Modifying the Heljan Drive Unit

This section describes the operations involved in modifying the drive unit for each bogie. In previous stages, we have removed the cosmetic sideframe mouldings and wheelsets, leaving the unit with its top cover in place and lower undertray removed.

Sideframe mounts Remove the pickup contact strips from the sideframe support pillars and then cut down the pillars to leave about 1.0 - 1.5mm projecting from the sides of the drive unit.

Top cover modified Cut the pivot hole tag from the top cover of the drive unit. File the remainder of the tag down flush with the top surface of the cover.

Shortened coupler fork Separate slightly the two halves of the drive unit and remove the coupler pocket. Shorten the forked spring mouldings associated with the coupler.

Undertray, flattened Remove the moulded bumps from the lower surface of the undertray; they may be taken down flush without making a hole in the moulding.

Anti-gape bolt and reduced axle arch Clip the undertray back in place. You may find that the two halves of the drive unit tend to gape apart at their inner ends. Should this occur, drill through the two halves 1mm, along the line of the stud and socket in their top inner corners (you may find it best to dismantle the unit completely to do this). Use the 14BA machine screw and nut, supplied in the kit, to secure the two halves of the unit together.

Reduce the depth of the arched shaped spacer mouldings around the top half of the axle slots in the drive unit (highlighted white in the photo here), aiming for a nominal overall width of 12.2mm.

The following operations check the match between the axle centres of the drive unit and the etched bogie subframe. It will probably not be necessary to relieve the axle slots, but we need to check just in case there is any variation in the wheelbase of the plastic drive mouldings.

Remove the assembled bogies from the loco. Unclip the secondary springs and remove the bolsters.

Take one of the bogies and its associated drive unit. Remove the undertray from the drive unit.

Placing drive unit over axles Orientate the drive unit, with the socket for the cardan shaft facing the inner end of the bogie, and place the drive unit over the axles.

Check that the arched mouldings on the drive unit have been reduced sufficiently to fit loosely between the axle bearings.

Unclip and withdraw the primary springs.

Axles captured in drive unit Invert the assembly and lift off the subframe, allowing the axles to drop from the subframe and remain in their slots in the drive unit. Clip the undertray back on to the drive unit.

Drive unit bottomed out in subframe Refit the drive unit and wheelsets into the subframe. Place the subframe on to a raised support so that the bearings can drop down to the bottoms of their slots.

Lifting axle to check free vertical movement Check that the drive unit and axles move freely up and down within the slots in the subframe, both when the bases of the unit and subframe are parallel, and when one axle is raised 1mm with respect to the other.

If the free up and down movement is not obtained, dismantle the components and remove the undertray of the drive unit. Relieve very slightly the inner or outer faces of the slots in the drive unit until free movement is achieved. There should be no need to remove more than, at most, a thin sliver of material.

Repeat the checks for the other bogie.