Restoring Atlas contacts

[peteski]

Wait... the axle/wheel's only point of contact with the entire rest of the engine is at the two axle points.  Those axle points are supporting the entire weight of the engine, so they are indeed performing a weight-bearing function.  They are carrying the engine.  A 100g 4-axle diesel means 100g are carried on 8 points.  12.5g per point.

What's the contact area of that axle point?  Even a square mm would be generous.  12.5g/sq mm = 2550 lbs / sq ft. = 17.7 lbs/sq inch.  And remember, that actual contact area is much smaller than a sq mm, so this is a low estimate.

Yikes.  I never realized it was so high.  I'm glad I let you guys talk me out of trying to use brass axles on my 0-6-0.   

Yup!  The contact area at each axle is probably more like 0.1 mm/sq, but you clearly got the picture.   

[jagged ben]

Well then, run your dry.  I've seen globs of whitish grease applied to those bearings at the factory many times.  The contact area between the axle point and the bearing cup is so minuscule that there is probably dozens of  pounds per square inch pressure there. 

Wait... the axle/wheel's only point of contact with the entire rest of the engine is at the two axle points.  Those axle points are supporting the entire weight of the engine, so they are indeed performing a weight-bearing function.  They are carrying the engine.  A 100g 4-axle diesel means 100g are carried on 8 points.  12.5g per point.

What's the contact area of that axle point?  Even a square mm would be generous.  12.5g/sq mm = 2550 lbs / sq ft. = 17.7 lbs/sq inch.  And remember, that actual contact area is much smaller than a sq mm, so this is a low estimate.

Yikes.  I never realized it was so high.  I'm glad I let you guys talk me out of trying to use brass axles on my 0-6-0.   

Lol, you guys crack me up.      You're so wrong about this, at least as it applies to Atlas diesels.  You should go look at some of the trucks again to see how it actually works. 

First, the weight of the loco is on the geared plastic mid-axles.  It's not on the axle points.  The only downward pressure on the brass pieces containing the 'bearing cups' is from the notches in the gear tower that hold them in place.  There is no general downward pressure on the axles at the bearing cups.  I suppose theoretically there might be some downward pressure from the contact strips, but I don't think it's significant enough to count.  The pressure holding the cups tight to the axles is basically all side pressure from the side frames that hold the cup pieces in place.  Who knows how much that pressure is; it's a function of the plastic the side frames are made of, not a function of locomotive weight.

Second, there's probably a lot of contact on the side of the axle point cones.  That's probably where most of the contact happens, not at the point.  Tolerances are loose enough, at least on Atlas, that the axle probably moves around the side of the cup quite a bit. (see: middle axle 'hop' in the Atlas HTC truck, as extensively discussed by Ron Bearden in past years).   

I admit I'm not really at all familiar with steam design so maybe things are different there.  Your calcs would also be relevant to rolling stock trucks, especially Kato's trucks for lighted passenger cars.    But not Atlas diesels.

[peteski]

Jagged Ben, you made me chuckle.  I have taken hundreds (yes, that many in my hobby lifetime) of N scale locos (steam, diesel, electrics, from the early N scale to the present designs, from a plethora of manufacturers).  I understand quite well how the low-friction trucks function.  Gears to not carry any load.  Axle ends do that.  You night want to do some experimenting if you don't believe me.  Remove the metal sideframes and see how the loco will run, and where the wheels end up.

As for the pressure, it is near the point end of the axle (since the point is actually rounded). It contact the internal conical surface of the cup at that very small area. That's because the angle of the cup's cone is different than the axle cone.

[Doug G.]

On the Pacifics, Mikados, and E8's I use...Oh or are you talking about slightly newer models?



Doug

[MK]

I can see this turning into a Physics thread.   

No matter what the truck assembly is or how the loco is sitting on what gear, the final wheels on the track are what is holding up the loco.  Therefore there are forces on the wheels and thus the axles.

[Teditor]

I have worn Atlas axle cones right through on some SD-9's and GP35's, thankfully Atlas had spares.

[mmagliaro]

I think you have a point.   Looking, at an Atlas RS-1, or a Kato F unit, the axles pass though plastic frames and that is where all the weight bearing is carried.  The points riding in the pickup strips only have the pressure of the contact strips from inside the engine on them.

I disagree on the contact surface at the axle point, however.  From my experience changing out pointed axles, both in diesels and steam tender trucks, the contact area is at the point.  The axle is tapered more gradually than the inside of the cup so that the point is guaranteed to reach all the way into the bottom of the cup.   If the taper isn't right and the sides of the cone-shaped point start rubbing on the inside of the cup, it binds.  In a steam tender, where the wheels are just rolling along (not driven by a gear), you'll see this immediately because the wheels will hang up and drag along the rail instead of rolling.  They have to touch only at the point.

[Kuchler10]

Thanks for all of the tips, I think I will try this DetoxIT stuff.

I figured this would turn into more of a chemistry thread than a physics one, but neither of these fields are my forte. Amazing what is involved in model railroading.

[GaryHinshaw]

I can see this turning into a Physics thread.   

(Caveat - I don't own any Atlas locos, so my comments refer to a Kato truck, but I think the design is similar.)  The question is: how is the weight of the loco transmitted to the axles?  Here is a Kato truck (with a broken gear tower tab) with the side-frames removed, with and without the axle cup contact pieces:





and a view of the axles resting in the frame:



The axles and truck frames are both made of slippery plastic (Delrin?) and a key point is that the trucks roll almost as well with or without the axle cups in place.  So clearly most of the loco weight is transmitted to the axles through the truck frame resting on the axles.  However, the trucks roll a little bit better with the axle cups in place, so I think the axle cups bear a fraction of the weight.  If they bore all of the weight, they would be prone to buckling.

