Axleboxes

The plans show a split axlebox design that follow a tried and tested pattern. Some builders use needle roller bearings or 'Oilite' bushes here. which have both advantages and disadvantages. Although these alternatives may potentially lasts longer, the difficulty of removing the wheels from their axles are a major disadvantage if there is a problem.

Split Axleboxes
There is room for modification of the split axlebox to improve lubrication by the provision of a larger oil reservoir in the top of the axlebox. It's probably a good idea to make a cover for this to stop dust getting in.

The plans show a single drill way at the top but this is at the point of maximum pressure so the design in the picture has two drill ways, one in front and one behind the axle centre line. There's also a tiny oil hole leading to the slot that engaged with the horns although this probably isn't necessary. It's only a small hole and it's uncovered when the oil level drops so it could never starve the axle bearing.

A single floating pin is shown in the plans, but these could be replaced with two placed either side of the centreline to avoid the tapped holes for the stay rods and to prevent the axlebox parts from pivoting about the single pin. The 3D model shows a split axlebox with offset oil holes coming from a large oil reservoir. The cover is not shown but would have a hole in it to line up with the oil pipe coming through the top of the horn.

The axlebox runs directly on the ground steel axle which is made from mild steel or medium carbon steel (EN8).



Alternative Axlebox design
As an alternative design to the original I designed an axlebox where the bearing bushes could be easily and cheaply replaced using 1"x1/2" brass bar also the likelyhood that any wear would be in the top portion of the axle box (the one with the weight on it) then just one half could be replaced quite easily by swapping them over. (Bronze would be a better bearing material than Brass)

I wanted to remain as close to the original design as possible with this in mind there are some parts of the axlebox that remain "standard" to stop the bushes moving sideways out of the box there is a simple key milled into the steel box and steel retaining block and a rebate milled into the brass.



The finished article no lubrication holes or spring bars have been fitted yet though. the hole assembly is held in place as is the original with a 3/16" bar. The center of the axle when fully loaded should be in the center of the piston 1 1/4" from the keep plates on the horn block seen just above the axle box.

Axlebox flanges
The plans show the axlebox flanges to be parallel with no clearance specified. The two drawings below show a 1.3 degree clearance beginning at the axle centre height both above and below. The intention is to allow for the axle to lift on either side by approximately 4mm while keeping the opposite wheel on the track. The narrowest point is a close sliding fit on the hornblocks, There is a radius at the axle centre height to increase the contact area.

The narrowest point is at the axle centre line to keep the axial movement along the axle to a minimum. On some types of locomotive, the inner flange is missing altogether.

The axlebox flange on the outside is thicker than on the plans because the back to back wheel distance on the plans doesn't conform to modern practice. The plans show this to be 4-5/8" whereas the norm is now 4-11/16".

The modern dimension gives thinner wheel flanges that are more capable of riding across points.

The flange therefore needs to be 1/32" thicker, making the whole axlebox 1/32" longer on the line of the axle.

There's no reason why both inner and outer axlebox parts can't be made a little longer in the axial direction so that they can be machined flush once the parts have been pinned together. Care must be taken to make sure that the faces of the assembly are at right angles to the face that engage with the horns else the axles won't be perpendicular to the frame if that face is used as the reference when boring the hole.

The axial clearance of the drive axle is to be kept to an absolute minimum to reduce strain on the valve gear,

The front to back fit of the axlebox in the horns is to be kept to an absolute minimum so that the action of the connecting rods doesn't move them in that direction..

The thickness of the outer flange is important because it controls the end float of the axle. Ideally this needs to be small on the driving axle, say 0.05mm, just enough to allow for the free movement of the axle when it's lifted through the 1.3 degree angle mentioned above. The idea is to keep the end float small so that the valve gear doesn't have to accommodate any more than is absolutely necessary. The locomotive still needs to be able to take a curve, so the front and rear axleboxes are reduced by approximately 0.4mm to allow them some end float. The connecting rods need to be able to accommodate this.



The picture shows the Machining of the tapered flanges on a CNC milling machine. A more common method is to machine parallel slot and then add the clearances wth a file.

