From Gazette Volume 19 No. 11 May 2016

By Bob Alderman

I believe there are number of steps that should occur before a loco reaches the stage of any extended running on a track. What follows are the stages I go through, with an occasional aside, to achieve a model locomotive that performs. I and others have written on some of this in earlier issues of the Gazette and references to the Gazette archive on the Guild web site will enable you to look them up.

Erecting the chassis
For a loco to run well, the first steps take place during the initial stages of chassis construction. When erecting the frames every care should be made to get them square and parallel. Details of how this can be achieved can be found in the Manual (Part 3 Section 6).

Free movement
With bearings in place, the wheels can be fitted and checked individually for free turning. Bearing bores, both square for horn guides or top hat, are often the same nominal 3/16in. diameter as the axle. This can leave a very small clearance between the axles and the bearings. This will be detectable by turning the wheel on its axle in the bearing. A certain stiffness may be present or it may just not feel free. A shaft in a plain bearing should have running clearance (in the engineering industry these are defined by a British Standard for Limits and Fits BS EN20286-1,-2 1993 or ISO 286-1,-2 1998). These standards go way beyond what we need to know, suffice to say is that we want something that turns freely. Experience says that the bore should be about 0.002in. larger than the nominal 3/16in. bore. A 4.8mm diameter hand reamer works well. A machine reamer is likely to be too short to cross a chassis. A 3/16in. reamer usually does nothing towards enlarging a hole because it is the same nominal size as the hole.

Ideally the bearings should be reamed in place in the chassis. The reamer is used across the chassis to open up both bearings together; hence a hand reamer. Crossing the chassis will help to align the holes. If there is a slight misalignment of one bearing relative to the other, this will be corrected by one bearing pulling the reamer into alignment with the other. The reshaping of the bores is miniscule and of no consequence apart from the positive alignment of one to the other.

Trial fit of wheels and axles
Check the rear of plastic spoked wheels for any moulding pips. These can pick up on the edges of the chassis. Remove them and smooth off. Fit one wheel to an axle and insert it in the first pair of axle holes. The wheel and axle should spin freely. If you have followed the guidance so far, experience says it usually does and requires no further work. Repeat for the other axle positions. Fit all the axles and wheels to each position. Bearings in horn guides do not need to have the springs fitted yet. Run the chassis back and forth; there should be no binding. If feels free then try it on a piece of sloping track to see if it runs down hands free; the shallower the slope the freer the chassis. It should be possible to run down a slope of around 1:10.

This note excludes ball or roller bearings. These are basically free running but for their installation the same issues of alignment apply. A mismatch across the chassis can lead to added friction in these bearings.

Rods and motion
Having achieved free running with a basic wheeled chassis there is the potential to lose this when the coupling rods and valve gear components are added. Add the motion in stages, starting with the coupling rods. If the chassis has been assembled using these with a jig or jury axles it should only be necessary to ensure there is a free fit on the crank pin bushes.

With the rods in place the chassis should still roll freely. However, binding may occur, and this will show in two places, at either the nine or the three o’clock positions. If the bind is on the left hand side only then that is the side that needs attention, similarly if the right then that side. This is where the rod centres match or fail to match the axle centres. If the rods are really tight then a serious error occurred when the bearings were fitted and it may be worth returning to them to position them correctly.

Usually any binding is only slight. Opening up the holes with a broach will normally suffice. When using a broach work on the holes from both sides to even out the taper that the broach cuts. Do not attempt to use a drill. The amount of material that has to be removed is small and the drill is likely to pick up and tear the rod from either your fingers or the vice. If it doesn’t turn into a propeller it can be bent into an interesting but useless shape! I have elaborated on the preparation of coupling rods in the Gazette Vol. 17 No. 4 which can be found in the archive. For a small inside cylindered loco this is usually the end of the process, but for a loco with outside cylinders and valve gear there is more. A lot will depend on how the loco is to be assembled. There is much variation across kits as to how the parts are assembled: cylinders permanently attached to the frames, cylinders that are bolted on, and similarly the fixing of motion brackets. Again I have written on this in the Gazette Vol. 15 No. 9 emphasising how to get freedom of movement in the system.

Plunger pick-ups are popular, but I have heard that once fitted they were effectively brakes. I would suggest that something has been overlooked in the installation. In kits the distance between the frames can be variable, some narrower than others and some near scale spacing. This will affect the plunger especially in the wider configurations. If the frame spacing is large enough it is possible that the plunger springs will bottom out leaving the ends of the plungers wider than the back to back dimension of the wheels, which is a very effective brake. To get round this, I have heard of modellers reducing the length of the spring but this can severely reduce the bearing pressure of the plunger, making electrical pick up ineffective. It is better to leave the spring as it is and reduce the plunger length. Some plungers have conical ends. File this off until the end bears on the wheels and movement is accommodated. It does not matter if the end of the plunger is flat.

