Resources/Information for Crank Lubrication?

A special section just for steam engines and boilers, as without these you may as well fit a sail.
Lionel Connell
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Re: Resources/Information for Crank Lubrication?

Post by Lionel Connell » Sun Mar 25, 2018 8:25 am

What difference does it make if the valve events are perfectly balanced for each direction of the stroke? because the cylinder/piston arrangement isn't balanced in both directions, nor can it be. The bottom piston face has a void in it's surface area where the piston rod connects. This causes an imbalance in displacement between the upper and lower cylinder and it causes steam engines to lope and there is virtually no way to remedy it. Practically or by worthwhile means anyways.
If this is the case with your engine then I am sorry to inform you that your engine is of poor design and or poor assembly. I am afraid that your statement above is incorrect with respect to what is possible.

The piston's position on it's rod can/should be adjusted to set the un-swept volume to match the swept volume at both ends of the cylinder so as to achieve a balanced compression ratio, this volume includes the volumes of the ports all of the way to the valve faces, and takes into account the volume that is displaced by the piston rod. If there is not sufficient adjustment available to achieve this, then the piston shape is incorrectly designed. It is quite easy for a fitter to measure and adjust the volumes and it was indeed common practice to do so. If the compression at each end of the cylinder is not put into balance, it is impossible ( probably should say "rather difficult") to set the engine's compression via the exhaust lap on the valve/s to achieve correct compression at the end of the stroke. If the compression is too high or too low, the performance of the engine, and it's life, can be greatly hampered. The volumes that I mention were simply measured using water, no computer or fancy simulation necessary. In larger marine engines, especially engines with insufficient counterbalance or no counterbalances at all on the crankshaft, the compression was made higher at the bottom than the top, and the lower inlet was opened slightly earlier, both to help balance the engine, but this is not really necessary in a launch size engines.

As i mentioned earlier, in loco practice it was common to work toward less than 1% error in valve events, this was done and can still be done without a computer. It was probably also common practice with the more professional marine engine builders/fitters to work toward similar tolerances, it is not much fun to run out of fuel in the middle of the Atlantic.

Steam engines are inefficient, some steam engines of seemingly identical design are a lot less efficient than others, but they don't necessarily need to be. A poorly set up steam engine can easily use twice the steam for the same output (or half the output for the same amount of steam) as the same design engine if it were set up correctly.

So yes, you are correct that if the boiler cannot keep up with the engine then the engine's ultimate power is limited. The part that you have left out is this, if the engine is wasting steam because it is incorrectly set up, this may be in fact be the cause of the boiler not keeping up.

While I agree that a hobby launch owner may not have sufficient interest or motivation to go chasing better efficiency or more power, it can be nice to only need to carry half the fuel for the same steaming distance and lighter boat is easier for the steam plant to push.

cheers

Lionel
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Re: Resources/Information for Crank Lubrication?

Post by DetroiTug » Sun Mar 25, 2018 3:06 pm

Not sure where all this is coming from, but if it's the "book" that's being referred to, I'd pitch it. :)

The imbalance I'm referring to has nothing to do with upper and lower cylinder volume, I'm stating the well known fact that due to the omission of the cross-sectional area of the piston rod on the lower piston face, the upper and lower cylinder can never be balanced in terms of force (which is what is important, all else is conjecture) in both directions on the same steam pressure. The reason I asked the question, what does it matter if perfect balance in valve event is achieved, when the cylinder is anything but. It's the same issue with hydraulic and pneumatic cylinders, they push more than they pull.


Wasted steam: Where does Mr Ashton suggest all this wasted steam is going? A factor of 50% of the steam wasted was referenced above (twice the fuel), how on earth does one make half of the steam disappear in a steam engine in satisfactory mechanical condition? There are really only four ways to waste steam in a conventional eccentric driven valve reciprocating steam engine. 1: slide or piston valve leaking directly in to the exhaust passage 2: Piston rings leaking steam past and out the exhaust 3: An engine horribly out of time, too far advanced or retarded, or valve rod length is way out of adjustment. 4: Poor thermal insulation. A steam engine is often defined as a steam expander. Any steam that makes it way to the cylinder is going to result in work.

