Motor choice

[Nick]

Now that I've made the decision in favour of battery electric rather than steam (not an easy decision but I won't bore you with the details), the question arises - what motor should I use? It's a 4-4-0, the prototype ran with trains of up to 8 bogie coaches but I will be very happy with half that number. I'm just looking for a motor because I will make my own gearbox. What do people use, or recommend?

In addition to straight forward dc motors, I see lots of geared motors of Chinese origin (i.e. cheap!) available with low shaft speeds and the right sort of power rating that would only require a simple bevel gear to the axle. I've used similar motors in small and very light duty applications, but does anyone have experience of them for the purpose of loco drive?

Nick

[cabbage]

Nick,
If you consult the G3WIKI then there are several articles on motors, wheel diameters and gear boxes. The main problem is matching the torque from the motor to the size of the driving wheel. The tractive factor of your loco is roughly 1/4.3 or for each kilo downforce you get about 200grammes of traction. The next problem is the rolling resistence of your axles. I use ball races and thus it is very low: ONCE IT IS MOVING!!!

Have a good luck at the MFA/Como drills spec sheets, no they are not that cheap, but they are exact. Technobots will provide MOD1 plastic (nylon or hostaform) gears to make your gearbox from. MOD1 may look clunky but think of the bearing surfaces and you will soon love them.

My standard motors are; 90p, £1.20 and £2.

The £2 motor is rediculous and can propel "Maude", (2-D0-2), plus four LMS coaches at over 100mph (scale). Aim for 12V to 14.4V working with around 4Ah of supply.

Regards

Ralph

[Peaky 556]

Hello Nick,
Re motor choice, I'm guessing that you are in favour of a single motor and gearbox to drive one of the axles, which seems pretty normal for steam outline, so taking the advice from the Wiki that Ralph mentions you are going to need a fairly robust and powerful motor.  I think the one that Ralph mentions for £2 is probably a 540 size (please correct me if I'm wrong Ralph, memory for numbers is not too great!), which is also the size mentioned in the Wiki article as typically used in car tyre pumps.  If you get stuck I'm very happy to send you one from my "come in handy box" for the price of postage, and I will also give you data on free running speed and currents at stall.  You would then need to make up a suitable gearbox once you have determined the approximate gear ratio needed, using the method outlined in the Wiki.

Re using a cheaply available Chinese motor-gearbox assembly, I have no direct experience but it should be interesting to try, providing you choose sensibly and have the patience to wait 5-6 weeks!  By sensible, I mean that you shouldn't choose a tiny unit aimed at toy use; as a useful yardstick your motor diameter should not be smaller than about 30mm to put you in the right ball-park for power.  The output shaft revs will be quoted as free running without a load, so ensure that about half of that speed (ie under heavy load) will give you enough scale speed for your wheel diameter.

Any help needed then just ask!

Have fun,
Tim

[Nick]

Thank you both for your replies. I can do the speed calculation (easy) but the torque calculation suffers from huge uncertainties in the final weight of the loco, the weight of the train, rolling resistance, gradients, curves ... you get the idea. That's why I was asking for others' experience. Unfortunately they all seem very shy, or maybe they have better things to do than read web forums  :-)

Anyway, here's my solution. A 37mm Pololu gear motor with a ratio such that the speed at maximum efficiency is just a bit above the intended running speed of the loco. Better too big than too small. Then a 1:1 bevel gear to the axle.

The motor packages very nicely in the lower regions of the firebox, leaving plenty of room above for the batteries and control gubbins.

Nick

[Peaky 556]

Yes Nick, judging by the physical diameter, and cited current limitation of 5A at 12V on one seller site, I would say that unit has adequate power for your purpose.

It is an interesting academic exercise to predict the torque needed to drive your train, and as you say there are a lot of variables, some difficult to quantify.  Instead one can use the experience of others with Gauge 3 models to obtain a ball-park estimate of the electrical power needed to run a typical train.  I have tried to gather some data over the last few years and my conclusion is that for mainline speeds and loads, and assuming the efficiency of spur gearing, we require something in the region of 50-80We.  Your unit is clearly in this power bracket so that's why I say it is likely to be adequate for pulling your short train.

Don't forget that maximum power is developed at close to half of the free running speed, whilst max efficiency is at around 80%. 

Regards,
Tim

[cabbage]

My solution being lazy... My favourite ESC is the Dimension Electronics "Syren" series. These can take an external processor card called "X2 kangaroo". This can be programmed for various functions when linked to a Syren ESC.

Regards

Ralph

[753]

Nick

I have used these motor /gearbox combinations available from several suppliers in China
https://www.ebay.co.uk/itm/DC-12V-2-1000RPM-High-Torque-Electric-Gear-Box-Motor-Speed-Reduction-Gearbox/302399971800?ssPageName=STRK%3AMEBIDX%3AIT&var=601236559876&_trksid=p2057872.m2749.l2649
Some have Uk suppliers depending on speed required, they have bags of torque and fit nicely between the frames.

Mike

[IanT]

That's why I was asking for others' experience. Unfortunately they all seem very shy, or maybe they have better things to do than read web forums  :-)

Or perhaps in the past, they've just looked through their 'spare motor' box and used the one that looked like it might fit. In which case 'they' (obviously not thinking of myself) probably wouldn't be able to give you too much expert advice Nick...   

Regards,

IanT

PS Very nice CAD by the way...

[MikeWilliams]

Nick, I also suspect that many take the easy but well tried route of using an off the shelf motor and gearbox which is tried and proven, albeit at greater cost.  I am thinking about the excellent Slaters units.

Mike

[Nick]

And after a long delay, here it is in the metal. On a rolling road (i.e. no load conditions) the maximum speed is a scale 70-80 mph, which is what I expected and probably all the prototype ever managed. There seems to be plenty of torque (I know, a very unscientific measurement). I suppose I could measure the drawbar pull, but if I did, what would I do with it? It is what it is.

The cylinders and rods are done, next up will be the valve gear. Most of the components are ready, so hopefully it won't be too long.

Nick

[IanT]

Very nice Nick - look forward to seeing more.

Regards,

IanT

[Peaky 556]

It's beautifully sculpted Nick, proper engineering!  As for drawbar pull, this does depend on the weight on the driving wheels, naturally, so I'd encourage you to build it heavy.  As a spot example I have a loco that exerts 1.9kg static, with the wheels just slipping, and that pulls nine Mk1 BR coaches at scale speeds with no bother.
Regards,
Tim

[Michael_Mott]

Beautiful workmanship on the chassis Nick. Exactly what loco is it?

Michael 

[Nick]

It's an LSWR T3 class.

Valve gear is now done. That completes all the moving parts, unless I decide to make the reverser work just for the heck of it.  The next job is the brake gear. With a large project like this, I like to divide it into smaller tasks so that I get a sense of progress when each one is complete. Helps to keep me motivated.

By the way, did anyone notice that the conn rod was assembled the wrong way up? Neither did I, but it's fixed now.



Nick

[IanT]

Nick has his own website which is most interesting Michael.

[urlhttps://www.nickbaines.me.uk/index.html][/url]

Btw - that's a very nice CAD drawing Nick - what software do you use?

Regards,

IanT