The method of pick-up with least friction is split axles running in insulated horn guides. It is particularly suited to tenders. I have found the easiest method to make split axles is that of progressively dividing the axle and filling the gap with epoxy resin. No special tooling or lathe is required. An article in the Gazette Vol. 16 No. 7 describes this. Looking at the archive, I see that no one describes what to do with the wheels, though. If Slater’s wheels are used, a shorting wire between the tyre and centre is required. I use 0.5mm diameter copper wire from telephone cable for this. If you wish you can flatten it to reduce its profile by passing it through a set of rollers.

At each end of a spoke drill 0.5mm holes in the tyre and in the brass centre. Ensure that the wire is not in the way of the axle end, preventing it from seating. Solder the wire in place with cored solder used for electrical work. If you are unhappy about this, first tin the wire with this solder and then attach it to the wheel with low melt solder. You may also feel that it is worthwhile to recess the back of the spoke to accept the wire.

Motors and gearboxes

The use of ‘off the shelf’ motors combined with a gearbox has little effect on the established free running of a loco. It is just a matter of installing them. A motor/gearbox that is built up from etched components needs to be assembled with care and set up to run well before fitting to a loco. This is described in the Gazette Vol. 17 No. 7. With a satisfactory assembly this can be installed without undue effect on the free running. \

Flywheels With today’s mechanisms and control systems, I think that using a flywheel is questionable. Modern motors are multi-pole and are free from the slow running cogging that the old three and five pole open frame motors had. In that case a flywheel helped smooth the rotation. There is an argument that a flywheel helps a loco cross an area of poor contact with the track. I would say look to the track and correct that. Ensure it is properly level and in common with DCC practice, every rail is connected by wires. Do not rely on fishplates to conduct the current from one rail to another. Even so there is the DCC feature of ‘stay alive’ running that combats dead sections or poor pick up.

A flywheel where the mass is concentrated at the periphery is better than a solid one. The best flywheels are fitted to the shaft with adhesive, not a grubscrew. A grubscrew can force the flywheel eccentric and ruin any chance of true running.

If a flywheel is to be fitted then it should be balanced. An out of balance flywheel does nothing for smooth running and will eventually damage the motor bearings. To balance or check the balance, temporarily fit the flywheel to a shaft such as a piece of silver steel. Allow the shaft to extend beyond the flywheel. Ideally a pair of engineering parallels or something similar should be set side by side to provide ‘rails’ to run the flywheel up and down. A lathe bed may do for this if you have one available. Set the shaft on the ‘rails’ and roll it along. If there is a heavy side it will come to rest at the bottom; do this several times to confirm it. Once confirmed, mark the bottom and then drill into the flywheel at that position to remove some material; a 3mm drill is good for this. Do a little at a time, checking by rolling until there is no longer a bottom, and the stop position is random confirms that it is ready to be fitted.

Test tracks
A yard of track is good enough for checking the general freedom of movement in a chassis. However the ability of the chassis to negotiate curves and track imperfections should be checked. The simplest track is a six foot length arranged in a flat S. Set the curves to your minimum known radius. Only fix the track in the straight portions, leave the curves free.

Both inside and outside of the curves can be lifted. The photos show some gross distortions of the track, but if a loco can negotiate this then it should be able go anywhere. The loco should be turned and checked in both directions. Footnote; I set this radius when I was commission building locos. I never believed the customer – “my track has minimum radius of 5ft 6in.” I made the curve tighter so I was certain the loco I was building would go around his track.

There is an argument that a test track should incorporate a crossover to check how a loco will cope with point work. The points here should match those on the layout, whether Peco or hand made. If they do not match the standard of points on the layout the check will not be valid. It could be a case of it works on the test track but not the layout. On this set up it is not so easy to introduce the ‘imperfections’ of the simpler track. I am not convinced that this is a really useful design.

Sources of 4.8 mm hand reamers. I only found these when I searched the web. I purchased mine from Toolex as they are local. Note they are not cheap. http://www.chronos.ltd.uk/acatalog/HSS_STANDARD_HAND_REAMERS.html http://www.toolex.co.uk/ The electrical meters are available from Squires Model & Tools – Electrical catalogue Simple track plans drawn on Any Rail https://www.anyrail.com/index_en.html