Author Topic: Simple slip eccentric  (Read 9926 times)

Offline arnoldb

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Simple slip eccentric
« on: August 23, 2013, 12:14:40 PM »
I've had a couple of questions after I mentioned a slip eccentric in another thread, so I'll add a bit more detail here.

Background:

A slip eccentric is one of the easiest ways of making just about any slide or piston valve steam engine that uses a single eccentric to drive the valve with reversible.
It's not often mentioned - most likely as it was rarely (if ever) used on full size stationary steam engines.  I found the first reference to slip eccentrics while browsing around garden railway forums; it's often used on simple 1:16 locomotives.

There are MUCH better valve gears around for making engines reversible; the slip eccentric is not adjustable like the others, so can not be used to control things like varying steam cut-off.  To reverse the direction on an engine using a slip eccentric, it is also necessary to turn the flywheel of the engine manually in the direction one wants it to go for at least 1/2 a turn, or in the case of a locomotive, to push the loco a short distance in the direction one wants it to run. 

So in essence it's a bit of a gimmick that can be added to small engines to make them more interesting, as a simple way to study valve operation, and lastly it's just a good little exercise in machining.

Operation:

A single fixed eccentric is normally fitted so that it is rotated 90o respective to the crank.  The direction it is rotated relative to the crank determines the direction the engine will run in; in order to change the running direction of an engine with a fixed eccentric, the eccentric must be loosened from the crank shaft, turned through 180o and fastened again.  The slip eccentric allows this 180o turn to happen without loosening the eccentric from the crank shaft.

In most basic form, it is made of two parts; one part is fastened to the crank shaft, and the other bit actually driving the eccentric rod is loose to turn, and restricted to the needed 180o movement.
Here's a simple one I made a while ago for my Elmer's #32 engine:


The part on the right is made to be fixed to the crankshaft with the grub screw that's visible at the bottom.  It has a 180o groove milled in it.  The part on the left is made to be a loose running fit on the crankshaft, and the pin mounted in it engages in the groove.  As you can see, this makes the "eccentric" bit; the flange left on it is to retain the eccentric strap on it from one side, and once assembled together on the crank shaft, the eccentric strap is held in place between the two parts.  Of course, some means of keeping the "loose" bit of the eccentric in position is also needed; in the following photo I simply used the bearing bush in the bearing column to keep the loose bit in place between the bush and the fixed part of the eccentric:


A short video of the eccentric in action; you can see that I manually turn the flywheel in the direction I want the engine to run - if you watch closely you will see how the eccentric "slips" relative to the crankshaft:
<a href="http://www.youtube.com/watch?v=WKGjK1VU4FU" target="_blank">http://www.youtube.com/watch?v=WKGjK1VU4FU</a>
I just videoed this on the kitchen counter, so please excuse the parrot chirping up in the background and any huffing and puffing; I just ran the engine on breath-power.

Some "Gotchas" to look out for if you want to make a slip eccentric:
Successful operation of the slip eccentric depends on a little friction in the valve train to make it operate, and as little friction as possible in the eccentric itself.  If there's more friction in the eccentric assembly than in the rest of the valve train, it won't operate, as it won't be able to "slip"
If you make a slip eccentric, and you can't get the engine to run, turn the fixed part of the assembly through 180o on the shaft.  The eccentric's slip and lock directions are directional; if the engine wants to start to run and the "slip" is in the wrong direction, it will just immediately want to reverse direction again.  Like I mentioned earlier, it's a good way to study valve gear operation on simple engines to learn how they work...

There are many different ways to make slip eccentrics; this was just one way that I happened to have on-hand to show.  If you want to fit a slip eccentric to an existing design with a fixed eccentric, the slip eccentric must be built with exactly the same offset as for the fixed eccentric.
Multi-cylinder engines with separate eccentrics for each valve can also be fitted with slip eccentrics; simply replace each fixed eccentric with the equivalent slip eccentric.

I intentionally kept this write-up as simple as possible; there are other factors in an engine's operation that can to some extent be influenced by changing the design of the eccentric a bit, but that's a long story  ;)

Hopefully this is of some use.

Kind regards, Arnold
Building an engine takes Patience, Planning, Preparation and Machining.
Procrastination is nearly the same, but it precludes machining.
Thus, an engine will only be built once the procrastination stops and the machining begins!

