Planet - Transferring the Load

[What About The Bee]

Translating Armengaud's fabulous period drawings into CAD continues. A massive task, to be completed one tiny step at a time.

I left off here last time with the boxed frame and the axle plates. Illustrated was the concept of the draw pin retainer, wrapped around the WOOD rear frame.  

How odd, I thought. Planet's tractive effort is applied through a wooden beam. 

But what is the tractive effort effort of a Planet class locomotive? The value reported ranges from as little as 1400 pounds up to 2000 pounds. Not a tremendous amount. ~30HP, far less than your automobile. The frictional load of carriage and wagons is known far more precisely and due to exhaustive empirical experiment by the LMR. 1/248 is almost entirely due to the primitive bearings and friction. One part in 248, for flat, straight track. 

In 1833, Penny wise magazine

https://books.google.com/books?id=cqDfAAAAMAAJ&pg=PA167 

documents Planet drawing 80 tons at 12 to 14 mph [top right column]. This required a tractive effort of 695 pounds. The tractive effort calculation does not account for curves or inclines, both of which are on the LMR. 695 pounds, or even 2000, will not rupture wood, but there will be some bending!

I decided to move on, and add the tie rods between the wheel plates and the frame. Here, I encountered yet one more oddity. The rear most bolt was huge! 

Why would they need that? Just above the bolt was the forward most pillar of the railing. Certainly, a bolt of the dimension shown (1" diameter) isn't required for a lightweight railing.

The last bit of strange design was that the draw pin drops down through the footplate. Not only a tripping hazard, but also a nasty pinch point, as the load is taken up by Planet. Golly, none of this made any sense.

And then the penny dropped.

The draw pin goes thru the ¾" thick footplate. This transfers the load from the draw bar to the footplate. The foot plate is connected to the rear diagonal strut, green, via that enormous bolt (actually 2, one on either side), transferring the load to that rear diagonal strut.

Transfer the load through the rear axle box link to the middle strut

Transfer the load through the front axle box link to the front diagonal strut.

And then on to the front beams via another massive bolt.

As the Planet class wheels turn, they apply a forward vector of thrust through the side sandwich frames and thence to the front beams. This thrust is transferred, via that massive bolt, to the front diagonal strut. Where the load is also applied! 

Orange arrow points to draw pin hole.

Much like the collar on a horse pulling a horse drawn carriage, Planet pulls its load from the front beam. The pull is transferred, via those diagonal struts, to the foot plate. Similarly, the load is applied to the footplate. A horse drawn carriage would have been much better understood in the 1830s. Now, not so much.

A great sigh of relief, as I understand Planet's design. It finally makes sense. Those diagonal struts transfer the load. 

Who knew the footplate was such an integral component of a Planet's class design and not just a place to stand? A ¾" thick plate to stand on.  Ha!

Bee

PS. I will no longer whinge about how long Hornby takes to produce a design. Surely their Mechanical Engineers are much, much faster than I. Yet the amount of detail to wade through is stupendous. It takes a very long time to properly construct a model!

[threelink]

Hi Bee.

Impressive CAD work.

I take it that some of the load is also transferred by the wooden framing and that the function of the diagonal struts may be two fold, first to transfer some of the load and secondly to create, with the wooden framing and the axle box horns, a sort of bowstring girder with all that that entails in terms of strength and rigidity of the entire frame?

[What About The Bee]

Hi Three Link

Thank you for the compliment. I'm working very hard to keep the CAD in line with the Armengaud drawings. Details make a model pop, and since the lad doing the labor works for free, I shall have all the details.

Wood has a low modulus of elasticity. Further, the material isn't homogeneous. It is certainly NOT an ideal material from an engineering standpoint. I'm not positive why Stephenson used sandwich frames, but they indubitably would bend and wear poorly.

The side wooden frames have outside metal plates, creating the sandwich of the sandwich frames. The stiffness of a beam goes by the cube of the height and linearly by the width of a beam. The plates opposing the load rods are ~180mm high, only ~10mm thick. So yes, the sandwich frames clearly prevent the load rods from straightening out by that bowstring girder effect.

The rod itself is ~1 inch diameter. That was my first real clue that something special was going on. A 1" rod, even in wrought iron, is very strong indeed.

Perhaps Robert Stephenson wanted to reduce the cost of the frame by reducing material costs (solid wrought iron v sandwich frames). It is far easier to machine wood than wrought iron, perhaps it was a labor savings. Perhaps it was a weight savings, as George Stephenson established an upper limit on weight. Solid wrought iron would absolutely be far heavier than wood.

I just never could understand why those rods were there, before I started this CAD model. Now, it is fairly clear. The load is transferred by them. Not exclusively, of course, the sandwich frames do contribute.

Bee

[threelink]

Hello again, Bee

I think the reason for the use of sandwich frames in early locos was the impossibility of producing frames of wrought iron at the requisite size. The technology required simply did not exist. I believe that it was the shipbuilders who finally produced the technology later in the 19th century - things like the steam hammer, first used I think to produce the prop shaft for the SS Great Britain.