Locomotive Kits - Chassis Construction

These techniques are of equal value for scratchbuilding and kit construction, and are recommended for the use of adult scale modellers only.

Introduction

It should also be remembered that soldering these materials requires the use of a phosphoric acid based flux. Always use in a well ventilated room and avoid inhaling any gas expelled during the soldering process. Always wash well any spills in contact with the skin. Always follow the COSH directions on the bottle. Remember that when soldering brass and nickel silver the heat is transfered to the metal and the metal becomes hot - very hot!

The techniques for locomotive body and tender construction are covered in the Brass, Whitemetal & Nickel Silver Kits section. See kit construction index.

Chassis construction often proves the down fall of many modellers. However if a few basic engineering principals are followed, much of the anxiety can be removed. Use a flat true surface eg plate glass to test throughout construction that the assembly is square. Never permanently solder any parts until you are 100% sure of their fit. Finally observe the prototype, particularly the valve gear. The same or similar principals apply.

FRAMES, BEARINGS & SPACERS.

Two basic frame types are available according to the preference of locomotive kit manufacturers:

Rigid Frames - No 'suspension' is provided ie the driving wheels or other wheels cannot move up and down according to undulations in the track/baseboard. This is the most simple and easy to construct method of frame construction.
Compensated or Sprung Frames - The most protypical method of frame construction and to many modellers the most effective as all wheels remain on the track (for electrical collection) particularly on small layouts with many slow loco manouvres. Selected wheels move up and down in a slot cut in the frames according to track/baseboard level. A more complex method of construction not recommended for the beginner unless practical with basic engineering techniques. Most manufacturers supplying compensated/sprung frames allow for the chassis to be constructed rigid.

Rigid Frames
Remember that frames are a mirror image and are 'handed'. The bearings need to be inserted with the bearing flange on the outside of the frames (unless otherwise directed). They should be a tight fit ie they require pushing in with gentle pressure and should not fall out. If the bearing hole in the frames is too small, carfully open it up preferably with a broach or reamer, or very carefully with a round needle file. If you make the hole too big, or move its centre, running problems may result later on. Remember that the coupling rods share the same spacing and centres as the frames; if the axle centres on the frames are moved, the rods will bind later on! With the bearings in place, some modellers prefer to solder them in place at this stage. Some modellers prefer to solder the bearings in place once the wheels have been tested with the rods. Others believe that if the fit of the bearings into the frames is good enough, soldering is not required. It is personal preference. We prefer to solder at this stage whilst the frames are single and on a flat surface. The assembly of the frames is the first test of your accuracy. Insert the spacers as directed by the kit instructions. If solder spacers are used, only solder into place once the kit has been checked on a flat surface. Screw spacers allow minor alterations to be made with ease to the frames. When you are happy with the fit, solder all frames in place to ensure accuracy. A useful trick can be several metal rods of an equivalent diameter to the axles. These can be pushed through the bearings to align both frames.

It is worth a brief discussion of motor and gears at this stage. Many kits have been on the market for over twenty five years! Motors and gears have come and gone. Always check that your motor and gears/gearbox will fit your frames prior to their assembly. You may need to cut the frames to accommodate a new motor, or add washers to frames spacers to widen your frames for a gearbox or mount. This is frequently required when using Portescap gearboxes. They are too wide for normal spacers and will not fit between the frames. It can be prudent at this stage to assemble your motor and gear combination to test in the frames before the frames are soldered together. If using a motor mounting plate as supplied in some kits, this will need soldering to the frames.

