walschaerts - full gear cut off - AJAX

[phild1]

Hi all,

quick question.  Am i right in assuming there is a relationship between the return crank throw and full gear cut off.

My loco is an Ajax and in full gear with die blocks travelling freely to the ends of the slots i am getting cut off at around 135 degrees - 70%

The loco is very happy here, never had any issues with it pulling and the chimney gives a good bark under load.  I can notch back to around 45% but no further

Would increased throw of the return crank and subsequent adjusting of the eccentric to match increase the time the ports open in full gear?

Thanks all

[phild1]

Sorry, that should be 125 degrees, close on 70% give or take

Thanks

[donashton]

Hi phild1,
I advise you to leave things alone until your knowledge is more comprehensive. Enjoy steaming your Ajax and chat with someone more advanced.

It's a large subject.
Take it easy.
Don Ashton.

[oldnorton]

Hi Phil

You have had a reply above from THE expert in valve design so you are honoured, and the rest of us will be too polite to say anything else :-)

The return crank has to be just the right length AND set in the correct place in order to move the valve (piston valve in the Ajax?) the correct distance. Cut off is when the valve ports close the steam inlet, which means that you have to know where the valve ports and head are in relation to wheel rotation. You and I might be referring to different things.

If you are interested in the subject then a good place to start is to read all of Don Ashton's writings (look up his website). I did that a short while ago; drew all the valve gear out for my Britannia in CAD; then looked at the simulation models of Bill Hall; and after much mental grunting, plus some helpful conversations with a couple of other experts, came to enough understanding to know what I could and should change on the Britannia. It took me a couple of months of odd evenings and I am really glad I had a go.

Norm.

[jma1009]

Hi Phil,

I had lots of dealings with one particular 'Ajax' namely the IWMES club loco 'Ron Groundsell' many years ago.

It is quite an old design, and when Walshaerts valve gear in miniature was 'designed' in a very rudimentary way, and to compensate for the inadequacies of Dick Simmonds' ability to work out return crank throw and how to work out the resultant travel from the full gear setting of the die block in the expansion link, the return crank was made just a bit too long so if properly set provided more throw than needed for the expansion link. (Don Young also did this a few times, as did Martin Evans).

Anyway, on the IWMES example (which we never had the drawings for) I carried out a lot of tests and measurements as the loco was not a good steamer in those days. Anyway, my own conclusions were the opposite of your own in that with the die block at top and bottom (with due clearance) and the reverser in the full forward notch, and full reverse notch, the valve was 'over running' ie way over accepted full gear cut off (it was something like 85% cut off).

For a 2 cylinder loco I aim for 79% full gear cut off. More than 80% is unnecessary, and less than 75% will cause problems in starting with a load.

I can't remember what I did to the IWMES example... I may have recut the reverser quadrant or blocked off the full gear notches so that the 2nd notch each end became the maximum for cut off and with a wider spacer added between the stand and quadrant. Or I may have raised where the reach rod engaged with the reverser lever.

Ajax has very generous diameter cylinders (dustbin size). The IWMES example had been re-bored out to a larger size a few years previously to eliminate wear on the bores.

The valve gear geometry itself isn't too bad on Ajax.

Your 125 degrees is rotational and not linear ie reciprocating. Cut off is measured linear ie when the piston is cut off to inlet steam as a proportion of it's stroke. I haven't worked out what 125 degrees would be as a linear measurement in percentage terms - I am sure others will help with this.

Cheers,

Julian

[donashton]

Hello Phil,

Some excellent advice for you there.

And thanks Norm - have reminded me that I have a website! The reason for that is a touch of radiotherapy.
Actually I ran through the machine to check out it's valve gear but it was too quick for me! Truly, I seem to have some bother, but recent work on FS suggests that sometimes the current accuracy is a little OTT.

Thanks again,
Don.

[phild1]

Thanks all for your replys. I have happily run this loco now for the last few years but bought it second hand and the build quality wasn't the best. I have been gradually fixing problems as and when time / work allows.

Only reason i asked about the return crank is because i recall seeing a post Julian contributed to where he suggested the throw of the return crank was too much. I wonder if the builder of my loco new this and therfor made it shorter?

Anyway, i havent scrutinised the timing yet again and made it work the best i can, the port openings are not eqaul but the cut offs are matched. I ran the loco for a couple of hours last night at a club event and i couldn't fault it, notched back all the way to last notch before neutral. No trouble starting a full train of 8 people.

thanks all

[donashton]

Hi Phil,
You cannot alter a single element without requiring the replacement of the other dimensions to match. Julian knows this full well - he's a master. Only one dimension of return crank is correct for a single situation. Imagine the main crank with the wrong connecting rod.

From your enthusing about your last night's duties don't touch a thing!
Best wishes,
Don.

[johnthepump]

May 14, 2019 20:05:21 GMT jma1009 said:

Hi Phil,

I had lots of dealings with one particular 'Ajax' namely the IWMES club loco 'Ron Groundsell' many years ago.

It is quite an old design, and when Walshaerts valve gear in miniature was 'designed' in a very rudimentary way, and to compensate for the inadequacies of Dick Simmonds' ability to work out return crank throw and how to work out the resultant travel from the full gear setting of the die block in the expansion link, the return crank was made just a bit too long so if properly set provided more throw than needed for the expansion link. (Don Young also did this a few times, as did Martin Evans).

Anyway, on the IWMES example (which we never had the drawings for) I carried out a lot of tests and measurements as the loco was not a good steamer in those days. Anyway, my own conclusions were the opposite of your own in that with the die block at top and bottom (with due clearance) and the reverser in the full forward notch, and full reverse notch, the valve was 'over running' ie way over accepted full gear cut off (it was something like 85% cut off).

For a 2 cylinder loco I aim for 79% full gear cut off. More than 80% is unnecessary, and less than 75% will cause problems in starting with a load.

I can't remember what I did to the IWMES example... I may have recut the reverser quadrant or blocked off the full gear notches so that the 2nd notch each end became the maximum for cut off and with a wider spacer added between the stand and quadrant. Or I may have raised where the reach rod engaged with the reverser lever.

Ajax has very generous diameter cylinders (dustbin size). The IWMES example had been re-bored out to a larger size a few years previously to eliminate wear on the bores.

The valve gear geometry itself isn't too bad on Ajax.

Your 125 degrees is rotational and not linear ie reciprocating. Cut off is measured linear ie when the piston is cut off to inlet steam as a proportion of it's stroke. I haven't worked out what 125 degrees would be as a linear measurement in percentage terms - I am sure others will help with this.

Cheers,

Julian

I would just like to add that the Ajax that Julian speaks of has been reboilered a couple of years ago. It was pulling well loaded trains the Saturday afternoon before last and performed well. I have known this engine since 1973.
Johnthepump.