acetal causing wear on steel guide rod in sliding application

[kjoiner]

Hello,

We have redesigned an aluminum part with nylon 66 bushings to a single piece acetal part. This part slides on a guide rod made of 1144 steel that is zinc plated. There is about 60lb of radial force on the acetal part and it slides back and forth 2.75 at a low speed. During the testing procedure, I have noticed wear marks on the steel rod visible as slightly flattened areas.

The bearing surfaces are Ø.500 and are .250 long each. They are separated by 1.3". The rods are Ø12mm. This is not a high precision application so fits are not tight.

I am very surprised to see an acetal part wearing a steel rod like this becuase the intent is to have the bearing surfaces molded into the part.

I do not have a specific resin spec but can get it from our molder if needed. The resin is not glass filled.

Has anyone experienced wear like this for sliding applications between acetal and steel?

Finally, we chose acetal over nylon becuase the overall part is 32.5" long and 2.75" wide and had concerns over the dimensional stability of nylon due its moisture absorption tendancies.

Thanks,

Kyle

 

[lumenharold]

By any chance is the zinc plate coming off the rod and getting deposited in the bushing? If so then it will be grinding off the surface as if it were sand paper. If that is the case you might consider a st st rod. Also, the fit of the bushings might be worth considering. Bushings function like screen door stoppers if the fit is too loose compared to the engagement.

Harold
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[IRstuff]

how many slide cycles before the wear is noticeable?

TTFN

FAQ731-376

 

[kjoiner]

Hello,

Thank you for the replies.

Regarding the zinc plating, I purchased a piece of ground and polished 416 stainless rod and installed it on the other set and will monitor its wear vs. the zinc plated rod. It could be that the zinc has embedded in the acetal. I'll take a look at the bushing surfaces and see if there is any zinc embedded in them.

The ability of a linear bearing to lock is something we considered in the original design. The nylon bushings worked well but if you twisted them enough, you could lock them. That would take and abnormal amount of twist. The parts also bear against a plastic strip on the front that helps keep the bearings from twisting too far out of position.

As far as cycles, I have reached about 100,000 cycles and noticed the wear somewhere around 90,000. Our 10 year life cycle requirements are at around 900,000 cycles.

Kyle

 

[IRstuff]

And, so, presumably, the previous material, the Nylon 66, was able to go the full 900,000 cycles?

TTFN

FAQ731-376

 

[IRstuff]

Your new material looks to be potentially harder and stiffer than the original nylon.

TTFN

FAQ731-376

 

[lumenharold]

Perhaps you can see what's inside the failed bushings by slicing them in half along the axis and looking at them under a stereoscope. The bushings don't need to be binding to be causing wear, just increasing the friction due to mis-alignment.

I haven't got a clue what the cost difference might be or how it might effect your prefered manufacturing method but I have had good performance with UHMW-PE for bearing surfaces.

Harold
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[Compositepro]

I can't imagine using zinc plating as a bearing surface. Zinc is soft and used as sacrificial corrosion protection.

 

[patprimmer]

I am really having some problem understanding this part. This is compounded by the mixed units and lack of units on some dimensions.

Have you measured the PV value for the bearing.

Have you compared it to the permissible limits for acetal.

Have you approached Ticona or DuPont. They both have a lot of data they may share.

The zinc will definitely complicate the situation.

What is the environment of operation re moisture, temperature, chemical exposure including exposure to water, exposure to UV light, exposure to grit, and possibilities of lubrication.

What is the size of the moulding and can a smaller bush be inserted where required so that an inappropriate material is not specified for a large part to fix a small area.

Regards
Pat
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[kjoiner]

Hello,

Regarding the zinc, it's not my favorite choice but cost is a factor and the carbon steel rod is less expensive than the SSTL. Having said that, the zinc is not performing well and I plan to look for a better material. I am testing with a SSTL rod and will compare it to the zinc plated rod.

During consultation with our molder, we decided that nylon would not meet our dimenional requirements so we went with acetal. The tool is already made and the dimensions of the part are well within our tolerances.

I have attached a PDF with a small area of the molded part. The area shown is a hat section and the rest of the part is flat with ribs. The part is 32.5" long and 2.75" wide so shrinkage over that long of a distance is a factor. We have two other rods that pass through the part. One rod passes through clearance holes and the other passes through one clearance hole and one hole that is tighter to serve as a guide.

As far as mixed units, the 12mm rod provides about .027 of total clearance for the Ø.500 hole. It is a loose fit but in the application and cost constraints, it performs well. The previous design was an extrusion that was then drilled and subsequently anodized. Becuase the extrusion had an open hat shaped section, dimensional control for width was a big problem. The acetal parts are much better.

Pat,

I contacted our molder and requested the specific resin used and from there I will be able to get the data for a PV check. The part slides back and forth. It travels 2.75" at about 1" per second. The projected areas for the bearing surfaces are .125in^2 each so it looks like the total bearing stress is 240psi.

