Disc Drivers Actually Hollow?

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Smokey N Steamer
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Disc Drivers Actually Hollow?

Post by Smokey N Steamer » Mon Dec 03, 2012 4:06 pm

There are three main types of disc drivers used on American steam locomotives from the 1930's on--Boxpok, Baldwin and Scullin. But there is an interesting aspect to their design that has me curious...
"While appearing [on the outside] to be solid, [disc drivers] are actually hollow, made of two thinner [cast-metal] discs fastened together."

-J Parker Lamb, Perfecting the American Steam Locomotive
I recall seeing in a book (I forget which one) a drawing of a cross-section of both a Boxpok and a Scullin driver that proves they are really hollow. However, in many of the locomotive construction photos I have seen on this forum and other websites, some builders ignore this, and use solid drivers instead. What is the reason? Ease of fabrication? Cost? Have there ever been examples of true hollow disc drivers used on a scale model? I want to know.

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Trainman4602
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Re: Disc Drivers Actually Hollow?

Post by Trainman4602 » Mon Dec 03, 2012 4:31 pm

I just mentioned this fact in my video on the NYC Mohawk I have the drawings for the Boxpox driver used on the locomotive.

here is the link to the video



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makinsmoke
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Re: Disc Drivers Actually Hollow?

Post by makinsmoke » Mon Dec 03, 2012 4:33 pm

I would guess since weight does not scale down, that one would want as much weight as possible on your drivers.

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Re: Disc Drivers Actually Hollow?

Post by Harold_V » Mon Dec 03, 2012 4:41 pm

Smokey N Steamer wrote:I recall seeing in a book (I forget which one) a drawing of a cross-section of both a Boxpok and a Scullin driver that proves they are really hollow. However, in many of the locomotive construction photos I have seen on this forum and other websites, some builders ignore this, and use solid drivers instead. What is the reason? Ease of fabrication? Cost? Have there ever been examples of true hollow disc drivers used on a scale model? I want to know.
My opinion?
Cost.
The cores required to cast the wheels in miniature most likely put the wheels out of reach for the majority of the builders. Scaled, the thin sections may be difficult to cast, too, and may come out chilled. Also, don't overlook the possibility that the added weight of solid drivers is most likely desirable. Still, I think they can be cast, and would be willing to try them in ductile iron.

I've noticed that the details of Boxpok wheels tend to be included, but inside details are placed on the outside face. At a glance, they *almost* look prototypical.

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LivingLegend
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Re: Disc Drivers Actually Hollow?

Post by LivingLegend » Mon Dec 03, 2012 4:50 pm

A couple of years or so ago, in response to a similar question, I posted a drawing of a Big Boy's Boxpok driver center... Also a portion of the cross section view from that drawing. Here's the link to that posting....

http://www.chaski.org/homemachinist/vie ... 03#p183403

Off hand, I can only think of one live steam loco up to 1.6" scale which has prototypical cored Boxpox drivers.... That would be Tom Miller's Big Boy.

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Pontiacguy1
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Re: Disc Drivers Actually Hollow?

Post by Pontiacguy1 » Mon Dec 03, 2012 5:51 pm

I would also think that if you made the drivers prototypical thickness, the wheels might tend to be a bit fragile. One really good derailment could quite possibly break one. Most of the tracks we run our trains on are pretty rough compared to the roadbeds that the prototype locomotives ran on.

Most likely it is still because of the ease of making the patterns and getting them cast.

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Trainman4602
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Re: Disc Drivers Actually Hollow?

Post by Trainman4602 » Mon Dec 03, 2012 6:57 pm

I would make the castings from ductile iron or mangnese bronze. Both materials would stand up to any derailment.

BTW someone on here said that they don’t core them because they want more weight.

FYI the drivers don’t give much towards tractive effort, just adds to the total weight.
Last edited by Trainman4602 on Mon Dec 03, 2012 10:12 pm, edited 1 time in total.
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Trainman4602
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Re: Disc Drivers Actually Hollow?

Post by Trainman4602 » Mon Dec 03, 2012 10:10 pm

As promised pictures of a Boxpok drive 69 inches
Attachments
016 - Copy.JPG
017 - Copy.JPG
019 - Copy.JPG
022 - Copy.JPG
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Loco112
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Re: Disc Drivers Actually Hollow?

Post by Loco112 » Mon Dec 03, 2012 10:29 pm

Even in 3" scale, 15" gauge they have only been cast with a front side, (outside) to them and no rear/inside section, and with full spokes in the middle to make up for the lack of a rear/inside structure.

EDIT: I just realized the above is not entierly true, Mike Weldon has a 3" scale Sante Fe 4-8-4 loco that has fabricated boxpok drivers, and they are done really well. They are hollow too, but its not visible as the core holes on the prototype are, still they look very good, link: https://www.youtube.com/watch?v=S4tpCLVRTzs
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Dick_Morris
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Re: Disc Drivers Actually Hollow?

Post by Dick_Morris » Mon Dec 03, 2012 11:14 pm

FYI the drivers don’t give much towards tractive effort, just adds to the total weight.
Dave - I don't understand why, could you elaborate? I would think that sprung versus unsprung wouldn't make a difference to tractive effort, as long as the weight is on the drivers and the drivers can follow the irregularities of the rail.

I have a vague recollection that someone, sometime, made their disk driver in two pieces, then machined the backs to fit into a recess and fixed them in place (silver solder??). This would avoid the need for cores.

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Trainman4602
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Re: Disc Drivers Actually Hollow?

Post by Trainman4602 » Mon Dec 03, 2012 11:44 pm

It is the weight ON the drivers not the weight OF the drivers.

The frame, boiler and everything else with the exception of the lead truck adds to the weight ON the drivers.

The trailing truck adds weight on the drivers thru the equalizing levers .
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Andrew Pugh
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Re: Disc Drivers Actually Hollow?

Post by Andrew Pugh » Tue Dec 04, 2012 2:02 am

Physically, I believe it is the force on the contact patch that is important.

Dave, I know those trailing trucks can get pretty heavy on some locomotives; is it typical on models to 'tune' the equalization to the point that the weight on the trailing wheel set is less than the weight of the truck components? If that's the case, wouldn't it be better not to have the trailing truck at all? (I realize that might make for a ridiculous looking model...) To my understanding, the primary purpose of the trailing truck on full scale locomotives is to carry some of the weight of the firebox/cab etc, and it probably helps some to how the locomotive behaves going down the track.

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