NB:- The semi-circular shape that's just showing above the vice jaw has only been roughed out, there's another 0.3mm to be removed from that

NB:- The depth of the slot is critical when the second side is machined. This is make sure that there is virtually no play in the hornblocks. It's probably a good idea to make the hornblocks 10 to 20 microns oversize on the depth. The gap in the horns can be measured with slip gauges and the axlebox can be set high enough in the vice to be able to get a micrometer underneath it. Measure both ends because it may not be perfectly parallel.

The machined axleboxes can be fitted to the horns using needle files or a small scraper to remove areas shown up as tight using marking blue. The sharp edges on the horns need to be chamfered and the matching corners of the axlebox need to be kept sharp to match them. The axlebox needs to be free to move and rock so that the tapered faces can reach the matching horn faces while still having virtually no clearance in the other direction. This is time consuming to achieve but worth the effort. Each axlebox is marked so that it stays with the matching horn from now on.



Axlebox inners
The outer part of the axlebox is quite flexible and it would be easy to distort that if the inner part was made a tight fit. Although the top half of the bearing is going to take most of the load, the bottom half needs to be a good fit too. Ideally the inner (lower) part should be a really close fit but without being either slack or so tight as to spread the outer part. Machining these perhaps 10 microns oversize so they would be a gentle push fit might be a good option. they can then be fitted by hand to the required fit. The picture shows one method of making these from round bar using a CNC mill. These ones are made from Leaded Bronze. The diameter of the bar was turned first and faced off so that the mill made them true to the top face. The finished part was parted off in the lathe.

This method can be used using manual machines but care needs to be taken to take the leadscrew backlash into account. Digital scales can help with that.

The axle bore has been rouged out leaving 0.3mm to finish when it's assembled in the top part. This has been done to aid visualisation of which way round the part goes while it's being fine fitted to the top part.

Boring the axle holes




It's important to make sure that all of the axles are in the same position when the axleboxes are fitted to the horns. One way to achieve this is to make sure that each pair of axleboxes have their hole the same distance from the face that bears on the horn. Don't assume that the flanges are accurate enough for this, and don't assume that the hole will come out precisely in the centre so which way round it goes won't matter.

Some builders set up the axleboxes directly in the 4-jaw chuck and and just loosen two jaws when swapping between axleboxes. This is not a particularly accurate method and some sort of fixture could be made whereby it's set up in the lathe and then used as the reference for all of the axleboxes.

The fixture in the pictures is one way of doing this. In the design shown, the RH inside face that engages with the horn is used as one reference, the two grub screws on the left push the axlebox against it, a shim being used to prevent damage to the opposite surface.

The axlebox is pressed against the back of the fixture by the clamp at the front.

Finally, the axlebox is held back against the face of the jig by the two white Delrin plugs that press against the inner part of the flange.

The design is a bit 'over the top' but was done this way so that the axleboxes could be fitted either way up and the outer surface was free of any clamps. This enables the exposed face to be machined later to adjust the end float of the wheelset.

The jig also allows it to be used on the magnetic chuck of a surface grinder and for the inner part to project slightly but still reference only the outer part against the jig face.

A simpler design could be used for just machining the axle hole. The key thing is to get all of the bores true to the hornblock face and each pair the same distance from say the FRONT hornblock face. This means that three if the axleboxes will be machined with their inside faces against the jig, and the other three with their faces pointing away from the jig!

The holes can be bored or reamed. The clearance over the axle is ideally between 10-20microns

Adjusting the axle end float
Once the axleboxes have been fitted to the horns and the axles made to run freely, the exact width over the outside of the axleboxes can be set if they have been made with a little extra thickness on the flange. Calipers can be used over the outside faces to measure the exact size. A depth gauge or another pair of calipers can be used to make sure that the outside faces of the axleboxes are the same distance from the frames on both sides. Allow for a total clearance of about 0.07mm on the middle axle which is just enough to allow one wheel to lift by 1.3 degrees without binding. The idea is to make the middle axle have almost no end float so that the valve gear doesn't have to accommodate more than it has to. Allow about 0.8mm total end float on the coupled wheels to allow it to take sharp curves. Make sure that the end float is equal on both sides of the centre position.