Polymer choice for wear pad? 3

[adamgnt]

Hello all.
I am looking to implement a polymer insert in a load bearing hook in an effort to address corrosion being worsened by fretting in the interface. The hook is constructed of stainless for stength and corrosion resistance in a submerged marine environment. The passivation layer on the stainless holds up well on non-stressed points, the problem only lies within the hook which interfaces with a glass-filled nylon covered shoulder bolt. I attached a REAL rough 2-second drawing of what the insert may look like just to help with the visualization.
I am hoping a small polymer insert will be a low cost solution by reducing friction and eliminating oxidized metal from the contact surfaces, which promotes the vicious circle of corrosion. The key properties of of the insert would be its ability to withhold the load without cracking (two separate products experience loads of 350lbs and 2500 lbs respectively), not succumb to deforming or cradling the load pin (operation includes freeing the pin by releasing the hook) and be able to stay in place (looking to use a high grade adhesive if possible).
Unfortunately I do not have as much design experience with engineering grade plastics. I have worked with Delrin, but something in my intuition tells me it is too soft and will deform. I have no hard evidence to back this up, but I am aware of much harder polymers which would seem to work better as long cost doesn’t get too prohibitive. A quick material property search with Delrin as the baseline (85 Rockwell ‘M’, Compressive strength 18ksi) yielded many results with hardness level above 110 Hardness and 30ksi Strength. Results included: Ultem (PEI), Celazole (PBI), Torlon (PI), Ketron (PEEK) just to name a very select few that I have at least heard of before. I was hoping this forum could give me some insight to these and similar materials beyond just the hard data sheet numbers. Also I am aware that nylon based composites can be used as self lubricated load bearings and wear pads. However, mechanical properties show them acting closer to Delrin. Could this mean my initial reticence about Delrin was misplaced? My brain would tell me there also may be some adhesion problems if the wrong polymer is used.
I know this a somewhat long post, but I have seen such excellent results, both for myself and others from these forums. Hey, if someone needs some hydrodynamics help down the road, I may be able to return the favor ?

 

[patprimmer]

Unless you want to spend a lot of money, acetal is the hardest best creep resistant plastic you will find in its unfilled state. The list you have is good, but prices start at about 4 times that for acetal and go to possibly 20 times.

Ultem would probably be my next choice after acetal. Ask Sabic for more details re abrasion, salt water, UV exposure if applicable, impact and creep resistance.

Regards
Pat
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[MikeHalloran]

What's the chance of making the load pin cover square on the outside (still round on the inside), so it will have a finite bearing area against the hook?

Even given that, I'm thinking of brass for your hook insert.



Mike Halloran
Pembroke Pines, FL, USA

 

[adamgnt]

Excellent Pat, thank you. I was leaning towards Ultem since I can't get over this hang up that the acetal will begin to cradle the glass filled nylon after being under load for 6 months. I will see what I can dig up with regards to abrasion and creep resistance of these materials, these are two columns that should be present in the comparison spreadsheet.

You mention SABIC, and a quick glance at the website seems to indicate they are a large supplier with huge selection of stock sizes. Have you had good experiences working with them in the past?

Mike, thank you for your input also. Your comment sparks me to revisit old design ideas (one of which was square) and take a second look over the calcs, especially for the high load version. I remember one concern was that if there WERE any deformation/corrosion, the hard angled interfaces were much less likely release succesfully. We have had success with the corrosion resistance of brass and Al-brass alloys alone in the past. However, brass and 316 stainless have different galvanic potentials (

http://www.corrosionsource.com/handbook/galv_series.htm

)and therefore I was under the impression these would form a galvnanic cell and promote corrosion the brass acting as the anode. I'm drawing back to old Materials Science days in college for this stuff, if you have tangible experience with the topic, I am more than happy to listen.

Thanks again guys.
--Adam

 

[MikeHalloran]

Line the hook's insert pocket with plastic, so the brass and stainless are not directly coupled.



