How locomotives are made video

[CMStPnP]

https://www.youtube.com/watch?v=WokCyQAsh-E

 

[BaltACD]

CMStPnP

https://www.youtube.com/watch?v=WokCyQAsh-E

One thing that surprises me - the wheels are only held to the axle by the designed interference fit established with the wheels being pressed on the axle.

I would have thought, in addition to the interference fit there would also be some form of a positive locking key.

[rixflix]

Her script could be fact-checked to pieces by our resident experts.

Rick

[CMStPnP]

BaltACD

One thing that surprises me - the wheels are only held to the axle by the designed interference fit established with the wheels being pressed on the axle. I would have thought, in addition to the interference fit there would also be some form of a positive locking key.

It looks like there is a opaque composite collar underneath as well and I am curious if the purpose of that is to prevent heat transfer from wheel ( when the wheel spins) to the axle OR does it make the seal tighter when the lube they used to put it on is absorbed by the collar.........or both.

[Lab]

2 parts being joined by an interference fit is not unusual. Sometimes the outer part is heated for an even tighter fit.

[BaltACD]

Lab

2 parts being joined by an interference fit is not unusual. Sometimes the outer part is heated for an even tighter fit.

The thing that surprises me - is relying on only a interference fit to transmit 500 to 1000 HP per axle hauling and/or braking train loads of 15K tons and more.  I suspect the bull gear is also a interference fit on the axle.  My mind leads me to think a keyed connection in addition to the interference fit would be more secure with the forces involved.

But that is just me.  

[ClassA]

Wouldn't the friction in the interference fit be more than at the rail head? It looks like a lot of more surface area between the axle and wheel faces than the wheel and rail faces. Wouldn't that result in the wheel slipping on the rail before enough force was created to slip the axled in the wheel? 

[BaltACD]

ClassA

Wouldn't the friction in the interference fit be more than at the rail head? It looks like a lot of more surface area between the axle and wheel faces than the wheel and rail faces. Wouldn't that result in the wheel slipping on the rail before enough force was created to slip the axled in the wheel? 

Tread braking puts heat into the wheel.  Heat expands the materials it gets applied to.  Heat 'could' expand the wheel to the extent it overcomes the interference fit.  Admittedly it is unlikely, but being unlikely doesn't mean it can NEVER happen and with catastrophic results.  We know railroads have written rules to prevent its occurrence, however, we all know that rules get violated - sometimes intentionally, sometimes not.

I have seen pictures of where steam engines have 'thown' their tires off the their wheels.  I presume the tires got heated to the extent that they enlarged byond their interference fit.

[Overmod]

There is a bit of 'cheating' in that interference fit and the material used in it.  The engineering issues here are better understood than you might suspect, including combinations of force that might induce wheels to slip gauge.

If it helps any, the amount of 'tread' heating necessary to heat a whole cast wheel up to the point the interference fit 'breaks' is enormous but you have to look at what's producing that heating, either in braking or the kind of unattended motoring that burns crescent holes in rails.  The GG1 problem was actually an Amfleet brake problem... and was only a problem there because of the comparatively small mass of metal being quickly heated. That happens far less easily with a cast wheel of comparable diameter, even if large cheek-plate area is used for rapid braking from very high speed.

There are issues when you try to produce keying systems that aren't 'worse than the disease' in critical stress-raising in invisible areas.  While there are interesting approaches to detect incipient axle failure, not one of them is guaranteed.  And the results would be far more catastrophic than any 'loss of traction' or slight change of gauge due to wheel fit slippage.  So anything that minimizes points of concentrated stress in the fit is, and should be, seen as a best practice.

[bogie_engineer]

Locomotive wheels and bull gears are pressed on in a hydraulic press, wheels are required to have a press tonnage of 95-145 tons recorded on a strip chart to observe the shape of the tonnage vs. distance curve. This insures a smooth continuous rise in the tonnage as the wheel reaches it's final position. The interference fit is .008-.010" on a 9.25" diameter seat so roughly .001/1" diameter. A lubricant is used, today a linseed oil base with some stuff I can't remember in it, up until the early 1980's it was white lead. 

The only issue with the wheels moving on the seat that occurred during my 47 year employment was on Amtrak SDP40F's on the Santa Fe where the back-to-back spacing of the wheels was observed to grow; it was eventually traced to a diamond crossing somewhere in the Southwest that was out of spec. As a result of that experience in 1974, the tonnage was raised to the levels I stated above, I don't recall the previous limits. 

As Overmod mentions, stress concentration due to splines or keyways or such would result in rapid fatigue failure of the axle. As it is, there is a very smooth surface finish required on axles to avoid stress cracks. EMD locomotive axles have a groove between the wheel seat and gear seat that is roller burnished to a mirror finish imparting a high compressive surface residual stress to avoid cracks forming in that area.

Many EMD export customers spec'ed hydraulic removal holes in the wheel hub where high pressure oil is forced into the joint to make the wheels easier to remove but I'm not aware of any NA customers who use that process, they just crank up the press and push the wheels off.