Port timing for piston valve, help needed

[bogstandard]

Can anyone help.
I know the position of the ports I require, what I don't know is how to calculate the positions on the valve rod of the grooves to give me the correct timing, and how much lift on the eccentrics is required.
Can anyone explain in very basic laymans terms how to calculate what I require.

John

[sandy_ACS]

 

Hi PD's,

John asks..

I know the position of the ports I require, what I don't know is how to calculate the positions on the valve rod of the grooves to give me the correct timing, and how much lift on the eccentrics is required.
Can anyone explain in very basic laymans terms how to calculate what I require.

The finite answers to your question depend upon several factors such as: -

Type of reversing gear to be employed.

Inside or outside steam admission.

whether Steam lap/lead and or/exhaust lap/lead are being employed.

However, assuming, and correct me if I am wrong, that you are envisaging a basic set-up, with no lap or lead, using a single eccentric per valve, and are thinking of port reversal as the means of reversing then the attached PDF drawing should explain all that is necessary.

This is showing a basic piston valve cylinder using outside steam admission, for rotation in a clockwise direction and employs no lap or lead for either steam or exhaust.

By reversing the steam and exhaust connections the engine will run in the opposite direction without any change to the valve settings.

From a steam used point of view this will be the same as an oscillating engine, in that the valve will remain open for the whole length of the stroke.

More complex valve gear, with dual eccentrics and with Valve lap and lead allow this to be altered, giving variable cutoff...but I don't think you want to go there just yet.

The piston valve drawn is for a hollow valve with a steam gland at the valve rod end of the valve chamber.
If you intend using an open end valve chamber (bottom end) then make the bottom end of the valve the same OD as the valve cylinder and cut a groove in line with the steam transfer hole shown (same width as hole dia, or thereabouts) but keeping the same width on the port face section....the hole into the inner bore should then be drilled through at the bottom of the groove.

Naturally, if you go this route then the valve chamber must be long enough  to ensure this groove remains inside the bore when the valve is in the extreme lower position...i.e with the piston at half stroke in the direction shown....otherwise you will get rapid steam loss....   :cry:  

The lower end of the valve is shown with a thread for attaching the eccentric rod....this will be necessary since you will need to adjust the position of the valve(to that shown) with the piston at TOP DEAD CENTRE and the eccentric at exactly 90 degrees ahead of the crankpin.
 A threaded fork with a lock nut would be excellent for this.


Hope this helps....if not ...just ask.

Best Regards.

Sandy. :luck  :computer  

[bogstandard]

Hi Sandy,
You've done it again and come thru with flying colours, I now know why you are such a highly respected member of PD's.
I am using a slightly different method of inlet/exhaust porting but the diagram explains everything I need to know.
I will be making the block out of cast iron with bronze pistons, just got to get the sash weights hacked down to size. As usual I will be doing a fag packet drawing, shame I haven't got a drawing board and I don't know how to use these latest computerized gimmicks otherwise I could share it with all.
Many thanks again, another project on the road.

John

[sandy_ACS]

 

Hi Again,

No problem John, glad to have helped, after all that is the whole purpose of PD's.

Keep us informed of your project/progress, I am sure lots of the guy's will be very interested.

As for fancy drawings.....nothing wrong with the good old fag packet/scrap paper...I use them all the time in the workshop...much quicker than CAD...I only use that for posh final drawings or for showing off...     ...in fact quite a few of my production drawings are still hand drawn sketches with dimensions...just never got round to putting them on CAD yet.

Best regards.

Sandy.  :computer  

[bogstandard]

Hi Sandy,
Glad you work sometimes the way I do. Many years ago at work I used to do all my own drawings but the job took twice as long, so at home now I make and modify as I go, and if anyone wants to make one they can strip it down and draw it themselves.
Do you think that other PD's would be interested in the build, starting from scratch, showing all the pimples as well? Maybe showing a few people that mechanical things can be built from scratch using a few basic raw materials.

John

[Eddy Matthews]

Do you think that other PD's would be interested in the build, starting from scratch, showing all the pimples as well? Maybe showing a few people that mechanical things can be built from scratch using a few basic raw materials.

