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Electric Motor and Gearbox Initial Considerations

DocRef Peaky/2b

Where to start

As mentioned elsewhere there appears to be a bewildering range of drive units used in our locos. These notes are intended to guide the beginner through this wilderness, such that a happy outcome results. Happily we can buy ready-made motor/gearbox units designed for G3 (currently sold by Slaters), or we can press into service a huge range of other motors and build our own gearboxes to suit.

Don’t be put off thinking the subject is far too technical, because there is no right or wrong answer for powering your model. It is always better to seek advice from others more experienced before treading a path, but if you do jump in, don’t despair if it ends up not quite right for you and your model. If your first attempt works but the model races around like a scalded cat, then you might resort to lowering the battery voltage or using a different or better Electronic Speed Controller (ESC). If it grinds along too slowly then you may get better performance by raising the battery voltage, but with some caution in case the motor gets overloaded. Either way you will have taken an important step and gained a lot of experience. If at first you fail, seek further advice, and, most importantly, persevere!

Firstly there are several important things to consider:

How much space do you have in your model for batteries?

  • If yours is a small tank engine like an 0-4-0 and you can only access the tank space, this might limit the number of AA cells to six or eight, so we are talking of around 7-9 volts available for the motor.
  • At the opposite extreme, if it is a mainline diesel or electric prototype, there is plenty of space in the boxy body for big capacity cells of 12V, 18V, 24V or more.

Will your model pull heavy loads like a rake of six or more bogie carriages or a dozen or more trucks?

  • Train weight (or number of axles), and the speed required, will give you an indication of the power and ruggedness needed by the motor.
  • A long train at passenger express speeds is going to need a power available in the region of at least 60 Watts, and preferably more to cope with the increased drag on curves. If you aim for 80 to 100 W then you wouldn’t ever be short of power. Your motor would need to be quite robust, not sourced second-hand from a flimsy toy.
  • Motors are often sold on both voltage and power rating. One motor type currently on sale on Ebay is rated at 24V and 120W, which clearly has ample capability for anything Gauge 3 can throw at it. This particular motor also runs very well at lower voltages such as 9 – 12V, but the power would be down.

Is your model a racehorse or a donkey?

  • Mainline expresses might have a real speed of anything up to 90 mph (unless you are building an HST125); but a branch line steamer or DMU might be happy at 30 – 40 mph, whereas a station pilot, shunter or banking engine might be quite realistic at only 10 – 15 mph. Speed has a marked effect on the power required.
  • Required train running speed is important to know for your gearbox ratio, and you will ideally need to know the free running speed of your motor.

Electric Motors

As you can see there are a lot of variables, but if you already have a motor that you want to press into service then quite a lot is already decided for you. If you don’t, then a few ideas below.

  • MFA-Como sell a wide range of motors, from small, geared and puny, to big and capable, at a wide range of prices. Their RE-850 model is normally the largest motor used by modellers at 52mm diameter with a ¼” shaft. It is rated at 80W and 12V, and so one of these should be fine for the biggest and fastest G3 model. Their 944D41 12V motor/gearbox unit is small and low duty, costing around £19, but they also market a very large and solid industrial quality motor/gearbox unit as the 995D41/24V, with around 60W capability and costing £130, discussed later. Some of their motors are supplied with "precious metal" brushes -these are the more desirable ones because of their higher torque values.
  • If you have a defunct electric car tyre pump, these are a useful source of robust 12V motors that would suit up to medium duty requirements.
  • Kids’ electric scooters have several styles and sizes of DC brushed motor, typically 12 – 24V, generally with a lot of power. These are widely available new on ebay for less than £20 (120W size), and you will often be told the running speed, voltage and power rating. These are generally suitable for anything big, heavy and fast in G3, but they can be quite bulky. They may also be suitable for people-pulling, if that is your fancy. The G3S Newsletter 105 has an article showing one such large motor mounted vertically to drive three axles in a power bogie.
  • Electronic hobbyist and surplus shops might have something tempting, but you might not know anything more than a suggested voltage. Prices could be low and they are often worth an experiment!
  • Defunct ink-jet printers may be a source of low-duty motors (and belt drives), see images in Newsletters 101 and 106.
  • Car windscreen wiper motors may have promise for a high-power application, and should fit within a modern-outline body.
  • The Slaters GBG3 series of motor-gearbox units are very well-engineered 18-24V units, but come at a price (£140 in 2017). These are discussed later.
  • Ensure your selected motors have ventilation slots, otherwise they will quickly overheat if used too hard, unless of a large and heavy construction.

