two steam engine questions - scotch yokes and beams

[crueby]

two fairly unrelated steam engine questions:

1) scotch yokes make for a visually interesting engine, but how does the efficiency of one of those as a crank to piston connection vs a standard crank/connecting-rod? The yoke can simplify the arrangement, but is there a friction or leverage loss with it?

2) why did so many of the early engines employ a beam, with the cylinder vertical? Seems like many of the same parts are there with the cranks/rods/etc, but with the beam in the middle - anyone know why?

thanks!

 

[Brian Rupnow]

Can't help you with #1, but as for #2---
This is the engine which started the industrial revolution. These steam powered engines were originally built to use as pumps in the coal mines in Wales, to pump water out of the mines. Instead of having a rotating wheel at one end, as you see in the model, they had a long rod which reached down into a well casing to act as a pump, with its “Up and Down” motion. These were called a “Newcomen” engine.
In the mid to late 1700’s, James Watt, in partnership with Mathew Boulton made improvements to the valving on these engines, and invented a mechanism which would use the rocking motion of the beam to actually rotate a wheel, thru a crank mechanism.
The cylinder is vertically aligned, and the piston is “Double Acting”, which means it gets pressurized steam from both ends, as it moves up and down in the cylinder. This back and forth motion is transmitted to the rocking beam, which thru a series of connecting rods and levers makes the crankshaft rotate.
This engine has a steam valve which is driven thru a “Bell Crank Mechanism” by an ‘Eccentric” on the crankshaft. It is the operation of this valve which shuttles the steam back and forth from one end of the cylinder to the other.
Now, for the very first time, if you wanted to build a mill, or a factory, you didn’t have to be located on a river to utilize the power of water turning a water wheel, and you didn’t have to depend on the power of wind to run a windmill. The fact that a steam engine could run anywhere that there was fuel available to boil water and turn it into steam meant that industry could be located anywhere. This directly affected the way manufacturing was then spread thoughout the civilized world.

 

[JorgensenSteam]

I am no Scottish Yoke expert, but a few ideas that come to mind are as follows:

1. If the crank pin is traveling in a slot, then the bearing surface is a line along the face of the crank pin, instead of the bearing surface being across one side of a standard crank pin acting in a round bearing. This will concentrate all the force along this line, and greatly increase the wear to both the pin and the yoke.

2. The Scottish Yoke slot would probably not wear evenly, since the forces on the crank pin vary depending on the position of the crank and valve. Rod bearings sometimes do not wear evenly either, but it would be much easier to make and/or replace a rod bearing than to try and resurface the slot in a Scottish Yoke.

3. I have a Scottish Yoke model, and it does run well, but generally anything works well in a model engine, but not necessarily in a full sized engine, where forces and balance become critical.

4. The design of valve gear, and specifically the suspension point of the Stephenson link was designed to adjust for the nonlinearity of the piston movement caused by the angularity of the rod. A Scottish Yoke engine does not have any distortions in piston movement, so this would have to be taken into account in the valve gear design for a Scottish Yoke engine.

5. The mass of a Scottish yoke would have been large, and the potential for the yoke to flex under large loads would be much greater than the standard connecting rod/crank arrangement. A Scottish yoke with its larger mass would cost more to cast and machine.

6. A large yoke mass would make balancing an engine difficult, and an unbalanced engine will not run reliably.

7. I don't recall seeing a commercial steam engine being produced with the Scottish Yoke, but perhaps some of the smaller engines had them. Perhaps the closest commercial unit that is similar to the Scottish Yoke would be the Dake steam engine. The Dake engine was a successful commercial design up to perhaps 20 hp, but the Dake was a specialty engine used for ship steering systems, hoists, capsans, etc., and was not a mainstream steam engine design.

8. The force of the crank pin acting on the end of a Scottish Yoke is going to produce a moment (torsional force) that will have to be supported, similar to the force applied by a long wrench. With a standard crank and rod design with pivoting bearings at both ends, you will not have this twisting moment.


I seems like the requirements for marine engines drove much of the development of the steam engine. Ships with a high center of mass are prone to rolling over in rough seas, and warships with engines protruding high about the top deck were vulneralbe to enemy fire. The side lever engine was an interesting adaptation of the beam engine, and helped lower the center of mass and reduce the engine frame structure. Once it was realized the beam was unnecessary, its use quickly faded. Early boiler pressures were low, and that caused the cylinder diameters to be large for a given horsepower. Once boiler pressures increased, the cylinder sizes reduced, and compound engines quickly dominated the marine world.


Pat J

 

[tel]

The scotch yoke allows the piston rod to deliver its impetus in a straight line without the use of a crosshead/connecting rod while still driving a flywheel for the obvious benefits thereof. In full size, a lot of in-line pumps and the like were set up that way.

 

[peatoluser]

coal mines in Wales?! I think there'll be a few cornishmen who'll take umbridge at that!
it was the tin mines in Cornwall that first used them.
as an aside they were known as atmospheric engines, were single acting and relied on condensing the steam in the cylinder to cause a vacuum to retract it.
there's a good animation of one here
http://www.animatedengines.com/newcomen.shtml
Watt developed the double acting cylinder, far more fuel efficient, but he would charge the mine owners a percentage of the money saved on coal
so the Newcomen remained popular long after it should have been retired.
they were surprisingly long lived, and I believe there is archive film footage of on in actual use (not as a preserved museum piece) from the 1920's or 30's.

 

[crueby]

Great info - thanks!

One thing I still dont get though - I can see how the beam worked great when driving a vertical pump into a mine, but why was it still used on things like ships, where it was used to drive a horizontal shaft/flywheel/prop? Was it to shorten up the engine to fit in the narrow hull?

Oh, and I'll stay out of the way of the welsh/cornish debate - dont want either of them steamed off at me!! I've already got enough mixed english/irish/german bloodlines in the family tree to cause wars!

 

[Brian Rupnow]

Peatoluser---My apologies. From way over here in the colonies, Wales and Cornwall (If thats where Cornishmen come from) ?? look a heck of a lot the same. ;D ;D ;D----Brian

 

[JorgensenSteam]

Beam engines were used in ships prior to the invention of the screw propellor. John Ericsson invented the screw propellor, and he was told by the great navy men of the time that it was a silly idea that would never work in a real application.

The first powered ships were an adaptation of sailing ships, and they were propelled either by sidewheels or a rear mounted paddlewheel, but generally still had sails.
The steam engine was not considered as reliable as wind power, and so the sails were retained. The first northern ironclad "Monitor" build by John Ericsson had written into its contract that in addition to its steam engine, it must also be fitted with sails. John Ericsson wisely ignored that part of the contract and used his low-profile twin-piston, single-cylinder trunk engine with a screw propellor. Once the first battle of the ironclads was over, there was no longer a concern for sails.

My statement #1 above I don't think is correct. The pin would have ridden in a crosshead that operated in the yoke slot, so I don't think wear would have been a problem.

The Scottish yoke seems to be just a crosshead stood up on edge, and moving.
The standard crosshead arrangement allowed the rod forces to be transmitted into the frame, since the standard engine has the crosshead bolted to the frame.
Standing the crosshead up in the yoke fashion and allowing it to move would create some support challenges, and would require some very rigid piston rods to keep the rods from deflecting.

 

[tel]

(If thats where Cornishmen come from) ??

Nah, it's where cornish pasties and bad jokes come from! Rof}