John

I'd certainly be interested John - I may not have much to say on the topic, as I'm not into engineering, but I've always had a slight interest and often wondered if I could ever get into it...

Maybe a detailed "warts an all" type article may just persuade me (and others perhaps) that it's worth investing in a lathe etc and having a go?

My only comment would be that it would need to be an article that kept things very basic - I have no experience of machining, so technical jargon, or glossing over simple items (simple, to you and Sandy perhaps) would leave me baffled

If you do decide to do it, I'm sure I wouldn't be the only person that would take an interest - I'd suggest you started a new thread to cover it though, rather than adding it to this....

[bogstandard]

Hi Eddy,
Glad you like the idea, but do bear in mind that to do every little machining exercise would need a novel, and I have machinery that would be very expensive to purchase just to make one model. But very good results can be obtained from a basic lathe, a vertical slide and a cheap drill press and of course some basic hand tools. It will have to be like this eg. "this piece was turned and drilled in the lathe at the same time to keep it concentric", keeping it basic but informative as well, if people need to know more, just ask the question, if fact I am just going through the build of my previous engine with a chap thru email including some pics, if he gets stuck he sends me an email and I try to talk (and can I talk) him thru it.
I will try to get on to it today.

John

[derekwarner_decoy]

Hi PD's.... Metric or Imperial :?: does it matter..,,, the physical distance may be the same but the mind dosn't know the answer :nose

I recently attended a number of meetings :gather  and a critical question was "what tolerancing should be used on a 240 diameter pin to suit a 240 bore spherical cylinder rod end bearing?"

All of the younger graduate engineers couldn't grasp the concept & suggested referring to a catalogue which provided answers of an interference fit.. so with age & cunningness on my side   [& as non graduate] I suggested

240 diameter is approx 10" diameter... so @ 1/2 a thou per " = 0.005" clearance on the pin... & I didn't even need a fag packet or a calculator or a  :computer

What I am really trying to say is I now that 0.005" = about 2 1/4 thicknesses of standard new print ....but if someone said 0.127 of a mm I wouldn't have a clew what they are talking about  :roll:  :shock: -   but good for a laugh  :wink:

[bogstandard]

Hi Derek,
I know exactly what you are on about, but the problem is we are being forced into using a system (metric),  the older generation is slightly unhappy about this due to we were brought up on the standard imperial system. But who is to say which one is the more difficult, I for one use both, but I use imperial to three decimals (sometimes four if I want to show off) and metric to only two. This gets me personally the accuracy I require.
It is accepted nowadays that metric is slightly more accurate for general use, and children now no longer think in imperial.
We Brits have just fought back and won against the might of the European Union. It is now legal to sell in pounds and ounces again (as well as kilos).

John

[derekwarner_decoy]

OK PD's .... just before my mate....our Sir Sandy Campbell  :respect2 of the West Highland   responds again..... I thought I would submit my humble 2 bobs   worth on thread forms

BA = British Asociation.... however the inventor decided to base the formula in mm & not inches or part there of  :rant

UN - when the Poms went to Yank land... they decided on 60 degree thread form & left the old 55 degree thread form back home

Japanese metric - 2.5 mm , 3.5 mm, 5.6 mm  :idea:

French metric - 1.0 mm, 2.235 mm, 7.37 mm

Russian metric - 1+1 =2, 2+3=5.76  :idea:  :roll:  :    

OZ = just don't let your nutZ get screwed up on anything not the right size  :nah

[malcolmbeak]

Just a bit more about the BA thread system. Whilst thumbing through my old 1962 version of Kemps Engineering Handbook, I came accross a Swiss thread system that looked rather familiar. In fact all the thread Numbers, diameters and pitches are the same as the BA series. The only thing different is the 60 degree thread form compared with the 47.5 degree for BA.
Although the sizes appear rather arbitary, on closer inspection there is more to it. The root area of the 2BA is half that of 0BA and twice that of 4BA and so on for the whole range. So from a load bearing point of view, it is far more sophisticated than going with fractional inch or whole millimeter sizes.
Ah well - I just thought it might be of interest to one or two folk

Malcolm

[bogstandard]

What are you worried about threads for, in the small sizes that we work with anything can be made to fit.
All you need are two big adjustable spanners, one of them imperial and the other one metric and a big hammer. Job done. Aussie style.

