Fan bearing lubricant was Re: WD-40 (again)

[Tapley, Mark]

On Apr 19, 2016, at 10:52 AM, Noel Chiappa <jnc at mercury.lcs.mit.edu> wrote:

> Speaking of orientation, though: these fans, like most PDP-11 fans, send air
> downwards. I was thinking of flipping them, to send the heat upwards (its
> 'natural' direction), but after pondering a bit, I'm not sure this is a good
> idea: the air-flow on the intake side is diffuse, whereas on the output, it's
> a concentrated, directed blast - better for cooling boards, etc.

I share the sentiment. However, in many cases thermal engineers actually had something to do with the original design and they also know about heat rising. I would be careful about second-guessing the design, at least on well-engineered systems.

I reversed the fan on my NeXT 040 Cube a long time ago. That way air goes out through the Optical drive port, and does not pull dust into it. Shortly thereafter, I started getting very occasional read errors on the SCSI bus. I thought through the air pathway, and sure enough the SCSI chip was now *down*stream of the power supply. I put the fan back to its original orientation, and no problems since.  (The optical drive doesn’t work anyway, but I understand that is not a rare problem and in this case I don’t believe it is related to dust.)
									- Mark