2 replies to this topic

[fountainbel]

Hi all,
Drawback of the existing Sheaffer piston head design is the fact the piston seal creates a high back-up pressure when pulling the rod out.
This is the reason why Sheaffer warned not pulling the rod out with a filled ink container, and in fact the major reason of failure of the rod seal.
I've seen many Vac-fill's on which the original rod seals were "flipped over" towards the filling knob -and leaked - due to this back-up pressure
The piston seal - which has to work as a check-valve- is not flexible enough to do so and pressure build up when pulling the rod out can generate leakage over the rod over time
My “shifting O ring “ design - which I’ve successfully used on my vac-fill pens and on my recent repair of the Rotring 1928 – completely overcomes this potential problem.
The O ring has a slight radial pretension in the existing barrel bore and is mounted on a conical seat
The conical seat groove is twice as wide as the O ring, and on its smaller diameter the cone is 0.5mm smaller in diameter as the O ring internal diameter/
On its largest diameter the cone is 0.3mm bigger as the internal O ring diameter
When pulling the rod out the Oring shifts to the smaller cone diameter till contact with the front flange of the piston .
Having a flat part on the front flange the ink passes underneath the O ring and there is no restriction at all for the returning ink.
When pushing the rod in, the Oring shifts to the larger diameter of the cone providing a perfect vacuum seal.
Filling rate is between 80 and 100% of the potential filling volume
After further tests on the need for an offset tail on the feed, the tail is definitively not needed using my new piston head.
A normal cupped piston seal features a radial pretension of 0.3mm in the barrel bore.
However the enlarged vaccum relaese chamber on Sheaffer vac-fills is only 0.7mm larger as the barrel bore.
This results in the fact that the piston seal only has a radial gap of 0.7-0.3 = 0.4mm when arriving in the vaccum release chamber ( being a ring opening of 0.2 mm all around )
This is logically not enough to provide a good inkflow towards the feed, so Sheaffer neccesarely opted for the the offset tail.
The shifting O ring n my new piston head only has 0.1 mm radial pretension in the barrel, resulting in a radial gap of 0.6mm ( being a ring opening of 0.3mm all around)
However there is also a complementary radial "check valve" gap of 0.6 mm between the inner bore of the Oring and the piston head. ( being a ring opening of 0.3mm all arround)
Further on the Oring does not remain concentric with its piston head seat when arriving in the vaccum release chamber, its moving to one side, creating a 'one side" radial gap of 0.6mm both on its inner and outer diameter.
As I've experiecced adding these two ink feed ringsurfaces there is no inkflow problem at all.
Your comments and critique are most welcome.

Francis




http://i62.photobuck...ernative002.jpg
http://i62.photobuck...ernative003.jpg
http://i62.photobuck...ernative004.jpg ( pulling rod out)
http://i62.photobuck...ernative005.jpg (pushing rod in)

[Teej47]

I think this is a brilliant solution Francis (as usual), and feel it's a shame that Sheaffer wasn't able to do this in the first place. A quick search tells me that the American patent for the O ring was filed in 1937 (about 40 years after it was first patented in Europe, interestingly)... so even if they had come up with this they probably couldn't have used it without risk of copyright infringement.

Anyway, this seems like such a logical pairing to the current standard packing unit replacement process of washer and O ring. You do plan on making this availble to those of us who are not retired engineers and machinists, I hope?

Tim

[BrianMcQueen]

Francis,

This is an amazing replacement piece! I think if you added another flat spot on the opposite side of the front flange, you would allow even more ink to flow through when pulling the rod out. It would also allow more ink to flow through when the pen is filled and you are writing with it. However, I might be concerned at that point with the strength of the parts that remain on the front flange. I would hope that they wouldn't break off as you draw the piston backwards. Is there much resistance exerted on that front flange when drawing the piston back?