V4 Oscillator Motor

[Ken I]

ACHiPo - you can use cast iron for the cylinders (like I did for the single cylinder) - I have no idea why I went Ally-Bronze - its pricy and difficult to machine.

If you go cast iron then make the piston brass.

Regards, Ken

 

[ACHiPo]

Thanks Ken!

 

[Ken I]

Update cylinder drawing with relieved face mentioned in the single cylinder video.

Regards, Ken

 

[ACHiPo]

ACHiPo - you can use cast iron for the cylinders (like I did for the single cylinder) - I have no idea why I went Ally-Bronze - its pricy and difficult to machine.

If you go cast iron then make the piston brass.

Regards, Ken

Ken,
Would Delrin or HDPE work for the piston? Seems like the friction would be low and there shouldn't be that much heat?
Thanks,
Evan

 

[Ken I]

Hmmmmm..... Delrin or HDPE would work for the piston, I often use it for piston rings in air driven models.

But.....

Its screwed onto the piston rod with locktite - that won't hold the plastic.

You could in that case you could use a nylock nut in a counterbored hole in rear of the piston.

I made my pistons cast iron (running in bronze) on the V4 and brass (running in cast iron) on the single.

I made them a few thou oversize, loctited them onto the piston rod and then clocked the rod dead true in a 4-jaw and finished the piston OD to suit the cylinder.

I don't see that process working well with plastic.

Note: The 2 small (undimensioned) "V" grooves in the piston are "labyrinth" seals - whilst they don't actually seal, they create turbulent flow in any by-pass - thus reducing it.

Regards, Ken

 

[Steamchick]

I like labyrinth seals. Discussed with a "gas" professor from Newcastle uni, back in mid-80s, when analysing a design of valve on a compressed-air motor for a HV circuit breaker. His theory - and computations confirmed that 4 to 6 square grooves was incredibly effective as a low friction seal. Not a total seal, but for a dynamic application 99% of pressure is retained as long as piston to cylinder clearance is minimal (standard sliding clearances). The seal relies on the bit of air that leaks to make the seal, as that makes the back pressure at the sharp corner between the piston OD and th open groove. On models a hacksaw blade can be ground to make a 1mm wide "parting tool" and be used to make labyrinth seal grooves 1mm deep with 1mm land between them. You can make the tool round ended, as on fine highly stressed pistons this will reduce stress concentrations at the bottom corners of the grooves. The sharp outer corner of each groove is the key to effectiveness, so no radius or relief of those corners please! Length of land between grooves needs to be at least 10 times Radial clearance between piston and cylinder. But with 0.002" radial clearance, the length of land at 1mm = 0.040" is good! Smaller clearances allow more smaller grooves that are closer. But don't chamfer the outer corners!

Enjoy!
K2

 

[Ed T]

If the engine will be run on air, the plastic may work out fine. If it's going to be steam powered, you may run into some issues with thermal expansion of the piston being greater than the expansion of the cylinder and things may get tight.

 

[Steamchick]

Hi Evan. Ed is right, but there are also other considerations. Glass filled nylon (typically 66% filled with glass) may seem soft, smooth and slippery to the touch, but can be very abrasive when used in any bearing application - eg. a sliding piston. The glass is dust sized filler powder and gives excellent mechanical stability to the nylon. Non-glass-filed nylon is less stable for size and shape. Nylon is also hygroscopic (??) - I think that's the word - I.E. It absorbs water and expands when I does so. So if immersed in water (eg under the waterline of a boat) or steam (model piston) it will expand and seize.
I have no experience of Delrin. Just read Hackaday on the subject - it may work?
Do let us know...
K2

 

[ACHiPo]

Hmmmmm..... Delrin or HDPE would work for the piston, I often use it for piston rings in air driven models.

But.....

Its screwed onto the piston rod with locktite - that won't hold the plastic.

You could in that case you could use a nylock nut in a counterbored hole in rear of the piston.

I made my pistons cast iron (running in bronze) on the V4 and brass (running in cast iron) on the single.

