Comparision of Mamod/MSS/PPS Reverser Valves

[dougrail]

I think next time I go steam I'll remember this...cheers. I ssumed that the cab reg became 100% regulation of steamflow, but seems not quite.

[dougrail]

I wonder then, hence my thread last night, if a =good= Mamod/MSS rev block might be better as it allows more steam through? The narrow channels of the IP/PPS seem to restrict the amount of steam that can logically pass through to the cylinders and having seen a supercyl'd engine run freely with an old but good block am pondering experimenting. Further to the 'physical proof' (Brit...) Dr Cairn's scientific approach seems to conclude that the PPS/IPE isn't as 'better' as thought.

Hmm.

[DolwyddelanLightRail]

Further to the 'physical proof' (Brit...)

Well, Brit still uses it's original regulator (now reverser). Never had to lap it, and the only mod done to it was to add an extra spring so that it could cope with 45psi of pressure, not 25. Never had any issues with it at all.

(If something now breaks on it because I've said that.....)

[dougrail]

Well yea, exactly, we've seen how Brit works and works good. Brit and SB have the same chassis - DS steelers, DS supercyls, both roll freely when off-loco, but SB struggles a little whereas Brit runs wild. This is even with the gas ramped up.

I wonder if the original block on Brit having wider channels allows for more steam to reach the cyls and thus more steam = more power and better running given how supercyls are stiff needing lubrication just to run.

[Narrow Minded]

Doug, I think you'll find that, after listing the similarities, the difference lies in the amount of running that Britomart does

[dougrail]

Going to experiment anyway. Also if 24th Feb goes well, the 'demonic pact' will ring true and we'll start from the drawing board.

[WWLR]

An interesting post Chris.

Looking at the group picture of the 4 rotary valves I get the impression that the first two are designed to start opening the exhaust passage first and then progressively open the inlet port using the pointed end of the curved channel for a controlled opening. The PPS one looks fully symmetrical, opening exhaust and inlet at the same time while the new Mamod one looks to me as if the solid part of the gasket is wider on the right side and narrower on the left side which, if the feed and exhaust pipes connect the same way as the others, would open the inlet before the exhaust.

Is the gasket glued in place in the rotating valve?

Regards

Will

[Chris Cairns]

Will,

Interesting discussion.

I think it is just the angle that the reverser valve was to the camera as I recall the gasket in the new Mamod reverser was symmetrical, and yes just to confuse one further the steam pipe & exhaust pipes are reversed on the new Mamod Mark I & IIs so the valve operates the other way. I assume the gasket is glued in, but I'm not going to pull apart a loco again to check - probably end up breaking something!

Yes the PPS Reverser has symmetrical slots, but I've not come across anyone who has just replaced the standard Mamod/MSS reverser with this one for full control - always seems to be fitted on a loco with separate regulator control as well. My little experiment fitting the PPS reverser to a standard SL3 did not convince me that I got better control for the extra cost of £20.

I do not have a good graphics programme and these photos are not to the same scale but may assist in your analysis.

Reverser in Neutral


Reverser in Forward


Reverser in Backwards


The old Mamod SLs have a reputation of running backwards better and faster than forwards, which apparently was something to do with the design of this reverser. Unfortunately the only spare reverser I have access to has the poorly cut slots so difficult to make any useful comparison with this reverser.

Chris Cairns.

[dougrail]

Okay, so if you got your 'good old' rotor from Mamod/Mss and carved out the pointy bit so it was rounded like the other size of the channel, would that a)improve running/steaming in forward (bearing in mind the better running in reverse) and b) a good idea?

[Chris Cairns]

Doug,

As both slots have that flat edge I cannot see why that would make a difference between forward & reverse. The 16mm NGM Mamod e-book suggests that it could be a function of the cut out in the front footplate of the smoke box casting but that appears central & symmetrical to me. Is it possibly that the operating lever on the rotary part of the valve is not actually mounted central (slightly offset)?

Given that these slots are larger than the ones in the PPS reverser I think opening out the flat end will just allow more steam to enter at an earlier stage thus more of a runaway loco.