Offline vcutajar

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Re: Simple slip eccentric
« Reply #1 on: August 23, 2013, 12:24:34 PM »
Thanks Arnold for the explanation.

Vince

Offline chucketn

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Re: Simple slip eccentric
« Reply #2 on: August 23, 2013, 12:48:31 PM »
+1 on the thanks, Arnold. Stored away for future use.

Chuck

Offline EmanMyford

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Re: Simple slip eccentric
« Reply #3 on: August 23, 2013, 01:02:32 PM »
Thanks Arnold for the explanation. This one is also filed for future use.  :cheers:

Regards.
Ewald

Offline b.lindsey

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Re: Simple slip eccentric
« Reply #4 on: August 23, 2013, 01:11:53 PM »
Very clever idea Arnold. I can see how it would add interest to existing forward only engines with minimal changes, as long as there is width available for the slightly wider slip eccentric.

Bill

Offline Bearcar1

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Re: Simple slip eccentric
« Reply #5 on: August 23, 2013, 02:21:45 PM »
BRAVO! I am familiar with the slip style method for reversing an engine and how they were constructed, but i've got to commend you Arnold for a terrific write-up explanation. Many will possibly not be aware of this method and the way you explained it can indeed be useful to to those builders. Nicely done.  :ThumbsUp:  The first time I ran across such a setup was when I purchased a Reeves Monarch, that was in dire need of some repair, at an estate sale. I thought it ingenious and went about the task of researching the notion and found it fascinating that something so simple could be so effective in getting an engine to reverse all the while being unobtrusive in nature as it was virtually invisible to the unsuspecting eye.


BC1
Jim
« Last Edit: August 23, 2013, 11:46:00 PM by Bearcar1 »

Offline Dan Rowe

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Re: Simple slip eccentric
« Reply #6 on: August 23, 2013, 02:34:04 PM »
Nice write up Arnold. :praise2:
For those scratch designing slip eccentric is included in the set of Dockstader's valve gear programs.

Dan
ShaylocoDan

Offline ths

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Re: Simple slip eccentric
« Reply #7 on: August 23, 2013, 10:29:56 PM »
Good of you to put that up, Arnold.

Cheers, Hugh.

Offline arnoldb

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Re: Simple slip eccentric
« Reply #8 on: August 25, 2013, 08:00:23 PM »
Thanks all who replied  :LittleAngel:

Like Bill mentioned, check if there's enough space to fit a retrofitted slip eccentric; if carefully made it can be made nearly as narrow as a normal one.

Thanks Dan; I forgot Charlie Dockstader included it in his valvegear program.

Kind regards, Arnold
Building an engine takes Patience, Planning, Preparation and Machining.
Procrastination is nearly the same, but it precludes machining.
Thus, an engine will only be built once the procrastination stops and the machining begins!

Offline TerryWerm

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Re: Simple slip eccentric
« Reply #9 on: August 26, 2013, 05:26:45 AM »
Thanks for putting that up, Arnold. Did I hear an African Gray in the background??
Terry

Making chips with old machines - there's nothing like it!

Offline arnoldb

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Re: Simple slip eccentric
« Reply #10 on: August 26, 2013, 07:41:11 AM »
Pleasure Terry.

Yes, its Shrek the African Grey chirping in.

Kind regards, Arnold
Building an engine takes Patience, Planning, Preparation and Machining.
Procrastination is nearly the same, but it precludes machining.
Thus, an engine will only be built once the procrastination stops and the machining begins!

Offline Dan Rowe

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Re: Simple slip eccentric
« Reply #11 on: August 26, 2013, 01:07:54 PM »
Arnold,
The reason I was excited to find slip eccentric in Dockstader's valve gear programs was it is really a single eccentric solution. The oval diagram shown in the curve section is the best you can do for a given port design and stroke and con rod length. The minor errors of con rod and eccentric angularity are in the graph. So that graph is really the best that can be achieved for the valve design.

Now if we add a reversing gear other than slip eccentric like Stephenson gear and compare the oval diagram to the slip eccentric oval diagram the closer we come to the original slip eccentric diagram the better. This way we can judge with a standard weather the valve gear is getting better or worse when changes are made to a more complex reverse gear geometry.

Dan
ShaylocoDan