Gibson LMS Compound
Posted: Sun Jan 12, 2014 2:22 pm
This may be the equivalent of a discussion of the sun's orbit around the earth. But here, from an era perhaps a very long time ago, before CSB, is a description of my LMS Compound compensated suspension. The model has progressed to the point where it definitely runs reliably, though much work remains to be done before it is visually complete. That will take me many more months...
In my previous contribution to the Forum "In Search of Smooth Running..." I described my lessons in making my first P4 loco, an 0-4-4 tank loco (ex CR Class 439), and particularly how I have found that more weight needs to be on the leading wheels (in either direction) than on the inner ones for good roadholding and reliability not to derail. So here I am assuming that anyone with the patience to read that far will have the motivation to refer back to the other post...
The model was acquired in a semi-built condition. The tender was almost complete, and the loco chassis was fairly near being a working proposition, mostly made as per the Gibson instructions and with the cylinders and slidebars installed. Just the cab of the loco was fixed to the semi-completed footplate framing.
With only the light springs in the hornblocks to positively keep the wheels all on the track, I was not satisfied with how I thought this arrangement would run, so the wheels, hornblocks, motor and gearbox were removed, but the chassis frames were not further dismantled. The rear wheels were arranged to run in a fixed bearing, and a new lower reduction 80:1 Hi Level gearbox was made up and a much bigger motor with flywheel attached. According to the HiLevel gearbox ratio planner this would give a top speed of 35mph, slightly higher than my normal aim of 30mph. The front hornblocks were replaced (although that was not so necessary) with MJT ones that I had already prepared for a currently abandoned project.
The boiler was now made up and fixed to the frames etc. The chassis with new motor arrangement was checked to fit into the boiler space and attention now turned to making a running locomotive.
The objective was to have 4-4-0 locomotive that ran as successfully as my 0-4-4T Class 439, but incorporating the opportunity a tender presented to get more weight where it mattered. As the tender could load plenty of weight onto the rear driving wheel, the bogie could load more weight onto the inner driving axle than on the tank loco, by making them compensated with each other. The tender would run as in Mike Sharman’s design with only the rear wheels taking weight, the other wheels lightly sprung to keep them on the track, the front of the vehicle resting on the loco but allowing swivel between one and the other.
The loco bogie would steer in the same way as the 439, and so the inner driving wheel would again not need much sideplay, just half a mm or so.
In addition I would aim to have the bogie wheels picking up current, and also the tender wheels: not by using wiper pickups but by making each axle live to one wheel. I realised that the easiest option would be to have the whole bogie live to one rail, and the tender chassis live to the other.
The bogie would be made as per Mike Sharman’s diagram for a 4-4-0 bogie, but with improvements – it would have a roller where the front of the loco rests on it rather than a rubbing plate, and the guidance fixing to the loco would not be a spring but an arm to give positive steering and directional control. Somehow this would have to be electrically isolating and yet also provide freedom for the bogie wheels to move up and down independently of the loco.
Additionally the bogie would have the weight from the loco arranged to slightly bias this weight to the front wheels.
The weight of the loco from a boiler filled with lead would rest on the bogie, which would be compensated to the front (inner) driving wheel. The aim would be for twice as much weight to be on the bogie as the driver. As there was not so much scope for weight at the rear of the loco the outer rear driving wheel would be lighter on the track than the inner front one, and this would not work on its own in keeping the loco on the track (i.e. it would derail) but with sufficient weight from the tender the rear pair of wheels would have more weight than the inner front pair.
The steering arm would be pivoted a little in front of the inner driving wheels.
The options were considered with the drawing and photos to see where the least visible places could be for the bogie steering arm pivot and the compensation fulcrum. I made two fixing points for the steering arm in case the outer one needed more sideplay than I installed in the front driver. The motion plate that ran through the model chassis between the ends of the slide bars was about half way between the front driving wheel and the centre of the bogie, so that looked like a good area for the compensation fulcrum. This was rather nearer the driving wheels than I had intended though still less than half way from the bogie centre, so more weight is on the bogie than front driver. The actual distances are 21+a half mm front bogie asymmetrical resting point to fulcrum, and 26+a half mm fulcrum to front driver, centres. The slide bars of the centre connecting rod plus crosshead and piston looked from photos as though they were in much the same place as the compensation beam would or could be.
So I made up a compensation beam to at least begin to resemble these elements (much improvement possible here!), and how I imagine the inside connecting rod to look – although this is hidden on this and any other photos I had, and so largely will be on the model as well, by the weighshafts and other gear. At the front a plate was arranged on which the bogie would actually rest. After this photo was taken a layer of Copperclad pcb for insulation was added. At the back I arranged a screw to give adjustment of the ride height of the loco.
The bogie as constructed by the original maker as per the Gibson instructions was dismantled. The side frames were used to construct an H shape arrangement of tubes.
Both axles can tilt. A roller was arranged asymetrically to bias the weight of the loco to the front wheels.
The bearings on the side frames were filed to just the thickness of the frame material so that they could allow the axles freedom of movement to tilt, as they now had only cosmetic purpose beyond restricting the tilt to a necessary amount of one or two mm.
Two bogie wheels were painted with electroconductive paint, but I found this did not conduct any current. Anticipating future unreliability, even if I could make the paint work, I abandoned this idea. Shorting wires from the EMGS (which are not a very satisfactory etch for the 2mm ones) were fixed to them instead. For this operation I removed the plastic wheel centre by immersing in hot water, and tinned the rim in two places opposite each other as lightly as possible. The shorting etch was cut till it only just reached the rim at each end, and soldered on after the rims had been reattached with Loctite - soldering as quickly/deftly as possible to avoid heating up the plastic. Care was taken to then remove any stray solder so that the flange thickness was undisturbed for running through pointwork. The following photo shows a modification to one axle which will be explained later.