It would also be difficult to open out that slot. The reverser valve is made of 2 pieces of brass soldered together, but if you de-solder them it is very difficult to get them realigned again. I had one of my reverser's where the cut-out portion had been mounted in reverse but after de-soldering I found it very difficult to get this cut-out portion mounted the correct way and at 90 degrees to the pivot hole. Despite a lot of honing I could never get it steam tight.

Chris Cairns.

[Warwickshiresteam]

Fascinating thread. Just a simple thought on the "runs better backwards than forwards" line; has anyone tried re-routing and swapping over the steam-in and exhaust pipes? This should in theory make locos run better forwards by swapping it all around, but if not perhaps would give an indication as to what causes it

[Chris Cairns]

In theory the steam pipe could be fitted on the other side of the reverser valve if the Z bend is adjusted accordingly, however the hole for the straight exhaust pipe is on one side of the smoke box casting (not central) so a different exhaust pipe would be needed.

Something to try out if I ever tinker with a plain Mamod/MSS in the future.

Chris Cairns.

[dougrail]

SCIENCE!!!

BODIES!!!

I have two 'plain' Mamods in my works at the moment. Could experiment on one for the community?

'Doug's Patent Steam Gear' Maybe?

[Aizoon]

Surely, a less drastic approach to reversing the input would be to mount the reversing valve upside down, so that the other cutout controlled the input steam. Or is that madness?

[Chris Cairns]

As I said over on the 16mm Yahoo group if you mounted the valve upside down you would need to shorten the operating lever so it did not foul the track, particularly crossing points. When time permits I may try this on my MSS kit built loco but it is still not run in yet - I'm trying to stay away from my Mamods just now to get other projects finished but it is not working!

I'm still not convinced that it is the reverser valve that is causing the speed differences.

Does anyone who uses a cab mounted regulator with a normal Mamod SL reverser valve notice any speed differences?

Chris Cairns.

[Lner fan Sam]

When I was talking to James about britomart I was impressed by it smooth running even backwards. Perhaps it one of those things that can be delt with just running in. James also mentioned that the loco has been ran backwards more than forwards over its career.

[DolwyddelanLightRail]

. James also mentioned that the loco has been ran backwards more than forwards over its career.

Yeah, that's cause the cab end buffer fell off and I never got round to sticking it back on for a while

In all seriousness though, Brit isn't a role model loco to bounce this theory off, even though it's still the original reverser, I did notice when it used to be the regulator that it ran better in reverse. Since having a cab mounted reg I haven't actually noticed a difference apart from the rather apparent cylinder cock impression it does in reverse.

[Chris Cairns]

That's the point, we are talking about a problem that affected a standard Mamod SL with no real modifications.

I think it may be more to do with the design of the cylinders and their mounting & timing. They have a central pivot bolt and the steam/exhaust ports in the cylinder are symmetrical, yet have they allowed for the effect that the reduced volume the piston rod will cause on one side of the piston. The cylinders are also mounted at an angle and drive the front wheels.

And is the weight distribution of the loco another factor (front heavy)?

But how many people still run an unmodified Mamod SL/MSS these days?

Chris Cairns.

[Narrow Minded]

Running "better in reverse" seems to be a general observation with Mamods - now, this might be completely crackpot but, since a Mamod in basic trim is inherently front heavy could this be a simple law of physics in operation?

"The force applied to move a body depends upon its effective weight."

("Pulling" the effective weight = mg-fsinx and "Pushing" the effective weight = mg+fsinx)

[Chris Cairns]

It will be found that the loco’s weight is concentrated at the front end, where a good deal of brass is used. This can cause the rear end to lift when starting a train cab first. Preference for running backwards is a fault of the Mamod loco, which hopefully can be corrected by following the previous steps.

The only previous step I can see that would affect the reverser valve is chamfering the exhaust hole from 2.5mm to 2.8mm.

The loco can be balanced by adding lead weights under the cab floor. About 160-180g is required. If the lead is cast into two blocks 80-90g each, and a suitable shape, it should be possible to add the blocks outside the frame, and disguise them as cab steps.

Adding a meths or gas burner, a driver figure or a bunker would have a similar effect.

It is an interesting discussion which has never been satisfactorily explained over the years because most owners upgrade their Mamods, and so the reverser valve is never used to the fully open position again.

Chris Cairns.