Model Railway Gearboxes

[Kayla Munl]

Ihad one mechanism of the correct wheel base and was, therefore, faced with building at least six new mechanisms. This was a rather daunting prospect and I decided to look for ways in which I could fabricate, in the quantity required, common parts for all the models. Fortunately I had previously had to make some axle bearings so I had taken the opportunity to manufacture 64 of these at one time. This operation had entailed making some simple tools to ensure a quick repeatable manufacturing process. I seem to remember that I had taken two days to make the tools and two hours to make the bearings! However had I tried to make them individually I doubt whether I could have made them sufficiently alike to be of use and would have had to make considerably more than that to cover wastage. It turned out that one of the tools I made at that time was of great use in making some of the parts which form the subject of this article so that the original time spent on the tools was well worth it.

One common part which takes the most time to make and has an important bearing on the running of the models is the gearbox. I use Romford 30:1 or 40:1 gears and anything built should be able to take these gears and mount them and the motor to give the correct mesh.

There are of course gearboxes available using etched parts but these are generally designed for 1/8th axles for standard 4mm scale models. I have always thought these as a little flimsy and as I use 3/16th axles as used in 0 gauge they are not really suitable. The gear boxes I have built for previous tramcars have used 1/8th bearings and a screwed assembly, so I decided to look in detail at a design which follows my established pattern.

The design I evolved uses just four parts (not including the 10BA screws) two of which are the same, and which could be made in reasonable quantity.I use AutoCAD to produce the drawings so that I can try the parts for size and fit before cutting any metal. The drawing of the individual parts is shown in Fig 1 and the assembly is shown in Fig 2.The parts are 2 off side bearings, 1 off back plate and 1 off motor bearing. I built a total of 10 assemblies for the seven models plus spares.How I made the parts is shown below and is not necessarily the only way but may give others some thoughts on how they may tackle the same or similar jobs.I have included a number of photographs which should fill in any shortcomings in my description.

The main advantage of using a needle of some sort instead of a conventional scriber is that it is much easier to see the exact place where a mark should be. I also use a similar arrangement to position the edge of parts in the lathe.

The photo shows a gramophone needle mounted in the chuck being used to determine the edge of the lower jaw of the machine vice. Again I use a magnifier clipped to my specs to examine the set up closely.

Do be careful when doing this and make absolutely sure that the lathe cannot start up when looking closely at anything mounted there. In the absence of a dial gauge it is possible to set the jaws of the vice exactly level in the same way.

The interval between the holes depends on how you cut off the individual parts from the strip. Initially I used a hacksaw and therefore left a larger space between the holes than if I had cut them up on the lathe.

This operation I subsequently found I could carry out using a tool made to assist in the manufacture of the main bearings referred to above. This operation is easier and wastes less material and will be referred to later. Drilling the holes is shown in the photo.

Clamp the assembly firmly in the jaws of the vice, ensuring that the first piece from the strip, with its scribed lines, is facing the operator. Make sure that the bearing pieces are exactly parallel with the axis of the milling cutter.

I use a square lightly clamped to the lathe face plate to ensure that the assembly is correctly clamped in the vice. This will ensure that the face that you are about to machine is at right angles to the sides of each side bearing.

Once this face is machined the clamped assembly can be turned over in the vice for the opposite raw faces to be treated likewise. This time you can use a simple parallel spacer i.e. a suitable piece of square or rectangular bar between the now machined face and the bottom of the vice to ensure that the two faces are machined parallel to each other.

These can now be removed from the machine vice, turned through 90 degrees and reclamped so that the correct non machined side is facing the headstock end of the lathe as in the photo.

Note that the assembly is now clamped between the faces that have just been machined and that the next operations are carried out on the faces formed by the original extrusion.

I then drilled out the holes using a 54 or 1.375mm drill to a depth of 3/8th. My Dormer hand book supplied to me as an apprentice states that a deep hole is defined as one whose depth is more than three times the diameter. This hole is certainly deep by that definition and feed speed, via the leadscrew hand wheel, should be low and the drill withdrawn to clear the small brass chippings from the drill flutes. Use the previously noted readings to return the drill to exactly the same positions as the centre drilled holes.

