Gland packing Graphite or PTFE?

[vyv_cox]

Another factor is that you need to be aware that you can have electrolytic corrosion using graphite packing with stainless steel shafts. Crevice corrosion is a potential problem with any packing, due to the exclusion of oxygen, even with a nickel aluminium bronze (NAB) shaft, as in my case. There are some relatively exotic packings available that are worth considering, many combining PTFE and graphite!

Close to scare-mongering, I fear. Galvanic corrosion is theoretically possible between graphite and stainless steels but the reality is that it is almost unknown, particularly where the graphite is such a minor part of the packing mix.

I have been researching crevice corrosion in stainless steels for some years and have never come across it in a packing, although there are photos of it under cutless bearings on my website. Considering how many hundreds of thousands of boats use packed glands without suffering this problem it cannot be considered a genuine risk.

So far as nickel aluminium bronze is concerned, the copper Development Association says this:
Any crevice corrosion of aluminium bronzes, however, takes the form of minor selective phase dealloying ....... which results in little reduction of strength and practically no impairment of surface finish. Aluminium bronzes are, therefore, very widely used for pump shafts and for valve spindles - situations where pitting corrosion in crevices makes stainless steels, for example, unsuitable.

 

[ksutton]

I thought I knew a little about gland-packing materials and, on reading this thread became increasingly confused (as the posters appear to be).
As with many things in the life we look for sound-bite answers, which are as likely to farther confuse than to elucidate. Hence, I suspect, many revert to the ultimate low-tech answer - greasy string.
I will confess that I have no answer for the OP - his question offers inadequate information and appears to be based upon a simplistic perception of a fairly hi-tech branch of industry;-

http://www.sealrite.co.uk/gland_packing.htm?gclid=CM3swPOTj70CFSUUwwod8GoArA

Greasy string or "Greasy Hemp" as its know admittedly more in southern hemisphere countries than northern is far from being a low tech answer. It has been used for decades, new materials principally developed because of the commercial competition in industrial use, they don't have to be necessarily correct for our boats. (If it wasn't broke, why did it need fixing) comes to mind.
As you can see from the posts here, many of us have tried PTFE packing, a large percentage of those have reported hot running and difficulty in finding a balance between being too tight and too loose.
PTFE and its lack of compressibility may be a factor, it does seem to go hard (or solid) and lacks give very quickly. Initially when installed with some kind of lubricant it seemed fine to me, and it was initially cool to touch while running. The problem with mine began after about 20 hours motoring it then got hot and began to smell, it then started extruding pieced of white PTFE pieces.
At that point it was no longer possible to adjust it either leaked or ran so hot it began to partially seize the prop shaft.
I was in Minorca at the time and had a conversation with a Kiwi marine engineer in a marina. He told me about greasy hemp packing and quoted that as being the preference with both NZ and Australian boat builders and maintenance companies.

This comment is based on an experience, I would never try to persuade or convince anyone, its entirely up to individuals, just in case you still think there is an element of confusion with responses.

 

[vyv_cox]

I believe the problem may be partly due to the very low thermal conductivity of PTFE. Once it begins to heat up, maybe due to a small rub or a shaft deflection, the heat accumulates quickly as it cannot be conducted away. PTFE also has quite a high coefficient of thermal expansion, which then exacerbates the problem.

 

[rob2]

I was thinking about the comment earlier that PTFE gland packing promotes excess wear of the shaft. I would imagine that this would be due to the fact that it is a relatively soft material and particulates of hard materials can become embedded in the surface, scouring the shaft.

Rob.

 

[ksutton]

I was thinking about the comment earlier that PTFE gland packing promotes excess wear of the shaft. I would imagine that this would be due to the fact that it is a relatively soft material and particulates of hard materials can become embedded in the surface, scouring the shaft.

Rob.

All the sales info written on PTFE seals suggest they cause less shaft wear. However further to my post above number 22, when I dismantled the gland with the PTFE seal after only 20 hours some of the PTFE had stuck solid to the shaft "in rings". I suspected then that the water injection cooling may have been blocked because they looked to have been running dry. I connected an external water hose and water gushed out into the dismantled gland, so it hadn't been blocked. It proved difficult to clean the baked on PTFE rings from the shaft which were 20 or so mm inside the gland casing. I disconnected the prop shaft from the engine and moved the prop rearwards out of the cutlas bearing and cleaned the shaft at that area thoroughly,thankfully there were no score marks or shaft wear.

 

[vyv_cox]

I was thinking about the comment earlier that PTFE gland packing promotes excess wear of the shaft. I would imagine that this would be due to the fact that it is a relatively soft material and particulates of hard materials can become embedded in the surface, scouring the shaft.

Rob.

A fellow student in my final year did a sponsored project on the wear associated with PTFE on stainless steel. Surprisingly the steel wore a lot and the PTFE hardly at all. I then joined Shell, who turned out to be the sponsors of the work. It was for the seats of very large ball valves, originally intended to be PTFE until it was found that the balls were wearing badly. I now know that raw PTFE is colourless but the white stuff that we mostly call PTFE is filled with powdered glass to improve its mechanical properties. It's the glass that wears the steel. PTFE intended for reciprocating duties, e.g. dry running gas compressors, is filled with graphite plus lead, bronze and other materials.