Hello and a part-built blowfly

[Jim]

The half size Simms and Jeffery traction engine building group sounded interesting but as you say circumstances don't permit, plus it would be a big project with some seriously big and heavy components. My 3" Burrell certainly pushed the limits for me with 10" 100 tooth gears to cut along with the bevel gears for the differential.
Your build is looking great and the workshop is a sign of someone building something.
Keep up the great work your doing.

Jim

[Rex Hanman]

Lisa, that's more or less what I did in my Rob Roy except that I didn't form a pocket for the o ring. I just had the conical recess from a drill and the gland nut was tightened until it just pressed on the ring, leaving no roll space.

I understand that this is wrong, o rings are not ideal in sliding situations and when used as such there should be some roll space.

I haven't touched them since they were fitted, I've not even tightened up the gland nuts. After over 20 years they still don't show any sign of leakage even on the coldest day when you might expect to see the condensation, and the engine must have covered well over 500 miles.

Everything else is noticeably worn however but I reckon you will have no problems. It isn't exactly a major job to replace them anyway!

[mbrown]

It isn't exactly a major job to replace them anyway! [/quote]


Not so sure about that. Replacing O rings means breaking the crosshead / piston rod joint.

I have had excellent results with packed glands in the prototypical manner, using PTFE tape rolled into "string" about 1/16" dia and fitted like traditional graphite packing in rings with the joints staggered.

True, my locos don't work as hard as many, but I haven't even had to nip up the glands after several years use.

For my money, a very occasional adjustment, or even replacing the packing rings, is far less hassle than separating the crosshead and piston rod.

But each to their own approach!

Malcolm

[Roger]

Nov 7, 2017 17:47:14 GMT Lisa said:

Playing around in CAD a bit tonight. Blowfly was designed with packing for the glands, which I'd much rather replace with o-rings, so I came up with this:

It's basically as designed, but with the thread shortened on the gland nut, and a pocket put in for an o-ring. The aim being to seal the o-ring on the diameter, and give it a little space for it to 'roll' along the piston rod; not much, the pocket length is 1.5*o-ring diameter. As such, the gland nut doesn't put any pressure on the o-ring, just traps it in place.
Anyway, I think this should work.

Hi Lisa,
Yes, I think this is the right approach, it's going to work. The interference only needs to be 10-15% of the 'O' ring cross section. Anything more just makes it stiff and wears more, it doesn't seal any better.
You'll probably need a generous clearance on the gland nut, it can be difficult to get the thread and the hole concentric.

[Lisa]

Nov 7, 2017 19:15:59 GMT mbrown said:

Not so sure about that. Replacing O rings means breaking the crosshead / piston rod joint.

I have had excellent results with packed glands in the prototypical manner, using PTFE tape rolled into "string" about 1/16" dia and fitted like traditional graphite packing in rings with the joints staggered.

True, my locos don't work as hard as many, but I haven't even had to nip up the glands after several years use.

For my money, a very occasional adjustment, or even replacing the packing rings, is far less hassle than separating the crosshead and piston rod.

But each to their own approach!

Malcolm

I've seen that ptfe tape string trick before, and did consider it, but ultimately I haven't used an o-ring for a cylinder gland before, and figured I'd give it a go.

Nov 7, 2017 21:49:56 GMT Roger said:

Hi Lisa,
Yes, I think this is the right approach, it's going to work. The interference only needs to be 10-15% of the 'O' ring cross section. Anything more just makes it stiff and wears more, it doesn't seal any better.
You'll probably need a generous clearance on the gland nut, it can be difficult to get the thread and the hole concentric.

Generous clearance would likely help with getting some oil through to the o-ring too I'd imagine, which would no doubt help with longevity.

[Roger]

Nov 8, 2017 5:58:39 GMT Lisa said:

Nov 7, 2017 19:15:59 GMT mbrown said:

Nov 7, 2017 21:49:56 GMT Roger said:

Hi Lisa,
Yes, I think this is the right approach, it's going to work. The interference only needs to be 10-15% of the 'O' ring cross section. Anything more just makes it stiff and wears more, it doesn't seal any better.
You'll probably need a generous clearance on the gland nut, it can be difficult to get the thread and the hole concentric.

Generous clearance would likely help with getting some oil through to the o-ring too I'd imagine, which would no doubt help with longevity.

