PTFE creep/flow 1

[apsix]

I'm looking at using 30mm thick PTFE as a bearing pad. Plan dimensions are 300 x 170 mm and compressive stress is about 150 - 200 kPa. Will creep/flow be a problem?

John

 

[rnd2]

PTFE is very prone to cold creep. At 0.2MPa what likelyhood is there of exceeding that pressure due to stress concentration due to loading imbalance? There are many other alternative materials to look at.
Re-enforced composites either dry or lubricated. Some of these rival PTFE for low friction yet can handle significant loads with zero creep.
ORKOT, MARITEX BEARINGS, TUFNOL, TEN MAT are some that can help.

 

[plastx]

As rnd2 stated unfilled PTFE can pose creep issues. Search for filled PTFE with inorganic fillers. Rulon is one at

http://www.rulon-meldin.com

but there are others to consider too.

Regards,
Mike

 

[gjm78]

Hi apsix,

As rnd2 states there are other options where creep may be a problem ... and it often is with PTFE.

Mica filled PTFE is available which improves creep resistance - marketed as Flourosint I think, though there may be others. It's also worth checking out other filled polymers which may have PTFE or MOS2 added. Vespel and torlon certainly exist in these forms. You can also try Ketron PEEK HPV which is a bearing grade PEEK composite.

Good luck.

 

[JimCasey]

Having a 30mm thick chunk of TFE for a bearing pad is an expensive option that invites dimensional instability. If 29mm of that is steel and a 1mm Engineered resin (RTFE, etc.) bearing surface, you will probably have a less expensive and certainly more durable and dimensionally stable bearing surface.

 

[arto]

how about a steel-backed teflon bearing/slide plate?
i.e., Fluorogold,

http://www.sepbearings.com/index.html

 

[apsix]

Thanks for the replies.
I should have said but the pads will be supporting bus bars in an aluminium smelter, and are also acting as electrical isolation, hence the thickness.
However, due to the response, we will be looking at alternatives.

John

 

[tothepoint]

Check out a product named Thordon. It is a polymeric type material with exceptional load bearing and lubricious properties.

 

[rnd2]

Is it correct to assume the busbars supply electricity to the pots and therefore are in close proximity? If this is correct, there is a very important other consideration and that is chemical degradation of most organic substances exposed for long periods to low levels of fluorine gas, a by product of the process.
PTFE group are one of very few electrical insulating materials that resist fluorine.
My 2C worth: Position a metal plate that completely covers the PTFE. The plates could be fixed to the busbars and act as load spreaders between busbar and PTFE, thus preventing point loads when the bars move. This wil reduce the risk of the PTFE creep.

 

[arto]

note that Thordon is limited to 140F [wet]

http://www.thordonbearings.com/pdfs/Engineering_Manual_4.0.pdf

 

[scooter911]

Just a note but remember teflon at high temperatures gives of a nasty gas, I think it's called fosgene, sp?.

 

[plastx]

Actually Phosgene isn't related to PTFE. Phosgene is used to make polycarbonate and is a nerve gas.

If you get PTFE over 900F you will start to release fluorine type gases which are nasty.

 

[BSQUAREDBUCKEYE]

RND2's idea of using metal plates to spread the load out sound like a good idea. You may however want to consider pultruded fiberglass instead of metal. It is not conductive and has been used in high voltage switch yards for years. It is available in polyester or vinylester resin for greater corrosion resitance.

There is a company on the state line of Bristle, TN and KY that is the largest FRP pultruder in the world by a factor of eight they say. They make the side rails for Warner FRP ladders. They have a complete catalog of FRP shapes that mirrors steel structural shapes. One needs to design by deflection instead of yeild since this product's yield and ultimate are about the same.

http://www.strongwell.com

Look at their pultruded plate material.

If you are looking for just a couple of pices call a company down there called Appalacian Plastics. They buy up Strongwell's scraps and can make you a hell of a deal on the material.

 

[rnd2]

BSQUAREDBUCKEYE
That is good information for sourcing FRP structural shapes. However, for this thread, if I am correct in assuming apsix's application an aluminium smelter potroom, regular FRP that uses epoxy, polyester or even vinyl ester for the resin matrix will have a (disastrously)short life. Very small concentrations of fluorine gas adsorb into and destroy the resin matrix suprisingly quickly. Fluorine gas is a by-product of producing aluminum metal.

 

[apsix]

Thanks to all for the comments.
Upon the advice of a manufacturer we have selected a 12mm thick glass-filled PTFE sandwiched between steel plates, which should cope with the heat (100deg C, higher if the ventilation breaks down) and fumes.

Also at the low compressive stress, thinner size and glass-fill should ensure that creep is not a problem.

John

 

[rnd2]

Suggest also that you specify a minimum continuous 30mm overhang of PTFE beyond the plate edges to help prevent metal dust arc.