Coal fired Garratt K1

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Re: Coal fired Garratt K1

Post by ge_rik » Wed Feb 23, 2022 12:32 pm

Absolutely astonishing what you've been able to achieve! As someone who struggles to get a neat fold in a piece of brass sheet (and a double fold is a wild dream), I take my hat off to you ..... :salute: (just imagine the hat)

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Re: Coal fired Garratt K1

Post by Trevor Thompson » Wed Feb 23, 2022 12:37 pm

ge_rik wrote: Wed Feb 23, 2022 12:32 pm Absolutely astonishing what you've been able to achieve! As someone who struggles to get a neat fold in a piece of brass sheet (and a double fold is a wild dream), I take my hat off to you ..... :salute: (just imagine the hat)

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Thanks Rick

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Re: Coal fired Garratt K1

Post by Trevor Thompson » Wed Feb 23, 2022 1:06 pm

Im starting to work on the valve gear. To get it to work correctly I have been studying the book!

So I need to work out how long the return crank should be - and that involves some calculation. I have checked them a few times and corrected some mistakes. So to explain what I'm up to:
IMG_2131.JPG
IMG_2131.JPG (1.18 MiB) Viewed 4832 times
This sketch shows the valve gear and the valve and its port face. If you look carefully at the valve itself you will see that the ports which let the steam into the cylinders are connected to the "wrong" end of the cylinders. This is part of the conversion from piston valves to slide valves. It is called "cross porting" - and I did that so that the valve gear could be a true copy of the real thing. This high pressure unit had "inside admission" - which you cant do with slide valves. The steam would push the valve away from the port face.

So to start the calculations I need to measure the actual valve and ports:
IMG_2132.JPG
IMG_2132.JPG (1.28 MiB) Viewed 4832 times
The important information here is that the valve is 9.85 diameter, and the outside edges of the steam ports are 9.05mm apart. "Lap" is half the difference between those dimensions. 1/2(9.85-9.05) = 0.4mm. Also note that the valve chamber is only 3.36mm wide - so that is the maximum that the valve can move without hitting the ends.

If I take the lap and add it to the width of the steam port - that is the distance the valve has to travel each way from the centre to fully expose each steam port. That is 1.5 + 0.4 = 1.9mm. That would involve the valve moving a total of 1.9 x 2 mm = 3.8mm - too much it hits the ends. It is normal ( according to the book ) to not fully uncover each steam port - so I will go for it opening it 80%. So 80/100 of 3.8 = 3mm.

To be continued shortly.

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Re: Coal fired Garratt K1

Post by Trevor Thompson » Wed Feb 23, 2022 2:48 pm

The next step is to work out how the valve gear is going to deliver this.

Let me take you back to the picture with the parts named. This type of valve mechanism uses the return crank to provide motion 90 degrees out of phase with the movement of the piston, and the combining lever to provide motion in phase with the motion of the piston. The first part means that the valve is fully open when the piston is half way between the ends of the cylinder , and the second part is opening the valve when it is at the end of its travel.

The diagram has the calculations:
IMG_2133.JPG
IMG_2133.JPG (1.08 MiB) Viewed 4829 times
The triangle relates the movement of the valve to the effects of these two motions. The full travel I want for the valve is on the hypotenuse of the triangle 1.5mm - and we are working in the movement of the valve from central to its extent here - half the total. The top side is the amount of movement to come from the combining lever. That is the "lap" of 0.4mm. I will come back to this combining lever and its effect later. I need to concentrate on the return crank first.

X is half the travel to be given by the return crank. 1.445mm.

The lower half of the diagram takes into account that the valve rod travel is less than the travel of the return crank rod. So the measurements are of the actual return crank. So to move the valve rod 1.445mm the return crank rod needs to move 3.356mm.