[mmagliaro]

Thanks, Gary.  Those photos really help.
After Jagged Ben's observations I looked at some trucks, and I see what you see.

But it really helps to observe what the drag is with and without the axle point contacts in place.
Clearly that points have to carry a "little" of the weight.  Otherwise, they wouldn't work.  But the bulk
of the load is on the plastic frame slots and axles.

(But it is true that in something like a Kato steam tender, the points do carry all the load because the axles do not pass through anything else.  Only the points carry the load, which is much lighter than an engine frame because it's just a tender).

[peteski]

(Caveat - I don't own any Atlas locos, so my comments refer to a Kato truck, but I think the design is similar.)  The question is: how is the weight of the loco transmitted to the axles?  Here is a Kato truck (with a broken gear tower tab) with the side-frames removed, with and without the axle cup contact pieces:



The axles and truck frames are both made of slippery plastic (Delrin?) and a key point is that the trucks roll almost as well with or without the axle cups in place.  So clearly most of the loco weight is transmitted to the axles through the truck frame resting on the axles.  However, the trucks roll a little bit better with the axle cups in place, so I think the axle cups bear a fraction of the weight.  If they bore all of the weight, they would be prone to buckling.

Still incorrect.  But this is an excellent photos which shows that the metal bearing/pickup plates are solidly attached to the central gear housing.  The axle opening in the gear housing are larger diameter than the axle. They are not bearings for the axles!  If the wheels were made from transparent material then you would see that the bearing cups support the axles in the center of the axle openings of the gear housing. The axle does not contacts the gear housing - it floats in the center.  If the axle openings in the gear housing were acting as bearings for the axles, you would see some rub marks on the axles - there will be none.

As a test, remove the idler gears and snap a wheelset in the axle opening without installing the metal bearings/pickup plates. Try to spin it and see how easily it turns.  Now  install the outside bearing/pickup plates so the axle is now properly suspended in them. Hold that assembly (upside down) with one hand and now try to spin the wheels. See how much better it spins?  It is becuase the axle no longer rubs against the wheel opening in the gear housing - it now rests exclusively in the bearing cones of the metal sideframe plates.

[rrjim1]

(Caveat - I don't own any Atlas locos, so my comments refer to a Kato truck, but I think the design is similar.)  The question is: how is the weight of the loco transmitted to the axles?  Here is a Kato truck (with a broken gear tower tab) with the side-frames removed, with and without the axle cup contact pieces:





and a view of the axles resting in the frame:



The axles and truck frames are both made of slippery plastic (Delrin?) and a key point is that the trucks roll almost as well with or without the axle cups in place.  So clearly most of the loco weight is transmitted to the axles through the truck frame resting on the axles.  However, the trucks roll a little bit better with the axle cups in place, so I think the axle cups bear a fraction of the weight.  If they bore all of the weight, they would be prone to buckling.

You can clearly see in the second picture that the wheelsets sit higher in the truck frame that the ones in the first picture. If you look at the top of the wheels you should see the difference.

[GaryHinshaw]

I stand corrected.  The fact that the truck rolls well without the axle cups is a red herring.  Here is a shot that shows the axle housing with the cups in place and it is clear that the axle does not make contact with the frame in this case.

[peteski]

I stand corrected.  The fact that the truck rolls well without the axle cups is a red herring.  Here is a shot that shows the axle housing with the cups in place and it is clear that the axle does not make contact with the frame in this case.

Exactly!  If the gear frame was used as a bearing for the wheelset's axles then it would no longer be a low-friction truck. It is a brilliant design which abandoned the old-school inside-bearing design.  It eliminates those higher-friction bearings *AND* the high-friction wheel-wipers by replacing them with  a combined needlepoint bearing/electric pickup unit.

[jagged ben]

Jagged Ben, you made me chuckle.  I have taken hundreds (yes, that many in my hobby lifetime) of N scale locos (steam, diesel, electrics, from the early N scale to the present designs, from a plethora of manufacturers).  I understand quite well how the low-friction trucks function. 

Atlas diesel trucks are not low friction.   

Gears to not carry any load. 

I never said they did.  I referred to the geared plastic mid-axles only to make clear what component I was referring to.

Axle ends do that.

You night want to do some experimenting if you don't believe me. 

I speak from years of working on the Atlas HTC truck, which unfortunately requires a lot of frequent repairing.  I will admit that I'm a bit less familiar with other Atlas trucks, perhaps they are not all exactly the same.   

Remove the metal sideframes and see how the loco will run, and where the wheels end up.

It would be rather difficult to run one of these locomotives without its electrical pickup.  But perhaps I'll give it a go just to see you're eyes bug out.     I have a feeling that an Atlas SD60 mech (for example) would run okay with a 9V battery taped on top, at least if you didn't try to run it through turnouts or crossings.  The wheels would have more side to side and front to back play than they should, but this doesn't prove what bears the weight of the loco.

As for the pressure, it is near the point end of the axle (since the point is actually rounded). It contact the internal conical surface of the cup at that very small area. That's because the angle of the cup's cone is different than the axle cone.

I see your point on this.