Sprocket question and others

[Harold_V]

McMasters sells rotary shafts that

are turned, precision ground, and polished to meet tight diameter and straightness tolerances.

It's 3/4" shaft has a (Tolerance: -0.003" to 0")

Excluding the need to mill a keyway, is it possible to anticipate a slip fit, press fit or you really don't know till you get them both together?

Even with that material in hand, you may not enjoy success unless you work with each individual piece, and if it's centerless ground (most likely is) it may not be straight, round, or uniform in diameter from piece to piece. Don't assume any of those features. Check the material. It should meet those specs, but it's all too easy not to, especially if the material starts life less than straight (most likely) and is not ground by one with experience (a centerless can very easily generate three sided material that measures round, but isn't---if not operated by one skilled in centerless operation).

Note that when one machines a keyway, the material is subject to movement, yielding a less than straight shaft. A short keyway isn't much of a problem, but a long one will definitely bow the material. That's due to uneven relief of internal stresses, created when the shaft is rolled.

It is commonly accepted that a half thou spells the difference between a press and slip fit.

Also, McMasters sells keyway stock that is either "oversized" or "undersized". Since it's only around $1.25 for 12" of each do you just buy both kinds and see what works best?

The very best scenario is for you to obtain shafting that is functional, then machine the mating piece(s) to fit according to your needs. Commercial shafting is made to fit commercial fittings---and may or may not be a size that lends itself to your particular project. There is generally considerable clearance to assure that the pieces will assemble. Hoping for a proper press fit would be unreasonable (especially with .003" tolerance).

Bottom line here is it all depends on how much you're capable of doing on your own. If you have no machines, you may be forced to buy what is available and hope you can make it work.

These are my thoughts. Others may or may not agree.

H

[Pontiacguy1]

Get your sprockets figured out and figure out what size of bore you need on those sprockets. I always want a nice shoulder on the axles so that I can press the wheels positively up against them and that will set the back-to-back dimension. So you want your axle center size to be large enough to leave a nice shoulder on there. As others have said above, if you use ball bearings, then you can use almost any type of steel and have it machined and it will work perfectly fine. On the switcher, I used 1" diameter Cold-Rolled steel for the axles. The 1" is a little bit oversize, I know, but i could easily get sprockets with a 1" bore, and it also left me plenty of shoulder. 1" diameter is also a very standard size, so any steel shop or sales yard should have it. The wheels were bored 3/4", and the bearing bore was 10 mm (0.394"). Made a LONG keyway slot for the sprocket, so that I could have some side-to-side adjustability if I needed it, then I pressed the wheels on and then the bearings on. Maybe I have a picture here.
I guess I didn't take a picture of just the axles after machining. This is the best picture of it that I have.

20210410_165155b.jpg (119.56 KiB) Viewed 1001 times

[Greg_Lewis]

...and if it's centerless ground (most likely is) it may not be straight, round, or uniform in diameter from piece to piece. Don't assume any of those features. Check the material. It should meet those specs, but it's all too easy not to, especially if the material starts life less than straight (most likely) and is not ground by one with experience (a centerless can very easily generate three sided material that measures round, but isn't---if not operated by one skilled in centerless operation).
...

Ain't that the truth. I once spent extra bucks for a piece of special alloy that was advertised as being ground to ±.0002 or something like that. I measured it with a mike when it arrived and it was ROTM (right on the money). But after cutting off a piece and spending several hours machining it, something wasn't right. I finally set it on some v-blocks and discovered just what Harold wrote — it was tri-lobed by about .003. And for what I was doing, turning it down to make it round wouldn't work as I needed that o.d. and that ground finish. So the extra bucks I spent were wasted and the piece of stock sits today in my rack where it probably will never get used.

[Pontiacguy1]

Any grinder worth their salt will do both 2-point and 3-point roundness checks during their setup and on finished product, as well as surface finish checks, straightness, hardness, etc... That tri-lobe (3 point roundness) won't be found by using a caliper or any 2-point measuring device, which is what we in this hobby are used to using for measurement. Have seen that before as well on ground shafting I've ordered. Unfortunately it happens more than what you would expect. You just can't trust that what you think you are getting is what you are actually getting. Also, in the bearing industry, you quickly learn that round is not round. You have to determine how close to round you actually need it to be and make sure you get it there.

[Bill Shields]

does that include 'round toits?

[Glenn Brooks]

One additional thought. Once you get your wheels assembled on your axles, plan on turning the wheel sets on centers- e.g. holding the wheels mounted on the axles with centers mounted in the headstock and tailstock. In the event shaft stress or other small anomaly shows up in the concentricity of the wheels, you can “square” them up to your desired “roundness” , even with small offsets appearing in your axles, or wheel journals.

How are you doing so far on constructing the loco? Construction Pics are wonderful to see here…

Glenn

[Erskine Tramway]

Here's what I'm doing on my 'Listeresque' 4-wheel gas/mechanical. The wheels I got from 'Real Trains' years ago have a 1" bore and keyways. So, I'm using a section of 1" diameter keyway stock as the actual axle. To get the back-to-back correct, I've cut and squared off pieces of tubing in the lathe that are a sliding fit over the axles. Because of the width of my frame, I'll be putting more pieces of tube between the wheels and bearings to center them and take any side thrust, instead of putting it on the set screws in the bearings. The tubes will also help with the stiffness of the axles overall. Here is a view of the bare axle assemblies.

2-19-15 wheelsets.jpg (130.37 KiB) Viewed 788 times

The frame is this wide because I needed it to fit the riding mower chassis I'm using for power.

3-20-15 weight bolted on.jpg (184.39 KiB) Viewed 788 times

The 'Lister' will work fine with my half-size 'Heywood' wagons.

3-25-15 Lister train.jpg (114.22 KiB) Viewed 788 times

Mike

[Bill Shields]

that will work.....ain't going nowhere....

[Glenn Brooks]

reminiscent of WW1 trench engine mules… impressive construction.

[Pontiacguy1]

I have disassembled a locomotive which had gearboxes made onto each axle, and that is how they did it. It looked like 1" axles, but was really 3/4" axles with a 1" OD sleeve pressed over it. The bore of the gear inside the box was 3/4", the wheel bore was 3/4" and the bore of the axle boxes was 3/4". This one used Woodruff keys where needed for the gear and the wheels. The gear box was put into place, then the sleeves were pressed in place, then the wheels pressed on and also set with a setscrew. This was made back in the 1950s, and it was a bear to get it all apart after all those years. It worked fine, and was really sturdy. What had confused me at at first was that the seals for the axles on the axle box were 1" seals, They sealed onto the sleeve. Worked pretty good. Box was filled with grease, not oil, so I never noticed anything leaking.

[Bill Shields]

KISS