The next diagram:
IMG_2134.JPG
IMG_2134.JPG (1.4 MiB) Viewed 4829 times
This tries to show the main crank rod in the circle. The bigger circle represents the crank pin as it goes round and round. The inner circle it the motion of the return crank pin as it goes round and round. On the left is the expansion link. the dotted line between the centre of the axle (and therefor the centre of the cranks) and the pin on the expansion link defines the position on the small circle which are important. The two points where this dotted line crosses the small circle (marked A) are the points where the expansion link will be central - the neutral position - The dotted nine at 90 degrees to this first dotted line defines the points where the valve will be fully open in each direction - marked B and C.

I have already calculated that the crank needs to move the valve a total of 2 x 3.356mm - and that is the diameter of the small circle 6.712mm. Note that since the line joining the pivot point on the expansion link and the crank centre is not horizontal, the line joining the points marked A is not vertical. That effects the length of the return crank.

Having drawn that out to scale in sketchup I can read off that the distance between the holes in the return crank needs to be 13.136mm.

So I can now make the new return cranks.

I will come back to calculate the length of the return crank rod, and where the pivot point in the bottom of the expansion link need to be after the return cranks are fitted.

Trevor
Last edited by Trevor Thompson on Fri Feb 25, 2022 3:07 pm, edited 2 times in total.

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Re: Coal fired Garratt K1

Post by SimonWood » Thu Feb 24, 2022 8:46 am

This is exciting. I'm looking forward to seeing this running.

The valve gear detail is going over my head right now, but I'm bookmarking this to return to in the future once my own understanding has developed!

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Re: Coal fired Garratt K1

Post by Trevor Thompson » Fri Feb 25, 2022 3:21 pm

The return cranks were made using both the lathe and the milling machine with a rotating table. Firstly a blank disc was partly parted from a length of brass rod. Then enough of the rod was parted off to enable the blank to be held in the chuck of the rotary table:
IMG_2138.JPG
IMG_2138.JPG (1.64 MiB) Viewed 4676 times
That enabled me to place the two holes in it at an exact distance apart, one 2.9mm (to ream to 3mm) and one 1.8mm (to tap 8BA). Then the sides to the disc were milled off, and the crank end "prettified", by turning the table through 5 degrees and thinning the sides. Then turning the table chuck through 90 degrees so that it was on its side I was able to drill the holes for the securing bolts , 1.8mm again for 8BA, and the top half opened out to 2.2mm:
IMG_2141.JPG
IMG_2141.JPG (2.06 MiB) Viewed 4676 times
Removing it from the chuck, I partly sawed through between the pinch bolts, tapped all the holes, and mounted it onto a mandrel.

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Re: Coal fired Garratt K1

Post by Trevor Thompson » Fri Feb 25, 2022 3:24 pm

Facing the back of the crank to clear the connecting rod:
IMG_2143.JPG
IMG_2143.JPG (1.96 MiB) Viewed 4675 times
The final crank in approximately the correct position:
IMG_2140.JPG
IMG_2140.JPG (1.89 MiB) Viewed 4675 times
Now for more calculations and drawing to work out how long the return crank rods should be.

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Re: Coal fired Garratt K1

Post by Trevor Thompson » Sat Feb 26, 2022 10:00 pm

I think I have values calculated to enable me to make the rest of the valve gear.

They were established by drawing the valve gear to scale in sketchup and lots of trial end error:
Screenshot 2022-02-26 at 21.37.56.png
Screenshot 2022-02-26 at 21.37.56.png (70.39 KiB) Viewed 4635 times
The circle on the bottom right hand side represents the main wheel crank, and the circle is the path it forms as it rotates. The inner circle with a line is the circle that the return crank makes as it rotates. Its diameter is the total movement required to make the valve move the required amount. The line joining them defines the distance between the holes in the return crank.

The circle on the top left is a representation of the expansion link. The circle is the path that the point where the return crank rod attaches to the expansion link makes as it rotates. The line across those circles shows the centre line of the expansion link.

There are 4 large circles between the two circles I have already described. Two of these are centred on the Point I called point A in an earlier picture - these circles coincide at the same point on the expansion link circle. They indicated that at those two points the valve gear will indeed be in neutral - with the valve fully covering the ports. The other two large circles show how far the expansion link will rotate when the valve is fully open at each end of the return crank motion.

