Bushings in POM/polyacetal

[SorenDK]

Hi

Does anybody have experience with making their own bushings for my '74 coupe...

- Springplate/rear torsion mount
- front A-frame - top/bottom
- Trailing Arm - top

Im looking for sizes and measures of the bushings before i start up the lathe...:-)... and the odd advice

What about the material - I am planning on using polyacetal/POM/delrin... anyone got comments on this.

I will share my experiences as I get along.

Happy New year

Søren

[DaveMcKenz]

Hi Soren,
Look at Rebel Racing to see the design they came up with. You will need to make the delrin sleeve plus a metal bearing sleeve plus a mounting system. The stock bushings were rubber and have no low friction bearing surfaces, so you will have to change the whole design. Elephant Racing has another system, that may give you ideas as well. You will not be able to just replace the rubber with delrin.
Good luck,
Dave

[chris_seven]

A-arm solid bushings | 9products

These guys make them in Solid POM for the Front Control Arms and the Weltmeister Polygraphite bushes are also virtually solid.

I have seen rear spring plate bushes in Delrin as well and again Weltmeister Polygrahite are fairly solid and seem to work.

You will experience some increase in vibration but if you control clearances well they will work.

I would tend to make them a tight fit into the housings so they rotate on the inner surfaces.

You will have a problem if you try to make the inner trailing arm bushes in Delrin as they need to allow camber change. If you fit a solid parallel bush the arm will be over-constrained and you are most likely to break the plastic which will allow the arm to float and it could be fun catching the tail if this happens.

[DaveMcKenz]

Thanks Chis,
I see what you are saying. Still, those solid bushings make use of imprecise bearing surfaces, and are less than ideal. I think the systems I mentioned are a more complete approach to the problem.
Good luck,
Dave

[fanaudical]

Regarding material choice, I would recommend using one of the PTFE or moly-filled polymers for friction reduction/enhanced lubricity.

Another option may be to look at available IGUS I-Glide bushings/bearings; they may have sizes/materials readily available (or close enough).

EDIT: Sorry - was going too fast when I put the above up originally. I meant to say to look at the POM/acetals that have some PTFE or moly modifiers to aid in lubricity. (i.e. - Quadrant Acetron and others.) I did not mean to suggest that PTFE itself would be a good material for this as the ultimate compressive strength is too low for the loads these bushings will see.

[ganun]

There is a Delrin AF, a rather slimy feeling delrin, I think PTFE would be too soft.

[chris_seven]

I would agree about PTFE as machining and it is difficult to be precise with the dimensional tolerance as they tend to move following machining and then lose some of the fit.

Ertalyte TX or Vesconite HiLube may be good.

The Nylons and Nylatrons which are Moly filled seem a good idea but are not dimensionally stable when wet. They can increase volume by up to 5% and this can cause some binding. They may generally keep dry but I would rather avoid this type of material 'just in case'.

We made several Delrin Spring Plate Bushes and they worked quite well but they really need to made individually to suit a specific spring plate. If you make them to a 'commercial' tolerance they will either be loose or bind from plate to plate and then they will wear.

We always process spring plates in a controlled pyrolysis oven to remove all the rubber and then chemically strip before re-plating normally in 'Yellow' Zinc.

The other issue is the length of the spring plate bush which can also be a problem.

We machine a step on the end face of the bush where it fits into the cover and then add a 5mm diameter O Ring - we fit the bush in the torsion bar tube with the same type of O ring.

These O rings take up the tolerances and locate the bushes quite well.



Because the spring plate bush is bonded on the majority of spring plates the surface of the 'pin' is normally quite good.

We now use a different 'High Impact' Polymer which has a small amount of compliance. It retains it toughness down to -100 degC and is much better than Delrin in this respect. Delrin has a glass transition temperature at -35 degC and a relatively poor KiC even at normal ambient temperatures and can break with impact loading.

The material we use takes a very small set so we can make a bush with a slight interference and press fit them into the housings and then with a very small interference onto the spring plate.

The material can be rough machined and then heat treated (low temperature) which stabilise the size and then given a light cut as a finishing op.

After 2-3 excursions of the spring plate they free up and provide a smooth operation without clearance.

We made the first set with this material 5 years ago and they are still in regular use and have never been lubricated.

We use the same material for control arm bushes and they work equally well.

If the arm has corroded on what need to be the bearing faces - not uncommon in the UK - we interference fit a steel sleeve which we electroless Nickel plate. The bush on the front control arms isn't bonded so the 'pin' is more likely to have some issues than the spring plate.

We have fitted around 50 sets now - they are effectively beer money and they work well.

We don't sell them as loose parts as they do need some experience to fit.

[cashman]

Originally Posted by ganun

There is a Delrin AF, a rather slimy feeling delrin, I think PTFE would be too soft.

Delrin AF is a 20% Teflon fiber filled Homopolymer POM.

The industry standard is Delrin AF Blend which is a 13% Teflon Fiber filled POM. We cut the 20% concentration down to 13% because you achieve 95% of the reduction in coef of fric at the 13% loading. The additional 7% is really only adding additional cost to the product and diminishing some of the mechanical strength. Like mentioned in other posts, Teflon is rather soft. Check out the data page below.

http://www.roechling-plastics.us/en/products/sustarin-acetal-sustamid-nylon/sustarin-h-af-delrin-af.html?eID=dam_frontend_push&docID=4164

Delrin AF is not a big European market, mostly in the US. You should be able to get some at your local plastic distributor or Mcmaster Carr.

I would be hesitant to try Moly Nylon due to moisture absorption issues.

[Driven97]

The materials science is beyond me, but racers edge sells both the a arm and spring plate bushings in some form of delrin.

I have only the delrin outer spring plate pieces and they fit really nice. Reasonably priced, too.

[chris_seven]

There can be some problems fitting the front A Arm bushes as the steel housings can deform with hard use and go slightly out of round and this can mean that these hard bushes need to be trimmed to fit.

There is also the small differences in the varying spring plates that have been made over the years and we have found 3 distinct 'pin' sizes depending on age.

I do believe that all of these hard bearings need to be a very good fit if they are to have a reasonable life and they they do need to be a press fit into the housings so they don't rotate on their outer diameters and cause wear to the housings.

The Delrin AF sounds like a good idea but I would be concerned about its notch sensitivity, which is why we switched away from normal Delrin.

Delrin has a notched impact strength (IZOD) of around 1.5 ftlbs and Delrin AF is 0.7 ftlbs.

In material science terms this means it is relatively brittle and could break from a sharp corner when subjected to dynamic loads.

Under normal loading it is unlikely to be an issue.

The material we selected has an Impact Strength of 18 ftlbs and would be significantly less likely to crack under impact loading was the main reason we moved away from Delrin.

We had not suffered from any failures but I had always been a little uncomfortable with this material's basic toughness and I spent some time looking for a material with 'better' properties in this respect.

The material we now use has virtually the same frictional behaviour as Delrin, is dimensionally stable when wet, it has similar Modulus and is good for use at up to 180 DegC.

Its disadvantage is that unless it is stabilised after machining it does grow slightly with time hence the two stage manufacturing process with the intermediate thermal treatment.

I would still, however, be comfortable with standard Delrin for the majority of applications and could accept that I am being very cautious in using an alternative which is more expensive.

[mspirito]

Does anybody have any bushings made up for the weltMeister sway bars?