Split axle boxes

I am in the process of machining the 8 split axle boxes for 2-8-0 5" loco question is should the insert part be a clearance fit, a push fit (hand) or an interference fit, I'm leaning towards a push fit.

I am also thinking of Loctite to hold the two parts while I make the two locating pin holes and machining the bearings and outer surfaces so that it is all retained true and square. Once done heat (in oven) to split the two parts and then clean up ready to assemble.

Any thoughts?

Bob

what insert part?

The axle box is in two parts an upper part and a lower part that fits inside the upper part something along these lines

now I understand - I make mine a close slide fit, but im sure others will prefer a tighter fit. Just make sure the retaining pin is a good fit.

I had thought that was the case as far as fit was concerned, I am also considering using tapered pins although the drawing shows 1/8" parallel ones (two in each box).

hi bob,

they should be a tight push fit with all machining marks removed. please note that 2 pins are better than one to secure same as prevents any tendency for the 'keep' to tilt. just 1 pin is very bad practice. the pins should be a press fit. i think there are some pics of the split axleboxes on my current loco build in my albums.

cheers,

julian

Hi Guys

I am busy building a LNER B1. I am also making split axle boxes and have used Sellock (spring pins) which seem to to working. They are quite easily fitted and taken out using a dowel punch

Ken

Not sure I'd be happy with spring (roll) pins so may well stick with 1/8 pins as there is in fact 3 pins holding it together, a third carries the rods for the springs.

Bob

The fit should be, that when the keep/insert is pushed in it does not expand the width of the axle box. You must also remember that when the hole for the axle is bored it will increase the possibility that the axle box will expand if the fir is to tight. I am afraid I am with fizzy on this one, an easy sliding fit, it is after all held in place by two pins.

I think this is now clearer some points I had not considered, like the keep causing the axle box out of shape.

Julian,

you mention removing machining marks, my problem with that is it tends to leave the centre high or at least using a surface plate and a sheet of 600 wet and dry does.

Bob

I am afraid I will have to disagree with Julian regarding the machining marks there is no need to remove all the machining marks it is the fit that is important not the look, lets face it no one is going to see it. There is of course allways a proviso in that the machining is of a fairly good finish, if so a gentle rub with a needle file is all that is required. Your problem with 600 grit paper on a surface plate is that with small area's the component rocks causing the effect you have noticed and of course the more you rub it the worse it gets. I would stick with a needle file, a few strokes and check and so on, oh and don't forget to break the corners on the keep where it mates with the recess in the axle box, it is almost impossible to get a sharp corner into a sharp corner. I forgot a felt tip pen is a must when you get a part of the keep to enter the axle box mark it with felt tip pen and stay away from it.

Edited By Dusty on 04/03/2015 16:40:47

I am getting a good finish at least IMHO, fast cutting speed and slow feed with a very sharp solid 12mm end mill.

Corners are sharp but a small clearance is not a bad thing all be it there is only 1/32" left on each side after the bearing is machined so just the minimal removed. May see how the fit goes as to how much to remove from the corners, as for getting the keeps parallel that's another story as it only takes a very small amount out of true and you can be 0.001" out.

I remember during my college days being given the job of making a 1" cube from a piece of round stock with only a hacksaw, file, rule and square. I cheated as there was a milling machine in the other shop!

Bob

i totally agree with Dusty that the keeps shouldnt be so tight as to spread the 'legs or sides of the axlebox proper.

i agree with Dusty and would also use a fine file to remove machining marks rather than wet and dry in this situation.

my comment about fit is because the parts will need putting together and taking apart quite a few times during the build. my own experience is that if made not tight enough, the axlebox and keep quickly become a loose fit during these operations.

incidentally for the felt pads i use the felt used in Church organ restoration projects. ordinary felt is useless IMHO.

cheers,

julian

Made a start on the axle boxes

I now have 8 blocks with inner piece fitted, they are a sliding fit, they will not drop in but a little pressure from a finger and they slide in. I used a deburring tool to take a slight chamfer off the bottom edge where it meets the corner.

Bob

Have completed the machining of all 8, I fitted the keeps, machined the outer faces and the holes for the axles. One thing I found was the axle boxed closed up a fraction as the keeps are a little harder to fit than they were but I can still push them together by hand. I marked each pair as they are matched so removal should nt mean they get mixed up.

The drawing shows a slight curve to allow the boxes to "rock" but does not say how much so put a slight angle on each end and dressed it to a sort of curve may be 0.020" wider at the ends than the centre. The leading and trailing boxes are 1/16"th wider than the middle pair to allow for cornering.

A further question is what should the axle clearance be? I settled on a couple of thou but not sure that is enough as the axles "rock" when traveling around the track. Once I have the axles assembled in to frame I will try it on the club track to see how it copes with the curves.

Bob

" what should the axle clearance be?"

can you be more specifific here please bob as to what you are referring to?

do you mean the axle box bore for the axle journals?

cheers,

julian

Axle box bore for the axle journals Julian I did it by measurement e.g. set a micrometer to the axle and set a dial bore gauge to machine the bores +0.002 but it is probably between 0.001" and 0.002" so hole reads 0.751" to 0.752".

They seem OK but not sure if it's enough.

Bob

hi bob,

i would regard 1/2 thou as enough, and 2 thou as Crewe as opposed to Swindon standards!

2 thou slop makes quite a difference when valve setting though otherwise probably ok though not what i personally aim for.

cheers,

julian

I think Julian sounds right.

Most plain bearing designers who run calculation programmes to work out clearances and bore profile start their runs with an input figure around 0.00125"/inch on diameter ( i.e. somewhere between 0.001" and 0.0015" per inch or metric equivalent ).

On smaller diameters of less than 2" or so it would be down to 0.0010"/inch. The final clearances are calculated based on type of lubrication, bore area which takes in length,, lube pressure, temp, method and viscosity., brg operating temperature etc,etc. At 0.00125/inch most bearings will run but longevity may not be maximised. The conditions mentioned will determine. These figures are not cast in stone and more or even less may be the end result after runs, along with a change in bore profile.

At around 3/4" diameter, 0.002" seems rather excessive and I would have expected it to be less than half of this required as well.