Rocker arm bushing source

[Lapkritis]

I had a nice response typed up here and then my web browser crashed so here's the cliff notes version. Engineers- feel free to grade my response below; I'm in telecom project management by trade so I'm talking way outside my scope.



The oil channel idea - I wouldn't try it on my own. If you cut a channel down into the bearing face where the two parts meet then you will reduce the surface area between the two. The remaining surface will have higher pressure per square inch than unaltered because there is less surface to spread the load over. This could potentially cause more harm than good.

The thermal expansion causing binding idea. This doesn't hold up as the rate of expansion for brass is much higher than steel. If the part is sized correctly to match exactly the OEM dimension then the new bearing would actually grow less than the original brass under the same conditions. See coefficients of linear thermal expansion above for table of materials and values. http://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

I suspect the wear is due to the teflon coating wearing off which exposes the steel material. Once the coating is gone you have an increase in bearing temperature through friction which only exacerbates the thermal expansion discussed above and causes further bearing wear. Basically, once the teflon is gone the bearing is toast.

Proposed solution: increase diameter of bleed through holes and degree of taper of these holes in the outside of the rocker to catch more oil. Confirm size of bearing and fit of rocker shaft; if they're tight when cold then you're going to have a serious problem when the heat and expand into each other.

[KTL]

Frank,

Do you have any pictures of your rocker shafts? Curious to see what metal transfer you experienced. My low usage shafts show some "shadowing" where the load is placed on them. But no indication of bearing failure.

[chris_seven]

Andrew,

I have been reseaching Rocker Arm bushes for some time now and I am not confident that DU/DP and other bushes of this type are entirely suitable for refurbishing old Rocker Arms.

If you look at data provided by Glacier then the Teflon Impregnated bushes were originally designed for 'dry' operation where their characteristics are described as 'excellent'.

Once you lubricate this type of bush their performance is described as 'good'.

I don't believe that you should 'hone' this type of bush and the recommended method for obtaining the correct clearance is to adjust the shaft diameter.

Glacier make the following statement;

The burnishing or fine boring of the bore of an assembled DU bush in order to achieve a specific clearance tolerance is only permissible if a substantial reduction in performance is acceptable. Fig. 24 shows a recommended burnishing tool for the sizing of DU bushes if absolutely required.

I must say I wouldn't be confident about doing this without significant testing.

I don't think expansion is much of an issue.

The bearing is very thin so expansion will be small and the housing (the bore of the rocker) is steel and will constrain outward growth of the bronze.

Obviously the clearance and fit need to be controlled to the correct amount and the data is readily available on the GG Bearings website.

The clearance and fit of shafts and bushes is also an interesting topic and it is normal to make adjustments to shafts as this is generally easier than honing bores but this process can be carried if the bush is designed for this procedure.

The shaft tolerance of a 911 Rocker is around h7 which is not too difficult to acheive.

I think these tolerances are a bit large for DU and would prefer to see shaft tolerance of around h5 with a suitable housing size.

Surface finish is also an issue and DU needs with an Ra of = or better than 0.2 microns for mixed film or hydrodynamic operation. If the shaft is rougher than this then wear could be quite rapid.

If you compare this to the values recommended for use with a Glyco Bimetal bush, used on the standard rocker you find a recommended Ra of 0.8 to 1.6 micron. This is to ensure that the shaft surface holds the correct oil film. If the surface is smoother than this level the bush will wear due to insufficient lubrication.

My conclusion is that there are detailed significant diifferences in the design requirements for Bimetallic and DU bushes which means that the two bushes are not readily interchangable. The devil is always in the detail.

I am sure if the 'housing' (rocker arm) is dimensionally correct and the shaft designed to suit then DU or one of its improved derivatives such as DP4or DP11 would be very good but I would not be confident that they would work well with housing and shafts designed to suit Glyco or other BiMetallic bushes.

As we are just concluding the final design and manufacturing stage of our new Rocker arms and shafts we have spent some time worrying about the choice of bush material.

It was tempting to use these new materials but then we would have to sell the shaft as well as the arm and this just pushes up cost and looks a little 'opportunistic'.

We have decided to use a Glyco bush so that our new rocker will work with standard shafts.

Glyco bushes are sold in a semi-finished condition so we can precision hone them to the correct size after they have been installed.

