Bachmann Class 117 and 121 DMUs

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ianpenberth
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Bachmann Class 117 and 121 DMUs

Postby ianpenberth » Mon Aug 08, 2022 5:13 pm

I had at first assumed that these conversions would be along similar lines to the Class 101/105/108 DMUs as in this thread with the Branchlines conversion pack, but it turned out that the powered bogies are quite different.

In consultation with Brian Osborne at Branchlines, for this conversion I used his split-axle pin point DMU wheelsets for the unpowered bogies, and 12mm solid-axle 'Black Beetle' wheelsets for the powered bogies. Give Brian a call and he will make up the packs of wheelsets as required.

Power Bogies

First surprise - two driven bogies in the Class 117, one in each driving car. One driven bogie in the Class 121. Each driven via cardan shaft by a chassis mounted motor circa 14mm L x 10mm D. Core of each drive bogie is a cast metal unit.

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Second surprise – they all run beautifully smoothly together, running in on an oval of OO track.

I converted the Class 121 single unit first to prepare these notes.

I found I had to remove the body and dismantle the interior and circuit boards to remove the drive bogie. However, it is possible to spring off the sideframe / undertray moulding without removing the bogie, which may be all that is strictly needed to do the drop-in conversion. (I can remove mine quite easily now it's been on and off a few times, but it's more tricky when new and not being able to see exactly what needs to be bent and where). Not forgetting to uncouple the rather neat speedometer drive too – the rubber moulding can be eased off the spigot at either end.

Using the Branchlines 'Black Beetle' 12mm wheelsets, insulated both sides (to be on the safe side, though insulated one side should be ok as far as I can see) this was my procedure for setting up the new wheelsets:

    Check back-to-back, then measure distance between inner faces of insulating bushes.

    Dismantle the wheelsets, remove pinpoint ends from axles and measure axle length.

    Measure the distance over bearings of the OO axles before removing them from the drive units.

    Remove wheels, spacing washers and bearings from the original OO axles.

    measure the offset of the drive gear from the mid point of the OO axle, then press the gear from the axle.

    use the measurements to determine position of the gear on the Branchlines axle and the number of spacing washers needed.

    The drive gears are a sliding fit on the new axles, so Loctite (603) them in place, having first put a concentric groove in the axle at the gear location to give the glue somewhere to be.

    Fit bearings, then original and additional spacing washers and refit the wheels.

Pickups – original width over pickups when free of wheelsets, 16.7mm: increase to 20.2mm by increasing the angles of the 'Z-bend' in each leg of the pickup strip.

Brake gear – not necessary to resite shoes, but do need to cut away the strips that go beside the wheels between the shoe hangers.
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Unpowered Bogies

The unpowered bogie can be removed from the car by simply undoing the one retaining screw. There is no need to dismantle the body. But, if there is a speedometer drive, don't forget to release it from one of its spigots.

Here I used the Branchlines DMU conversion pin-pointed half-axle and wheel assemblies, fitted into the centre plastic insulating muffs from the Bachmann OO wheelsets. I shortened the muffs from 9.5 down to 9.1mm to get the required back-to-back, by hand turning in a lathe.

Distance between inner faces of the plastic sideframes is 23.6mm. The Bachmann pickup strips with their integral pin point bearings can be retained (the Bachmann OO wheelsets are a very loose fit between these). I did however grind a notch, using a slitting disk, leading downwards from each bearing on the inside face of the strip, to ease the path of each pin point into its bearing.

You can see in the photo that I cut off the brake mouldings and will have to refit the shoes and their hangers (having first removed the strips joining them that would foul the wheels) on new locating pins. I think that I could have left them in place and just cut off the strips in situ; I will confirm that when I come to do the rest of the cars.
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Ian
PenBits Model Railways - Diesel bogie springing and detailing

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