In my previous contribution to the Forum "In Search of Smooth Running..." I described my lessons in making my first P4 loco, an 0-4-4 tank loco (ex CR Class 439), and particularly how I have found that more weight needs to be on the leading wheels (in either direction) than on the inner ones for good roadholding and reliability not to derail. So here I am assuming that anyone with the patience to read that far will have the motivation to refer back to the other post...
The model was acquired in a semi-built condition. The tender was almost complete, and the loco chassis was fairly near being a working proposition, mostly made as per the Gibson instructions and with the cylinders and slidebars installed. Just the cab of the loco was fixed to the semi-completed footplate framing.
With only the light springs in the hornblocks to positively keep the wheels all on the track, I was not satisfied with how I thought this arrangement would run, so the wheels, hornblocks, motor and gearbox were removed, but the chassis frames were not further dismantled. The rear wheels were arranged to run in a fixed bearing, and a new lower reduction 80:1 Hi Level gearbox was made up and a much bigger motor with flywheel attached. According to the HiLevel gearbox ratio planner this would give a top speed of 35mph, slightly higher than my normal aim of 30mph. The front hornblocks were replaced (although that was not so necessary) with MJT ones that I had already prepared for a currently abandoned project.
The boiler was now made up and fixed to the frames etc. The chassis with new motor arrangement was checked to fit into the boiler space and attention now turned to making a running locomotive.
The objective was to have 4-4-0 locomotive that ran as successfully as my 0-4-4T Class 439, but incorporating the opportunity a tender presented to get more weight where it mattered. As the tender could load plenty of weight onto the rear driving wheel, the bogie could load more weight onto the inner driving axle than on the tank loco, by making them compensated with each other. The tender would run as in Mike Sharman’s design with only the rear wheels taking weight, the other wheels lightly sprung to keep them on the track, the front of the vehicle resting on the loco but allowing swivel between one and the other.
The loco bogie would steer in the same way as the 439, and so the inner driving wheel would again not need much sideplay, just half a mm or so.
In addition I would aim to have the bogie wheels picking up current, and also the tender wheels: not by using wiper pickups but by making each axle live to one wheel. I realised that the easiest option would be to have the whole bogie live to one rail, and the tender chassis live to the other.
The bogie would be made as per Mike Sharman’s diagram for a 4-4-0 bogie, but with improvements – it would have a roller where the front of the loco rests on it rather than a rubbing plate, and the guidance fixing to the loco would not be a spring but an arm to give positive steering and directional control. Somehow this would have to be electrically isolating and yet also provide freedom for the bogie wheels to move up and down independently of the loco.
Additionally the bogie would have the weight from the loco arranged to slightly bias this weight to the front wheels.
The weight of the loco from a boiler filled with lead would rest on the bogie, which would be compensated to the front (inner) driving wheel. The aim would be for twice as much weight to be on the bogie as the driver. As there was not so much scope for weight at the rear of the loco the outer rear driving wheel would be lighter on the track than the inner front one, and this would not work on its own in keeping the loco on the track (i.e. it would derail) but with sufficient weight from the tender the rear pair of wheels would have more weight than the inner front pair.
The steering arm would be pivoted a little in front of the inner driving wheels.
The options were considered with the drawing and photos to see where the least visible places could be for the bogie steering arm pivot and the compensation fulcrum. I made two fixing points for the steering arm in case the outer one needed more sideplay than I installed in the front driver. The motion plate that ran through the model chassis between the ends of the slide bars was about half way between the front driving wheel and the centre of the bogie, so that looked like a good area for the compensation fulcrum. This was rather nearer the driving wheels than I had intended though still less than half way from the bogie centre, so more weight is on the bogie than front driver. The actual distances are 21+a half mm front bogie asymmetrical resting point to fulcrum, and 26+a half mm fulcrum to front driver, centres. The slide bars of the centre connecting rod plus crosshead and piston looked from photos as though they were in much the same place as the compensation beam would or could be.
So I made up a compensation beam to at least begin to resemble these elements (much improvement possible here!), and how I imagine the inside connecting rod to look – although this is hidden on this and any other photos I had, and so largely will be on the model as well, by the weighshafts and other gear. At the front a plate was arranged on which the bogie would actually rest. After this photo was taken a layer of Copperclad pcb for insulation was added. At the back I arranged a screw to give adjustment of the ride height of the loco.
The bogie as constructed by the original maker as per the Gibson instructions was dismantled. The side frames were used to construct an H shape arrangement of tubes.
Both axles can tilt. A roller was arranged asymetrically to bias the weight of the loco to the front wheels.
The bearings on the side frames were filed to just the thickness of the frame material so that they could allow the axles freedom of movement to tilt, as they now had only cosmetic purpose beyond restricting the tilt to a necessary amount of one or two mm.
Two bogie wheels were painted with electroconductive paint, but I found this did not conduct any current. Anticipating future unreliability, even if I could make the paint work, I abandoned this idea. Shorting wires from the EMGS (which are not a very satisfactory etch for the 2mm ones) were fixed to them instead. For this operation I removed the plastic wheel centre by immersing in hot water, and tinned the rim in two places opposite each other as lightly as possible. The shorting etch was cut till it only just reached the rim at each end, and soldered on after the rims had been reattached with Loctite - soldering as quickly/deftly as possible to avoid heating up the plastic. Care was taken to then remove any stray solder so that the flange thickness was undisturbed for running through pointwork. The following photo shows a modification to one axle which will be explained later.