Multiple Motors

If you are using just one motor for the loco then all of the previous advice applies. On the other hand if you are using multiple motors, such as the Axle-Hung Motor Gearbox (AHMG) in motor bogies, you may have four or six small motors driving your loco; or two motors, one in each bogie. I even know of one “Peak” model using no fewer than 12 small, cheap, motors. This means of course that each motor can be of lower power. A good example is the ‘Powermaster’ hair drier motor, recently in circulation with Gauge 3 Forum members, known as “HQ7P”. This has not currently been fully evaluated but would seem to need at least four and preferably six of them to drive a mainline express. The G3S Newsletter 105 has an article showing three of these built into a ‘Peak’ power bogie, issue 104 shows two in a Bo bogie, and issue 107 shows a single HQ7P per bogie. The HQ7P hair-drier motor is an example of a low-power motor, obvious from two aspects; the brush gear is rather crude, and it has no ventilation slots. An article showing two small MFA-Como motors in a bogie is in Newsletter 109. Some plans in current circulation (Loco Design Co) show an MF-Como geared motor type 944D41 driving each axle in a ‘Peak’ diesel, but be warned, even with six motors the performance is not sparkling.

Gearbox Introduction

The motor needs to be considered along with gearbox design, and this subject is overviewed next. Gearboxes are required solely to transfer the drive between a high-speed motor shaft and the lower speeds needed for our loco axles. Once you have selected your motor then find out how fast it spins at your chosen voltage. If not detailed on the box then others may know the speed, or have a tachometer to measure it for you. At this free-running speed, however, your motor develops no useful power; it has to be slowed down to produce driving torque. The usual best efficiency point for a motor is to run it at about 80% of its free revs. This is often still very fast, and often not achievable without a fancy, multi-stage gearbox, but in practice we can use any lower speed as long as we don’t stall the motor. Keep in mind that 80% speed though, as an upper limit, as we can’t easily run faster than that. Now you know your upper motor speed, the loco driving speed wanted, and driving wheel diameter. Next consult the speed/revs chart below:

Rotation of axles (RPM) at scale speeds for wheel sizes.

Scale MPHMetre per Sec30mm35mm40mm45mm50mm55mm60mm65mm70mm75mm80mm85mm90mm95mm100mm

Read off the RPM for your wheels at the speed you want, Most importantly reduce the speed of your motor to about 80% of its free-running speed, as this will be roughly the best power and efficiency from the motor. Divide the big number by the smaller one and you have a starting point for gearbox design.

Here’s an example to show what I mean:

Suppose you are building a steam outline loco like a ‘Black Five’. Driving wheel diameter in G3 is about 80 mm. Supposing you’d like to reach a maximum scale speed of say 70 mph, and you can see from the speeds chart, using a bit of guesswork, that you will need the axle to turn at up to about 320 rpm. Now turning to your motor, say the MFA Como RE-850, the max efficiency speed at 12 V is already given for us at 8,311 rpm. If we wanted the gearbox to perfectly match it would need to have a ratio of 8,311/320, which is about 26:1. This then is the ‘ideal’ ratio for this motor and model. Every other model is likely to be different, ie may have a different motor speed, or different wheel sizes, and perhaps a totally different running speed, so they will have different ‘ideal’ gear ratios.

The next stage is to think if your ideal gearbox is available. Generally the answer will be NO, as a large ratio like this will be multi-stage, ie a complex and expensive unit, and thin on the ground. Don't despair, however, as we can overcome these apparent difficulties!

Proprietary Motor/Gearbox Units

Let us evaluate some of these to see how they might suit our models.

The Slaters G3 units are designed specifically for model trains and currently come in alternative ratios of 30:1 or 50:1. The 30:1 unit (GBG3) comes with a 26V Mabuchi motor and is intended to give appropriate speeds for mainline diesel expresses with their smallish wheels. The motor runs free at around 17,000 rpm, and the best efficiency speed is about 14,500 rpm. The axle will turn at around 480 rpm at best efficiency speed, and using the chart above we can see that for 50 mm wheels we can get about 63 mph, or 55 mm wheels would give about 70 mph. These speeds are of course scale speeds, rather low for a mainline express but perfectly adequate for those making suburban sets or with smaller railways. The 50:1 unit (ref GBG3-50) has an 24V Canon coreless motor and is aimed at steam outline locos with large wheels, but I have not characterised the running speeds.

