I'm just a layman trying to understand some information and a chart I'm reading about viscosity as it relates to practical lubrication choices.
There's an article here at MachineryLubrication.com
on how to choose the proper grease.
First, I would like to make sure I am reading this chart
from that article correctly (ignore the arrow pointing at ISO22).
At the top left corner of the chart, at ISO grade 5, we have viscosity suited for a narrow range of applications, those with an operating temperature from approx -40C to 25C and a DN factor of ~190,000 to 1,000,000; and at the bottom right corner of the chart, at ISO grade 460, we have viscosity suited for a narrow range of applications, those with an operating temperature of roughly 100C to 120C, and a speed-rating of roughly 10,000 to 25,000; and in the middle of the chart, at say ISO 32, we have viscosity suited for a very broad range, from 20C to 110C and with a speed rating of 10,000 to 1,000,000.
Second question. The author writes:
For bearings, speed factor and operating temperature can be used to determine the best consistency or NLGI grade for a given application. It may seem counterintuitive, but higher speed factors require higher consistency greases.[my emphasis]
Why is a thicker consistency required for higher-speed applications? (I read "higher" = thicker, less "runny".) Is that because higher speeds generate more heat and the thickness is meant to keep the grease from getting too "runny" . Or is it a question of force and not heat?