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machines which I have made, am making, or intend to make, and some other stuff. If you find this site interesting, please leave a comment.

Tag: steam engine timing

Timing the Trevithick Dredger engine-4

Success!!

After the debacle of the valve sleeve rotating because I had not secured it, I secured it with a grub screw, and had another attempt at running the engine today.

But before that I shortened the valve lever blocks by 2-3mm.  I had noticed that air was entering the cylinder well before top and bottom dead centre.  Shortening the bars delayed the entry a bit.   Still a little bit before TDC and BDC, but watch the videos to see the result.

 

So, it is still not finished, and the timing will need some fine tuning.  But it works!   Phew..

It works in forward and reverse, depending on piston position when it starts, or if the flywheel is given a initial swing.

In the above videos the engine is not bolted to the box, and some of the noise is the cranks hitting the box.  The valve arm being hit by the striker is also contributing to the noise.

I will work on the gas burner next.

 

Timing the Trevithick dredger engine-3. How embarrassing….

As planned I removed the engine from the boiler, to discover why it would not work with compressed air yesterday.

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After yesterday’s failed attempt with compressed air, I removed the cylinder and piston and valve assembly from the boiler to check the steam passages, throttle and rotary valve. 

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These components were removed so I could see what was going on.

I wish that I had checked this before going into print yesterday.

I discovered that the bronze cylindrical insert with the steam ports had never been fixed/Loctited into position.  Clearly it had rotated with the first engine movements yesterday, causing all steam passages to be blocked.  No wonder I couldn’t get the engine to do anything!

And worst was that I had mentally made a note to secure the insert, when I inserted it, a month or so ago.

So today I drilled and tapped a hole for a stainless steel grub screw to hold the insert into position.  I decided not to Loctite it at this time, but will do so when I am sure that the timing is correct.  I might need to alter the shape of the ports, and the insert would need to be removed to do that.

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Tapping the thread for the grub screw using the shop built tapping head which I made to Mogen Kilde’s design a couple of years ago.  I have never broken a tap while using this tool.

After reassembling the components I ran another test with compressed air.

Somewhat to my surprise, at 20psi the engine ran for a few revolutions, before  settling down into a back and forth motion.  It still needs to be timed, but hey, it moves.  But by this time it was becoming hot in the workshop, so I left the timing for another day.  Maybe tomorrow pm.   Not Thursday because it will be HOT.  42ºc  (107.6ºf)

 

Timing the Trevithick Dredger Engine_ 2 making the fittings

Today I made the fittings which would enable timing the dredger engine…

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I needed access to the boiler for compressed air, and a temporary installation of a pressure gauge.  I decided to add a valve, so that I could shut off or adjust the compressed air.

I had made a safety valve, with a lead weight, but I had no idea of what pressure the valve would release, hence the gauge.  A spring had been specified in the plans, but I did not know whether it was actually required, so initially I left it out.

So, with the above setup completed, I turned on the compressor, and waited to see when the safety valve would vent.

It vented at about 20psi, a bit low.   So I added a spring.   That pushed the venting pressure up to 30psi.   At 30psi, the engine was still not turning over.

I was expecting that the engine would move at 30psi, but that the timing would need adjusting.   After an initial movement back and forth…. nothing.

The engine is still a bit tight, so maybe more pressure required, so I installed a heavier spring.

Nothing.

So I installed a heavier spring.  Pressure rose to 50psi before venting.  Still no movement in the engine.   Something is wrong.

Possibilities…

  1. The throttle is not working properly.  Maybe the plans are back to front, or I have made the throttle back to front.  I have made both of those errors in previous builds.
  2. The steam valve is back to front.  Yes, I have done that before too.
  3. There is a blocked steam-air passage.  Yes, I have managed that one before too.

Next steps…   re-examine the plans.    Do a tear down of the throttle and steam valves.   If no obvious cause for the failure of movement…. call for expert help.   Yep.  I have done that before too.

Timing the Trevithick Dredger Engine – 1. theory.

The plan is this…

  1. Finish constructing and installing all of the components, including gaskets, seals.
  2. Install a pressure gauge, in a temporary position.  Trevithick did not have pressure gauges available, but the current model boiler regulations insist on one.  So my plan is to have one installed eventually on the base, out of the viewing public sight, but where the operator (me) can see it.  For the purposes of the timing, which requires some pressure in the boiler, the gauge location will be on top of the boiler, but moved to its final position later.
  3. Install a fitting to admit compressed air to the boiler.  And a valve to adjust the flow and turn the air on and off.

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The lever with the hole controls the entry and exhaust of steam into each end of the cylinder. (The other handle is the throttle.)

The timing is adjusted by removing material from the blocks above and below the lever.  The blocks were made, deliberately, a few millimeters too long.

I will put some compressed air into the boiler, at about 20psi, and slowly move the lever until I hear air entering the cylinder at each end.  The lever position at which  air enters each end of the cylinder will be marked and measured.  Then the blocks will be machined or filed to the correct length.  I will assume that if the entry point is correct, then the exhaust will automatically be correct.

That’s the theory.

I have started making fittings for the pressure gauge and air couplings, and hope to get to the timing in the next day or so.   I will also be checking the pressure at which the safety valve lifts.  I will be aiming for 55-60psi.

How to time a Model Triple Expansion Steam Engine

The daunting aspect of timing the triple delayed the completion of mine by at least 6 months.  In the event, it was not difficult.

If timing a steam engine is not a particular concern of yours, I suggest that you turn off now.   Otherwise this will be particularly boring.  This post is in response to a request by a reader.

