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valve timing

5" firefly

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Pearl Parker 131/01/2015 12:10:43
5 forum posts
I am rebuilding a Martin Evans 5" G Prairie (Firefly) and I need information about valve timing. I have the drawings c 1961 but require the written details for setting the valve timing. I am enquiring if anyone might have this information or which issue of ME had these details from the 60's and 70's.
Many thanks for your help.
Ken Parker (Swindon) Tel No. 07710 515 507
mick H01/02/2015 08:12:22
766 forum posts
28 photos

Ken.....first, what sort of valve gear is fitted to the loco?


Anthony Kendall01/02/2015 09:43:00
153 forum posts

Hello Ken.

ME October 5, 1961

Page 420

Pearl Parker 101/02/2015 10:32:15
5 forum posts

Hi Mick,

Stephenson's launch


Ady101/02/2015 11:45:47
5091 forum posts
736 photos


Manofkent01/02/2015 21:33:27
142 forum posts
29 photos

This is a problem I have wrestled with in the last few weeks. Now, I do exactly as the details say, but which way do you turn the eccentric to match before securing? If I get it wrong the valve will travel the wrong way - ending up with one forward and one reverse! So - is there a golden rule for the direction and placement of the eccentric please?

John Olsen01/02/2015 22:09:22
1250 forum posts
94 photos
1 articles

When you are setting the valves for forward gear, you should be turning the wheels in the forward direction, and the valve should be just starting to open just before the piston reaches top dead centre. It will then stay open until nearly bottom dead centre when you are in full gear.

Similarly, when you are setting up for reverse gear, you should turn the wheels backwards. Again the valve should start to open just a whisker before top dead centre, and remain open most of the way down the stroke.

The actual position of the eccentric will depend on the valve, eg whether it is outside admission or inside admission. Slide valves are outside admission, piston valves can be either but are usually inside admission. For the slide valve case, the eccentric will be advanced relative to the crankpin in the direction of rotation by a bit more than 90 degrees. Bear in mind that for reverse, advance will be in the opposite direction.

So for a slide valve loco with the piston at top (forward) dead centre, the forward eccentric will be at the bottom position (near the track) and behind the axle centre and the reverse eccentric will be at the top position and also behind the axle centre. The amount they are behind the axle centre will depend on how much lap the valve has, but it may typically be 15 to 30 degrees or so. This is what you set up when you follow the procedure given in the article.

The idea is that when you are trundling along, the valve opens just a little before top dead centre so that steam starts flowing by the time it is needed. It keeps being admitted until the valve cuts off. When in full gear, that will almost be at the bottom of the stroke. Once the loco is rolling you can advance the cutoff, so that although steam is admitted at about the same point, it is cut off before the piston has moved far and the steam already in the cylinder expands for the rest of the stroke. This give more economical working.


julian atkins01/02/2015 23:00:52
1254 forum posts
353 photos

hi ken,

'Firefly' is one of martin evan's early designs. he didnt actually design the valve gear for it - this was done for him by the late fred cottam of the harrow and wembley club. i have fred's original drawings for same somewhere.

unfortunately martin's description of how to set the valves is a bit erroneous.

for any given arrangement/design of stephensons gear there is always an optimum or 'correct' position for the setting of the angle of advance of the eccentrics. this angle isnt stated on the drawings or provided in the construction notes because martin didnt know how to calculate it.

his description of altering the length of the valves is completely wrong and what he describes in Ady's very helpful page from ME shows how little martin understood stephensons valve gear in those early days (he did get better later on). if you alter the length of the valves you also alter the lap and unequal laps is a recipe for disaster for good valve events, good valve setting, and equal cut offs. equal cut offs and exhaust was how GWR locos had their valves set rather than equal leads.

for the arrangement of stephensons gear used by martin for Firefly there is no reason why the forward gear events should not be the same as those for reverse gear (see martin's erroneous comment to the contrary in Ady's ME extract).

martin guesses where lead should be 'nil' as a percentage of cut off. this can actually be very accurately worked out with precision and a bit of maths.

if you want to get the best out of your Firefly's valve gear can i please recommend valve gear supremo Don Ashton's booklet on stephensons gear? my copy is very well thumbed and has been used to design the gear for a number of my locos.

if you want some more detailed help please feel free to private message me and i will do my best to help you.



Edited By julian atkins on 01/02/2015 23:03:08

Manofkent02/02/2015 06:29:55
142 forum posts
29 photos

John (Olsen). Thank you for this. Most helpful. Mine is an outside admission piston valve, so I am relying on a crack of air from the drain cocks to tell me when steam will be first admitted. Your description of the eccentrics relative to the track etc is great, and I will give this a go. Are the opposing eccentrics (front and back) going to be at 180 degrees to each other as a general rule?


julian atkins02/02/2015 10:28:53
1254 forum posts
353 photos

if i might be permitted to correct a few points made by john (olsen)?

references to 'top dead centre' and 'bottom dead centre' are not very helpful on a steam locomotive and can lead to confusion. it is customary to refer to front dead centre (FDC) and back dead centre (BDC). the first part of any valve setting procedure is to establish these positions as accurately as possible with the driving axle in its running position. the valve is moving very quickly at these positions and accuracy here is essential.

john's reference to 15 to 30 degrees angle of advance can also be misleading. simplified somewhat, the angle of advance is a factor of the lap and type of expansion link. launch type links will have a greater angle of advance than loco links. typically the angle of advance of stephensons gear fitted with loco links will be around 7 or 8 degrees. quite a bit less than john's 15 degrees.

rather than refer to the angle of advance (very difficult to measure when setting the eccentrics) the eccentrics are best set with a jig. Don Ashton describes all this in his excellent booklet.

the GWR stephensons gear as fitted to Firefly produces a large increase in the amount of lead as the gear is notched up from fullgear. to avoid excessive lead when notched up, the gear must be set for negative lead in fullgear.

generally the valve gear should not be set for equal port openings as the front port should not open quite as much as the rear port to give equality of events. also it is not necessary to provide for full opening of the ports in fullgear, 3/4 opening being ok. in any event the cut offs in fullgear should be checked.

every time you alter one of the eccentrics when setting the gear, it upsets the events for the other eccentric which will also need a corresponding adjustment. this is why it is better to set the eccentrics via a jig.



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