Quartus

[PaulR]

It is looking really great and more like a robot every day; looking forward to see this one powered up!

[PaulR]

I'm afraid to say this isn't going at all well. I don't know whether it's a problem with the design or my workmanship but it won't complete a single cycle without locking up. The piston moves freely when the rods are disconnected and it pops up vigorously when the the valve is rotated to allow admission. The valve and wheels spin freely when the rods aren't attached. Maybe it's fractional differences in the distance between the holes in the connecting rods causing the piston to rotate... I've spent the last two workshop sessions fiddling around with it and my patience with it is wearing thin! I'll try again tomorrow but if that doesn't work I'm going to shelve it.

Thinking about it, I might just try connecting the piston crossbar and the crank pins with some simple wire 'hooks' in case the alignment of the connecting rod holes isn't right and that's causing the problem.

[redhouseluv]

Oh dear, I have no advice to give apart from walk away from it tonight and look upon it with fresh eyes tomorrow. I have had to teach myself to do this in order to not make rash decisions which have made things worse.

I hope you get it sorted 

[Jasonb]

Shooting to one end of the cylinder and not wanting to move from there is usually a timing issue. I think just having holes in the shaft means it is not open to air or exhaust for long enough. Or you may need to set some lead by not having the crank pins lined up with the shaft holes

Let me know the diameter of the shaft and the diameters of the air holes and I'll see if I can model it and get an idea of what is happening.

[Jasonb]

basic timing should be as follows

With the holes in the shaft vertical and running 12 o'clock to 6 o'clock the crank pins should be at either 3 or 6 o'clock. This will have the passages fully open at mid stroke and closed at the end.

Now for duration. A typical slide valve engine will have the exhaust open for about 180deg of crank rotation and the inlet a bit less say 150deg. With just small drilled holes and subject to their size in relation to the diameter of the shaft you may only be getting 60deg of opening for both. Probably not such an issue for the inlet but for the exhaust it will create resistance as the piston is moving down but can't expel the air which is now trapped in the cylinder until the port opens and then again will be trying to compress what air is trapped at the end of the stroke.

let me know those sizes and it should be possible to improve things without scrapping any parts, just a simple alteration. Looks like 6mm shaft & piston dia and 1.5mm passages.

[PaulR]

basic timing should be as follows

With the holes in the shaft vertical and running 12 o'clock to 6 o'clock the crank pins should be at either 3 or 6 o'clock. This will have the passages fully open at mid stroke and closed at the end.

Now for duration. A typical slide valve engine will have the exhaust open for about 180deg of crank rotation and the inlet a bit less say 150deg. With just small drilled holes and subject to their size in relation to the diameter of the shaft you may only be getting 60deg of opening for both. Probably not such an issue for the inlet but for the exhaust it will create resistance as the piston is moving down but can't expel the air which is now trapped in the cylinder until the port opens and then again will be trying to compress what air is trapped at the end of the stroke.

let me know those sizes and it should be possible to improve things without scrapping any parts, just a simple alteration. Looks like 6mm shaft & piston dia and 1.5mm passages.

Thanks Jason. As you thought, the shaft is 6mm but I made the passages 2mm. I seem to recall someone telling me years ago that getting the exhaust out is usually the bigger problem - I have a feeling that opening out the ends of the holes (countersink style) might help a tad. Turning the thing over by hand still results in binding now and again; I think that may be due to press-fitting the bottom cap on the piston which might have squeezed it out a bit. I'm working on that and it seems to be getting better. I should have some time later today to have a good look at it.

[PaulR]

Oh dear, I have no advice to give apart from walk away from it tonight and look upon it with fresh eyes tomorrow. I have had to teach myself to do this in order to not make rash decisions which have made things worse.

I hope you get it sorted 

Cheers, yep that's exactly what I did  I'll revisit it later

[Jasonb]

A countersink risks getting a bit close to the otjher hole in the valve block.

Image 1 shows how far round the crank has moved before the valve starts top open

Image 2 shows how far it still has to go before it closes

Image 3 shows a flat added, this can be done with a square needle file or just the edge of a larger file, just make sure you don't drift towards the middle but Ok if the flat is wider and extends towards the end. I have shown the depth of the flat as 1mm which can easily be measured from the opposite side with digi callipers as 5mm.