I made them a few thou oversize, loctited them onto the piston rod and then clocked the rod dead true in a 4-jaw and finished the piston OD to suit the cylinder.

I don't see that process working well with plastic.
View attachment 121290
Note: The 2 small (undimensioned) "V" grooves in the piston are "labyrinth" seals - whilst they don't actually seal, they create turbulent flow in any by-pass - thus reducing it.

Regards, Ken

Ken,
You answered my question very well. Thanks!

I'm just a newbie--this will be my first engine after a few stalled attempts at other designs. I can stick to CI and bronze.
Evan

 

[Ken I]

Evan, don't get stalled just keep asking - I'd like to see this to completion.

Regards, Ken

 

[Steamchick]

Hi Evan,
We all have loads of stuff we have learned in our careers - and outside work - and are willing to help. As I get older, I feel I must pass on "the knowledge" before I get to old to remember it all - so write "the technical stuff" - for anyone who really wants to understand the Engineering. And there are "better men" to correct me when I am wrong as well! So stay in touch through this site - and show us you models as you make them. So - Welcome to "the club"!
K2 (Short for Ken2 - to avoid confusion with Ken I from Cape town. He is VERY helpful! - one of the "better men"!)
K2

 

[ACHiPo]

Ken and K2,
Thanks. I have a couple weeks off coming up over the holidays. Hope to make progress on this since we won't be going anywhere.

I'll move my further comments/questions to Ken's post on the single wobbler as I've decided to tackle that one first.

Best,
Evan

 

[Ken I]

That's a good idea.

Regards, Ken

 

[holmes_ca]

I haven't been following this thread so I am reaching in from outside the box I am in the process of building Fizgig a hot air engine and for the piston, I am using Graphite would that material work for you? just a thought,

Edmund...........Alberta

 

[ACHiPo]

I haven't been following this thread so I am reaching in from outside the box I am in the process of building Fizgig a hot air engine and for the piston, I am using Graphite would that material work for you? just a thought,

Edmund...........Alberta

Graphite might work. Not sure of its properties when it comes to being threaded or Loctite? Sure would have low friction!

 

[holmes_ca]

Here are a few pictures of my Graphite piston, it might give some clarity in design, of course, if there is the possibility of oil getting on the piston then it will fail completely,

the white plug is Delrin and a close fit in the bore and the flat head is self-explanatory,

Edmund..........Alberta

 

[ACHiPo]

Here are a few pictures of my Graphite piston, it might give some clarity in design, of course, if there is the possibility of oil getting on the piston then it will fail completely,

the white plug is Delrin and a close fit in the bore and the flat head is self-explanatory,

Edmund..........Alberta


View attachment 121321View attachment 121322View attachment 121323

How is the rod connected?

 

[Steamchick]

Amazing! A new material for me. I was amazed recently to see it used as the material for foundry crucibles. - I thought it would burn.... but maybe too little free oxygen for it to burn? Please explain more about this interesting material - Where do you buy it? Any data properties? Are there different grades? - I assume so having seen artists charcoal in different grades. Why does it fail with oil? I assume it is basically graphite powder simply compressed to form a "cold sinter"? Must look this up on Wiki or wherever.. Well done with your application (and machining!). some nice pictures explain a lot.
Thanks,
K2

 

[Steamchick]

Stupid me... I now remember (slow brain - with age?) that as a lad the place I worked part-time (cleaning machines and machining & workshop training) maintained other peoples' Broom Wade Compressors. These were "oil free" reciprocating compressors, 10" bore and about 20" stroke with 3-piece (stepped) piston rings made from carbon. We also serviced dentists' compressors at the small end - and rotary blowers that had carbon "blades" mounted in the rotor, rotating in an eccentric space. (Not much pressure, just 5 or 6 bar, but lots of air volume). Anyway, the Broom Wade piston rings were the same basic application as your piston. - So I should have thought of that earlier!
K2

 

[holmes_ca]

ACHiPo, a few pics of the disassembled piston, I believe contact with any type of oil will turn it into a messy sticky black goo,