Quote: "If this is the case with your engine then I am sorry to inform you that your engine is of poor design and or poor assembly. I am afraid that your statement above is incorrect with respect to what is possible."

When you get finished with your boat, bring her on over this way and I'll give you a shot at outrunning the Tug. You won't, and I'll do it on half your steam pressure. :D

-Ron
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Re: Resources/Information for Crank Lubrication?

Post by gondolier88 » Sun Mar 25, 2018 6:59 pm

DetroiTug wrote:Not sure where all this is coming from, but if it's the "book" that's being referred to, I'd pitch it. :)

The imbalance I'm referring to has nothing to do with upper and lower cylinder volume, I'm stating the well known fact that due to the omission of the cross-sectional area of the piston rod on the lower piston face, the upper and lower cylinder can never be balanced in terms of force (which is what is important, all else is conjecture) in both directions on the same steam pressure.

-Ron
This is an oft' overlooked issue- and one that is very easy to solve- guides on piston rods were used on locomotives, traction engines and marine engines- but marine engines seemed to make the least use of them- perhaps because gravity was working on the piston in its direction of travel and the guides were an effort to balance the force on the cylinder walls in horizontal applications so designers and engineers were happy to overlook the balance in working surface area balancing, or...

... They were happy to overcome it by other methods; it can also be achieved by machining the piston with more surface area on the bottom to match the small amount of area taken up by the piston rod (in cross section), though I've not come across many asymmetrical pistons on locos or traction engines.

That said, it is rare to find a marine engine design that balances the working surface area of the piston- relatively crude methods such as longer lead times on the BDC go a long way to helping matters- personally I am quite tempted to try piston rod guides on the top covers of my 6A when it comes to rebuilding it- sooner rather than later I'm hoping!!! The fact they look rather nice (I think so anyway) also has something to do with it.
Don't get heated...get steamed up

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Re: Resources/Information for Crank Lubrication?

Post by Oilking » Sun Mar 25, 2018 7:49 pm

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This came from a 1946 US Navy Pub on propulsion steam engines. Thought it might fit with the on going discussion.

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Re: Resources/Information for Crank Lubrication?

Post by DetroiTug » Sun Mar 25, 2018 8:07 pm

Dave,

Thanks for posting that and it is just what I had suspected, the suspension link on the end of the quadrant nearest ahead valve rod and eccentric. The longer suspension swinging a larger radius has less side motion.

Well, that explains why they did that on auto and marine engines that spend most of their time spinning one direction and why a Locomotive which can spend equal times in both directions needs it center suspended.

-Ron
Last edited by DetroiTug on Sun Mar 25, 2018 8:57 pm, edited 1 time in total.
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Re: Resources/Information for Crank Lubrication?

Post by Lopez Mike » Sun Mar 25, 2018 8:26 pm

That's probably why my suspension link is so long. It has to reach all the way across the back of the engine to get to the other end of the slotted link so that it's lined up for forward motion.

Good clear writing in that publication. Also true that locomotives spent a surprising amount of time operating in reverse. I have to back away from a slip and do a bit of backing and filling to turn in tight quarters. The rest of the time I'm running hooked up and cruising along.

Time spent fiddling with my engine is taken away from enjoying the occasional bow babe. The reality is that a surprising number of women have seen The African Queen and have no problem asking to take the helm as Rosie did. I had a 12 year old girl take the helm once and start singing Bogart's drinking song about the Brave Fisherman.

https://www.youtube.com/watch?v=Z3MjCkv29JI

She had seen the movie four of five times and had been dreaming about a steamboat ride. I wanted to sign the boat over to her. I felt like a millionaire.
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Re: Resources/Information for Crank Lubrication?

Post by cyberbadger » Mon Mar 26, 2018 12:35 am

Alright, how about videos of all this in action?

http://www.youtube.com/watch?v=38q48mUqzo8


Disclaimer: Nothing is perfect, but my old Toledo Engine was doing this on Lake Chautauqua 2017 - and thats all that counts to me. Amazing to have a 115 year old anything work so well.

-CB
Lionel Connell
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Re: Resources/Information for Crank Lubrication?

