5 inch 0-4-0 Shunter

Hi guys,

I am new here so firstly I would just like to say hello.

I am designing a 5 inch electric 0-4-0 shunter type loco and gathering all the information I can. I,m sure I will have a lot of questions I hope you can help with.

To start, I am planning on using 2 x 12 volt - 145 watt - 2400rpm motors, one driving each axle, I am hoping to pull one adult and two children. Will these motors be able to cope with that load, from what I have read and seen I,m hoping they will.

The wheels I was planning to use have a tyre circumference of 10 inches. If my calculations are correct then a wheel rpm of 528 would be required for 5 MPH.

With the motors having a maximum speed of 2400 rpm this would mean a gear ration of 1 to 4.5 would this be ok for good running ?

Appreciate any help or comments on this.

Regards

Ron

Rolling resistance can be made quite small so the main power requirement is on starting when the motors will be drawing a high current so make sure your speed controller can cope and protect itself and the motors.
One option is to make the design with as large as possible large gear and ability to use a variety of pinions in case.

Design in a mechanical or electrical max speed limit for when the children are driving that they don't know how to over-ride, and a fail safe electrical plug that pulls out if the driving trolley becomes disconnected.

Make the battery leads so short that they can only connect when the batteries are in the right way round. The link between the two batteries can be mounted on a pillar that holds it one side of the space to help this too.

Thanks Bazyle for the tips, very useful.

At the moment I am leaning towards chain drive and the option of different size pinion gears will be relatively easy to do. That is a really good idea, to be honest I hadnt given that a thought so thanks.

I am also looking at having just a single 12 volt battery feeding both motors via the controller in parallel, dont know if that as any problems but that was my thoughts.

Regards

Ron

Two more questions if I can.

I am purchasing the wheels and axles and the wheels are to GL5 profile, can you tell me what the back to back measurement should be. Is it a set dimension for 5 inch or can other factors give some variation..?

Also I guess that chassis suspension makes for better running but is it essential on a 0-4-0 shunter type..?

Many thanks

Ron

My 3 1/2" loco has an 80 watt motor, and pulled me when I was 14.5 stone (less now ) but definitely prefers my own built driving trolley than the huge, heavy trolleys most clubs have.

290W isn't much more than three times the power I have, so you may find it's marginal if you have to pull a large, heavy trolley.

That said, since investing in good quality deep discharge traction batteries, the limiting factor is often adhesion rather than power - I added a fair bit of lead to get max weight and good weight distribution.

You can estimate the power you need. This was my calculation but it's only about finding the 'ballpark':

Hi Neil, by "A.U.W." do you mean all up weight of locomotive only?

Or did you add some % of trolley + user weight to that A.U.W. number in your estimate?

Just checking in case I need to do the calc myself on a future project.

All up weight of the loco.

The drawbar pull you need is roughly the weight of the train (driver, passengers and trolley) multiplied by the steepest gradient and an allowance for friction. So for 200kg of train and a 1:50 slope you would need 4 kilos of drawbar pull. For a ball raced trolley with suspension or fairly large wheels, I'd increase that by about and by a lot more for plain bearings or small, unsuspended wheels.

Neil

Interesting Neil, a lot more to it than I thought, how do you calculate what drawbar pull a loco has..?

Ron

Hi Neil, thanks for this. In your last sentence I think you forgot to state something, you said "I'd increase that by about" but didn't say how much.

I had 20-30% but thought I changed it to 20% rather than deleted it.

Drawbar pull is typically limited to 25% of the weight.

drawbar pull (in newtons= kg 89.8) x speed (in metres per second) gives actual power in watts.

our trolleys will freewheel down a 1 in 100, which means rolling resistance can't be more than 1/100 of mass. They actually accelerate, so that must be a top side estimate

The Reference page on SMEX has some articles on calculating rolling resistance and estimating loco tractive effort, although that is based on steam locos but i imagine the power/mass/traction dynamics would be similar. It also offers articles on a 'speed/gear ratio calculator' and 'electric motors for traction' among many others. I've not used any of these so cannot comment on their accuracy, suitability, or useability (is that a real word??).

I'm finding calculating drawbar pull from motor power to be quite a challenge. It's not that simple because the sums vary with weight and acceleration, not to mention that a real 80W motor is unlikely to produce 80W at all speeds, or that the transmission has losses, or that the wheels might not have good adhesion with the track. Is there a gearbox etc?

Theory tells us more power is needed to start and accelerate the train than to cruise at a steady speed, and pulling a train uphill also takes more power. In full-size practice, trains are sized so that the locomotive is powerful enough to restart the train if it stalls on the steepest section of the route. Or a second locomotive is kept ready at steep parts to temporarily double head the train. (This is an interesting problem in railway economics; locomotives are hideously expensive and it doesn't pay to have them hanging around on the off-chance they might be needed, nor is it a good idea to run trains light if that can be avoided. Is it cheaper to replace double-heading with a tunnel?)

Complications abound. However, it's relatively easy to calculate the power needed to maintain a train at a given speed on level track. Tables are available giving the Rolling Resistance of various surfaces. Note that the calculation is optimistic - it does not allow for stiff bearings, grinding on bends, sticky brakes or any other practical embuggerances that occur on a real railway.

Here goes: the calculation was done in Smath Studio for which I am indebted to Mr Duncan Webster of this parish. All mistakes are mine.

The sums show an 80W motor can be expected to be well capable of keeping a 200kg train in motion at walking pace on flat track; actually it has power available to either go faster, or to pull a heavier load. Starting is a different matter.

