First, I am not an expert in these things. Most of what I have learned has
been on the job experience making equipment work in rather hostile
environments. (Like near or on the ocean, under the ocean, amid spraying
water, baking in direct sunlight, etc.)
Second, when Hearthware started recommending to other people on the list
that they lube the bottom bearing and my precision unit started screeching,
I decided to just try lubing it.
Third, after looking at the guts I can see why it is likely the bottom
bearing goes before the top one - the shaft basically rests in this bearing
assembly, so you not only have the effect you have with a through shaft on
the bearing, but the weight of the assembly pressing down on it. (You can
see this by wiggling the shaft a bit. There is play in it and it rides up
and down in the bearing slightly so when sitting up, gravity puts downward
force on the bearing. It is also somewhat distant from the motor assembly so
any distortions in the shaft will also cause a chatter effect at this distal
end of the shaft.)
Fourth, although it should be a self lubricating bearing, my experience with
such things when they are stressed even slightly beyond their design
capabilities they have to have some lubricating help. This happens where
excessive heat and/or vibration exists. Both probably apply to this bearing,
though I suspect vibration to be the biggest culprit here.
Fifth, the unit only costs about 100 dollars to replace and I already have
most of a year of hard use out of it. (several hundred roasts, probably
averaging more than two a day) This means to me that if I can fix it myself
I should. (that is not necessarily true for everyone)
With all that in mind I used an oil I have had some success with for quite
some time. One part light machine oil (the kind used to oil industrial
sewing machines) and two parts duralube oil. This duralube stuff was/is sold
by the people who developed one of those engine oil additives to reduce
friction and heat effects in car engines. I suspect a fifty-fifty mix would
have been better for this, but I already had the 2 to 1 mixed up.
Two drops with a syringe into the bottom of the bearing assembly while the
unit was apart and upside down, wait one hour for it to fully sink in, then
two drops on the shaft as it enters the bearing from the top as I put the
assembly back together again, wait another hour before using. For about a
month now, not a single squeak, let alone a screech.
From past experience I know the bearing will go, but this will buy a lot of
time on the unit. It does not help to OVER lube. All that does is collect
dust and grit and increase friction and wear. I expect in about three to
four more months I will have to lube it again, so I may just do it on a
three month basis so I don't have to get back to the telltale screech before
Any good machine oil of light character should work. The stuff I am using
has penetrating characteristics which also help and some unusual friction
additives, which may result in better free spin on the bearings, but is
probably not essential. The oil has to be light enough to get into the
assembly and have enough body to stay there for a reasonable time. WD40 is
absolutely NOT recommended for this. It is ok for getting things unstuck and
cleaned of debris, but it is a horrible long term lube. 3in1 machine oil or
a good gun oil will work. Oil designed for high speed parts is what you
really want. The sewing machine oil by itself would likely work fine, but
would not penetrate as well.
P.S. If you can use a screwdriver, figure out how plastic tabs fit in
plastic slots and remember the sequence of what you did to take something
apart, you can do this.
Good luck and happy roasting.