Author Topic: Model Compressor---Maybe  (Read 24186 times)

Offline Brian Rupnow

  • Full Member
  • *****
  • Posts: 7573
  • Barrie, Ontario Canada
Re: Model Compressor---Maybe
« Reply #75 on: December 05, 2017, 09:42:25 PM »
Each cylinder has it's own piston now. The pistons are a lapped fit into the cylinders----as much as it can be. Lapping aluminum is a bit like trying to polish a turd.--It's too soft to lap properly. The pistons are at a point where they won't fall through the cylinders, but can be pushed through with a finger. I'll find out how good the fit really is when everything is finished.

Offline Brian Rupnow

  • Full Member
  • *****
  • Posts: 7573
  • Barrie, Ontario Canada
Re: Model Compressor---Maybe
« Reply #76 on: December 06, 2017, 08:37:15 PM »
Today was yoke day. Nothing too exciting, just milling, drilling, sawing and tapping. The piece of 1/4" rod stuck through the yoke is going to become the piston rod. I still have to square up the inside corners.

Offline Brian Rupnow

  • Full Member
  • *****
  • Posts: 7573
  • Barrie, Ontario Canada
Re: Model Compressor---Maybe
« Reply #77 on: December 07, 2017, 08:35:09 PM »
The pistons, piston rods, yoke and slider are finished. The piston rod is one solid piece right now, and is Loctited and cross-pinned to the yoke. I will let it set up for 24 hours, then cut away the center of the rod. I assembled the two piece yoke with the slider in place, because I didn't want to have to disassemble anything after I cut the center out of the piston rod.

Offline Brian Rupnow

  • Full Member
  • *****
  • Posts: 7573
  • Barrie, Ontario Canada
Re: Model Compressor---Maybe
« Reply #78 on: December 09, 2017, 07:05:18 PM »
This morning I reclaimed some of the 1/2" x 1 1/4" aluminum flatbar from my auto-reversing project and used it to make up the two cylinder supports. I don't know how well it shows up in the picture, but both pieces required 4-jaw work to turn the 1" diameter x 0.100" long "spigot" that fits down into the end of the cylinder. Its funny, 4 jaw work used to terrify/frustrate me but just as everyone said "Do it enough times and it will get easier." They were right of course. Now the biggest pain in the butt is changing chucks whenever I need to do something in the 4 jaw.--And no, I'm not so accomplished at it that I will do all of my work in the 4 jaw. The square corners have been machined/filed into the yoke and the bronze slider installed, as well as cutting the piece of 1/4" shaft out of the center of the yoke. I'm not really sure about these aluminum pistons. They seem "grabby" when slid in and out of the cylinders but that may change with some "running in".

Offline Brian Rupnow

  • Full Member
  • *****
  • Posts: 7573
  • Barrie, Ontario Canada
Re: Model Compressor---Maybe
« Reply #79 on: December 10, 2017, 09:00:16 PM »
It has been a snowy miserable weekend here and good wife has a cold, so I've spent more time than usual playing in my machine shop. The compressor is taking shape and beginning to look like the 3D model. The gears do mesh, and the pistons do slide back and forth, although somewhat reluctantly. I will finish up the baseplate and the drive disc that moves the yoke back and forth tomorrow, and then a bit of 'running in' on my bench set-up to loosen things up a bit. Once I get it moving freely, I will start on the 4 valves.

Offline Brian Rupnow

  • Full Member
  • *****
  • Posts: 7573
  • Barrie, Ontario Canada
Re: Model Compressor---Maybe
« Reply #80 on: December 11, 2017, 09:21:28 PM »
Here is the "running in" video.
<a href="https://www.youtube.com/watch?v=waGnhHYoS4o" target="_blank">http://www.youtube.com/watch?v=waGnhHYoS4o</a>

Offline Roger B

  • Global Moderator
  • Full Member
  • *****
  • Posts: 6133
  • Switzerland
Re: Model Compressor---Maybe
« Reply #81 on: December 12, 2017, 08:30:15 AM »
Coming along well  :ThumbsUp:  :ThumbsUp: I do like scotch yokes  :)
Best regards

Roger

Offline Brian Rupnow

  • Full Member
  • *****
  • Posts: 7573
  • Barrie, Ontario Canada
Re: Model Compressor---Maybe
« Reply #82 on: December 12, 2017, 08:45:05 PM »
My Goodness--What a horrible day outside. We're getting lake effect snow coming in off Georgian Bay on the east side of Lake Huron and it's snowing so hard you can barely see. So far today I've ferried two grandchildren from school on the far side of Barrie to their home on the far side of Barrie. They couldn't both get the belly-ache at the same time. One in the morning and then a different one in the afternoon. I am still "dicking around" with the compressor to see if it's going to "free up" enough to be driven by an o-ring drive belt. I had it all loosened up, then took off the cylinders to install gaskets at each end and forgot to match-mark things before I reassembled it all. On re-assembly, I found it to be as stiff as it was at the beginning. I still don't have a real feel for whether I am going to o-ring the pistons or not.

