Acetal, UHMW, LDPE, Bronze, ETC.....Bushings - Which to Use on Vertical Guide Posts.

I've just spent something like 4 hours researching various materials to use as bushings in a lumber sawmill I am building. I have two chrome plated 3 1/2" diameter x 74" guide posts that will be installed vertically. The posts will guide the sawmill's 4 bushing head up and down the posts. The idea in my design can be seen in the photo. The Cook Sawmill shows two vertical posts which the head rides upon. They are the two light colored, vertical posts behind the bandsaw. The pictured posts are far smaller than the ones I just bought since Cook's mill needs to be portable and manufacturing costs maintained.

Keep in mind, my mill be will be far more potent. It will run a 30HP electric motor, be permanently installed and the head, band-wheels, pulleys, framework, motor, etc.... will weigh in around 1500 pounds. But all that weight is kind of irrelevant to the load bearing needs of the plastic. The only real pressures applied on the bushings will be "slight" when the mill runs through the log. There is no need for substantial bearing load on the bushings. They and the posts are strictly there to maintain the head's vertical/horizontal positioning. AND.... I'd like to protect the guide posts since they will be a very HARD item to replace.

My question............. What plastic should I use? And what fits should I aim for with a 3.5" post? I was originally going to make the bushings in bronze (which I have in stock) since I was going to use steel rounds and live with greasing the posts. But after the arrival of the real guide posts, which I bought sight unseen for a song and dance..... I found them to be beautifully chrome plated, ground and ready for the job at hand. I started looking at Acetal, etc... in order to protect the posts. BUT am completely ignorant of plastics in machine design. The more I read.... the more confused I have become.

I will be making the bushing holders and bushings in my shop. So, they can be designed as needed to hold any size bushing and there will obviously be 4 bushings. SOOOoo... the bushings can be slim, thick, long, short, etc.... I have no idea yet. But in my mind, it will be easiest to make round holders and bushings on the lathe. Being a sawmill...... I want to keep the use of oils and greases to a minimum.

What reasonably priced material should I use for these bushings? Keep in mind, I want the bushings to wear of course, not the posts and I have no issues with replacements in the future. AND.... for the next year +-..... the mill will be in the weather until I can produce the lumber needed to build a building. It will then never see rain again.

Thanks for the help,
Pete

Delrin (acetal) machines very nicely, and is reasonable for light loads and relatively low friction; this application in vertical operation (for the 4 guide points) would probably be fine with that. I assume your design is 2 bushings per guide rod, separated by some reasonable vertical distance. Acetal DOES absorb some water, in the 0.2-0.9% range, depending on whether it is ambient humidity or saturation. This would be something to consider for diametral fit. There are acetal grades containing TFE that make things a little more slippery, but still mostly the same for water absorption.

UHMW has almost zero water absorption, but is a less stiff material, and more subject to creep under pressure, and softening at lower elevated temps than Delrin. It is also easy to machine, and considerably less expensive than acetal.

As far as fits go, I think this is largely up to your execution of the support structure for the guide rods, and the qualities of the rods themselves in terms of straightness and diameter variation. The water absorption is a factor here as noted, especially for something living outside. I would design the bushing holders so that the bushings are exposed to the minimum solar UV possible, as well.

I'd go for PTFE loaded Acetal, that is super slippery, yet very little different from std Acetals rigidity bit more $ but imo worth it Ensinger

+ 1 on the ultraviolet protection.

I'd steer clear of UHMWPE it's too squidgy, poly prop is little better on that front as is PTFE, nylons out because of water absorbsion which and PEEK, and that is really spendy

You could look at Igus bearings - see what's off the shelf and not gonna break the bank http://www.igus.co.uk/iglidur/plain-bearing

i could see some alignment issues , but plastic bushings wouldn't gall your expensive uprights without oil, and would
be fast , easy , and relatively inexpensive to remake. if specfab's moisture issue is a concern, PET has similar
mechanical properties as Delrin ,but is less prone to absorb moisture .

i'd be more concerned with it getting gunked up with sawdust and wood sap. green wood has all sorts of corrosive stuff-
acetic acid, tannic acid , phenols...whatever. that attack certain metals. maybe some research is in order.

I made the vertical bushings on my sawmill out of UHMW, sunning on cold rolled steel because it was cheap. I grease them as well. I have not had a problem

Because of the possibility the guide rods may not be exactly parallel I suggest using 2 bushings on one rod and 1 on the other.

specfab said:

UHMW has almost zero water absorption, but is a less stiff material, and more subject to creep under pressure, and softening at lower elevated temps than Delrin. It is also easy to machine, and considerably less expensive than acetal.

Click to expand...