The part is installed in machine that is in an air conditioned space so moisture and temperature are well controlled. I checked some small samples of acetal we use for another part with some of the lubricants we use such as Lubricone and Tri Flo. I soaked the parts in these two lubricants and saw no issues with solvent attack. From the initial data I looked at, chlorine or similar products pose the greatest risk to acetal and those types of products are not used on our machine.

Another test I should try is to install another part that has never run against a zinc plated rod and let it run against the SSTL rod and look for any signs of wear.

Thank you again for the assitance. If you need more information or clarification I will be glad to provide it.

Kyle

 

[patprimmer]

If it is in an air conditioned lab with humidity control, nylon should have been OK. In fact it's shrinkage is more predictable than acetal. Normally the complication is the time to normalise from moisture absorbtion to equilibrium and the variations from varying humidity.

I was not suggesting that the acetalwould be attacked by lubricants, simply that they will reduce wear.

Chlorine AND acids both destroy acetal and nylon.

You can get a PTFE filled grade of acetal that has improved bearing properties. Silicone oil masterbatch can also be obtained to add oil to the acetal compound. This also improves bearing properties. They do work well together as they work by different mechanisms.

Unfortunately these additives would be present through the entire moulding to only be required in a small area.

A small bush would most likely be more economical.

Polyesters can probably be moulded on size in the same mould and may perform better.

Other options may be to powder coat the rods with nylon powder. Nylon and acetal combine to make a very good bearing surface. Ticona and LNP (now part of Sabic I think) and DSM Engineering Plastics and RTP should all have data on this.

As to how long before the nylon peels of the rod is another matter, but that can be easily tested.

If bushes are necessary, PTFE, or PTFE filled PET or bronze filled nylon all make very good bushes.



Regards
Pat
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[patprimmer]

If it is modified so bushes can be used you might save a lot by going to PP instead of acetal for the main moulding, that is provided that PP meets the other criteria. It shrinks in the same ballpark as acetal.

Regards
Pat
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[kjoiner]

Pat,

Thank you for information about the other resins. My molder is checking on the specific acetal resin used for these parts and I will pass this information on to him. Regarding your mention of the PTFE and silicone fillers they are intersting. I'll see what impact they have on overall cost. Their presence in the entire part is not an issue unless they drastically change the appearance.

I have noticed that the ouside edges of the bushing bosses are supposed to radiused and they are sharp. I will need to check on how the side actions were constructed because they should be in there.

One piece of good news is that since I changed the guide rod to 416 stainless, I have put about 19,000 cycles on the machine and rod seems to be holding up better than the zinc plated version.

By the way, I made an error in stating the number of cycles. There are upper and lower sets of doors so when I let the machine run for N# of cycles the top and bottom sections of the machine really on see half the amount each.

As far as humnidity, the parts would be molded and delivered to us where they may sit in our warehouse for a while. We are in the southeastern U.S. so humidity can be a problem. At 32.5" long, I was afraid that moisture absorption for nylon would cause issues because the parts are assembled into the machine in an air conditioned shop and the parts would start moving around again.


Kyle

 

[kjoiner]

Hello,

My molder specified Celcon M270 for the parts. It is listed in the Ticona documentation as a copolymer bearing grade resin that has good flow properties (rating of 27) for thin walls and complex shapes. Since the tool is made in China and the first shots were produced there, he is checking with them to verify that is what they used. Eventually the tool will be shipped over here and parts made in my local molder's shop.

I took a look at the PV ratings. The limiting PV is 4000. If I run that using my data of 1"/sec (5 fpm) and 240psi I get a PV of 1200. The motion is not continuous so that should help.

I am starting to lean toward the determination that the zinc plating is causing problems: 1. It is applied over 1144 rod which is fairly low carbon material, and 2. After zinc plating, the surface is not as smooth as the bare rod and 3. Zinc is soft and wears off, embeds in the acetal and then as Harold mentioned, could be abrading the steel in some manner.

Kyle

 

[patprimmer]

M270 is a low molecular weight std injection moulding grade. It has no special bearing properties beyonf what could be expected from any grade of acetal. For improved properties YF10 could be used. Also silicon oil master batch can be added.

Regards
Pat
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[kjoiner]

Pat,

Does the silicone additive leave a film on the guide rod? One requirement we have on the overall design is that the parts run dry because our machine is used to dispense textiles which generate a lot of lint and if there is an wet lubricant present, the lint has a tendancy to stick to the guide rod. We do apply a dry silicone lubricant, Lubricone to be exact, to the guide rods, but this wears off over time. The silicone additive, if it does not affect the dimensions and other properties, might be a good alternative.

Thank you again for the continued advice. I'll talk to my molder about the silicone additive.

Kyle

 

[patprimmer]

The silicone migrates to the surface of the acetal. I have never noticed deposits on the mating surface, but I never really looked for them, so that only means they were not obvious, not that they don't exist i n a subtle way. I just don't really know.

Regards
Pat
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[kjoiner]

Pat,

I'll contact the manufacturer and see if they can provide some information on deposits.

Kyle