A Real Dirty Job

Where users can chronicle their builds. Start one thread and continue to add on to it.

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Carrdo
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Re: A Real Dirty Job

Post by Carrdo »

Some further work on the drawbar and the safety bar.
Attachments
434 End Rounding.jpg
435 After End Rounding.jpg
436 Milling Down the Centre of the Offset Safety Bar.jpg
437 The Partially Machined Drawbar and Offset Safety Bar.jpg
Wolfgang
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Re: A Real Dirty Job

Post by Wolfgang »

Don's draw bar is a classic case of an eye bar design, which was used in many bridge designs over the last 100 years or so. With some failure of individual bars in the area that is not apparent to the un-schooled eye.
The area in question is the transition between the parallel shank of the bar and the radius of the eye portion of the pin ends,
For fatigue resistance this area should be drawn as a tangent line to eye radius, and a 3 in 1 taper with respect to the parallel portion of the bar.
I think it was the Oakland Bay bridge where inspection disclosed the failure of these bars. The eye bars had the outline as shown in Don's design. w
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Greg_Lewis
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Re: A Real Dirty Job

Post by Greg_Lewis »

Drifting a little from the topic, here's an interesting vid of a bridge collapse that used such an I-bar design.
https://www.youtube.com/watch?v=8w5Fjouvma8
Greg Lewis, Prop.
Eyeball Engineering — Home of the dull toolbit.
Our motto: "That looks about right."
Celebrating 35 years of turning perfectly good metal into bits of useless scrap.
ccvstmr
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Re: A Real Dirty Job

Post by ccvstmr »

...and as we all know Greg, a bridge is ONLY as good as its weakest link! :oops: Sorry, couldn't resist.

Believe there was a bridge built long before the subject bridge, where cast links were used. As you can imagine, one or more of the links broke. Whether due to brittle material or fatigue, the end result was a bridge failure. Not good! Carl B.
Life is like a sewer...what you get out of it depends on what you put into it!
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Carrdo
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Re: A Real Dirty Job

Post by Carrdo »

Wolfgang thank you for the latest reply pointing out the weakness of the existing eye beam design.

Yes, I am aware that the intersection of the eye and the beam in both the drawbar and the safety bar represents a stress raiser and a fatigue point if there is not a generous transition radius between the two.

But you should see what exists now (a single 3/16" thick based out strip piece drawbar). Yet, there is no sign of any cracking or metal failure to date but yes, it could happen.

One thing which I am going to do is make it much easier to inspect the two drawbars. Right now this is impossible with the existing setup.

One problem encountered was the Installation of the cradle drawbar pin in the engine's cradle drawbar pocket. Everything has to be done blind here and can be an endless exercise in frustration unless one marks the position where the drawbar hole will align with the hole in the cradle drawbar pocket as seen in the second photo. The masking tape is temporary but without it there was no way I could get the pin to drop.

Finally, the gap between the engine and the tender as seen in the last photo. With the existing drawbar there is about a 5/32" gap between the engine and the tender chaffing plates at their closest points and about 1/2" gap between the engine and tender floor plates. I have no idea whether this is good, bad or indifferent so any operating experience here would be welcome.
Attachments
438 The Engine and Tender United with the Original Drawbar.jpg
439 The Temporary Marking of the Existing Drawbar to Assist in Locating and Dropping of the Cradle Pin in the Cradle Pin Pocket.jpg
440 The Gap Between the Engine and the Tender Chaffing Plates.jpg
Wolfgang
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Re: A Real Dirty Job

Post by Wolfgang »

Hi Don. Methinks the stresses, and stress reversals, in model equipment are much lower than in the full-sized hardware such that fatigue failure is much less of a problem.
In my opinion there should be no gap between the the tender buffer and the locomotive buffer. The locomotive CYCLOPEDIA shows a very powerful spring box that takes up any slack here. I incorporated this feature into my locomotive & tender. Seems to work very well.
Letting the draw bar take up any compressive loads between tender and loco may lead to derailments because of the sideways forces that will appear if the drawbar is not perfectly axial with the compressive loads which, of course, rarely if ever happens. Think of a column with pin joints at the top and bottom. w
Aside: In Bracebridge ON is a railway bridge over the Muskoka river which has an eye bar chain for the bottom chord. Very rusty as the railways don't paint their bridges.
Carrdo
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Re: A Real Dirty Job

Post by Carrdo »

Following Wolfgang's advice, I decided to make a test drawbar from a scrap piece with the two chaffing plates just touching.

It turns out that the centre to centre distance on this locomotive with the two chaffing plates touching is 4-7/8" where as the Langworthy drawings suggest 4- 11/16" but to check the actual distance before finishing these parts.

Sometimes, as the Better Business Bureau advises, " you should always investigate before you invest". Well...

When I first made the offset safety bar, I looked at the drawing and figured 1/8" extra length would be plenty. So now I was 1/16" too short.

To do it the difficult way again I then sawed out and machined a 3/4" + wide by 9/16" thick by 5-3/4" long hot rolled steel block chewing up a lot of time and effort in the process not to mention the wear and tear on my bandsaw blades, end mills and nerves.

Do not use cold rolled steel here if you are going to start with a solid block as the offset part is long and thin and will warp badly as most of metal will be machined away in the process. 1144 Stressproof steel would be ideal here but it is expensive.

Also, the drilling of the second pair of pin holes should be done as a matched pair (with the first pair of pin holes set together on a common pin) as both end holes in both bars will ultimately ride on a common pin with only a little bit of clearance.