A friend of mine used uhmw for rudder bushings on a large boat. They worked super but it was a good thing he had double-oversized hydraulic steering, when you'd turn the wheel the whole boat would groan and shudder because the uhmw did absorb water and swell up. At the first haulout he pulled them, took ten thou out of the bores, and all was well forever after. Boat's twenty years old now, the rudder bushings are still fine.

to protect the bearings from dirt and to keep grit out of the bearing shaft interface you can add rod wipers like on hydraulic cylinders.

If it were me I think I'd go with Oilite bronze bushings rather than plastic for something that that big. Stronger, won't swell, permanently lubed and very inexpensive:

http://www.oilite.com/bearings.asp?type=oilite

these work well
DU(R) Self-Lubricating Bearings, DU(R) Bushings - Metal Polymer | GGB

I'd go with the bronze or Oilite, because you can machine the clearance you want, and it will be stable, no matter what the humidity level is. Too much clearance (such as you might have to do for plastic bushings) can impede smooth sliding if the bushings are not widely spaced relative to the offset weight of the head.

Protect your chromed shafts, too, while they are out in the weather. Microcracks in the plating can allow moisture to get under the chrome and make rust, causing the chrome to peel in spots.

Protection is probably more important than bearing material as pretty much any of the suggestions will work fine with appropriate installation and clearance. Sawdust, especially the fine dust, is just plain horrible around anything that might be called a bearing. It clumps up. Its pretty much impossible to get rid of it all. It absorbs stuff. I'd design in wipers and concertina style "rubber" tubes as belt'n braces to keep it out of the working surfaces. Gotta accept tho' that whatever you do on inspection two years down the line you will find sawdust inside.

My engineers head says Oilite but the practical had says pick up the phone and call Igus with details of what you intend to do and they will be able to look up appropriate material, size and clearances that are known to work in such applications. Tapping into collective experience via a wide range supplier is always the best way to get decent, reliable results from a fairly commoditised application. Odds are you could engineer better for your specific job if you put the effort in but whether said "better" gets you anything or real practical import seems pretty moot.

Clive

Secound igus, that stuff is just magical in a pplication like this. Forget oilite, its rubbish and is best left in the past.

I've used lots of Igus. They really are the goto people for this.

So your trying to fabricate a large die-set with those (2) posts and bushings ?
And just how will you meet the required accuracy to eliminate binding ?


" The Cook Sawmill shows two vertical posts which the head rides upon. They are the two light colored, vertical posts behind the bandsaw. The pictured posts are far smaller than the ones I just bought since Cook's mill needs to be portable and manufacturing costs maintained".

Are you sure that the one your copying is done this way ? You only have this one picture ?

Have you seen this unit in person ? A laying on of hands might prove fruitfull.

Ya, why not just use some Torlon, would be perfect for your application.

I found some 1.5" rod but I'm sure if you searched you'd get some 4" material.

1.5" is available at McMaster-Carr, only $327/foot!

McMaster-Carr

Pictures of your mill when done, please!

Good question..... I have already thought about it too.

The need for this set up is to maintain the head in a stable fashion so it does not "walk" down the track as the head pressures through a log. The idea is to hold the blade as perpendicular to the travel as possible..... which is an easy scenario to perform.

The binding issues will be present only in the center line distances from post to post being mismatched top to bottom. So...... if a person were set up the posts in such a way to allow a slight float perpendicular to the head's travel but not parallel, all will be good and the design meets the need.

Pete

digger doug said:

So your trying to fabricate a large die-set with those (2) posts and bushings ?
And just how will you meet the required accuracy to eliminate binding ?


" The Cook Sawmill shows two vertical posts which the head rides upon. They are the two light colored, vertical posts behind the bandsaw. The pictured posts are far smaller than the ones I just bought since Cook's mill needs to be portable and manufacturing costs maintained".

Are you sure that the one your copying is done this way ? You only have this one picture ?

Have you seen this unit in person ? A laying on of hands might prove fruitfull.

Click to expand...

I've rebuilt hydraulic cylinders and the use of PTFE or PTFE like seals and bearings is common. A cylinder seal on a 3" or 4" cylinder is often 1/4"+- in thickness and can be bent into a kidney shape for installation. Bearings are often strips that are wide yet thin and very easy to manipulate.

Does anyone have experience with bending sheet PTFE. Say 1 1/2" to 2 1/2" wide strips in the 1/8", 1/4" or 3/8" thickness range??? My thinking is to cut as wide (and as thick) a strip as possible of PTFE and bend it into a relief cut on the carriers ID.... just like a cylinder's ID seal accept far wider.

I'm sure you all get my gist. If the PTFE goes south down the road, maybe I could tinker with other materials/carriers.

Thanks
Pete

You're not asking the right question. The right question is: what bearing system does woodmizer use on their columns? Don't reinvent the wheel when there are people pumping out a few dozen of these a week.