Model Compressor---Maybe

[Brian Rupnow]

Time to ask a question. I have looked a great number of Scotch Yoke engines on the internet, and all of them are single acting. This doesn't mean that there are non out there, but if so I haven't found any. Ergo, there may be some very good reason for that. It may be that there is too much bending moment imposed on the small diameter piston rod when the rotating member is at the extreme bottom or top of the slot in it's orbit. If I change my design to single acting, where I don't have to have a seal around the rod at the end closest to the center, I can run a much more robust rod. I could also dispense with the inboard cylinder caps and half of the valve bodies. It would pump air at only half the rate of the original design, but then again it's only work would be blowing up a balloon. comments please.

[crueby]

Most of the scotch yoke engines are a horizontally opposed layout, so even single acting it is operating on both directions in that line. Making it double acting would double the force, but not give any help for self starting like a 90 degree offset crank would. Thats why the crossed twin version with two yokes still has single acting cylinders, it behaves like a double acting twin with offset cranks.


One good engineering question: what pressure does it take to inflate a balloon?

[Jasonb]

Brian, you obviously did not look at the engine Bob posted a photo of as that is double acting. As for making your pump single acting and doing away with 4 valves, I suggested that earlier. You could also reduce the dead space by screwing the valve bodies into the top and bottom of the cylinders thus saving the volume from the L shaped passages and no holes to plug.

Very little pressure above atmosphere as you are only having to overcome the elastic properties of the balloon material

Another double acting scotch yoke design here http://www.vapeuretmodelesavapeur.com/telechargements4/h-tandem-scotch-yoke-metrique-09-02-11.pdf

[Gas_mantle]

If I change my design to single acting, where I don't have to have a seal around the rod at the end closest to the center, I can run a much more robust rod. I could also dispense with the inboard cylinder caps and half of the valve bodies.  comments please.

When I asked about the cylinder sizes my gut reaction was to think that twin cylinder double acting would be overkill for what you need and possibly put too much load on the small engine given the pump cylinder sizes.  Admittedly I'm no expert and it is only my gut reaction - I don't know what sort of gear ratio you are intending to drive through.

If you did go for single acting and dispense with the inward cylinder covers won't there be a tendency for the pistons to want to twist in the cylinders without some other guide ?

[Gas_mantle]

I wonder if angling the yoke gives smoother running or maybe help to reduce stresses ?

<a href="https://www.youtube.com/watch?v=zk4sg7-j7lo" target="_blank">http://www.youtube.com/watch?v=zk4sg7-j7lo</a>

[Jasonb]

Angled yoke returns the pump ram quicker to the bottom ready for the slower upstroke that lifts the water, probably gives a slight mechanical advantage to the motor when under load on the up stroke too.

[Brian Rupnow]

Jason--No, I hadn't see the little double acting motor that Bob posted. Thank you for calling my attentions to it. This site only notifies me about a post in one of my threads about half the time. And yes, there is nothing preventing the pistons from turning, except that the yoke plate with the slot sets right up against the face of a large gear which keeps it from turning.

[Gas_mantle]

Angled yoke returns the pump ram quicker to the bottom ready for the slower upstroke that lifts the water, probably gives a slight mechanical advantage to the motor when under load on the up stroke too.

Hi Jason,

I don't see how an angled yoke will return the piston quicker in one direction than the other unless the yoke is angled differently either side of the centre line ?   Surely in the video I showed if we imagined it as an opposed twin pump the inward stroke speed of 1 cylinder is matched by the outward stroke speed of the other.

Both pistons take the same time to travel in both directions - I can see they may travel faster at the middle or ends of the stroke but surely whether we are talking a single or a twin the piston travel speed is the same on the inward or outward stroke isn't it ? Or am I missing something here 

I'm sort of considering making a boiler feedpump so the subject is or interest to me 

[MJM460]

The effect of that angled yoke is not quite what people think.  It is easily analysed with a little trigonometry.  I put the formulae in a spreadsheet so the computer could recalculate for each 22.5 degrees of crank rotation, and the computer also plots the graph.  I added the piston displacement for a standard 90 degree yoke.  It did not take long, but probably would have taken half the time with fusion, for those who can drive that.

My iPad would not let me attach the PDF file with the maths, but I think I have the graph.  Apple knows best what I want to attach!

I hope at least my picture will come through.  The stroke is actually INCREASED by that angled yoke.  The motion is still exactly sinusoidal, but the phase is retarded by twice the yoke slope angle.  The two sine waves are displaced by that phase change, but I printed two copies, held them against a glass window displaced for one to lie over the other and found they are clearly both sinusoidal, just different amplitudes.  Also can be overlaid mathematically, but the print of that does not illustrate the phase lag.  I for one did not expect that result.  But I would definitely stay with the standard square design.  I am not sure why the angled yoke was used on that pump, but I did not continue through to calculate the torque.  There may be a small torque advantage.  However, that increase in stroke affects the cylinder length, so it is not interchangeable when you need to limit the clearance volume.

Brian there is no reason not to make a larger diameter piston rod so it is sufficiently stiff.  Just size the gland accordingly.  Two Pistons, the rods and yoke make one rigid moving part.  Each piston stops the other from twisting, and the two resist the sideways force of friction against the yoke, but a little ball race could be used on the crank pin instead of a sliding element.

You could easily gear the action down if motor torque is insufficient, it needs only one larger great on the crank shaft to mesh with the top one on the large pulley instead of those two intermediate gears.  Single acting or double acting simply alters volumetric capacity, but does increase the torque required of the driver.

I have a balloon pump and was going to try and measure the pressure, but unfortunately no balloons.

MJM460

[MJM460]

I think I have it this time, sorry it is on its side.

MJM460

[Jasonb]

Interesting that you found an increase in stroke, I'll have a better look later but to me the stroke should be the same in both as at TDC the pin is central above the piston rod, the same a BDC so as the angle of the slot passes through that same centre  line it should have no effect on stroke.

J

[Gas_mantle]

In the angled yoke situation I'm not convinced that TDC and BDC are when the crank pin is laying along the piston rod centreline. In the video of the pump my guess is that TDC will be very slightly before the crank pin reaches the 12 'o' clock position and like wise BDC will be just before 6 'o' clock (I think the TDC / BDC will be when the crankpin is in the tangenital postion of the yoke  )

It seems to me that the angled yoke has the same effect as altering the piston rod length. I'd say that we naturally assume the stroke is twice the crank pin throw but in an angled yoke that isn't necessarily true as the effective piston rod length is made longer by the yoke action.

I can sort of see how the stroke can be longer by the same amount at either end but the motion curve remains the same sinusoidal profile.

[crueby]

Sounds like we need a side-by-side (or overlapped) animation of perpendicular and angled yokes....

[Jasonb]

Just uploading the video now

[Jasonb]

Not the best as it is a bit hard to look at camera and screen while moving the mouse around but it does show that the angled yoke gives more movement for the same throw of the cam pin.

1" throw on cam gives total 2" movement of the straight yoke and I set the two blocks at the extent of the movement. You can clearly see that the orange angled yoke has more movement as it goes beyond the blocks at each end of the stroke by 0.153" to be precise.

Not sure if there is some mechanical advantage on the signle acting vertical pump but it may be a bit like those oval chain rings on a bike that give a better mechanical advantage where it is needed in the rotation of the pedals.

[youtube1]https://www.youtube.com/watch?v=ObfIp1Ew8iE&feature=youtu.be[/youtube1]