These holes can then be tapped 10BA using the tap held in the lathe chuck which is mounted in the headstock of the lathe. The chuck however should be loose in the headstock taper to ensure that, as the thread is cut, the chuck and tap are free to move towards the parts clamped in the lathe.

The thread is cut by turning the chuck by hand only. DO NOT use any other power. As the thread is cut you can take up the slack by gently operating the leadscrew hand wheel. However when withdrawing the tap, you will have to remember to reverse the process before the chuck is forced back into the headstock taper.

This is shown in the photograph where the curves introduced by the shearing action of cutting the strip is clearly seen on both the top and the sides of the upper strip. Contrast this with the straighter more rectangular outline of the brass flat, the lower of the two parts.

Before cutting the brass flat to length from the strip form in which it is supplied I scribe a line down the centre of the strip which I use later to show the centre line of the individual backplates. I did on some early ones mark each plate but it is considerably easier to get an accurate centre line over the whole length than one by one.

I then cut the brass flats up into pieces of the required length in the lathe in a similar manner to the way the bearing plates were cut up. The drawing shows that the length of each backplate as 22mm. The length is not critical and I actually cut mine 22.25mm i.e. 7/8th inch since, having lined up the end of the strip with the saw blade, I then back it away from the saw blade with the cross slide screw and move the cross slide towards the chuck seven revolutions of the lathe feed screw plus 40 thou (the thickness of the saw blade). My approach to measurement might be thought odd since I tend to work in a combination of metric and imperial measurements as it suits the components and the tools in use. As most model railway scales are expressed in mm to the foot or some very odd ratio I am sure that most modellers will be more than able to cope with the two units of measurement.

Having cut the requisite number of back plates these need drilling with four 10BA clearance holes. Again I mount each of the plates in the machine vice and, after ensuring that I know where the zero position is, I drill out the four holes using the small centre drill followed by the clearance drill in the same manner as for the side bearings. Remember to always approach the drilling point from the same direction to eliminate backlash.

Since the gear ratio at this stage is not fixed I do not drill out the hole for the motor shaft bearing since the distance between the centre of the worm and its corresponding wheel varies with the ratio. I do however mark the centre line of the axle on the centre line of the back plate to assist in marking up the back plate when the gear ratio is finally determined.

The last remaining part is the motor bearing which is soldered into the back plate when the ratio is finally fixed. Since this is a simple turning I made up a small number with 3/32nd bore and will turn others up if I find that I need to use motors with 2mm shafts.

When gears mesh together, they run with what is known as their pitch diameters (PD) almost touching and separated by the clearance between the gears. The problem is determining what the pitch diameters of the two gears are from what we can measure, which is the outside diameter. This is not easy to determine accurately since we need to careful to measure across the tips of the gear teeth and not across the notional flats between two adjacent teeth at the ends of a diameter. We then have to count the number of teeth on the wheel which we can usually take as the larger number of the gear ratio. However beware of two start worms, i.e. those with two spirals in parallel round the worm. A worm and wheel set having a two start worm and a 25 : 1 ratio will have 50 teeth on the wheel. If in doubt count the teeth!

The distance between the axle and the motor shaft centre lines is the sum of the two radii plus the clearance which in this case is 9.55mm plus the clearance, say a total of 9.75mm which is not too hard to measure and mark with a clear rule and fine scriber. If you use a 30:1 ratio with the same outside diameter as previous the distance between the two axles centre lines is 9.5mm. The 0.25mm (10 thou) does make a difference.

Having marked out the centre of the motor shaft bearing hole I drilled it out first using the small centre drill and then opened it out to 4.76mm (3/16th). The motor shaft bearing can then be inserted and the gears tried out for mesh. If necessary the motor bearing hole can be filed gently to adjust the mesh of the gears prior to soldering the motor shaft bearing into place on the back plate.

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