Post by Lionel Connell » Mon Mar 26, 2018 1:14 am

Wasted steam: Where does Mr Ashton suggest all this wasted steam is going? A factor of 50% of the steam wasted was referenced above (twice the fuel), how on earth does one make half of the steam disappear in a steam engine in satisfactory mechanical condition? There are really only four ways to waste steam in a conventional eccentric driven valve reciprocating steam engine. 1: slide or piston valve leaking directly in to the exhaust passage 2: Piston rings leaking steam past and out the exhaust 3: An engine horribly out of time, too far advanced or retarded, or valve rod length is way out of adjustment. 4: Poor thermal insulation. A steam engine is often defined as a steam expander. Any steam that makes it way to the cylinder is going to result in work.
If you don't believe how much difference there can be between a poorly setup/designed engine, and a very good one, then I am sorry but that is not a result of it not being true, there is plenty out there to read on the subject. A bad engine and a good engine can easily be 2:1 different in efficiency. There are plenty of examples where the imbalance between one end of the cylinder and the other cause the two to be detracting from each other at various points around cycle.

An example:

When the engine is linked up to 50% cut off then the travel of the valve is reduced, this results in the admission port opening being greatly reduced as it will be partly covered by the valve even at full opening. When the Stephenson's link (any motion) is out of balance not only will the cutoff at both ends of the cylinder be in a different place, but the end which has the shorter cutoff will also have a smaller total port opening area. This causes a greater drop in pressure across the valve. So what we have is one stroke with higher pressure as well as later cutoff that the other. The unseen and very often unrealized additional effect that happens simultaneously is that the exhaust cutoff for the two ends of the cylinder is also effected by the same imbalance which creates a higher compression of the steam at one end of the cylinder. The end of the cylinder that has the earlier admission cut off is unfortunately pushing the exhaust out of the end that has the earlier exhaust cutoff and this creates a large inefficiency. These inefficiencies are well documented and enormous effort was applied to understanding them and reduce them. Stephenson made one of the great leaps with his valve gear, the first at that time to manage to keep everything in balance through the greater range of the expansion link adjustment. Others later made further improvement by managing to maintain better consistency of the admission timing when the shutoff is adjusted.
A steam engine is often defined as a steam expander



There have been millions words published on the subject of the losses involved with unnecessary pressure drop during it's flow through the engine and how it should be avoided at all cost, a number of bright engineers won awards in the early days for producing valve arrangements which reduced drawing of the steam resulting in notable and celebrated improvements to efficiency. Getting the steam into the cylinder is not enough, it needs to be done without loosing ( perhaps better put as "unnecessarily using") a great deal of it's energy via bad engineering, and it needs to be converted to mechanical energy in an efficient manner. Just because the energy makes it into the steam chest does not mean that it will ALL make it to the wheels or propeller. There are many ways to waste the energy between the steam chest and the cross head.
When you get finished with your boat, bring her on over this way and I'll give you a shot at outrunning the Tug. You won't, and I'll do it on half your steam pressure. :D
Be careful what you wish for.

Lionel
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Re: Resources/Information for Crank Lubrication?

Post by Lionel Connell » Mon Mar 26, 2018 4:56 am

US Navy M,

I don't think that they needed to reverse very far, so why use a center suspended link?

Image

US Navy B, also with center suspended link

Image

Image

Perhaps the US Navy was confused about what worked well.

Lionel
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Re: Resources/Information for Crank Lubrication?

Post by steamboatjack » Mon Mar 26, 2018 7:31 am

Mr gondoliers comments about the use of tail rods, to use the correct term, is strange. These items were/are fitted to guide the piston or guide rod rather than equalise the net force on the piston etc. There are many reasons why the up and down forces on the piston can never be totally even, many due to the angularity of the various rods and to be frank this is not something I lose any sleep over.
As for Mr Connell, most engineers would tell him that vast amounts of steam entering a cylinder is "lost" to work by being condensed into water, the reasons are many.
I build engines with the link suspended in the centre and at the ahead end; I have built quite a few. I would suggest that people get off their computers and into the workshop!
Regards
Jack
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