After futzing around for most of the week on the above, this is where we are now.
Attachments
443 The Drawbar Safety Bar Assembly and the Original Drawbar (middle) and Scrap Piece Test Drawbar (top).jpg
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Harold_V
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Re: A Real Dirty Job

Post by Harold_V »

Carrdo wrote: Thu Jul 09, 2020 8:24 pm Do not use cold rolled steel here if you are going to start with a solid block as the offset part is long and thin and will warp badly as most of metal will be machined away in the process.
That condition is easily addressed by proper roughing and finishing. When one takes balanced cuts, alternating sides, the end result is likely to be as good as when using hot rolled.
1144 Stressproof steel would be ideal here but it is expensive.
Stressproof acquires its properties by cold working. To my knowledge, it is not available in flat bar, so one would have to start with round material. It has great properties, of that there is no doubt, but they can be duplicated by using chrome moly, which is available in flat bar. It does not machine as nicely as Stressproof, unfortunately.

H
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Wolfgang
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Re: A Real Dirty Job

Post by Wolfgang »

If you're going to machine a part "all over" you might as well start with hot rolled steel as I think it is less expensive than cold rolled.
Stressproof also distorts when machined unsymmetrically, as for example for crank shafts; however, the beautiful machinability of this steel, plus its strength and hardness (similar to 4140 heat treated and stress relieved) more than make up for this minor deficiency. w
Rwilliams
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Re: A Real Dirty Job

Post by Rwilliams »

In full size practice the spring loaded tender buffer was always kept in a condition so the contact surface of the tender buffer would always be in contact with the chafing plate of the locomotive rear frame crossmember. One needs to account for curves from side to side in scale models for the most part. In full size, the movement was such that vertical curvature was also a major requirement. Lubrication of the buffer plates was also important lest they became hung up which could cause a derailment of the lead tender truck.

Had a model steamer one time that did not have the correct final assembly of the tender buffer and it constantly got hung up on curves and immediately derailed. After the assembly was taken apart and modified to work like the full size buffer, no more problems with derailments.

Many locomotive designs in our field of models had the tender/locomotive buffer ignored as part of the drawing package in years past. The Allen Mogul was one example where a buffer assembly could have easily been included during construction and one would not have the constant slack action that is common without the spring loaded buffer.

Some of the full size designs were rather simple in design and easily included on a model rebuild. Just use metals that will play well together with some contact pressure. Keep the buffer well lubricated and all should be well.

In full size operation, crack inspection of the drawbar and the pins is part of the yearly certification for operation. Also gives a chance to inspect the buffer surfaces for wear which can become problematic with extensive use.
Carrdo
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Re: A Real Dirty Job

Post by Carrdo »

A different method for the milling of the centre portions of the eye bars.

This came about as I chose not to exchange my regular 4.5" 25 lb. milling vise for the 6" 85 lb. heavy duty milling vise without some sort of lifting assist which I don't have. Normally, one would just set the end pins in each eye of the drawbar and locate the pins on the top of the vise jaws which have been ground to exactly the same height in my case and clamp the part along its entire length.

However, the alternative setup does exactly the same thing although it is not as rigid, so only light cuts with a sharp end mill are taken. The 4 round plugs have all been surface ground to the exact same height and turned to the exact same diameter. Normally, they are used to align work to the tee slots in the table of the mill but they can be used this way as well.

After finishing, the drawbar and safety bar were installed on the engine as seen in the third photo. Just more fiddling here but everything finally dropped into place very nicely.

Then it was found that the drawbar and safety bar twin pocket casting on the tender was about 1/8" too high to allow both drawbars to enter the tender pockets for some reason which I will have to investigate and rectify. However, it is not due to the drawbar machining as both drawbars were made exactly to the Langworthy drawings.

Next, it is on with replacing the cab floor rear support sheet with one like the assembly shown in the last photo as that is what it really looks like. The large central opening was for the automatic stoker tunnel conveyer.
Attachments
444 A Different Method for Finish Milling the Centre Portions of the Eye Bars.jpg
445 The Finished Drawbar and Safety Bar.jpg
447 Engine End Installation.jpg
448 The Existing and the Replacement Cab Floor Rear Support Sheet.jpg
Carrdo
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Re: A Real Dirty Job

Post by Carrdo »

The first step to produce a new cab rear support sheet was to draw it up, full size, in cardboard.

The Hoffman Hudson cab rear support assembly (second photo) is constructed, in my case, from custom machined miniature HR steel angle pieces attached to the rear support sheet. They are only 1/4" by 1/4" by 1/16" thick as this was the thickness of the rear support sheet and cab floor on this engine. Since I was unable to locate any source carrying such small angle, in steel, I made my own. The finished assembly is more than adequate both with regard to strength and deflection as there is lots of other cab support/bracing added elsewhere (not shown).

In the case of the Langworthy Hudson, I used slightly thicker 5/64" thick (0.078") HR sheet steel as I had a large sheet of it available so the angle support pieces will be made this thick to suit.

To produce the custom miniature angle, one can start with the smallest commercial HR steel angle which is sized to do the job and cut it down. In this case, to match the existing bolt holes, I started with 3/4" by 3/4" by 1/8" thick stock. It will ultimately be cut down to 1/4" by 3/4" by 0.078" thick and attached to the inner face of the rear cab support sheet.
Attachments
449 The Cab Rear Support Plate Drawn Full Size in Cardboard.jpg
450 Machined Miniature Steel Angle Pieces.jpg
451 Cutting Down a Piece of HR Steel Angle.jpg
452 Miniature HR Steel Angle Milling.jpg
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