Insert string of expletives here....

Entry posted by Pugsley August 19, 2011

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As you may have gathered from the title, things aren't going quite according to plan with the 37 bogies, I've discovered a problem with the design of the plate that links the 4 secondary suspension points and the central bearing. If I mount it in the way I'd planned, the travel of the bogie rests on the top of the middle traction motor when the axle is at its highest travel. Seeing as the normal planned position for the central axle is towards the top of its travel, this causes a bit of a problem.

If I set the assembly up as intended, as shown below, the top of the bogies, where they meet the slide plates, will be nearly 29mm from rail height (give or take a flange depth ). This doesn't sound too bad, until you consider that the bottom of the body needs to sit 25mm above the ground in normal running, and the slide plates are only a millimetre or so above the bottom of the body. I can turn down the parts that fit into the secondary suspension springs, but that brings the clearance problem into play.

As things are now:

This closer view shows the problem with the bottom of the plate and the traction motor - if the height is reduced by the required 3mm, the plate will be sat on the top of the motor, stopping the movement of the suspension.

It also shows that the bar arrangement to stop the motor rotating under load needs a bit of attention - I needed to wedge a screwdriver underneath to get it to sit in the right position! I think I'll have to fix the bar at the bogie end, and open out the hole in the motor to a slot, so that the motor can move up and down, but not rotate. The movement is constrained by the current arrangement, but it can still move too much at present.

The parts that fit into the secondary springs have been turned from brass, drilled 1.8mm and tapped 8BA, so that there is a form of ride height adjustment incorporated, as there is no equalisation on the secondary suspension and the loco will look a little odd if anything is out with the ride height. I decided that I couldn't work to the level of accuracy required to get everything identical, and a screw adjustment seemed the easiest way of levelling everything up at the end.

The secondary springs are made from 0.7mm soft brass wire.

The solution to the problem is going to be to raise the bearing part of the plate up to fit much closer to the floor of the loco. There is quite a bit of room to allow this, as shown in the next picture. It also means that I can use the etched packing pieces, rather than having to turn up new pieces to fit.

Ironically, this brings the design of the bogie even closer to that of the real one! The 3mm or so gained by doing this should offset the clearance problem discovered earlier, meaning that everything should be able to move as intended. I plan to cut the existing plate into 3 sections, then re-attach the central bearing part, spacing it upwards in the process. I may also take the opportunity to slim the central section down slightly.

The threaded bolt protruding from the chassis will be cut down as much as possible leaving just enough clearance to slip a nut on the end, to retain the bogie when the loco is lifted up. There will be enough movement for the suspension to work, and the force will be transmitted through the central bearing, just like the prototype.

Unfortunately, this minor setback means that the bogies definitely won't be ready for Telford, but I'm hoping that I can get at least one finished for the S7 Society meet up at Mark, in October.