Offline Brian Rupnow

  • Full Member
  • *****
  • Posts: 7573
  • Barrie, Ontario Canada
Re: Model Compressor---Maybe
« Reply #83 on: December 12, 2017, 09:42:00 PM »
Decision has been made. The aluminum pistons simply aren't going to cut it. The problem I'm running into is too many pieces bolted together to allow perfect alignment. I have to think on this now. I have the viton O-rings. I can make a new set of aluminum pistons and turn them 0.010" undersize, but I don't really want to use aluminum again. The current pistons are too short to get a grip on with my lathe chuck, and they only have a 3/16" center bore, so re-using them isn't an option. Steel pistons sliding in a steel cylinder don't sound like the greatest idea to me either. I do have plenty of cast iron "stubs" left over from other jobs that I could probably make new pistons out of. I also have a piece of type 901 blue nylon that I could use. Maybe even UHMW.  I will think on this overnight, and post whatever I do tomorrow.

Offline Gas_mantle

  • Full Member
  • ****
  • Posts: 1365
  • North Yorks - UK.
    • My Youtube channel
Re: Model Compressor---Maybe
« Reply #84 on: December 12, 2017, 09:55:57 PM »
How deep are the pistons Brian ? I still think some sort of inner cylinder heads will help with alignment.

Have you considered graphite string as piston rings,, I'd have thought they'd do for a low pressure model.

Offline Brian Rupnow

  • Full Member
  • *****
  • Posts: 7573
  • Barrie, Ontario Canada
Re: Model Compressor---Maybe
« Reply #85 on: December 13, 2017, 12:15:59 AM »
The pistons are 3/8" thick, with one side hollowed out for a 3/16" nut to hold them in place against a shoulder on the 1/4" diameter rod.
I have two things going on here. The maximum diameter of a new piston will have to be about .010" to 0.015" smaller than the cylinder bore to allow a clearance to prevent binding. However, a Viton ring (which is quite compressible) will have to have about .005" diametric interference to create a good enough seal to pump air.--Should be doable. I'm leaning towards cast iron for the pistons, with a slight radius on the leading and trailing edge to keep them from trying to "dig in" to the bore of the cylinder.

Offline Jasonb

  • Full Member
  • *****
  • Posts: 9463
  • Surrey, UK
Re: Model Compressor---Maybe
« Reply #86 on: December 13, 2017, 01:47:57 PM »
I think I would be looking for the cause of the binding rather than treating the symptoms by making undersize pistions.

Could be a number of things -

-Pistons not screwed onto rods concentrically, I always finish turn the piston when it is on the rod
-Bores not lined up in square cylinders, are ctr to edges equal, can be measured with a ball bearing and micrometer
-cylinder bolting face not square to bore should have been done at same setting
"Head plates" not parallel due to ends of inverted "T" not being perfectly at right angles to face

J

Offline Gas_mantle

  • Full Member
  • ****
  • Posts: 1365
  • North Yorks - UK.
    • My Youtube channel
Re: Model Compressor---Maybe
« Reply #87 on: December 13, 2017, 04:17:37 PM »
I know this is going to sound crazy but does it run better if you have the cylinders vertically ?  The first engine I ever made had a horizontal piston in an open fronted cylinder and I found gravity alone would cause binding. It ran far better on its end.

I wonder if a round pin instead of the dieblock acting in the yoke may reduce the tendency for 'piston twist'

Offline Brian Rupnow

  • Full Member
  • *****
  • Posts: 7573
  • Barrie, Ontario Canada
Re: Model Compressor---Maybe
« Reply #88 on: December 17, 2017, 12:09:22 AM »
All is well in compressor world. I managed to squeeze in an afternoons work and replaced the aluminum pistons with nylatron pistons of 0.990" diameter and added an o-ring to each piston.  This made an amazing difference in the torque required to turn the large pulley. Of course with O-rings and gaskets the "compression" is quite significant. I also fitted each cylinder onto a mandrel and squared up the mounting face to the bore. It wasn't out by much at all, but every little bit helps.  Soon I will make the four valves that are required.

Offline b.lindsey

  • Global Moderator
  • Full Member
  • *****
  • Posts: 13860
  • Dallas, NC, USA
    • Workbench-Miniatures
Re: Model Compressor---Maybe
« Reply #89 on: December 17, 2017, 12:53:22 AM »
Nylatron might be just the thing Brian. Good wear properties and natural lubricity from the added moly di should work well in that application as it apparently is  :)

Bill

 

SimplePortal 2.3.5 © 2008-2012, SimplePortal