Aluminum rod in nylon bushing produce much heat?

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That's my two-stroke 21.2 CC grass trimmer.
I'm replacing the outer shaft and the inner driveshaft, with a
straight aluminum tube and a 1/4 inch diameter aluminum rod for
the driveshaft.
The rod will be rotating inside of nylon bushings. The nylon
bushings will be stuck inside of what Lowe's describes as "1/2" ID
PVC Reinforced Braided Vinyl Tubing". So that the bushings are
held snugly inside of the aluminum tube.
FWIW. Another way Lowe's describes that spacer tubing is "1/2 ID
Braided PVC Tubing". It's a flexible clear plastic tubing with
reinforcing material inside.
Question: Is the aluminum rod spinning inside of the nylon
bushings going to cause the bushings to get hot enough to melt the
plastic tubing, with everything closed inside of the aluminum tube?
Thanks.
Reply to
John Doe
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What is the rpm of the shaft? How hot is it when you are using it? How long do you intend to use it at one time? Are you using any lubricant? In short you do not provide enough information for someone to answer your question.
=20 Dan
Reply to
dcaster
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Just in case you want to see a picture of the bushings and spacer.
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Reply to
John Doe
If you would break down and buy a small lathe you wouldn't be constantly having these problems.
Reply to
Jim Wilkins
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Your design is not really clear. Are there two bushings? How are the bushings held in alignment? If they are not well aligned, the friction will be quite large and the bushings will wear.
However Nylon is generally only used for low RPMs. You might have a problem with a line trimmer. Here is a link that has a table of maximum speeds and loads for Nylon and Delrin bushings
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Another problem is nylon expands in wet environments and will make your bushing loosen up.
Reply to
anorton
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Spin it fast enough then yes, of course.
Reply to
Richard
Soft steel or stainless steel, as well as all non-ferrous metals do not run well with plastic bearings, even those with a socalled =91=91self-lubricating=92=92 filler. It is only a question of load, speed and time until wear increases rapidly, leading to premature failure
Reply to
Bob Engelhardt
I thought that was a property of UHMW or HDPE plastic, not nylon.
Reply to
John Doe
Backwards. Chemically Nylon is a protein, polyethlene is a wax.
Reply to
Jim Wilkins
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Depend on how the nylon is constrained. The swelling due to water absorption can tighten the bushing leading to seizure. My neighbour had this happen on a prototype using a nylon bearing and while all was well when installed after sitting in a damp environment over a weekend the mechanism couldn't be moved due to swelling of the nylon.
Reply to
David Billington
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Well if you're planning on running the rpms anything over idle the bushings might last 10 seconds before they melt.
You would be better off using round rod and small bearings. Easy enough to square the end with a file.
Reply to
Steve W.
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Thanks!!!
Gunner
-- "Confronting Liberals with the facts of reality is very much akin to clubbing baby seals. It gets boring after a while, but because Liberals are so stupid it is easy work." Steven M. Barry
Reply to
Gunner
Running aluminum in nylon without lubrication will result in very short bushing life. Aluminum running in nylon even with lube will still have a pretty short life. I know you are trying to keep the weight down but for long bushing life you really need to run a hard material against the nylon. A hard anodized aluminum shaft might work but you're not gonna be paying to get your shaft hard anodized. Can you use a dowell pin in place of the part of the shaft that runs in the nylon? Frankly, I wouldn't use nylon in your application. I would use Delrin AF. You can get sample bushings for free from some manufacturers. Can you Loctite a hardened or even soft steel bushing over the aluminum shaft where the shaft runs in the nylon? And are you really stuck using nylon? That shaft is gonna be spinning about 10,000 rpm which translates to 654 feet per minute which translates to sliding some aluminum across some nylon the length of a football field in less than 30 seconds. Eric
Reply to
etpm
[re bushings for drive shaft for inline skating push stick]
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As others said, (a) tell us the shaft RPM and (b) from following URL, running aluminum in nylon bushings is problematic.
Also, if you have a setup to measure melting points, tell us the melting points of your materials. From wikipedia links below, nylon melts in a 190?350 C range, and PVC in a 100?260 C range, so it might be possible to get some nylon that melts at a lower temperature than some PVC. In any case, heat transfer through the nylon will be slow, so I think your nylon bushing will melt down before the PVC does, even if your PVC has a low melting point.
Have you looked through all the sintered Oilite bushings available at local hardware stores? Typical Ace HW and TruValue HW stores carry assortments of dozens of different sizes of bushings like those shown at and such a bushing is worth considering if you can't get miniature ball bearings.
Reply to
James Waldby
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Could be worth looking at the Igus site
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as they have a number of bearing materials and can supply shafting in various forms in SS, anodised aluminium, chrome plated etc. They have design info also.
Reply to
David Billington
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those engines are rated to about 10000 rpm. I see your bushings melting or vibrating as they wear. We're talking a screeching vibration as the shaft rolls backwards on the bushing surface. A slender tube and rod are the perfect setup for such this.
I'd just get some cheapo radial ball bearings and sent this into your tube. they can handle the speeds with no problem.
Reply to
Cydrome Leader
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Using a 1/4 inch aluminum rod driveshaft, how many ball bearings per foot?
That sounds fine. To most people, like for production, cost of ball bearings over some bushing material would be the main concern? In other words... Why don't grass trimmer manufacturers use ball bearings?
Thanks.
Reply to
John Doe
Good point! What do they use?
Reply to
Bob Engelhardt
I'd plan on one every 8" or so unless the rod is alloy that is stiff.
That depends on who made it. The higher end commercial units do have ball bearings and solid drive shafts. The low end home owner stuff usually doesn't.
Reply to
Steve W.
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Have fun!
Reply to
Jim Wilkins

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