Walthers Proto 0-8-0 Cracked Gears

[peteski]

I think the problem is the splines. They cause unnecessary points of pressure on the inside of the axle sleeve. A smooth axle with an interference fit into a smooth sleeve is fine. It actually works really well and has been proven to last for decades. If the tolerances are close.

I agree, but smooth axle can easily rotate out of quarter, so that is not an optimal solution. One of the better and simpler solutions is in Kato Mikado. They used smooth hex-shaped axle. That assures proper (and easily repeatable) quartering with no additional stress on the plastic parts.

I have also seen, in other brand models (like BLI), clever solutions of driver assembly design, but those are a bit more complex.

[Doug G.]

I think the problem is the splines. They cause unnecessary points of pressure on the inside of the axle sleeve. A smooth axle with an interference fit into a smooth sleeve is fine. It actually works really well and has been proven to last for decades. If the tolerances are close.

Bachmanns don't have splines and they readily crack.

Doug

[Doug G.]

It's not that hard to understand. Plastic is plastic and is not immovable/unmalleable. And, there are always weaker spots in any molded plastic part, usually at what are called "parting lines" or areas where flow meets earlier flow that isn't as hot anymore, than when first flowed.

Either results in weaker areas and when you force a metal axle, which is immovable dimension wise, into the plastic hole, the plastic will eventually crack at the weaker spot(s). Forces are trying to make the hole in the plastic, bigger.

Making the hole and axle smaller in relation to the diameter of the plastic axle can be a remedy but not always practical with small assemblies. The much better way to do it is to have a solid plastic axle stuffed into a hollow metal axle as in the old Con-Cor/Sekisui PA-1s. The hollow metal axle is never going to crack.

Or, you can cement the metal axle into a plastic hole without outward forces on the plastic around the hole but not much will stick to the plastics usually used in these assemblies.

Doug

[Jimbo]

Different coefficients of expansion between plastics and metal certainly don’t help either.  How many of us store our items in places where temperatures remain constant?

Now I need to check my own 0-8-0, which has never been a top performer.  (First run.)

Jim

[peteski]

It's not that hard to understand. Plastic is plastic and is not immovable. And, there are always weaker spots in any molded plastic part, usually at what are called "parting lines" or areas where flow meets earlier flow that isn't as hot anymore, than when first flowed.

Either results in weaker areas and when you force a metal axle, which is immovable dimension wise, into the plastic hole, the plastic will eventually crack at the weaker spot(s). Forces are trying to make the hole in the plastic, bigger.

Plastic (as a polymer) does have some elasticity to it, so it should withstand pressure from a slightly oversize axle.

As far as mold partying lines being the weakest point, that can be true, but most of the axle tubes I have examined in detail do not have those because they are molded in tubular shaped molds.  But even the way the liquid plastic enters the mold can weaken certain areas.

[garethashenden]

Atlas and Kato press metal axles into plastic sleeves with molded gears for their diesels and they don’t fail. So it’s not an inherently flawed system. Like Like just didn’t do a great job here. They also seem to have felt the need to gear all the axles, so steam engines clearly aren’t their forte. You need to model the connecting rods anyway, make them work for a living. Otherwise they’ll fight with the gearing.

[peteski]

Atlas and Kato press metal axles into plastic sleeves with molded gears for their diesels and they don’t fail. So it’s not an inherently flawed system. Like Like just didn’t do a great job here. They also seem to have felt the need to gear all the axles, so steam engines clearly aren’t their forte. You need to model the connecting rods anyway, make them work for a living. Otherwise they’ll fight with the gearing.

Looking at all the early Atlas then Con-Cor (Rivarossi) loocs, they had all of their drivers geared (often using metal gears).  Same goes for the old MRC (Rowa made) Mallet (which also suffers from cracked gears).   Also looking at other German manufacturers' models (even current models), quite a few have all the drivers geared. To me it isn't inherently bad design - just difficult to service.

To me the best geared driver design was what was done in a TT gauge 0-8-0T switcher.  There are no idler gears where slop increases at each of them. The geared drivers are all driven by a long worm located over them. That way the slop is equal on all the drivers.

Kato is not perfect either - there are examples of cracked axle tubes (last was in their GS-4). The problem was so bad that they redesigned the drivers for FEF-3 and revised the GS-4 drivers using that FEF-3 design.

[Doug G.]

I always get challenged when I make my argument about a metal axle stuffed into a hollow plastic axle being a poor design, The fact is, it IS inherently a poor/inferior design. If it weren't there wouldn't be all these cracked axles in many different models.

In cases where there aren't failures, it is because they are leaving more plastic around the metal axle to add strength, which I mentioned in my post.

Poor engineering. Plastic is not always the miracle material.

Doug

[mmagliaro]

Of course... manufacturers could make engines from the factory with metal sleeves over the axle tubes.  But nobody does that that I am aware of, and they probably won't because of the manual installation step.

[peteski]

Of course... manufacturers could make engines from the factory with metal sleeves over the axle tubes.  But nobody does that that I am aware of, and they probably won't because of the manual installation step.

BLI seems to actually use a variant of what you described in their steam locos.  It is actually pretty clever design.  It impressed the heck out of me. I'll see if I can take some photos later.

[Lemosteam]

Of course... manufacturers could make engines from the factory with metal sleeves over the axle tubes.  But nobody does that that I am aware of, and they probably won't because of the manual installation step.

This could be easily done using an overmold process. Insert the shaft and two rings into the mold cavity  before injecting the plastic. Happens all the time in the auto and electronics industry.

[mmagliaro]

This could be easily done using an overmold process. Insert the shaft and two rings into the mold cavity  before injecting the plastic. Happens all the time in the auto and electronics industry.

Since it is a known automated process, now you've gone and made me mad that they don't do it!