Part four by Tony Sprent

Part two here

Part one here

Part three here

Model drawings start here

Work was started on the upper chassis. At Julius’ suggestion, I decided to make this as one unit rather than three separate parts which would be bolted together. As the upper chassis is actually three castings which I didn’t have the facilities to make, it seemed a good idea for it to be a single piece and easier to construct. As most of the materials I had at my disposal are imperial sizes I didn’t machine them down to metric units.

The chassis was made up of two side plates of 38 X 3.175 brass flat, 448 long with pieces of 12.7 x 6.35 fixed to the top of the side plates with countersunk screws from the inside. Six 38 x 6.35 cross members were attached to the side plates with 3mm countersunk screws from the outside of the plates. The heads of these are below the surface of the sides and will be filled in, smoothed and painted. 

The main crane bearing block consists of a 44 mm diameter cylindrical section drilled and reamed to take the main bearing shaft. This will have bronze bearing inserts to take the travel drive shaft. Two machined surfaces on the cylindrical section were cut and fixed to the two central cross members with 3 mm bolts.

The back jib stay points were drilled and silver soldered to the side plates.  These will take 4.5mm diameter guys with threaded ends for adjustment as does the original.

The forward jib hinge bracket was machined from a block of aluminium. This forms the front-end cross member of the chassis and is fixed to the side plates in the same way as the as the other cross members. I am considering whether to attach bolted vertical sections on the sides of the side plates to make it look like the chassis is made out of three castings bolted together, rather than a single fabricated piece. Not being a model maker, I would be interested to hear any comments about this practice in model making.

I made a jig to hold the cross members at the correct distance apart so that I could drill and ream the holes for the bearing mounts of the tapered rollers. These roll around the circumference of the ring gear and take the weight of the whole upper crane. 

I found a small error in Julius’s drawing of the ring gear and tapered rollers. Given that the roller’s outer radius is 21mm, and the distance to the centre is 165 mm the taper angle should be 7.25 degrees, not 4.5 degrees. The cross members were placed back in the chassis and reamed to ensure a good fit of the bearing mounts and shafts. The bearing mounts will be fixed in final position with Loctite. 

I turned the rollers from brass bar, held in the chuck for machining the taper and drilling and reaming the central bore, then parting this off. It meant that I did not have to hold the tapered roller in the chuck.

There are still a few parts of the upper chassis that need to be completed such as the central top plate and the base plate under the rear section.I will then start fabricating the side walls from 1.6 mm sheet and 9 x 10 angle section.

Machining the jib mounting hinge.
Drilling the roller bearing plates.
Reaming the roller mounting plates.
Jib hinge assembly.
Underneath view of upper chassis.

To be continued.

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