The key requirement for the valve to work the same way in forward gear and in reverse gear is that the angles through which the expansion link rotates through are the same on each side. You can see that I have added a dashed line where each of these circles intersects the locus of the expansion link pin. For the valve opening to be the same in each direction these angles need to be the same.

I suppose I should emphasise that as the backset is changed then the crank rod length also changes. Hence the need for a trial and error approach. It can be done by calculation - but I bottled out at the prospect of that - after all it was difficult enough to visualise what I was doing this way.

In this case they are not quite right - so I need to increase the "backset" and try again.

When they are equal I can read off the backset and the length of the return crank rod.

Then I can adjust these components properly.

Trevor
Last edited by Trevor Thompson on Sat Feb 26, 2022 11:09 pm, edited 2 times in total.

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Re: Coal fired Garratt K1

Post by Trevor Thompson » Sat Feb 26, 2022 10:50 pm

I have modified the drawing in the last post, adding something called "back set". That means moving the point where the return crank rod attaches to the expansion link 1 mm back towards the rear axle. You can see this as a dashed line which has an arrow pointing to it:
Screenshot 2022-02-26 at 22.08.23.png
Screenshot 2022-02-26 at 22.08.23.png (60.9 KiB) Viewed 4627 times
Repeating all of the constructions on the last diagram, the angles that the expansion link is moving through are now closer to being the same. Not quite the same but closer. I will modify it again and try 1.5mm of backset.

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Re: Coal fired Garratt K1

Post by Trevor Thompson » Sat Feb 26, 2022 10:54 pm

So I kept on trying different amounts of backset, and found that I actually needed "Frontset" to get closer to equal openings on each side of the central position. The final result indicates that the correct dimensions are:

Backset minus 1.4 mm (as in "frontset"?)
return crank rod length 34.12mm
return crank 13.21mm

So I will try making the components to these dimensions and see if they work!

Trevor

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Re: Coal fired Garratt K1

Post by Old Man Aaron » Wed Mar 02, 2022 3:45 am

I think I'm just barely keeping up with you. There's clearly a lot needing attention to make it run properly, and it's great to see someone putting in the hard graft to get it perfect. :salute:

This reminds me, I really ought to buy a set of reamers.. :roll:
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Re: Coal fired Garratt K1

Post by Trevor Thompson » Mon Apr 04, 2022 10:42 pm

I have completed rebuilding the rear unit, and tested it on air. It now seems to work faultlessly both forwards and backwards.

The calculations worked out OK - after I had corrected some silly arithmetic errors. I will correct the errors in the previous posts shortly.

The back end is now reassembled to the body, and the radio control is re-connected. I hope to be able to steam test the water pump and rear unit before the weekend - and I hope to be able to add a video of that.

Then on to the front unit and the compounding valve.

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Re: Coal fired Garratt K1

Post by Trevor Thompson » Sun May 29, 2022 2:14 pm

Believe it or not I have been working on this since I last posted.

That steam test didn't work out - not because of faults on the steam system, but I just couldn't get the coal to burn.
So I tried it in the multifuel burner in the house and it wouldn't burn there either. Not even in a roaring log fire.
Trying to run K1 on bits of smashed up BBQ fuel just didn't get steam to be generated fast enough - at least that is what I think was happening.

So I am in the process of making a gas fired burner for it. Not that I have given up on coal firing - but I need to get small amounts of proper coal and charcoal for this purpose. So duel fuel seems a good idea - if only so that I can separate out the issues.

So a gas tank - as big as I can get into the space previously occupied by a dummy air receiver:
IMG_2202.JPG
IMG_2202.JPG (1.8 MiB) Viewed 4058 times
All silver soldered and tested. Just needs its valve making.

In its final position:
IMG_2203.JPG
IMG_2203.JPG (1.88 MiB) Viewed 4058 times
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Re: Coal fired Garratt K1

Post by Trevor Thompson » Sun May 29, 2022 2:22 pm

I have made a ceramic gas burner to fit into the space where the coal grate and ash pan fit. Held in place by the same two pins which hold the grate and ash pan in place.