This is probably true. Someone mentioned to me that they had arranged for a supply of oil to the 'O' ring, but I can't remember exactly what was said. I have a feeling it was an oil hole and small reservoir on the top that fed just outside of the 'O' ring.

[Rex Hanman]

Nov 7, 2017 19:15:59 GMT mbrown said:

It isn't exactly a major job to replace them anyway!
"Not so sure about that. Replacing O rings means breaking the crosshead / piston rod joint."

Not all designs are the same but on a Rob Roy the piston rod is held to the crosshead with a taper pin. To replace an o ring simply position the piston at "back dead centre", tap out the pin, slide the piston and rod to "front dead centre", undo the gland nut, swap the o rings and re assemble. I could probably do both in twenty minutes.

I don't claim that o rings are better than graphite yarn, ptfe tape or whatever. I agree with you Malcolm, each to their own.

[terrier060]

I love 'o' rings - use them everywhere I can including as piston rings and never had any blow past! If using as piston rings the groove has to be by manufacturer's recommendations, otherwise you may loose power by binding.

[Lisa]

A bit of late night (early morning?) lathe work, and the right rear cylinder cover is ready to be turned around and have the front machined. I'll get the other one to the same state first though, as the front will be done in the 4-jaw chuck, so that I can get the piston rod hole running true.


I think ideally the chucking spigot would be on the inside of the casting, as then all of the important jobs can be done in the one setup, but this works with a bit of fiddling around.

The alignment spigot is a very nice push fit in the cylinder bore:

[Jim]

Very neat work Lisa. You should be pretty happy with the way it's all coming together for you, I certainly would be if it was me.

Cheers
Jim.

[Lisa]

Rear cylinder covers are gradually getting there. I did a bit of maths to work out what I could turn the slidebar support down to without taking too much off; then did the maths again because I stuffed it up the first time!

Just the o-ring pocket, and gland thread to make, then it's over to the mill to clean up the rest, and finish the slidebar support to size.


I've started to use the first one to finalised the process, then film the second one; so here's a video of me turning the inside face of the second cover: www.youtube.com/watch?v=iAjLXKzPJxc

[andyhigham]

How do you machine cast iron and keep your hands and fingernails so clean?

[Lisa]

My fingernails are currently painted pink, which makes them look cleaner than they are.

But they do end up in quite a mess; a good scrub afterwards with ordinary dishwashing liquid and a good stiff brush helps.

[Jim]

4.22am!!!! I thought I was an insomniac thinking about the next bit to make.

[Lisa]

It's only 3:30am, we don't mess with the clocks here.

I had a nap earlier, but I need to be up now in case dad wakes up and doesn't remember where the loo is. Will get some more sleep in an hour or so.

[Jim]

Well done Lisa, you're doing a wonderful job caring for your Dad as you do.

Jim

[Lisa]

I finished off the turning operations for the right rear cylinder cover today.
It's starting to look like a loco cylinder:

The gland thread is ½"BSB (26tpi), partially because that's what the plans say, but also because the tapping drill is a bit bigger than the bore of the o-ring pocket, so the o-ring can be slid into place without tearing it on the threads. Also I have a bottom/plug tap for that thread.
Dad was watching on while I was cutting the thread, commented "don't turn it too hard, or it'll break," and gave me a pat on the back when I assured him I was being careful. It's nice when he has these little moments of understanding what's going on; and they pretty much always happen in the workshop.

Meanwhile this turned up in the mail today, which will be handy.

[Lisa]

Nice weather the past few days (actually cool this morning!), so did a little bit in the workshop. Just finished up the turning operations on the rear cylinder covers, over to the mill with these now, once the smokebox saddle is out of the mill.



Also, there's that 26tpi thread.

[Roger]

Feb 2, 2018 21:01:41 GMT Lisa said:

Nice weather the past few days (actually cool this morning!), so did a little bit in the workshop. Just finished up the turning operations on the rear cylinder covers, over to the mill with these now, once the smokebox saddle is out of the mill.



Also, there's that 26tpi thread.

Just a thought on that thread, it's so close to a 1mm Metric thread, can't you just make it Metric instead?

[Lisa]

Feb 2, 2018 22:29:57 GMT Roger said:

Just a thought on that thread, it's so close to a 1mm Metric thread, can't you just make it Metric instead?

I've already threaded the cylinder covers, though I could open it out to M14x1.0 I suppose; but at this point I'm mostly interested in it because of the complexity, doing things the hard way for the fun of it.