GL5 Forum

I am making a O8 shunter but how heavey does a freight train weight if i was on pilot duties

thanks

Hi you should not have any trouble with a loaded 20 wagon train, remember maximum speed for an 08 would be 15 mph. Regards Ballan. :..D

Hi Ballan,
The reason i was asking is so i could work out what size motor i would need
cheers

Most people that have an 08, have bought the kit and that one has 4 motors driving 3 axles. The traction morors are normally mounted between the frames, parallel to the axles. with the batteries above the chassis inside the engine and cab casing. If you want one big battery , the could be mounted above the frames with chain drive to the chassis axle. and the coupling rods would connect the non driven axles. But the would not leave a lot or room for the battery power, so they could be in the driving wagon,feeding the loco via heavy duty wiring. Have a look round and see what others have done and take your pick, according to your pocket and time. Regards Ballan.

Hi, its Ballan again, it should read bid motor, not battery. Regards.B

Whilst Ballan is quite right when he says that model 08s use four axle hung motors, you must also remember that the top speed is 15mph, as he said. According to the Charlatan drawings, it was designed to pull 8 adults at 8mph. But we don't want anything like that. For our purposes, it would be much better to throw away 3 of the 4 motors, and gear the other one down appropriately.

8mph is equivalent to about 90mph full size, six times the scale speed. So, if a further 6:1 reduction was put in, you would get a scale top speed, but even more torque. Even better, you would reduce your slowest speed to a sixth of what it was, so you could really inch a loco up to a train. This is one of the problems with the single reduction almost universally used, the inability to move slowly, smoothly, and controllably, ie just like the real thing.

Asking about how much power you need is a very good question. I built a contraption to test drives and motors, and as you might expect, didn't write down the figures. However, it pulled the 13 Mk1's out of Erimus yard at Gilling and into the headshunt by itself. It struggled, and was very slow, as it was geared to give a scale top speed of 8.5mph. But it only used a Graupner Speed 500E motor which has a stall current of only 10 amps, so it's maximum input power was only about 120 watts. Allowing for electrical and frictional losses, the output may have been only 20 watts, possibly less. Obviously you need more than that, but pulling trains isn't just a case of putting more horses under the bonnet, it's what you do with those horses.

Lets do some sums. Say you have an 08 which weighs 100lbs. You on your driving truck weigh another 200lbs. You're pulling 20 loaded 12 ton wagons, about 600 lbs. So the loco must move a total weight of 900 lbs, that's pushing half a ton! Let's say it's going at a scale 10mph up a 1 in 60 gradient, quite a job. It's actual speed would be 10/11.3 mph, ie 78 feet/min, and so it's vertical speed would be a sixtieth of that, ie 1.3 ft/min. As it's pulling 900 lbs, the work done would be 1.3x900 ft lbs/min, that is 1170 ft lbs in the minute. If I've got my conversion right, that's about .035 horse power or 26 watts!

Now we must remember that that is work done in raising the train, and doesn't take into account any losses. What these losses may be I have to admit I don't know, but lets say that 90% of the input power is lost to friction and electrical losses. That suggests that a 260 watt motor would do the job. Admittedly, you don't want the motor to be working at it's limit, as any heating of the motor is wasted energy, so, on these figures you may choose to use a 500 watt motor running a bit lazily. You can get that from a 600BB Turbo, and I've got one to put into the contraption next. Watch this space, but don't hold your breath.

I suggest that you do similar calculations, choose an appropriate motor, and then design suitable gearing to give a scale top speed based on the motor's top speed. And now you'll want to know how to get a big gear reduction. TIP, the last cordless drill I bought contained a double epicyclic gearbox with a 25:1 reduction.

Q. How long do the gearboxes in cordless drills last?

A. Longer than the motors.

PS. Please check the sums in case they are wrong, then let us all know.

An excellent reply from Dave, I can't add much but would like to reinforce the point about scale speed. Unless you need to use the loco elsewhere for passenger hauling I would gear it for an appropriate scale speed as the battery shunters I have driven have all been quite difficult to drive smoothly at low (realistic) speeds which also seems to make them more prone to wheelslip as they are trying to put too much power down too quickly.

The other thing to pick up on from Dave's email that he hasn't made a point of is weight, he has taken a figure of 100lb as an example but I would say the heavier the better from an adhesion point of view. I have recently done a bit of refurbishement work on the Ryedale Class 10 shunter and out of curiosity took the opportunity to weigh it as I am thinking about building a shunter myself in the future and was wondering how heavy I should be aiming for. Unfortunately like Dave I didn't write my figures down but from memory it weighed 85lb when I brought it home and having done the work which included fitting the largest batteries we could fit inside the body got the weight up to about 100lb in an attempt to boost traction.

I cannot quantify the increase in traction as no tests could be done before due to the old batteries being beyond redemption but it can move a full rake of wagons out of the bunker at Gilling now although on the day I tried it I couldn't get it up the gradient from the bunker due to wheelslip on a greasy damp rail that hadn't been used for a few months over the winter.

The motors on the Class 10 have no markings on them so I can't help with a power figure but it has 3 axle mounted ones.

Another point to consider is the wheels, from conversations with steam owners at least a couple of them have said subjectively that their locos definitely gripped better with steel tyres fitted rather than cast iron as originally fitted, now I have no experience of this with battery locos and it may not be as worthwhile with the much smoother torque from an electic motor but I would be interested if anyone can comment on this.

You are perfectly right Richard, weight needs a mention.

Lets do some more sums. The coefficient of friction between steel (rail) and steel (tyre) is about 25%. If a loco weighs 100lbs, then it can not develop a pull at the drawhook of more than 25% of that, ie 25lbs, it will just slip. If we imagine the loco to have only one wheel, it will make calculations easier, it saves dividing everything by six in this case, then multiplying by six at the end.

The wheel has a radius of, lets say, 2.4". If running at a scale 10mph, it will rotate at 62rpm. The work done in a minute will therefore be
the wheel circumference x rotary speed x force at the rim
4.8 x pie (3.142) x 62 x 25 = 23373 inch pounds per minute = 1948 ft lbs/min
This is 0.059 horse power, or 44 watts.

This isn't a million miles out from the figure we got previously. Again we need to allow for frictional and electrical losses which are probably quite large, but we're still not talking of a lot of power.

As a last guess, the 08's were rated at 350 horsepower I think. This may be pushing it a bit, but, if we built an exact scale train, it would weight 1/11.3 cubed, or 1/1441 of the weight of the real thing. Would it be reasonable to assume then that, very roughly speaking, the power required to move a scale version of an 08's maximum train, would be something like 350/1441 horsepower which is 0.24 horsepower or 181 watts. That would be the diesel engines output which is the power input to a fairly efficient generator/ electric motor system. If we assume the system to 50% efficient (that's a wild guess), then the output at the wheel would be a maximum of 90 watts.

That's a rather bigger figure than the others, and really not far removed from a guess, but it still seems that 100 watts at the wheels would be more than enouigh, probably much less would do.