A second easy calculation would show how much power would be needed to pull 200kg at 1.4m/s up a given incline. It's relatively straightforward to calculate how fast an 80W winch would lift 200kg up a lift-shaft. Too difficult for me is to calculate how much weight an 80W locomotive could move from a standing stop. Even with full information about the locomotive; torque, wheel diameter, motor efficiency across the full speed range, transmission efficiency, and adhesion etc, the maths are too much for me. Perhaps a clever chap will have a go?

Reading about locomotive development in Victorian times, it seems that getting the optimum balance between weight and power in loco design was difficult. Over the years many different approaches were tried, with experiment often showing the theory was insufficient. Many good ideas didn't work well in practice. I don't know how modern locomotives are sized. Probably based on mathematical analysis much tuned by a couple of centuries worth of much studied experimentation.

Rough and ready methods aren't to be disparaged. Another way of getting a feel for what an 80W motor might be capable of, is to compare it with your good self. The average fit man can deliver about 200W continually, much more in bursts - professional footballers peak at about 1.5kW. Even unfit chaps should manage 200W in short bursts which is a benchmark. Don't over do it - an ambulance would be needed if I had to produce 200W for more than 5 minutes!

Dave

The sums are not that hard!

Use SOD calc to work out rolling resistance plus gradient resistance. If you are on a 1:100 slope with 200kg load you need 40N tractive effort just to keep going, multiply by speed in m/sec, this gives power. Say 2.2 m/sec (5 mph), 88 watts. Then add say 40% to allow for acceleration, the accelerating force is 40*0.4 = 16 N, which will give acceleration of 16/200 = 0.08 m/sec/sec and get you to running speed in 2.2 / 0.08 = 28 seconds.

Transmission efficieny using gears/chains/tooth belts will be high, so can be ignored. However it's not that simple (it never is). At slow speed a given voltage will pass more current, because the motor is producing lower back emf. This depends on the motor's short term rating, and I bet you won't find that information for the sort of motors we deal with. Provided your controller can deliver the current, you can probably reduce the 40% allowance quite a bit. A temperature probe on the motor would allow you to know how hard it was being worked, but that might be overkill. The sums then become harder because you don't have a constant motor torque. As with many engineering issues, the best way is to find someone who has a loco with adequate performance and use its motor as the basis for design

Edited By duncan webster on 03/05/2018 13:38:54

Thanks Neil, Duncan, Perko and Dave, I have learnt a lot from your posts and it is much appreciated.

I have had a change of mind re the drive and the motors I was going to use. I was going with 2 x 12 volt 145 watt motors and chain drive but as Duncan suggested I have been looking at successful designs and seeing how they are powered.

Over the last couple of days I have been reading up and watching lots of video on the Maxitrack Planet 0-4-0 loco which seems to be a very capable little engine..

The Planet is powered by a single 12 volt battery driving 4 x 60 watt motors with 2 driving each axle via spur gears fixed to the inside of two of the wheels. Maxitrack say it achieves 6mph and can pull 4 adults.

Most of the time I will be pulling myself and our two great grand daughters which including the trolley and the loco I estimate will amount to 220kgs.

So basing this on Maxitracks figures and the calculations and workings you guys gave me I am hopeful that 240 watts should be more than adequate for good running.

Regards

Ron

Just a thought but a 2-speed gearbox would be helpful on an electric shunter. A low ratio to get started shifting to a higher ratio as soon as the train is moving. Probably tricky to make a simple home-made box that didn't crunch horribly, but might something off a foot scooter or go-cart be suitable? Like this example perhaps.

I vaguely remember reading big DC motors could be fitted with a pair of field coils that were switch connected in either parallel or series. I think this allowed the motor to run at low speed/high torque or high speed/low torque. Could this be true? (Might not help - I've never come across a DC motor that gave access to the field coils!)

I would not worry about starting - torque is at its maximum when stopped for a brushed motor.

My view is gear to suit the expected load and gradients you want to cope with, and everything else will be OK.

My loco struggled on steep hills and I made a new drive pinion for greater power, but it became irrelevant when I fitted a proper traction battery that could supply more current (than my ex-burglar alarm battery!)

Neil

The motors 4 x 12 volt 65 watt, axles, wheels and gears have arrived.

I have to modify the axles and gears, fit the pinions to the motors and spur gears to the back of the wheels.

I have no machining facilities apart from a small pedestal drill but a friend is doing the basic chassis side frames and motor mounts, the rest is then down to me.

Looking forward to making a start.

Ron

Edited By Ron Laden on 13/05/2018 09:20:18

To put the power consumption estimation into perspective, an elite athlete is capable of a power output over short periods of over 750W, and up to 500W sustained power over extended periods. Assuming we are 'not quite' as fit as an elite athlete, with a power output capability of less than 50%, we should be able to pump out around 200W without going into cardiac arrest. I'm a reasonably fit 63 years old, and i can relatively easily move a 7-1/4" gauge riding car carrying 4 adults at a brisk walking pace around our club track which has some short but stiff gradients while hardly raising a sweat. So, your 240W electric motors, if geared correctly, should be easily capable of the task you describe. Just don't expect sports car performance. In most cases adequate traction is harder to achieve if your loco weight is a little on the lean side.

I,m not expecting sports car performance Perko, just hoping my calculations are correct and if so it should give me 6-7 mph. I am going with sprung axles so that should help a bit with the traction. As best as I can tell at the moment the dry weight should be around 10-11 kgs and the leisure battery I have my eye on is 17 kgs so a total of 27-28kgs.

Also from the calculations I have gone with 6.25 to 1 gearing which again I hope will be ok considering the motors I am using.

Well I guess there is only one way to find out and that is to get it built and give it a test.

Cheers

Ron