Compensated or Sprung Frames
Compensated or sprung frames provide a form of 'suspension' that aims to ensure the maximum wheel contact with the rail thus assisting electrical collection. Most compensated chassis operate on the driving wheels although some enthusiasts prefer to use the system across all wheels. The basic principal is that the driven axle is rigid allowing vertical movement of the remaing driving wheels using hornblocks or similar. Square/rectangular cut outs are etched into the frames based around the rigid axle centres. These cut outs provide the 'guide' to allow the square hornblocks or similar to slide up and down upon using a groove machined in the hornblock. A specially selected spring located on a 'stud' at the top of the hornblock provides the 'suspension' to push against. This system requires the coupling rods to be split to allow the axles to move vertical. Other systems involve the use of 'pivot' beams where a beam runs between the two moving axles and locates on a central pivot. The moving axles usually fit into the chassis cut outs using slotted bearings. As one axle lifts, the pivot beam lowers its partner axle thus ensuring contact with the rail. It should be noted that this is only a basic guide for beginners and therefore much background reading is recommended for beginners attempting any form of compensation.

Wheels & Crank Pins

Locomotive kit wheels generally fall into two basic types across the various manufacturers, gauges and scales: Self Quartering Wheels and Non-self Quartering Wheels

For the benefit of the beginner with limited knowledge of the prototype, it should be noted that the wheels on a locomotive are not fitted in the same crank postion on both sides of the locomotive. This is potential mistake number one! Observe a ready to run model or the real thing. If the nearside wheel set is showing the crank at 12 o'clock, the offside wheel set will be a quarter leading or trailing depending on the locomotive (ie three or nine o'clock), hence the term 'quartering'. It is essential that wheels are fitted EXACTLY in the correct quarter otherwise good performance will never be achieved. For this reason some manufacturers supply wheels with a simple quartering system.

Self Quartering Wheels
Self quartering wheels are the most 'user friendly' wheels on the market and are recommended for the beginner. Fast simple quartering can be achieved by the manufacturing of a square boss or similar on each end of the axle. With a 'square hole' manufactured in the centre of each wheel, quartering is achieved by pushing the wheels onto the axles in the quarter required. The wheel is then secured using either an axle nut eg Romford wheels, or a retaining screw eg Slaters wheels. It is recommended that the axle nut or screw is fixed using a retaining compond as continual use will gradually free the fixing. NEVER use super glue unless you intend buying new wheels and axles! This method of fixing allows for the relatively easy removal of the wheels for maintenance/repair at any time in the future. Romford wheels are supplied either as Insulated or Non-Insulated ie the insulated wheel contains a thin 'brown' insulating layer between the wheel rim and the centre casting. Remember to use the insulated wheels on the same side of the locomotive (usually the offside). Some modellers prefer to use insulated wheels on both sides of the chassis thus requiring collectors to the wheel rims on both sides of the loco as opposed to one side.

Non-Self Quartering Wheels
Wheels from manufacturers such as Alan Gibson and Ultra Scale do not provide a method of self quartering. They reply upon the modeller pressing the wheel onto the axle ensuring correct quartering using either a wheel alignment jig, or by sighting through the wheel to align the spokes and cranks. The use of a lubricant eg washing up liquid is recommended as the wheel is pushed on to allow a small amount of movement. However great care must be taken to prevent serious rotation if the wheels contain plastic spokes as these will bend. As the spokes are usually plastic with a metal tyre, the wheels are insulated from both rails. Collectors are therefore required to the wheel rims on both sides of the loco. These wheels are not recommneded for the beginner unless you are very confident.

CRANK PINS
Crankpins require very little attention other than to say they fit either from the rear of the wheel, or from the front. It is essential that the crankpin is fitted into the wheel using a retaining compound as operation will gradually free the crankpin causing future problems. Rods are retained on the crankpin using either a soldered washer or a nut. Always allow a small amount of movement for the rod up and down the crankpin. Usually half a millimetre is sufficient. When a nut is used again use a small amount of retaining compound to prevent the nut 'working' loose. A small 'blob' of paint will offer similar protection or general purpose adhesive eg Bostick/UHU. Soldered washers require a little more care as it is easy to solder up the whole assembly. The good old fashioned method is as good as anything. Before placing the washer over the last rod, place a small piece of newspaper to allow spacing and to soak up any excess flux. Then quickly solder the washer to the pin.

Motor & Gears

The type of motor selected for a locomotive kit will be governed by the type of gear drive/motor mount required, and the space available for the motor within the locomotive body. Many locomoive kits, particularly 00 gauge date back almost forty years! Motors have come and gone in that time and reference is often made to motors no longer in production. Generally two types of motor are available: Open Frame Motors and Can Motors.

