Post by suctionhose on Jul 23, 2024 12:23:27 GMT
Copying this from my New Zealand 4-6-0 Build Thread and happy to entertain discussion here.
Recognising that the 9m radius curves I want to negotiate are much tighter than scale, I have decided on the following for reasons to be explained.
Many a set of side springs on front bogies have been removed when thought to be the cause of derailments. Considering this and the usual 'sliding block' bolster set up, I theorised that the required side displacement for tight 5"g curves may exceed the functional range of springs subject to direct action upon the bogie centre. Maybe they became coil bound or over compressed, the spring rate of a short coil may be too steep...
To reduce the change felt by the spring when accommodating large side displacements, I have introduced a 3:1 lever (the tee bar piece). Thus, for a side displacement of 12mm, the spring length only changes by 4mm. Pic below shows the 'tee bar' pushed to one side and the springs: one extended and the other relaxed.
Interestingly, for any arrangement of springs opposing each other, compression or extension, equilibrium is established when the opposing forces are equal. It does not matter how much preload is applied - a lot or a little - because they provide equal and opposite forces neutralising each other at the centre position.
It is the 'spring rate' that determines how much force and how readily it is applied to recentre. By using fairly short strong springs, and reducing their deflection to 1/3 of that occurring at the centre pin, I hope to provide a smooth, functional self centring action suitable for the circumstances.
Recognising that the 9m radius curves I want to negotiate are much tighter than scale, I have decided on the following for reasons to be explained.
Many a set of side springs on front bogies have been removed when thought to be the cause of derailments. Considering this and the usual 'sliding block' bolster set up, I theorised that the required side displacement for tight 5"g curves may exceed the functional range of springs subject to direct action upon the bogie centre. Maybe they became coil bound or over compressed, the spring rate of a short coil may be too steep...
To reduce the change felt by the spring when accommodating large side displacements, I have introduced a 3:1 lever (the tee bar piece). Thus, for a side displacement of 12mm, the spring length only changes by 4mm. Pic below shows the 'tee bar' pushed to one side and the springs: one extended and the other relaxed.
Interestingly, for any arrangement of springs opposing each other, compression or extension, equilibrium is established when the opposing forces are equal. It does not matter how much preload is applied - a lot or a little - because they provide equal and opposite forces neutralising each other at the centre position.
It is the 'spring rate' that determines how much force and how readily it is applied to recentre. By using fairly short strong springs, and reducing their deflection to 1/3 of that occurring at the centre pin, I hope to provide a smooth, functional self centring action suitable for the circumstances.