In the last post I explained how we fitted the bearing brass flanges. Once this had been finished the next job was to fit the outside diameter of the brass to the crown of the axlebox, the aim being to get a reasonable area of contact between the two parts. Although the brasses have been turned accurately there is, inevitably, a certain amount of wear in the crowns due to movement of the brasses (fretting) whilst the locomotive was in service with the NCB and this is not uniform; hand fitting is the way this situation is dealt with.
A reasonable area of contact between brass and ‘box is necessary for two reasons; to stop the brass fretting when the locomotive is working and to prevent the brass distorting under the weight of the locomotive which could lead to problems with oil distribution etc.
The process involves using engineers’ blue in a more conventional manner than in the previous post.
This photograph shows the crown of the axlebox after it has been coated in engineers’ blue and before the brass has been tried in place. The studs are there to maintain the sides of the ‘box at the correct distance apart, this particular one has a tendency to close in slightly for some reason. (Copyright John Dunn Engineering).
The crown of the ‘box is initially coated in blue and the brass lowered into position. The brass is then tapped down with a mallet and a piece of wood to ensure that it is making as much contact as possible before being removed with the aid of a small lever.
This photograph shows the high-spots after the first trial fitting of the brass.(Copyright John Dunn Engineering).
The pattern of the transferred blue on the outside diameter of the brass indicates the points of contact (high-spots) between the brass and the ‘box. These high-spots are preventing the two parts making any further contact and need to be removed if the area of contact is to be increased. You would normally use a scraper to remove high-spots but I found this an awkward way to do things on the convex surface of the brass. It was much easier to use a brand new smooth or second cut hand file; this ensured a sharp file with a safe edge to protect the sides of the flanges on the brass.
Part way through the process and the pattern of the blue is beginning to change, extending over a larger part of the surface. (Copyright John Dunn Engineering).
Once the high-spots have been filed off, the crown of the ‘box is given another coating of blue and the brass is again tried in place and the high-spots checked. The process is repeated over and over until the necessary fit is achieved. I was aiming for around 70 to 80% contact with the main areas being along the top and upper parts of the brass and some along the bottom edges.
This is time consuming, laborious, back-breaking work. The brasses weigh over 3-1/2 stones each at this stage (23kg) and they have to be lifted off the bench, carried down the ‘shop, dropped into the axlebox which is on the floor, manipulated in position, lifted back out and then returned to the bench. They are then filed and the whole process begins again, and again, and again, ad nauseum. Thankfully this stage of the work is now behind us but it has been something of marathon.
This shows the work almost completed. There is a reasonable area of blue over the whole of the outside of the brass. A bit more work will see the blue spread a liitle more and that will be this one completed. (Copyright John Dunn Engineering).
I must apologise but I have neglected to take a photograph of one of the brasses with the final coating of blue on but I think the pictures give a pretty good idea of how the process evolves.