Image 4 shows how much earlier the valve starts to open

Image 5 how much closer to TDC the crank is before the valve closes.

Do the same to the inlet and set the crank as I have shown at 3 or 6 o'clock when the valve holes are vertical, worth marking that with a line on the end of the shaft.

[PaulR]

A countersink risks getting a bit close to the otjher hole in the valve block.

Image 1 shows how far round the crank has moved before the valve starts top open

Image 2 shows how far it still has to go before it closes

Image 3 shows a flat added, this can be done with a square needle file or just the edge of a larger file, just make sure you don't drift towards the middle but Ok if the flat is wider and extends towards the end. I have shown the depth of the flat as 1mm which can easily be measured from the opposite side with digi callipers as 5mm.

Image 4 shows how much earlier the valve starts to open

Image 5 how much closer to TDC the crank is before the valve closes.

Do the same to the inlet and set the crank as I have shown at 3 or 6 o'clock when the valve holes are vertical, worth marking that with a line on the end of the shaft.

Thanks for this, I'll add the flats on the valve and let you know what difference it makes. I think I made an engine with valves like that years ago ie with flats and passages in the valve - maybe one of Elmer's?

It feels a bit better mechanically now I've filed some flats on the connecting rods where they touch the disks - I think I made a miscalculation in the plans which was pushing the rods outwards. I think the disks could have been bigger and shaped like flywheels to get more momentum to get it 'over the hump'.

[PaulR]

Spent a couple more hours trying to improve things but still getting nowhere 

Even without air, for some reason I can't fathom, the piston tends to rotate which makes one of the connecting rods tend to twist or get pushed outwards at on end, which makes everything so tight I can't even turn it by hand. The problem is it's not consistent, sometimes it's one side then it's the other. I've tried it without the guide in place but that makes no difference, I've double checked that the holes in the rods are the same distance apart and operated the piston by hand which seems good enough. Maybe a small difference in the setting of the disks is causing it but I've tried many times to adjust that but to no effect.

All I can think of doing is maybe fastening guides to the outside of the cylinder to stop the piston twisting but that might just introduce more problems and certainly more friction which probably won't help given the lack of decent sized flywheels.

I'm admitting defeat with this one for now, at least 'til I can summon up another reserve of patience!

[uuu]

Could you amend the design so that the cross bar can twist in the piston, but is fixed to the connecting rods?   That way the connecting rods would have to remain parallel to one another.

Wilf

[PaulR]

Could you amend the design so that the cross bar can twist in the piston, but is fixed to the connecting rods?   That way the connecting rods would have to remain parallel to one another.

It's an idea but I can't think of an easy way to implement it off the top of my head. If I was certain of the cause of the twisting motion I might be able to fix it (probably with guides either side as mentioned) but I'm not 

[crueby]

I've had the same issue in the past - turned out to be that either the crank pins were not square to the main shaft (tilted just a bit) or they were not in line with each other, one disc was rotated slightly from the other, causing it to walk side to side. Another possibility is that the to side rods have the holes slightly different distance apart end to end, or one of the holes is not in line with the one at the other end of the same rod. Takes very little to throw it all off. Locomotives are very sensitive to this issue, if the side rods between wheels are not the exact same length.

[PaulR]

I've had the same issue in the past - turned out to be that either the crank pins were not square to the main shaft (tilted just a bit) or they were not in line with each other, one disc was rotated slightly from the other, causing it to walk side to side. Another possibility is that the to side rods have the holes slightly different distance apart end to end, or one of the holes is not in line with the one at the other end of the same rod. Takes very little to throw it all off. Locomotives are very sensitive to this issue, if the side rods between wheels are not the exact same length.

Thanks Chris, I'm sure the problem is in there somewhere.

Anyway, I took off one rod completely, removed the custom made hex headed screw from the other side and replaced it with a stock screw which is a looser fit. I removed the two brass washers between the disks and the valve block, stuck in the the bench vice at the dirty end of the workshop, hooked up the air and in two minutes it was running like fury... it will even self-start if in the right position!!

Might keep it as a one-legged Quartus or gradually reintroduce parts to see where the problem is. Maybe a bit more slack in the holes in the rods might help as well.

<a href="https://www.youtube.com/watch?v=xntJ1MOnJY0" target="_blank">http://www.youtube.com/watch?v=xntJ1MOnJY0</a>