Touchdown Mechanisms

[3rdlakerobert]

I think whem the touchdown plunger is depressed, the sac is first compressed, then near the end of the plunger's travel, something happens and the pressure in the barrel is released allowing the sac to expand and draw in ink.

 

My question is what exactly happens to release the pressure in the barrel?

[viclip]

Richard Binder explains the touchdown filler process in his description of the Snorkel:

 

http://www.richardspens.com/ref/anatomy/snorkel.htm

[3rdlakerobert]

Richard Binder explains the touchdown filler process in his description of the Snorkel:

 

http://www.richardspens.com/ref/anatomy/snorkel.htm

 

Thanks, yes, Richard's explanation confirms what I believed to be the case. The "dimpled groove" in the TD tube is the means by which air is released from the barrel.

 

I have a fat Touchdown pen that has the right parts--I've compared it to other fat TD pens.

 

Here's what happens: I have the barrel out of the pen, a new O ring in the barrel, a seal under the blind cap screw, inside the TD tube. I pull out the TD tube with my finger over the open end of the barrel and I get a vacuum I can definitely feel with my finger. It pops when I pull my finger off the end of the barrel. So far so good. I have good seals.

 

Then I push down the TD tube and feel the pressure against my finger as I hold it again over the open end of the barrel. I expect the pressure to release at the bottom of the the tube's travel but it doesn't. That's what I don't understand. All the other TDs and Snorkel's I've restored release at the bottom the tube's travel as that dimpled groove reaches then passes the O ring. This pen won't fill with ink because it never releases the air pressure in the barrel. How come?

[Ron Z]

It may be that the end of the groove is not being pushed past the 0-ring. The fat TD pens has a star lock washer in the end between the TD tube and the blind cap to keep the blind cap from turning. The rubber washer was inside TD tube between the screw head and the tube end. We often remove the lock washer on the outside and the seal around the screw inside, and then insert a rubber seal on the outside as they did on later pens.

 

If you've already removed the lock washer the tube will be that much shorter, i.e. farther back towards into the blind cap so not traveling as far forward as the blind cap stops against the barrel. Not always a problem, but sometimes it is. Its also possible that you have a good seal under vacuum (as you do with your test) but not under pressure. For either possibility, try stacking two rubber seals in the end of the blind cap, tighten snug, and see what happens. You'll have both a good seal, and the TD tube pushed a bit towards the nib in relation to the blind cap threads.

[3rdlakerobert]

Thanks Ron. Tried raising the TD tube to make sure the groove goes past the O ring. Still have the pressure.

[Ron Z]

Odd. I'd want to play with the pen a bit. Maybe too much silicone grease so the vent is getting blocked? There should be a small vent hole just behind the flange at the front end of the the TD tube. If this is closed or blocked, the air may not be a be able to vent out of the inside of the tube.

 

Is the TD tube a snug fit in the barrel? I have seen cases where plastic of the barrel has shrunk to the point where the flange on the TD tube not only presses against, but actually presses the barrel wall OUT as the tube is moved up and down. Normally this is a loose fit, but if it it is tight (so making a seal) and the vent hole in the front of the TD tube is blocked, the pressure won't be released at the end of the stroke - it'll be held captive in the barrel.

 

Are you certain that there IS pressure in the barrel? I've seen cases where the pressure seems to be fine when you hold a finger over the end of the barrel and push the tube down. But when the section is screwed in a small crack can open up enough to vent the air, or air gets through a crack somewhere in the section of the barrel. You should be using thread sealant on the section threads on assembly. It often makes a difference in the way that the pen fills.

[3rdlakerobert]

Odd. I'd want to play with the pen a bit. Maybe too much silicone grease so the vent is getting blocked? There should be a small vent hole just behind the flange at the front end of the the TD tube. If this is closed or blocked, the air may not be a be able to vent out of the inside of the tube.

 

Is the TD tube a snug fit in the barrel? I have seen cases where plastic of the barrel has shrunk to the point where the flange on the TD tube not only presses against, but actually presses the barrel wall OUT as the tube is moved up and down. Normally this is a loose fit, but if it it is tight (so making a seal) and the vent hole in the front of the TD tube is blocked, the pressure won't be released at the end of the stroke - it'll be held captive in the barrel.

 

Are you certain that there IS pressure in the barrel? I've seen cases where the pressure seems to be fine when you hold a finger over the end of the barrel and push the tube down. But when the section is screwed in a small crack can open up enough to vent the air, or air gets through a crack somewhere in the section of the barrel. You should be using thread sealant on the section threads on assembly. It often makes a difference in the way that the pen fills.

 

It was a snug fit because the barrel had shrunk. I see quite a few Fat TDs and TMs that have shrunk. I fixed the problem by peening a metal rod to create a "hook" like that on a woodworker's cabinet scraper. I shaved some plastic off the interior of the barrel, enough to allow the TD tube to move easily through it and made certain the open end of the TD tube was perfectly round.

 

I've recently been thinking that the area at the O ring may be so shrunk that there's a seal created there without the O ring, but I checked that by running the TD tube in and out of the barrel end with no O ring in place and there's plenty of play there--no seal.

 

I know what you mean about the vent hole possibly being blocked with silicone lube, or shavings from my scraper, but it's clear.

 

And yep, there is pressure in the barrel for sure.

 

I'm really stumped. That pen isn't worth the time I've put into it, but I want to know. In fact I've already stolen its Sentinel cap for another pen and put the nib section/sac protector in the Fat TD parts container.

[kirchh]

Thanks Ron. Tried raising the TD tube to make sure the groove goes past the O ring. Still have the pressure.

 

How did you raise the TD tube?

 

--Daniel

[3rdlakerobert]

Just the way Ron suggested.

Edited October 5, 2015 by Robert111

[Ron Z]

He raised the blind cap on the TD tube by adding an extra rubber washer. The effect being that the tube is pushed forward towards the section relative to the edge of the blind cap. Inverted, the tube is "raised" towards the section.

[kirchh]

He raised the blind cap on the TD tube by adding an extra rubber washer. The effect being that the tube is pushed forward towards the section relative to the edge of the blind cap. Inverted, the tube is "raised" towards the section.

I saw you suggest that, but his wording wasn't clear, so I requested a clarification.

 

--Daniel

[3rdlakerobert]

I saw you suggest that, but his wording wasn't clear, so I requested a clarification.

 

--Daniel

 

The writng police.

[kirchh]

 

The writng police.

 

No; I wanted to be sure that you had placed the additional seals in the correct location to produce a lowering of the Touchdown tube, because you used the term "raising."

 

--Daniel

Edited October 6, 2015 by kirchh