The MFA Como 995D41/24V has a 4:1 epicyclic gearbox mounted in line so that the output shaft is effectively on the same axis as the motor shaft. It is very large at around 200 mm long and 70 mm diameter, weighing 1.6 kg, but is competitively priced at around £130. Because of its shape it will have to be mounted longitudinally on the bogie or in the model body, requiring a means of dropping the drive to the axle(s). Its output shaft turns at around 1,150 rpm for maximum efficiency, and I think this speed would be fine without further gearing for a diesel express, but would be rather too lively for a steam outline loco with big wheels.

Worm Drives

Let’s pause here for a word on worm drives. A ratio like 26:1 is very easily achievable, just a single start worm and a 26 tooth gearwheel. Despite this seeming the obvious way to go, worm drives have two disadvantages:

  • There is a lot of friction caused by rubbing, so lubrication is important and there will be less power available at the wheels. Rapid wear is common and efficiency is rather poor.
  • They do not ‘run on’ when the power to the motor is cut. If the motor is asked to stop by the controller it will probably stop dead and the loco wheels will lock up. It then skids to an undignified and abrupt halt! The reason is because the worm drive is so inefficient that the momentum of the train will not cause the gearbox to keep turning after the power is cut. In other words the worm will not ‘back drive’, so even if the wheels want to carry on turning, they cannot turn if the motor has stopped.

There are ways around these problems, such as using a more powerful motor, using a proper shaped wormwheel rather than a spur gear, putting a guard around the gears so that they can be liberally greased without flinging it everywhere, and adding a very heavy flywheel to the motor shaft, so that it does not stop abruptly. Of course careful driving and progressive slowing using the controller will overcome some of the inherent ‘jerkiness’. On the whole though, the worm drive is really only suitable for slow speed, low-duty operation, such as with a shunter or railbus. Walsall Models can supply worm drive gearboxes with various ratios, on demand.

Finding or Making a Gearbox

If you decide to use a ‘conventional’ gearbox (by that I mean NOT a worm drive), you need to source a gearbox with the nearest to ideal ratio that you can find. Using the example ratio of 26:1, you might get away with 15:1, 10:1 or even 7:1, but the penalties will be:

  • Slow speed running may be poor or unachievable.
  • The acceleration might be sluggish, (which could be very realistic).
  • The top speed possibly too lively!

Slow speed running is generally most noticeable when you are driving light engine, as a loco that will only set off like a startled cat will never look good. If you will only ever pull a long train, or yours is a set of permanently coupled multiple units, their weight might cushion the starting and be acceptable. What matters though is how you drive the locomotive; a cautious driver will never race around at full throttle, but gradually apply power until the ‘right’ speed is reached.

Making a bespoke gearbox will be discussed under a separate section.

Electric Motor Suppliers

1. MFA/Como Drills.

2. Technobots.

Gearbox design and Construction

DocRef Cabbage/

Getting the power to the rail is one of those subjects like "skinning a cat"... Gearbox plates can be made from plastic or metal but I would advise some form of X-Y movable drilling platform to do this on. Your shafts will require some form of bush or bearing, these can be PTFE lined Bronze or small ball races. If using a worm and spur set up you might have to design for a thrust bearing as well.

Here are few "do’s and don'ts".

Worm and Spur.

This is more common on smaller scales rather than G3. The main problem with worms at this scale is that they need to be supported at both ends otherwise a destructive vibration sets in and the worm destroys the spur teeth. Ideally you should a have a shaft between two worms with a motor at either end running in opposition. The main downfall of Worm and Spur is that the worm "locks" the spur into position when it is not turning. If you can only fit ONE worm then it may have to have two "starts" to spread the driving force over. These are not common and the price reflects this! The cheapskate method is to use two "single start" worms as above. The main advantage of Worm and Spur is the very high level of reduction possible in a small volume. To work out the reduction using MOD1 gear tooth size is to take the spur gear tooth number as the reduction factor thus a 20 tooth spur give 20:1 reduction. Given the very high levels of torque that are easily attainable, some some thought should be given to how the spur is securely coupled to the axle.