A bit speculative in that the intermediate pipe which takes the steam between the two bogies might foul it - but it's worth a try. Its just a copper tray silver soldered up, with 4 pillars sticking up into it - on which the ceramic insert will sit:
IMG_2199.JPG
IMG_2199.JPG (1.85 MiB) Viewed 4058 times
and in position in the fire box:
IMG_2200.JPG
IMG_2200.JPG (1.7 MiB) Viewed 4058 times
I have spotted the hole in the soldering and have corrected it.

I have also completely rebuilt the front unit in the same way that I did for the rear unit. That now runs smoothly and freely on 10psi even with a valve chest leak. So a leak to solve as well.

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Re: Coal fired Garratt K1

Post by Trevor Thompson » Tue Oct 04, 2022 3:27 pm

Since the weather has deteriorated somewhat lately I have started to try to get K1 running properly. There are so many variables and so many bits I can't get at that I have stripped the locomotive down and I am going to test and reassemble as I go. So I am starting with a bare boiler on its frames, supported on wooden blocks:
IMG_2511.JPG
IMG_2511.JPG (1.92 MiB) Viewed 3312 times
I have abandoned the gas conversion for now - I managed to buy suitable coal in very small lumps. Having taken the cab off, and dismantled the radio control I worked out quite quickly that the regulator and other valves taking steam from the manifold were not fully closed as I had set them up with the radio control:
IMG_2514.JPG
IMG_2514.JPG (1.52 MiB) Viewed 3312 times
Now the fire lit easily, and I was able to maintain 50 psi or so for an hour at which point I was concerned about water level and let the fire go out.

So to get the pump running off the loco. I had made the manual filler point part of the feed to the steam pump, thinking it was simpler that way but I don't think it worked properly like that - and with the pump removed I couldn't fill the boiler manually. The arrangement has been modified so that both pumps feed into the clack valve. The manual filler point is a bit "in my face" but I can improve it if it works:
IMG_2512.JPG
IMG_2512.JPG (1.55 MiB) Viewed 3312 times
with everything dismantled I have temporarily mounted the rear tank (which contains the electronics as well as a water tank) near by and reconnected the water level indicator:
IMG_2513.JPG
IMG_2513.JPG (1.86 MiB) Viewed 3312 times
I'd played with drain valves on the pump - but that didn't seem to help so I took them off and blanked the holes off. Lighting the fire again using charcoal soaked in meths 20psi came very quickly. At 40psi I started the blower, and opened the steam valve to the pump. That bright light from the water level indicator does mean I can't fail to see when it needs more water.
Having pushed the pump piston rod back and for a few times it started up on its own. At this stage the pump isn't connected to anything - I'm just trying to get it to run on steam.

The next stage is to prove it runs mounted on the loco and actually pumps water.



So there is some video showing it running on steam. The first part was the first of it running - very quickly - and lots of steam leaking. After closing the valve I just tightened the bolts holding the end caps and the lid on the valve chamber - which got rid of 90 percent of the steam leaks. The second part of the video shows the result running much more slowly , and with little steam escaping (there is still some so some new gaskets might be in order eventually):

https://www.dropbox.com/s/a2jostpwid45m ... 1.mp4?dl=0

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Re: Coal fired Garratt K1

Post by Trevor Thompson » Tue Oct 04, 2022 3:44 pm

I have sorted out a few leaks - whistle valve and manual filler - and I have remounted the pump onto the chassis, and connected it to the clack valve as well as a temporary water supply. As things are set up I can connect compressed air to a fitting mounted instead of the safety valve. I have proved that the pump is actually pumping water by removing the manual filler and letting the pump run on air while pumping water out of the filler hole.

So lighting the fire again this time with some coal as well as charcoal, I had it running for another hour - and again let the fire go out. Another bit of video with it pumping water into the boiler:

https://www.dropbox.com/s/9n0hn9nye9cqv ... 2.MOV?dl=0

So I'm now setting up the rear bogie to get that to run.

