have some ex-RC 1/8th scale drive shafts to couple things together. I think that with some covers made for the motors the changes should be fairly invisible.

The hope is that with around 4 times the torque and steering that doesn't rely on brakes the off-road performance should be improved. Yes, I know it is getting away from the originality of the design but to be honest I'm prepared to make that change for the potential improvement in performance and anyway the ambulance is already modified!

Pete

[Graham P Hough]

Interesting. So what battery will you use for two 18v motors and will you still be using the supplied speed controller?
I like this idea since it removes the need for two servos, gives more power to tow my 25pdr and limber.

[Graham P Hough]

....sorry I hadn't registered the battery comes with the drill, bit bulky where will you put them?

[Peter Withers]

The battery will actually be a single 18V 4.0Ah battery from a different drill! I will either do an Arduino based controller myself or buy one of the many 2 channel speed controllers that are available for robotics. Because the engine bay is now empty there should be enough room to fit a battery in there. I will only have the battery, receiver, audio system and speed controller to fond homes for. I'm tempted to make the battery fixed and have a charging socket rather than have to remove it all the time.

Pete

[Tim Page]

I must admit Peter that i was a bit surprised that a 20kg tracked model was being run on a single 550 motor with a 7.4v 2S LiPo.
I guess that setup is fine on a hard surface as shown in the official video, but it does on paper seem a bit under powered and might struggle on some rough surfaces as you have indicated.
Lack of space and an open vehicle are likely reasons but a 12v or 18v twin motor setup sounds like the way to go.

I had a pair of 600 motors running on 12v in my old plastic M5 Stuart and that was less than half the UC weight.

Good luck.
Tim

[Peter Withers]

Thanks Tim, that is all the encouragement I need It was OK on smooth concrete but it didn't need much to upset it and short grass was a definite no-no. I'll keep you posted with progress.

[Dennis Jones]

Looking good Peter, but we shouldn't need to be going to these lengths to achieve a successful model.

Dennis

[Armortek]

I would not normally comment on a thread like this and it is great to see people like Peter making modifications to their kit as that is what this hobby should be encouraging.
But when people who have not driven nor own a model comment then I think it is only right to set the record straight.

To be clear - driving where I was driving this afternoon and the speed at which I was driving will not be good for your model - but to say that the motor/ gearbox does not have enough torque is plainly not true.

[Peter Withers]

Thank you Armortek for the response.

That is similar to the performance I was seeing on short grass except that in my case there was a much greater reluctance to turn due to the tracks being unwilling to slew across the grass which surprised me. Maybe longer grass is actually better! Gradients definitely caused an issue for mine regardless of surface. Also, your motor/gearbox sounds happier than mine. Having looked at my original gearbox again there is significant drag as compared to the similar ratio drill gearbox I'm intending using. The motor sounds like it is struggling a bit to spin the gearbox off-load. The bottom line is that I wasn't happy with this level of performance, hence the mods.

Please be assured that my modifications are intended only to satisfy my own personal desires and are not in any way intended to be a criticism of the original kit which I do believe to be very good. My desire is to have better performance than the kit can provide out of the box and to improve on some of the aspects of the design which I personally feel will benefit from further development.

My own view is that twin motors is a better bet for this kind of radio controlled vehicle if you are after significant off-road performance. This is based on years of robot wars experience (yes, I was on the telly) and, more recently, RC models of all kinds including a steam powered car.

Pete W

[Graham P Hough]

All this is interesting. The two motor system is the same as in the bigger brethren like the Tiger. I note Armortek's post and video and Peter's comments and this system does seem to have potential. My preliminary thoughts are to put two motors across the body and use belt pulleys to drive the sprockets independently. I don't see the need for a 90 degree gear box it also eliminates the need for two servos.

Confirming Peter's comments I agree this is a brilliantly engineered model and offers a great opportunity for us to try alternative methods of propulsion.

[Peter Withers]

Some progress on the twin motor setup in the UC ambulance. Having carefully measured the space around the rear axle and made the 90 degree gears to fit I found that I couldn't actually install them! As they are currently configured it is impossible to fit them in one piece or assemble them in situ. It will require a largish chunk to be cut away from the vertical plate at the back of the chassis or the removal of the rear drawbar plate, neither of which I particularly wanted to do at this stage.

I have therefore gone to plan B which is to pick up on Graham's idea above and have the motors crosswise in the chassis driving the axles via chains (I had the chains and sprockets from a previous project). This means I can try out the performance relatively easily without major surgery on the chassis. If all looks good I can then decide on how to fit the gears which is still my ultimately preferred route. Hopefully I will get some photos uploaded over the weekend.

In the meantime I have to say that the Wickes own brand 18V drill motors and gearboxes are remarkably smooth and quiet. For £45 you get a motor, 2 speed gearbox, battery and charger and a simple speed controller!

Pete

[Jerry Carducci]

Continually being the old fuddyduddy that I am I still believe driving most tracked models in tall_ish vegetation is not optimal...

I've not begun construction of my UC yet so I'm keeping an eye on threads like this. Now I'm wanting to examine the drive system
to see what's what. I'm sure the power of a 540 applied efficiently will be more than enough for me. Not certain what the power rating of the supplied motor is (if it was mentioned elsewhere I've forgotten) but 540s generally run from 80- 240 watts or so. We're talking fractional HP. Some of my other models weigh in at 22 to 45kg and are perfectly good performers with excess power using a .5 -.7hp power plant. The UC being lighter than these should be good with a 540 again assuming minimal power loss.

Peter you mentioned 'there is significant drag ' in your original motor drive - wouldn't that be worth investigating? Might save some extra effort...

Jerry

[Peter Withers]

Some pictures of the interim chain drive twin motor setup. Bear in mind that this is very much a test bed at the moment so the workmanship is not all it could be. Also, the paint is taking a hammering! Anyway, I have powered up both motors at 18V and get very smooth running. One potential issue is the skinny 6mm pitch chains but like I said, this is a test bed setup. Hopefully I'll get some useful running before they let go! They are rated at 3.2KN so they are not that weak.

Jerry - the supplied motor is a 550 size, rated at 12V but run at 7.4V (with a 2S battery pack). Looks like a generic Chinese 550 motor with flux boosting sleeve. The motors from my Wickes drills are much smoother and quieter running. There is no doubt that it has the grunt to move the UC around quite effectively as shown in Armortek's video. However, my own personal view is that it is working quite hard to do so and the somewhat on-off nature of the OEM steering brakes makes for dramatic direction changes. My original brake modifications made the steering more progressive but I still felt that power was being thrown away by braking the differential. At part throttle it really bogged down, hence the idea for twin motors. My gearbox is draggy due I suspect to manufacturing tolerances. Planetary gearboxes can be a bit variable in this respect. We use planetary gearmotors in equipment that goes down oilwells and believe me, even motors from a good supplier (aerospace, military etc) can suffer from surprising variations.

The bottom line for me is that I enjoy the effort . I feel that the UC could be a better performer than out of the box but as I have made clear before this is not a criticism of the product, simply an attempt to make my example behave as I want it and have some workshop fun in the process..

Pete

[Phil Woollard]

Nice mods, as you are going the mile you must change out those split pins in the tracks?

Kind regards Phil

[John Clarke]

Impressive. I like it Peter