[Eddy Matthews]

All you need are two big adjustable spanners, one of them imperial and the other one metric and a big hammer. Job done. Aussie style.

Actually I think you'll find you need four adjustable spanners - You forgot about those pesky bolts with left hand threads!

[sandy_ACS]

 

Hi PD's,

John Say's....

"this piece was turned and drilled in the lathe at the [size=18]same time [/size]to keep it concentric",

    ...so thats where I am going wrong....And please meester, what does CONCENTRIC MEAN???....  

Just so you all know.....yer can't do this when the part is called an ECCENTRIC.....for these you either need a special set of offset point drills...or you have to get pretty cute with the lathe cross slide to keep up with the wobbly bit....I use the former method...plays havock with yer cylinders though, if you pick up the wrong drill...

Derek...

240 diameter is approx 10" diameter... so @ 1/2 a thou per " = 0.005" clearance on the pin

Not to bad for an OZZIE.....you are obviously one of those people who, like me sometimes, makes full use of the NUFF scale...as in NEAR - E-  NUFF or CLOSE- E - NUFF.

The actual range would be 0.0042" - 0.0048" depending upon first selection or second selection of fit being chosen.....WELL DONE....


Talking about Imperial V Metric......As John has already said, we, in the UK, are to a large degree being forced to go more and more METRIC....in fact it is getting much more difficult to obtain IMPERIAL sizes in a lot of materials now, which is not necessarily good news if you are just coming in to the model engineering hobby...the trouble is a lot of the designs and drawings currently available were drawn up long before METRICATION and call for things like 1/8" or 1/2" material.....The nearest generally available equivalents now tend to be 3mm and 12mm respectively.

In a lot of cases this does not make a lot of difference, however, when the size of one part determines the size of mating parts, such as e.g the  side frames on a paddle engine and the spacers (stretchers) between them to hold them at the correct outside spacing....then a correction must be added to the length of the spacers(stretchers)...in this case you would need to add 0.138" to the length.

Now this is fine, if, like John and myself, you are used to working with both systems...we just calculate and add/subract as required......the poor youngster, who has never been taught IMPERIAL...is faced with a much more difficult task....even more so in that he/she would probably have difficulty understanding the dimensions on the drawings in the first place, let alone converting them.

On the other side of the coin....what if the particular design has to be kept exactly to the sizes as drawn.....say 1/2" dia...this could probably mean purchasing 14mm or 15mm dia material, and having to turn it down to 1/2"....

ENOUGH ALREADY..... :music  :nah

OK John...I agree with Eddy, and the other guys....I am sure many PD's would find such an article interesting....I for one enjoy seeing how other people go about things....there is always something to learn.

Eddy...for those pesky metric left handed bolts...you use the imperial shifting spanner in the other hand and vice versa for imperial ones.....  


Malcolm....the more you consider the BA range, the more you can understand why model engineers, at laest the older ones, are so reluctant to discard them in favour of the more modern Metric threads...they are just so much stronger size for size.


Ok guys' thats all for now.

Best regards.

Sandy.

PS...John....part 1 looks good  :no1 , much better working drawings than mine....WANT A JOB?....keep up the good work.

BTW....looking at your proposed layout...side view, with number 1 cylinder on the left.......when you come to fit the eccentrics...remember that if number one cylinder is rotating clockwise, when looked at from the number 1 end, then number 2 cylinder will be going anti-clockwise when looking from number 2 end....hence...number one eccentric crown should be 90 degrees to the RIGHT of the crankpin at number 1 top dead centre but number 2 will be 90 degrees to the LEFT of the crankpin at number 2 Top dead centre.....but you have probably sussed this already.

     :vacat  

[sandy_ACS]

 

in this case you would need to add 0.138" to the length

OK you spotted the deliberate mistook...this should of course read 0.0138"


JUST TESTING.

Silly old SCOTTISH  *******

Bye.

Sandy.