Walschaerts Valve Gear Help Needed

[Rob Pulham]

I wasn't sure where to post this but as I hope others will learn from my experiences this is perhaps the right place.

I finally got around to assembling the other side of the A3 valve gear tonight and I am having a few problems. the main one being that the reversing crank on one side wont stay tight and after travelling half way around then reverses itself.



This side works as it should rotating around the crank pin.



This side starts off tight and in the same position as the one on the other side and when going in reverse it seems to behave as it should. However when going forward it goes so far then, reverses itself and comes loose.

In order to get the clearances for the rods on the two from axles I have had to do away with the crank pin nuts and reverse the threaded crank pin bushes tightening them onto brass washers up against the wheel boss. I have though of soldering the reversing crank to the reversed crank pin bush but wanted to solve the problem before taking such a step - I don't want the crank pin bush to unscrew as well.

It is also worth mentioning that while going forward in spite of the reversing crank coming loose and only doing half the circuit it does run quite smoothly. In reverse it also runs quite smoothly for about 8 revolutions before it all tightens up with no visible reason why. All this is done with the motor loose on the axle and turned by hand.

All suggestions gratefully received before it turns into an unmanned aeroplane

 

[Scale7JB]

You might just have a little tight spot somewhere that will loosen up with some running in..

I would try a little loctite thread lock before anything drastic..

JB.

 

[Dikitriki]

Have you made sure that the bearings are wider than the rods; ie when all is fully tightened, the rods still have some sideplay?

 

[Rob Pulham]

You might just have a little tight spot somewhere that will loosen up with some running in..

I would try a little loctite thread lock before anything drastic..

JB.

Thanks Jonathan, I have already tried that one

Have you made sure that the bearings are wider than the rods; ie when all is fully tightened, the rods still have some sideplay?

Thanks Richard, that's one I haven't checked and looking at it while I type you may have hit the nail on the head.

Thank again to both of you for the input - I will let you know how I get on.

 

[Simon Dunkley]

Have you made sure that the bearings are wider than the rods; ie when all is fully tightened, the rods still have some sideplay?

Thanks Richard, that's one I haven't checked and looking at it while I type you may have hit the nail on the head.

Can I just add that this is really important for all of the rods, coupling, connecting, and/or valve gear related.

Don't ask me how I know this.

 

[Rob Pulham]

Thanks once again go to Richard his diagnosis was right on the button. When I examined the offending crank bush it turned out that despite not having anything taken off it it was still too short to fit through all the rods with clearance.

To remedy the situation I got another spare crank pin bush which was also tapped 10ba. I then filed this down to just above the thickness of the main connecting rods. I then screwed this onto the crank pin (I also filed square flats on to each bush to allow me to use pin vices as sockets to tighten them up). I then added the main connecting rods and a brass washer opened up to go over the bush. Then I added the connecting rod that is attached to the piston (apologies the correct names for these escapes me at the moment) and fitted the original bush back on collar outwards screwing this up to meet the first one and ensuring that the washer had passed over it. I then added the return crank and the return crank rod and applied some loctite to each nut/bush.

Then a short interlude of lunch and fixing some fence rails that had come loose over the winter and I gave it a test.

The result is a complete success, here are a couple of videos.

Forward

Reverse

 

[Scale7JB]

But the reversing rod isn't at the top of the expansion link when it's in reverse... How does that work?

Running very nicely...!

JB.

 

[Rob Pulham]

But the reversing rod isn't at the top of the expansion link when it's in reverse... How does that work?

Sorry mate but you may as well be speaking (typing) in Serbo-Croat for all the sense it makes to me

 

[iploffy]

Looks good mate is it me or is the mechanism running in and it's still a little tight, by the way the PC starting in video 1 up is a good sign

 

[Rob Pulham]

Hi Ian, yes it was a bit tight in the video, all it had run before the video was about 30 seconds to confirm that all was turning over smoothly under power after al the testing being carried out by hand.

Last night I had it running an hour in each direction so it should be much looser now.