Mike Halloran
Pembroke Pines, FL, USA

 

[adamgnt]

Ahh Sabic is the manufacturer of "Ultem" brand PEI. I was under the impression it was a regional polymer supplier.

One other thought. I can't imagine I need more than a 0.1" thick inert (low load version) to accomplish what I'm looking to do. Perhaps someone could point me towards an appropriate "creep" or "indentation" equation to help document this decision?

 

[Pud]

Are these inserts to be moulded or machined from stock?

Rgds

Harry

http://www.tynevalleyplastics.co.uk

 

[adamgnt]

The current idea would be have them machined, then apply the adhesive (not settled upon yet) on our assembly floor. Somebody brought up that machining could restrict our material choices since certain glass filled polymers like G10 experience water retention problems once the external seal is broken. Neither products are high volume (no more than 100/year) so I don't feel as if it would justify the cost of a mold or die.

 

[Pud]

Fot that small amount, i would be tempted to use a choice from the following:

Glass reinforced:
PEEK
Torlon (PAI)
Any reinforced Phenolic.

Properties available anywhere.

I would hesitate on using PES (Ultem) as it's an amporphous material and will creep more than crystalline materials.

The idea of a metal insert appeals (MikeHalloran) - maybe this stuff:

http://www.hillmanbrass.com/Products/Naval-Brass.htm

Cheers

http://www.tynevalleyplastics.co.uk

 

[unclesyd]

If you are going to use a plastic insert make sure you have an adhesive lined up as some of the materials mentioned are relatively hard to bond..

Have you looked at the Nitronic alloys mainly Nitronic 60 or 30 for your complete part?
Nitronic 60 shouldn't have any wear or galling problem in your application.

http://www.hpalloy.com/alloys/descriptions/NITRONIC60.html

http://www.hpalloy.com/alloys/descriptions/NITRONIC30.htm

 

[patprimmer]

Sorry I presumed you were injection moulding.

Also you do need to obtain the data yourself from the manufacturers data sheets. I would never trust data I got of a discussion forum as you don't know who you are talking to.

I really do not have time to do your data searches for you. I can give impressions and directions, but not hard data.

A company called Victrex makes PEEK.

Solvay make the largest range of exotic plastics. You should also contact them.

You should remember that nylon permanently in water will absorb about 9% water and swell somewhat and softem dramatically. Glass will reduce the trend but not eliminate it.

Glass filled PET might also be worth a look.

I have no idea of the loads as you quote force but not area or duration or temperature or allowable tolerance or number of impacts or energy of impacts or alternating load or operating cycles.

Regards
Pat
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[PhoenixInc]

PEEK is the most expensive plastic on the market, with varying grades going from $45 pp. I would take a second look at PEI in the $6-$8 range.

Bryan Gebhart
Phoenix Recycling Inc.

http://www.plasticscrap.us

 

[vettedude05]

adamgnt,

I am not sure if you found a solution to this application, but have you considered using a thermoset composite bearing material? I recommend trying “Orkot”. It is a fabric reinforced thermoset composite material. It has a a low coefficient of friction(STATIC CO EFFICIENT OF FRICTION 0.05 - 0.10 DRY AT 15N/mm2 / 2180 lb/in), a high load capacity(COMPRESSIVE STRENGTH Normal to Laminate 280 N/mm2 40600 lb/in),( IMPACT STRENGTH 120 kJ/m2 0.079 kJ/in2)… It has good chemical resistance and operates in fresh and salt water without lubrication. It has virtually 0 swell and does not encourage any corrosion. It has minimal thermal softening and is realitivly cheap compared to PEEK.

 

[dFarley]

Glass Filled Epoxy sheet would probably suffice too. The first few materials mentioned. PEEK, Torlon, Celazole and even Ultem are fairly expensive resins.

Epoxy and phenolic sheet give you plenty of bang for your buck. Epoxy would probably give you the highest compressive strength and the lowest moisture absorbtion, which tends to soften materials. Sheet is manufactured in gauges down to .030" thickness.

 

[xqv]

If you are still looking a glass fiber filled phenolic molding compound would do nicely.