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Re: Coal fired Garratt K1

Post by Trevor Thompson » Sun Oct 23, 2022 3:04 pm

I am still working on this - I haven't just given up!

So running the rear unit on steam.

Each time I light the fire in this I get more used to the technique of getting the fire to light and stay in. Im also managing to control the water level in the boiler - manually most of the time. But more of that later.

In the following videos you can hear the exhaust of the weir pump - in fact you cant miss it!

A short section of video with the rear unit running in ahead gear on about 50psi, and the throttle half open:

https://www.dropbox.com/s/eje9dgmsugylf ... 9.MOV?dl=0

There is still some steam coming from somewhere but it is running, and continues to run with the throttle only slightly open, once it has warmed up. Those are 15mm diameter pistons by the way.

I transferred the steam supply to the front bogie, and repeated the exercise:

https://www.dropbox.com/s/xhz649f6li5xk ... 0.MOV?dl=0

You can see that it is running quite erratically - which is exactly what the rear unit did at this stage. That is in reverse - and this next bit of video shows it in ahead:

https://www.dropbox.com/s/p09vzud82lriu ... 1.MOV?dl=0

This time you can see definite hesitation and a plume of steam in time with the hesitation. So that is piston stem seals in need of repacking (They are already tight). But it is running and consistently down to 18psi. (These are 20mm diameter pistons so it should run on low pressure).

So I will correct the stem seals and try the front unit again tomorrow.

Back to running the boiler. I have been using the fan to provide the draft, rather than the blower. It is just easier to be able to concentrate on the fault finding rather than running the boiler. So for example I can use the water bottle to pump water into the boiler even though it makes the pressure drop. I can keep the fire glowing red with the fan.

The water pump is running nicely, and I have had it going constantly as I have been testing - it only stops when the boiler pressure drops below 20psi. It is pumping water - I can see the level in the dish feeding it dropping. However it isn't keeping up with the water being converted to steam, so every often I have to stop playing and pump water by hand (and then wait for the pressure to rise). So that is a pretty fundamental piece of information.

A redesign required there I think. I wonder how much bigger I can make the pump piston? I have LBSC's drawing for his weir pump, so Im going to look at the relationships between steam piston diameter and water pump piston diameter.

Still I am making progress.

Trevor

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Re: Coal fired Garratt K1

Post by ge_rik » Sun Oct 23, 2022 4:40 pm

I can't profess to fully understanding - I could hear the sound on the video but not sure I detected any movement. It did sound as if your fix worked, though.

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Re: Coal fired Garratt K1

Post by Trevor Thompson » Sun Oct 23, 2022 7:54 pm

ge_rik wrote: Sun Oct 23, 2022 4:40 pm I can't profess to fully understanding - I could hear the sound on the video but not sure I detected any movement. It did sound as if your fix worked, though.

Rik
I suppose I was trying to express the idea that the boiler was creating enough steam for the pump to run at the same time as it was supplying 15mm diameter cylinders running fast. It had to be running in the videos even if you couldn't see it because you could hear its distinctive exhaust sound. You can see and hear it in this clip:

https://www.dropbox.com/s/6qi02j42s23xd ... 4.MOV?dl=0


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Re: Coal fired Garratt K1

Post by ge_rik » Mon Oct 24, 2022 9:05 am

Trevor Thompson wrote: Sun Oct 23, 2022 7:54 pm
ge_rik wrote: Sun Oct 23, 2022 4:40 pm I can't profess to fully understanding - I could hear the sound on the video but not sure I detected any movement. It did sound as if your fix worked, though.

Rik
I suppose I was trying to express the idea that the boiler was creating enough steam for the pump to run at the same time as it was supplying 15mm diameter cylinders running fast. It had to be running in the videos even if you couldn't see it because you could hear its distinctive exhaust sound. You can see and hear it in this clip:

https://www.dropbox.com/s/6qi02j42s23xd ... 4.MOV?dl=0


Trevor
:thumbright:

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