Open Frame Motors
These have a visible armature and magnet with the facility of easy maintenance particularly to change carbon brushes and springs. Older types frequently referred to in 00 kits and now discontinued include the Romford Bulldog, Hornby X03/X04 and MRRC 1001. These motors have been replaced by Asian manufactured motors by companies such as Mashima, Hanzoi, Tenshodo etc. Trade names include DS10 (H1024, Cheetah), D11 (H113) and D13 (H11). One motor brush usually earths through the chassis (not DS10) to the non insulated wheels (if this method of collection is used). The other brush is insulated and requires a cable from the collectors (insulated wheel) to complete the circuit. Mounting fixings are generally on the base of the motor although some eg DS10 style have face fixings. Where these motors earth on one pole to the chassis, they are more difficult to install DCC decoders. It is likely that this type of motor will slowly become discontinued with a move towards the more efficient can motors. The D11 and all its varients are slowly being discontinued.

Can Motors
Very easy to recognize as they appear as a sealed can (sometimes flat on two sides ie 'flat can') with a motor shaft extending from one or both ends through a sealed bearing. Two electrical connection tags extend from one end face of the can, with tapped fixing holes on the opposite face. Can motors are becomming the modellers choice as they usually offer more power from a smaller motor. Manufacturers include Escap, Mashima, Sagami, Buhler and many more. They are sold either by a trade name eg Puma/Panther, or more often by their external dimensions (width first) eg 1620 or 1833. Can motors are sealed and therefore offer no facility to service or repair. Both motor brushes are isolated from the can chassis.

MOTOR MOUNTING METHODS
Mounting Plate - a simple plate soldered to the chassis in the correct position to mount the motor and mesh with the gears on the driven axle. This type of mount is sometimes supplied by kit manufacturers and requires the use of the simple worm and wheel gear set.
Motor Mount - a self contained etched sheet or lost wax casting forming a 'channel' shape designed to fix onto the motor using the mounting screws supplied with the motor. The mount locates between the frames of the chassis as it contains axle holes in the correct position to obtain a basic mesh between the worm and wheel. The unit when assembled can be used to test the accuracy of the gearmesh off the chassis.
Gearboxes - Over the past ten years the mounting plate and motor mount have lost favour to gearboxes. Gearboxes usually increase the torque at axle level and improve the running quality by removing the fine adjustments often required to obtain a satisfactory mesh when using mounting plates and motor mounts. Gearboxes are also manufactured to allow motors to be located within a locomotive where they would have previously been visible. Some manufacturers supply ready assembled gearboxes, or gearboxes already located on a motor eg Escap.

When using mounting plates and motor mounts it is essential that the modeller spends time adjusting the mesh to achieve the best performance. The mounting plate usually forms an integral part of the chassis, thus any adjustment to the mesh can only be achieved by 'warming' up the plate and carefully adjusting its position. Motor mounts allow a little more freedom as they can be assembled and tested off the chassis. Adjustments to the gear mesh can be made by adding or removing thin packing strips between the mount and the motor. A little engineering knowlege with worm and wheel gears is useful to obtain a good mesh. Some of the principals apply to self assembley gear boxes.

GEAR MESHING - WORM & WHEEL

The gear ratio of a locomotive should be selected according to the required performance and wheel size. A heavy freight loco would generally have small wheels and be expected to run at slower speeds than an express freight with larger wheels. A gear ratio of between 30-45:1 is generally acceptable for express stock, whilst 40-60:1 is acceptable for freight locomotives. The lower the gear ratio, the easier the gears are to mesh as the cut is more coarse. When meshing gears follow the following basic beginner tips:

File a small flat on the drive axle and armature shaft where the worm and wheel locate using a small grub screw.
The gear wheel located on the axle should have a tiny amount of travel from side to side ie the axle should move a tiny amount with the gear fixed to it between the frames. If the axle is tight in the frames with no movement, performance will be poor. Gear wheels vary over time and between manufacturers. If the top of the gear is cut flat, more travel of the axle is possible. Some gear wheels however are tailored to suit the worm and are 'scooped' out to allow improved meshing. This often occurs on higher value gear sets. Only a minimum amount of axle travel can be allowed with scooped gears to prevent the worm 'chewing' the gear wheel.
Always allow a tiny gap between the worm and wheel ie mesh at about 75% of the tooth depth. This small space allows the gears to run free. If the mesh is tight, running will be poor.
Ensure the worm sits perpendicular to the wheel and does not cut across the worm at an angle.
When locating the worm on the armature shaft, ensure the armature can move backwards and forwards a tiny amount as again if tight running will be poor.
Use a quality gear oil eg Hobby Lube Gear Oil on the worm and wheel.

Valve Gear, Cylinders, Crossheads, Motion Bracket & Rods.

Probably the area of locomotive construction that is most 'feared' by beginners and may be the obstacle preventing modellers from actually having a go. With a little patience, a little knowledge of the prototype and a few basic tips for each stage of assembly, the obstacle can be over come. Remember you are dealing with some of the most fragile parts of a locomotive kit, and thus they should be handled accordingly.

Cylinders, Slidebars & Motion Bracket

This section covers the stationary structural components of the valve gear assembly which usually form part of the chassis frames as they are attached by either screws or glue/solder. The construction and location of these components will vary substantially between manufacturers according to scale/gauge, materials and age of the kit; however once assembled their location is the most influential factor upon performance.

CYLINDERS - Generally the location of the cylinders is fixed by mountings on the chassis. On occassions mistakes in the development of the chassis kit are found and it is recommended that the cylinders are only 'tacked' in place until the rest of the valve gear has been tested. It should be noted that the inclination of the cylinder is of vital importance. The cylinder iteself should be inclined towards the centre of the driven axle ( a few exceptions do occur, always check photographs) extended through the line of the slide bar. Ensure that the cylinder assembley is 'clean' internally to allow the location of the slide bars and the travel of the piston rod. Fit vlave guides if applicable. Also check the clearance of the driving wheels as this may effect the final location of the cylinders and their finished shape if material has to be removed according to the selected track radius required.
SLIDE BARS - Always check the manufacture of the slide bars to ensure the smooth travel of the crosshead along their length. Shape and fix the slide bars into the cylinder before locating the cylinders on the chassis. Where slidebars are supplied 'rough' ie their length is not cut precisely, never trim their length until the crosshead - connecting rod assembly has been tested on the crankpin to ensure the required length is correct according to the amount of travel.
MOTION BRACKET - As with the cylinders, the motion bracket location is usually marked on the chassis. This is however not always accurate as the position of the motion bracket is usually determined by the accuracy of the cylinders and slidebars. It can be easier to assemble the valve gear onto the motion bracket before fixing the motion bracket in place.

Coupling Rods

The initial make or break on performance once the motor/gears have been correctly set is the coupling rods. Once the wheels (complete with crankpins) have been fitted to the chassis with the bearings reamed to ensure the wheels rotate freely (a round needle file can be used), the next task is to locate the coupling rods over the crankpins. Ensure the wheels are in the correct quarter. The coupling rods are often a very tight fit with the crankpin holes on some kits appearing to show little resemblance to the location of the crankpin. It is important not to 'jump in' with the file as this will probably cause damage to the rods. Some thought is required to ensure the correct action is taken. The temptation is to randomly increase the diameter of the crankpin holes in the coupling rods until the rod fits over the crankpins resulting in rods that will lock solid. Only increase the diameter initially if the crankpin will not pass through the hole, then only sufficient to allow a 'comfortable' fit ie a tiny amount of movement of the rod over the crankpin. Any additional opening of the holes should be done laterally ie elongate the hole as a horizontal slot to allow the rod to fit correctly. Once the first rod sits on the chassis, repeat for the opposite side. When complete rotate the wheels until the crank is at nine o'clock. Check for tight spots. At this position the rod should have a small amount of lateral play (side to side). Rotate the wheels and repeat for the opposite side. When fixing crankpin screws or solder washers, always allow a small amount of travel down the crankpin. If the rod is held rigid, poor running is likely.