Bevel Gearing.

This is a very simple method but the main problem is that the reduction factor is not very high typically 1:1 or 2:1. Muffet produce a 5:1 in moulded Hostaform and HPC Gears produce a steel 4:1 -but the price is not for the faint hearted... It is normal for the motor supplying the torque to have a some form of step down gearbox before it reaches the output shaft. MFA/Como Drills, (see above), supply such motor gearbox combinations. Which type of bevel gear you use is debatable. Some members have used the metal Helical type as found in Angle Grinders from sellers on eBay, others have used the simple Radial type made of Gear Grade plastics. The helical type seem to be quieter and lose less power, but the radial type is capable of far higher torque before teeth breakage occurs and is far more tolerant of alignment.

Spur Gears.

These are the most common. The transmission gear can be clamped, keyed or pinned to the axle, or bolted through a driving wheel. Means of attaching gears to shafts or wheels are suggested in an article in Issue 108 of the G3S Newsletter. It is recommended that you use MOD1 gearing as this makes the calculation of sizing a lot simpler and the tooth size is big enough to ignore some amount of grass that it will normally munch on! Here it is again a choice between Gear Grade plastics and Metals. The advantages of plastics I now feel far outweigh those of metals as they are cheaper, lighter, and in some plastics as strong as metal. The principle plastics to look for are Pocan, Nylon, Hostaform, and Delrin. The latter two may be classed as members of the Acetal group of plastics.

Chain and Sprockets including Timing Belts and Cogs.

These are very reliable and easy to design with. However the one thing you have to remember is that chains ONLY pull... Therefore the force will be taken on one side of your sprocket depending on its rotation. The other side can therefore flop around and some form of tensioning device will have to be devised -even if it is only a "rubber" made from springy brass strip. If you have multiple axles driven in series by chains and sprockets you must beware of the "tensioning time" as each axle takes up the slack of the chain and passes on the torque to the next chain and sprocket.This may be seen as a slight wheel slip as each chain is tensioned. For perfect transfer of power each sprocket should have at least 16 teeth. If you have a step down system of an 8 tooth to 16, I would advise you to have two sets of 8 tooth and 16 tooth sprockets each at half tooth settings. You will only need Simplex Chain in a G3 loco, but you WILL have to use Duplex Chain if the loco serves dual G3S and N2.5GA duties...

large chains Replaces Page

The rules for Timing Belts and Cogs are similar to those of the above. BUT the timing belt is capable of taking far more power than a chain. It is not as flexible, so some design work has to be done to get good straight tangential lines to the cogs. They are quieter than chains and do not need the large amount of grease and cleaning that these require. Shaft centre-to-centre distances need to be set very precisely, as there is no stretch in these belts; too close and they jump teeth, too far apart and they cannot be assembled.

Flexible Rubber Belt Drives

Flexible rubber belts can be used with vee pulleys. For low to medium powers then belts of 1.2mm square section ex DVD players can be used. For higher duties rubber O-rings of 2mm section have been successfully used. They do need to be stretched very tightly such that no slippage occurs at motor stall, otherwise they burn out in seconds. They are best placed as the first stage of a gearbox, directly onto the motor shaft. One modeller has successfully used twin drive belts arranged such that the belts pull in diametrically opposite directions, which takes a great deal of load off the motor bearings. To achieve this he uses the motor midway between two gearbox input shafts. The great advantage of this type of drive is that shaft centre-to-centre distance does not need to be set with precision, unlike with toothed belts or spur gears.

Gear Sprockets and Chain Suppliers.

These people supply a VERY large range of gears and sprockets etc. However their catalogue is aimed at the commercial buyer -this means you HAVE to know what it is that you want!

Muffet supply most of my gears and have most of the MOD types and diameters that are most usable in G3. They have downloadable PDF files for you to look at and think about. The moulded Hostaform MOD1 and MOD 0.7 range of spur and bevel gears are the best for our uses.

Gears and Sprockets, A Branch of Technobots (Rugby)

It has the same look and feel of the main Technobots site with a large number of items -but quite a few of these are "special order" and there can be a slight delay in delivery.

MFA/Como Drills -see above.
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