[burgermeister]

If you alter the rocker bore or the bushing length of the roll, the press fit won't work, so that's not an option.

But I'm not a guy who got the Du bushings to work (though I did nothing except press them in), so I am probably not the right guy to converse with!

[burgermeister]

Kevin, unfortunately no pics of the shafts or worn bushings. It was summer, I was in a hurry, so I got the rockers rebushed (with bronze) & refaced, polished the shafts a bit, and plopped it all back together without photographing a single thing. The bushings were disposed of at the rocker refurbisher.

Based on Chris_seven's info (and since the motor is apart once again for valve guides) I have ordered all new rocker shafts, as it sounds like the polished ones are not good with the bronze bushings ...

[Lapkritis]

Chris - thanks for the response. Would scuffing the rocker shaft with an abrasive to return that ra be beneficial prior to install for longevity purposes? Any other suggestions?

[chris_seven]

Originally Posted by Lapkritis

Chris - thanks for the response. Would scuffing the rocker shaft with an abrasive to return that ra be beneficial prior to install for longevity purposes? Any other suggestions?

If the bush you are showing in your photograph was a PTFE DU type bush then I am sad to say that is is completely worn out.

The guidance is that when 50% of the surface shows the bronze backing material it is 50% worn and at 70% visible bronze it needs to be replaced.

Running the bush in this condition will probably damage the shaft.

I decided that the best course of action with rockers and shafts has been not to mess with them as doing the detailed calculations of life and service factors just became too worrying.

We are just about to make a batch of 'Motorsport' style rocker shafts to complement our new rockers and at the moment we are just going to use the standard design while we test and develop some of the new ideas we have been looking at on a different thread.

We are centreless grinding the surface to the recommended Ra and we are using a sub-contractor with a Talysurf so we can correctly inspect the surface finish.

We are then going to use a Nitrotec process to surface harden the shaft to around 1000Hv.

Nitrotec is said to be oleophilic as it is slightly porous.

The cost of making these shafts even in modest quantities is so attractive it just isn't worth the risk and the time mucking around with used parts.

We can also control the tolerance of the shaft to higher level than the standard part and ensure we can optimise performance with our new rockers.

By using a Bimetallic semi-finished bush we can optimise the shaft fit into the housing and still have the correct bearing clearance to allow for temperature.

I have recently seen a video on U-Tube where the suggestion was made that it is good practice to 'stone' the OD of a used rocker shaft to remove any scoring or pick up marks and then re-use.

I have to say that I couldn't endorse this practise and any shaft that was scored I would throw away.

I looked quite deeply into both DU and DP4 bearings in the 'Du B and DP4 B' forms.

Both these materials are triple layer = PTFE/Bronze/STEEL and PTFE/BRONZE/BRONZE.

1. Burnishing or Honing to Size

Removal of 0.025mm by Burnishing reduces life by 20%
Removal of 0.025mm by Honing reduces life by 40%

Removal of 0.05mm by burnishing reduces life by 70%
Removal of 0.05mm by honing reduces life by 90%


Not great news.

The details of the materials and sizings are a bit detailed but for anyone interested were the basis of the decision not to use PTFE style bushes.

Standard Porsche Data.

Shaft Diameter 17.992 -18.000

Bore Diameter in Carrier 18.000-18.018

Clearance 0-0.026mm

Obviously if bore is at minumum and housing is at maximum it will be quite difficult to fit the shaft and damaging the surface will be quite difficult.

By centreless grinding the shaft we can easily tighten up the shaft tolerance and avoid the interference and still ensure that we have some potential to deform the end of the shafts.

We can make the shaft to an h5 tolerance and use a maximum of 17.998.

If we use DP4B bushes we would need a shaft of 17.948 - 17.966

This is about 0.001" smaller than the minimum shaft size specified by Porsche and could cause some issues in clamping the shaft into a housing with a maximum size of bore.

This part of the design is already troublesome and increasing clearances just doesn't seem wise.

To make the DP4B work it also assumes that the the housing has a diameter of 20.000 to 20.021otherwise there could be other issues.

I came to the conclusion that using a Bimetallic semi-finished bush with a honing allowance built in gave me the best scope to do the right job.