Crossheads & Connecting Rods

The crosshead and connecting rod form one assembly once they are linked together by a rivet or small bolt. Ensure the link allows the required amount of rotation as the crank rotates. Some crossheads are supplied with the piston rods needing trimming to length. Never guess at this. Using a temporary connection eg a blunt pin, join the crosshead to the connecting rod and insert the crosshead into the slide bars. Place the crank end of the connecting rod over the crankpin (ease out the hole gently to allow this if required, then rotate the driving axle marking the maximum and minimum travel of the piston rod assuming internal access within the cylinder is possible. If not, gently trim the front of the piston rod until it can travel inside the cylinder without hitting the piston covers on the front of the cylinder. Where applicable it is recommended that the union link assembly is fitted to the crosshead prior to fixing.

Rods & Rivets

The contents of this section apply to all rods including connecting and coupling rods. When removing the rods from their etchings, always cut them on a flat hard surface to prevent damage. Clean the rods gently using a fine file removing all etching process debris. Open all holes very carefully to the minimum diameter to clear crankpins, rivets or bolts/screws. Manufacturers often allow insufficient material around the locating holes on rods to prevent them splitting when opened up. Always file or drill in small increments to try and reduce the chance of damage. When using valve gear rivets always proceed with caution. Remember they are tiny so start tapping with a light hammer very gently. If the rivet does not start to open up, you can always hit a little harder - with caution. Leave sufficient play to allow the rods to rotate. If the rivet is too tight performance will be poor.

Collectors

Another stumbling block for the beginner. The essential fact of collectors is that they must ensure electrical connection with the rail at all times via the wheels. Collectors function in one of two ways:

CONTACT/FRICTION COLLECTORS
These rub on an insulated wheel using either a wiper or sprung plunger. Depending on the wheel manufacturer and/or modeller preference, contact connectors may be required on one or both sides of the chassis depending on the insulation of the wheels.
Wiper Collectors. These usually function using a small strip of copper laminated board fixed to the chassis to solder the wipers to. Using either thin phosphor bronze or brass strip/rod, select the best route to the wheel rim of sufficient length to allow the collector to place a gentle pressure on the wheel. This pressure should allow the wheel to move, but also allow the collector to follow the movement of the wheel. Care should be taken to adjust the wipers to ensure a continuous circuit. Electrical cable is then used to link the laminate board to the motor.
Plunger Collectors. Using an isolated cup that sits in the chassis frames, a sprung plunger is gently pushed out onto the wheel rim to collect the power. A wire then connects all the plungers together as they pass through the insulation cup. Efficiecy can be improved by fixing small pieces of copper laminate on the inside of the chassis with a thin copper strip soldered to the laminate board and then gently pressing on the plunger rod. This removes the drag of the wire from the plunger as the motor wire connects to the laminate board rather than the plunger itself. Some manufacturers may provide locating holes for plungers within the chassis. If they are not present simply mark the outside circumference of the wheel on the chassis, and select a suitable position to drill a locating hole inset enough for the plunger to run on the wheel rim.

AXLE COLLECTORS
This system uses the axle itself to pass the power through the chassis to the motor. This can be achieved in two ways:
Split Chassis/Axle - the most complex method of collection not found in kits; requiring both the axle and the chassis frames to operate as electrically isolated units. No item eg cylinders, motion brackets etc can cross from one chassis frame to the other. Frame spacers have to be non conductive. Axles are then fabricted with an insualted central boss. Power is collected by connecting the motor contacts to the two sides of the chassis.
'American Style' - For use on tender locomotives by having insulated wheels on opposite sides on the locomotive to the tender ie insulated wheels on the nearside on the loco, and offside on the tender. The motor simply earths to the loco chassis and uses a wire to collect power from the tender. Remember the draw bar and any other contact between the locomotive and tender must be insulated.

Kit Construction - Hints & Tips For The Modeller

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