The engine needs to be pretty much completed and assembled.   Everything fitting.  Crankshaft rotating.  Valve rods tightened.  Stephenson’s reversing mechanism assembled and working. Cylinder drains installed.

Next I suggest that you make or buy a 360 degree protractor, and attach it to the crankshaft at the high pressure end.  Like this.

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Note that top dead centre (TDC) of each piston is marked (H,I,L), there is a pointer, big marks at 120 degree intervals, and identifiable marks at 10 and 5 degree intervals.  I added a rotation arrow later, because it is easy to mistake clockwise and anticlockwise directions when making adjustments.

Next, decide where in the cycle you want steam to be admitted.  On expert advice from a marine engineer who is also a model engineer, I decided to admit steam at 10 degrees after TDC. (thanks Rudi!).  I also decided to cut off admission of steam at about 70% of the  power stroke. (pretty standard).

The easiest valve to time is the low pressure valve.  It is on the end of the engine.  It is the biggest, and there is not much engine stuff getting physically in the way.  Despite that, I decided to start with the high pressure valve.  It also is on the outside end of the engine.  The reason is that I wanted to follow the passage of the steam flow, in order to understand what was happening.  Each cylinder is timed separately, independently.  So the order is, high pressure, intermediate pressure, low pressure.  Forward direction first, then reverse, for high, then F & R for IP, then F&R for LP.

The timing is adjusted by 1. changing the distance between the crankshaft and the valve, usually by adjusting the length of the valve rod and 2. by changing the position of the eccentric on the crankshaft.

Firstly, the valve must move equally over the steam inlet slots. (the top and bottom ports). The point at which the inlet slot starts to open is noted on the protractor for both steam inlet ports.  The number of degrees before or after TDC is noted for the top port, and the procedure is repeated for the bottom port.   For the bottom port Bottom Dead Centre (BDC) is the reference point on the protractor.  The angle should be identical for TDC and BDC.  If it not identical the length of the valve rod needs to be adjusted.  On my machine, that was done by adjusting the nuts holding the valve rod to the valve bracket, but it could be the valve rod to the eccentric strap.

Determining the point at which the steam inlet port starts to open is easy.  Remove the valve chest cover, bolt the valve chest to the cylinder block, and rotate the crankshaft by hand until the port is obviously visually open.  Cut a sliver of paper 5-10mm wide, (I used copy paper), measure the thickness of the paper (0.1mm in my case), insert the paper into the open port, rotate the crankshaft to close the port until the paper is jammed, then while applying tension to the paper, slowly rotate the crankshaft to open the port, until the paper just starts to move.  At that point the port will be open by the thickness of the paper.

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The valve cover is off, a sliver of paper is pushed a centimeter or so into the port, the crankshaft is rotated to jam the paper then the crankshaft is rotated in the direction that is being adjusted until the paper is just released.   At that point the port is open by the thickness of the paper.  I calculated that 0.1mm was equivalent approximately to 3 degrees of crankshaft rotation.  So whatever was displayed on the protractor, I subtracted 3 degrees to get to the exact point of port opening.

when the valve moves exactly equally up and down over the steam entry ports, the point of opening is noted on the protractor relative to TDC of BDC, depending on which is being measured.

The eccentric grubscrew needs to be loosened, and  rotated on the crankshaft to bring the point of port opening to 10 degrees past TDC.  Then the grubscew is tightened.  BDC will automatically be correct if the centering process has been done accurately.

I had bored a hole in the eccentric strap to allow access to the grubscrew from underneath the engine.  That meant that the crankshaft had to be in a certain position to allow access to the grubscrew, not necessarily TDC or BDC or whatever.  That does not matter.  What matters is that the eccentric is rotated a certain number of degrees on the crankshaft.  I did this by using the Allen key to loosen the grubscrew, then using the Allen key to hold the eccentric fast, while rotating the crankshaft.  Then tighten the grubscrew, being careful to not move the eccentric.   The measurements need to be rechecked of course.   With practice, it is not difficult, and can be accomplished first go in most cases.

If this all sounds complicated and difficult, it really is not.  But I did need to make a record of every step and measurement and direction.

For the intermediate cylinder, the HP cylinder block needs to be removed.  The HP valve chest can be retained, just swung out of the way, retaining the previous settings..  You have to be careful, but this method does save a heap of bother.

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One thing I would suggest.  When the opening points of both IP inlet ports are determined and set, I suggest that before the HP cylinder block is reassembled, that the IP valve rod is measured above the IP valve chest.  And that the measurements are recorded and placed in a secure vault.  Those measurements can be used for any future adjustments of the IP valve, without the time consuming and very fiddly necessity of removing the HP cylinder block.

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I have determined the opening points for the intermediate cylinder using the paper method.  With the depth micometer on the valve rod above the valve chest, I am measuring and recording those positions, for possible future use.

And I have a confession.  The next photo shows the HP upper cylinder drain, and the same view at top dead centre.  As you can see, at TDC the piston blocks the drain.)!*!)  Read on.

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There is another method for determining the opening point of the valves.

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Plastic tubes are pushed on suitable points for the cylinder to be set, in this case the HP. The valve is pushed against the valve face, in this case with rubber bands.  One blows into the appropriate tube while rotating the crankshaft.  When the port starts to open, you can hear your exhaled breath coming through (if your hearing is OK, which mine is not).  The protractor reading is recorded, and used as before.   Note:  the drain cock passages MUST NOT be occluded by the piston at TDC or BDC.  In my case, this proved to be a problem, hence the use of the strip of paper method.

So, I hope that this is of some use.  If my description is jaberwocky, please send a message and I will try to help.    John.

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