Another Atkinson Differential build

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dsage

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The o-rings are green. One to seal the adapter to the hose end and one to seal the adapter into the model cylinder head.
The black o-ring does nothing in this application . It's for the original seal to a standard plug hole. Bronze because it's what I had around and the original end was brass.


1593396464427.jpeg
 

Ken Brunskill

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Well, I fabricated all the parts as Dave P. had designed them, assembled everything less the rings and all operated smoothly, then observed a fair amount of stiffness with the spanking new rings installed, so the thought expressed by strangemitsku might turn out to be true. (BTW, Fishing was OK lastnight, catching not so much😏). Will be doing some E motor run in starting Monday, with the suggestions and comments here, think I'll try to power it on its own after each hr. of E motor breaking in. Will keep posting as progressing.
OK, ran into an issue doing the E-Motor run-in, as mentioned here the stiffness has caused the pivot pins (which were a light press fit in the 'ARMS'), and being catilevered enough to cause binding, (I think caused by the stiffness) which after 10 min. of relatively low speed worked one of the the pins out of it's 'ARM'. Analyzing options & fix to this. My suggestion to anyone thinking about building to this design, make the components out of steel vs. aluminum.
I know my holes were dead straight/square and to size, feel the angular pressure in soft material (6061 Aluminum) is a major factor. The mechanism operated smoothly until I installed the rings, which were built to Dwight Giles methodology.
 

dsage

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By chance did you fit a ring into the cylinder (without the piston) to ensure you have enough ring end-gap?
And also that there is enough space behind the rings in the piston groove?

Of course if either of those is not sufficient you probably wouldn't have got the piston into the cylinder.
What are the ring dimensions? Did you use two thin rings per piston ring groove as you once said you were going to do. Or just wide one?
 

stragenmitsuko

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The pins are a press fit in the arms and they pivot in the frame ?
I thought it would be the other way around .

How about pressing / bolting / loctiteing the pins in the frame .
I would thread them , add a small shoulder and loctite them in place .
And th use an extended bronze bushing in the arms wich could also be bolted or locktited to the arm .
 

Ken Brunskill

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By chance did you fit a ring into the cylinder (without the piston) to ensure you have enough ring end-gap?
And also that there is enough space behind the rings in the piston groove?

Of course if either of those is not sufficient you probably wouldn't have got the piston into the cylinder.
What are the ring dimensions? Did you use two thin rings per piston ring groove as you once said you were going to do. Or just wide one?
Dave, made a dummy cylinder exactly the same I.D. as the cylinder (.750"), made sure I had .001"-.002" end gap, rings on the Power Piston are .050" wide (1/groove), Pump Piston rings had same end gap, but since there are no side ports, just a cylindrical surface I'd gone with (2 rings/groove) as my 1st try. Did a relatively fair measurmemt of the load required to get the rings to close, near as I could measure they were consistent, and about as expected, the .025" wide rings came in about 1/2 of their .050" wide cousins. The angular load on the ARM pins in aluminum, & related losening, has set me back a bit, before I remake the ARM'S I am going to try cross through pins - as perpindicular to the implied load as I can make them. Might just be a simple enough solution.
 

Ken Brunskill

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The pins are a press fit in the arms and they pivot in the frame ?
I thought it would be the other way around .

How about pressing / bolting / loctiteing the pins in the frame .
I would thread them , add a small shoulder and loctite them in place .
And th use an extended bronze bushing in the arms wich could also be bolted or locktited to the arm .
Like your suggestion 'strangemitsuko', the ARM's piviot on bearings, that are mated to posts in the FRAME, they are secure there being a light press fit to a shoulder and tightened to that shoulder from the far side of the frame, a good design there! The culprit pin is a 3/16" diameter pin press fit into the 1/4" thick ARM. So I think I will invest in a 7/32 -32 Tap & Die (Sure that will come in handy in the future - or that will be my explanation to my lady :)) then do as you suggest including the loctite, since this particular pin requires a 3/8" Dia. spacer I'll use that as the shoulder.
I'll make drawings of the new pins and have them available for anyone interested in building this model.
 

jquevedo

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Ken:
I made a small change in the design of same engine, made the arms a one piece element, started from a 0.500 thick Al plate and machined all critical dimensions ( holes) then machiend profile and top surface to allow for the two ARM and Small ARM pieces to be made as one piece, then made the groove/space for the piston arm to be inserted, the pin that will hold the arm to piston Arm is a 0.125 in with external retaining rings.
Engine has very good compression, no binding and seems like it should run..
Working on ignition and water and fuel tanks now.
 

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jquevedo

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Coolant entry and exit, they connect to a water tank and fill a void pocket behind the main cylinder body, thermal circulation is expected to happen, top one is hot / exit and is at a 30 degree angle
 

Ken Brunskill

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Ken:
I made a small change in the design of same engine, made the arms a one piece element, started from a 0.500 thick Al plate and machined all critical dimensions ( holes) then machiend profile and top surface to allow for the two ARM and Small ARM pieces to be made as one piece, then made the groove/space for the piston arm to be inserted, the pin that will hold the arm to piston Arm is a 0.125 in with external retaining rings.
Engine has very good compression, no binding and seems like it should run..
Working on ignition and water and fuel tanks now.
Very nice looking work, you are to be congratulated!
I have stripped the paint from mine, and am tumbling them in just plain gravel, to provide a matt finish. Then back to understanding the compression issue.
 

Ken Brunskill

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Please do , I'm always interested how others approach this .

@jquevedo
what are the two brass pipes for on the side view ?
Coolant maybe
Here is a photo of the tap, I used (3) .024 wires for making the tap, carefully making sure to maintain ZERO on the cross feed and note the reading on the compound, then made the screw thread of the mating part imediately afterward, ended up with a Class 1 fit. The chip relief was milled with a . 125" Ball End Mill deep enough to achieve a flat face or .030" for this 32 TPI thread. Then the tricky part was back to the lathe to pick up the thread (done with dykem, 10X Optivisor) and just barely remove the dykem & burrs from milling the thread relief.
 

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