Unusual Valve Timing?

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Unusual Valve Timing?

Post by Old Steamer » Tue Mar 13, 2018 4:26 pm

I am restoring a launch whose Stuart Turner 6A appears to have unusual valve timing. The engine was built in 1978 by Anthony Bever and was one of the first (if not the first) of the 6A's reintroduced at that time. The only significant modification from new is a conversion to balanced valves in 1986 and, until now, I doubt if it has ever been stripped down since built.

Having cleaned, adjusted etc., I checked the valve settings expecting the usual positive leads of 6 deg. before TDC and 7 deg. before BDC. Instead I found negative lead with the valves opening about 7 deg. after TDC/BDC. The eccentrics are all firmly keyed to the shaft and thus adjustment is not possible without further modification.

I have heard of negative lead being used with short eccentric rods but I don't see it happening here without affecting smooth running. Anthony Bever was a well-known designer and builder who is very unlikely to have mis-timed an engine and yet, as I have said, I doubt if the engine has ever been stripped. Do I have a historical anomaly and/or do other owners of similar vintage engines have the same configuration?
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OS
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Re: Unusual Valve Timing?

Post by Lopez Mike » Tue Mar 13, 2018 5:21 pm

Have you had an opportunity to run the engine? I have seen this once in a while and in the one case where I observed the engine in use, it didn't seem to make any difference. Near the ends of the stroke the physical distance that the piston travels in a few degrees is remarkably small.

I suppose it might make a difference in the loads on the wrist pin at higher r.p.m. Our small launch engines run at such low piston speeds that the differences can be academic.
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Re: Unusual Valve Timing?

Post by gondolier88 » Tue Mar 13, 2018 8:38 pm

I remember an article in an old Funnel- perhaps by A. Bever- it was one of the articles on how to make a 6A quiet, one of many!

In my mind it is a poor substitute for properly designed pistons with proper clearance at BDC/TDC- in the 6A I'm rebuilding at the moment I found the clearance at BDC on the HP cylinder to be 0.0010"! In line with the Funnel article by John Hodgkinson 'Silencing the 6A' I have redesigned the pistons with conical tops, symetrical clearance (including swept volume of steam passages in the calculations, as well as piston rods on bottom clearance) and redesigned the cylinder covers.

There was a lot of evidence to back up the findings the John Hodgkinson had found in that the compression at the end of the strokes was such that the valve was getting lifted off the valve face.

I haven't tested it yet as I'm waiting on new crosshead castings to come back from the foundry, but I'm confident the entry and exit of steam into both cylinders should be much better- I'm very interested to hear how much negative lead your engine has been using as this would approximate to quite a sizeable difference in swept volume clearance at steam entry, as well as being in partial vacuum rather than on compression stroke.

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Re: Unusual Valve Timing?

Post by Old Steamer » Tue Mar 13, 2018 11:09 pm

I've not had the chance to run the engine as I'm on the last lap of rebuilding it. Checking the valve settings was one of the final tasks to be done.
The evidence indicates that the engine has been running with 7 deg. negative lead (or something close) since first built. As the first of its kind for many years and built from scratch, was this how Anthony Bever saw the best way to set it up and was he proved right?
I am loth to start re-cutting keyways to produce an engine with more conventional valve settings but the whys and wherefores of the present set-up are nagging away at me.

The timing diagram shown below is from the plans dated June 1977 that came with the engine.
Cheers,
OS
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Re: Unusual Valve Timing?

Post by TahoeSteam » Wed Mar 14, 2018 3:33 am

There are offset keys readily available or you can make your own if you've got a wild hair and would like to experiment without making permanent modifications.
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Re: Unusual Valve Timing?

Post by Lopez Mike » Wed Mar 14, 2018 3:40 am

Why didn't I remember that? I've used them many times in automotive engines. Do you think they are available in such small sizes?

A small consideration might be that in the application I am familiar with the key has fairly minor loads on it. It just determines the relationship. In the 6A I'm not sure if the key takes the loads. I suspect that the valve gear forces aren't enough to be a big problem.

As an aside, more than one person has needed to improve the H.P. valve rod design on these engines. A larger diameter and/or a stronger guide. One of our boats broke down due to this weakness. Perhaps wear led to misalignment.
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Re: Unusual Valve Timing?

Post by Old Steamer » Wed Mar 14, 2018 9:46 am

Wesley/Mike,
Thanks for your replies. By an "offset key", I assume you mean a key which is a sort of 'piggy-back' key in profile? I have some doubts that a key enabling a shift from -7 deg. to +7 deg. is feasible. The eccentrics are a 'push-fit' on the shaft and there is a grub screw that I assume simply locks the eccentrics against longitudinal movement. Thus, for lack of anything else other than friction between shaft and eccentrics, the drive must be transmitted by the keys.

As I've mentioned, I don't think the engine has been fully stripped since completion of build in 1978 and so I have to assume it has run to its owners satisfaction in that period. It's just nagging at me that the lack of a steam cushion at end of stroke must do little to reduce noise/bearing loads and improvements could be made. I have evidence that this is the first Anthony Bever ST 6A that he ever built. Did he change things later?

On inspection, all the journal bearings were in good condition and needing little attention. Only the crosshead guides were found in any way worn and a change of shim sorted that. Thus, it has worn well over the years, timing notwithstanding.

In respect of the weakness in the HP valve rod, this engine was modified with balanced valves in 1986 which must do something to reduce the valve rod load. I note that the stainless steel valve rods were renewed at the same time although these appear to replicate the originals.

I think I will just box her up with the valve settings as-is and see how she goes after seven years in lay-up before I took her over. Fingers crossed!
Regards,
OS
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Re: Unusual Valve Timing?

Post by Lopez Mike » Wed Mar 14, 2018 9:53 am

Balanced valves, especially on the H.P., are the real fix. I hesitate to recommend it for most people as they are seldom machinists.

As to inertial loads you should be fine running it as is. As I have harped before, our small engines scale very well from full sized engines as to do with mechanical stresses. I wish they did as well with thermodynamic issues. The old mouse v.s. elephant problem.
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Re: Unusual Valve Timing?

Post by Lionel Connell » Thu Mar 15, 2018 12:53 am

Are you certain that you are checking the ahead timing?

If the engine has been built with the original side suspended links then the forward timing and reverse timing will very likely be different to each other. I CAD modeled the entire 6A and I can run the CAD model and measure the valve events for any setting, the balance of the valve events across the range of the Stephenson's link on the standard 6A is just terrible. Using the side suspended link and Stuart's original dimensions, forward and reverse events will be nothing alike. The engine's balance of timing also varies considerably as the link moved toward the center. That is why most 6As only run well at one expansion setting.

You may well find that by adjusting the expansion link position a little that the timing of the opening position changes nearer to where you expect it should be.

After CAD modelling the 6A as per Stuart,s drawings and studying the valve events I was more than a little disappointed. Mr Don Ashton wrote a very good book on the subject of Stephenson's valve gear. I built my engine with center suspended links and correctly positioned suspension pins.

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Re: Unusual Valve Timing?

Post by wsmcycle » Mon Apr 02, 2018 1:48 pm

it is quite difficult to tell whether you are looking at leading or lagging. i have a 6a also and queried which direction was forward until i was schooled here that it was intended to rotate in the direction of compressing the stanchions.
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