[911 tweaks]

excellent here chris...just spot on...I can't wait to see the prototypes & here the trial results...I for one am 1 of many I am sure that will buy these as they are needed these days...
you are making a correct part to address what life (time) has done to these parts...thx for sharing your engineering research which looks very promising to be a solution to this issue...

[Porshaah]

Originally Posted by chris_seven

Glyco bushes are sold in a semi-finished condition so we can precision hone them to the correct size after they have been installed.

Is there a recommended hone grit size for the Glyco bushings?

TIA

[chris_seven]

We spoke with Federal Mogul but they don't provide any data or information but clearly the finish needs to be very fine and we opted for a diamond hone.

We use a two stage process with a 220 grit to remove the bulk of the material.

Final honing is then 400 grit.

Clearance is also important and as we are making shafts with a close tolerance we hone the bush to suit.

Machining is not a bad option and we used a tip radius of 0.7mm with an Approach Angle of 30 degrees and a Primary Angle of 10 degrees. Speed was 2.5 metres/second with a feed of 0.1mm per rev.

With plenty of lubrication the results were quite good. This approach was lower cost than buying a couple of diamond honing tools but quite time consuming and would possibly have generated more scrap.

You could also ream if you have a suitable reamer.

[911 tweaks]

sounds great chris...any pics a/o parts now available or is a case by case basis = you need to make parts per ea application = size them...??

[docrodg]

Could a pressure burnisher do this? I know they are used in high pressure hydraulic units to polish bores and pistons for dampers in metal stamping. I don't remember the RA acheived but it was very small, I think in the 20s, definitely under 40.

[chris_seven]

You can certainly burnish a Glacier bush, the method is shown in the Brochure for their SP material and this is relatively similar to the Glyco.

I don't have experience of burnishing so we bought the honing tools and we achieve better than 20.

[HawgRyder]

Is it possible to get the bushing too smooth?
My thinking...little depressions would hold oil...if it were too smooth and very well fitted (minimal clearance) there would be almost no oil in the gap.
Bob

[chris_seven]

Bob, I agree but it is more conventional to grind a shaft with the correct Ra to hold an oil film which is what we have done with the shafts we have made.

The grinding has been controlled and surface roughness checked.

Also the Nitrotec finish we have applied holds oil in the slightly porous surface produced by the diffusion based hardening process which should help.

The oil film thickness seems important and I believe we have the correct values.

I have seen the bushes with oil pockets which are a fairly recent development but have no practical experience with them.

It believe is difficult to size 'dimpled' bushes and it is conventional to use them 'as supplied'

The problem with this is that you have to grind the shaft to the correct size to produce the clearance. So if the bush is on the small size when fitted you would have to grind the shaft and this may reduce the degree of 'fit' into the housing which could allow shafts to walk.

We have tired to design our parts using a slightly different approach.

We have manufactured the rocker shaft to be a close fit to the bore in the cam carrier as this will provide the most reliable seal and it should lock without over tightening the bolt. We make the shaft towards the top end of the Porsche Spec and grind to a very close tolerance.

By using the Glyco semi finished bush with a honing allowance on the bearing surface we can produce not only the correct finish but the correct clearance for the shaft that we have manufactured. We used a very smooth finish on the bush.

This should give us a predictable and reliable result using conservative 'known art'.

[Porshaah]

What is the recommended hone grit size for the Glyco bushings when using OEM shafts?

TIA

[chris_seven]

Glyco/Federal Mogul don't provide any information that is the problem.

They neither recommend a grit size nor a surface finish so you need to make the engineering decisions based on experience.

I would consider 400 grit on a diamond hone as being OK.

[HawgRyder]

Chris...would there be any advantage to grooving the pin..just in the middle area where the bushing rides?
Either a straight groove or perhaps a slightly diagonal (length wise or side to side) on the bottom (bushing pressure area).
I am thinking it would spread the oil film full width on the bushing.
Bob

[chris_seven]

Bob,

I am not sure that shaft/bush wear is a major issue and I am not sure that introducing a new variable that will need testing and could cause problems is worthwhile so I have no plans to make this change.

I would be happy to make a set for you if you want to experiment.

[Lapkritis]

If you were to create a groove on the shaft then I would suggest parallel with the taper grooves. Otherwise you would have the harder shaft sawing on the softer bushing each time the edge worked over it if it were not parallel. Having a leading edge present under tension from the valve spring could have a windshield wiper effect on the oil film layer.