That's interesting, Bob, that the second lawn spreader didn't have the same problem, especially since it had the same plastic against metal setup. On the first spreader, I wonder whether grease would have been a better solution than WD40. Out there on the forums, there is anecdotal evidence that WD40 can hurt some plastics.
The problem with using that stuff, WD40, is that it is a good solvent for removing whatever was used to lubricate and prevent rusting. It is fine for removing dirty hardened grease that is keeping something from working right, but it is worthless for preventing rust in the long term. cheap gear oil is a better choice, or anti-corrosion grease, or even anti-rust grease, which is probably hard to find today. My 5 pound can of it is very old.
While plastic bearings may not need lubrication to function correctly, the steel part of the assembly certainly needs something to keep it from rusting. Probably the ones who designed the mower didn't know that. IT is one area of details that don't seem to be mentioned in engineering classes any more. Even good plastic bearings will usually benefit from a bit of the propper lubricant. Sometimes findin out which lubricant is the best choice is a challenge, though.
Through the years I have been involved in making dozens of parts for lawn mowers and similar consumer products. But due to material costs, I can count on my thumbs the number of times I have been asked to make those parts from stainless steel. It is much more likely the parts were expected to be zinc plated.
One problem we run into quite often in producing stamped parts is intolerance for lubricants. In days of old strips were oiled as they ran through the press and the parts produced also had an oil film. Hence they were not so likely to rust between operations. Some were washed and then plated or painted according to use etc. Therefore, no matter what, there was a natural barrier to rust for a time. Today, our customers do not want any oily parts and OSHA has dictated that many of the stamping oils we used are hazardous to somebody's health. Today virtually all of our stamping compounds must be water soluable and so are very vulnerable to humidity, particularly with changing temperatures. The same conditions that cause dew on your lawn encourage rust on unfinished steel parts, even in warehouses. Just think of how many times you unpack steel parts today and somewhere in the packing material you will find a pack of silical gel, treated paper or something similar. If oxidation starts it will just be covered by painting or plating, unless it is removed beforehand. Eventually the rust will surface. So that rust may have been in place long before the assembled lawnmower was shipped.
I would have used thin grease and I do not make it a habit of spraying water into bearing surfaces.
@Larry M. I read your comments about nylon with great interest, but came away a bit puzzled. I have fishing reels with some nylon components. I have used these for over thirty years and at the end of each season I dismantle, clean and oil all of them. I have never noticed any degradation in the nylon. I am sure that there are numerous grades of nylon so maybe your scenario only pertains to a select grade. Also in my reels the mating parts are usually brass, so that may be a factor. I know very little about nylon so I would appreciate any explanation.
I have used WD40 for a multitude of lubrication applications, usually with good short term results, but as you say light grease will perform better in the long term. However, as a rust preventative I have found it works well, unless it is physically removed. The product was originally developed to displace water in electrical applications (WD = Wire Drier and this formulation was the fortieth attempt), so it is not water soluable.
@Larry M: What you're talking about (reducing the amount of molecular entanglement and allowing polymer chains to slip relative to each other) is called plasticization. Polymers can be plasticized by solvents that are chemically similar to themselves. So non-polar solvents, such as the hydrocarbons that make up many lubricants, will tend to plasticize non-polar polymers. But nylon is a polar polymer. This makes it resistant to non-polar solvents. (On the other hand, it is plasticized by water; it readily absorbs water out of the air, and the higher the moisture content, the lower the strength and stiffness).
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