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Right Angle Cranks

Posted by frustrated 
Right Angle Cranks
October 21, 2021 06:37AM
One of the more unique aspects of the traditional 2-cylinder, double-acting steam car engine is the crossplane, two pin crank. The world is full of two pin cranks having pins at 180 degrees ... and a smattering of cranks with both pins on the same side... but right angles was sort of unique. That was before the advent of the 2-cylinder engine with a 270 degree crank.




Of course, these engines have a center bearing rather than a sprocket, and they also have outboard main bearings. This is definitely a stronger setup, and runs much more true than a built-up crank. (based on industry standards, they are probably within 6 microns of design spec).


Re: Right Angle Cranks
October 25, 2021 06:35AM
Hi Ken,
So what is better for a double acting 2-cyclinder steam engine? Keep in mind a standard Stanley engine set up with 2-bearings, cantilever and inside the crank throws along with center gear direct drive on the rear axle.

A Doble was 180 Degrees...right.

Thanks,
Rick
Re: Right Angle Cranks
October 25, 2021 07:43AM
Hi Rick,

I once ran across a lengthy article on crankshaft design that went into calculating stresses on the crank's various components -- part of it breaks down into simple leverage calculations. It didn't take very long to find out just how much stress you get with an overhung crank. When you have a crankpin supported on both sides, the load is split among two bearings. Besides that, the crank arms are now under a compression load, which they can easily resist. With a single bearing, the connecting rod is outside of the bearing and it applies a twisting force to the crank arm -- which more readily produces fatigue. It also tries to twist the crankpin itself. Think how much a diving board bounces up and down and then think about how much more steady it would be if supported on both ends. Of course, these twisting forces are also applied to the crank bearings. This is why even small, single-cylinder modern engines have bearings on either side of the crankpin rather than using an overhung crank -- even though that would be a bit easier to make.

This probably didn't matter as much on a Stanley engine because the open framework construction already invited a certain amount of twisting and flexing, but a number of the more advanced steamers avoided such construction either by adopting something more like ICE design or by replacing the open framework with solid, unitary castings.

Honestly, I rather like the design that Abner Doble's father and brother were using for the small truck project. They placed a double acting V-2 engine right behind the cab and extended a driveshaft to the rear axle. This gave them a simple and cheap crank with just one throw ... but having bearings on each end made it more rugged. The layout gave them exactly the same kind of torque distribution as you could get with a Stanley -- 1 impulse every 90 degrees. The primary unbalance for each cylinder combines into a rotary force that can be cancelled by crank counterweights leaving just a secondary side to side shake that is partially cancelled by about 30 percent.

As I recollect, the cranks on the 2 cylinder Doble-Detroit and the Series F engines were set at 90 degree angles. The Series is a cross compound and therefore a bit weird.
Re: Right Angle Cranks
October 25, 2021 09:13AM
A lot of Stanley engines when needing rebuilding and the crank pins have been Magnafluxed show cracks under the pins. This could be caused in the forging of the crank web and pin in the same press or the years of use, but is your rebuilding a Stanley engine have the pin and web Magnafluxed.

Rolly
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270 degree crank.png 167.2 KB open | download frustrated 10/21/2021 Read message