"harry" wrote in message
...
On Saturday, 29 February 2020 10:45:56 UTC, Dave Plowman (News) wrote:
In article ,
harry wrote:
The viscous clutch on a viscous fan slips more and more with engine
revs. This is because the fan is unnecessary at speed. The forward
motion of the car is sufficient.

The fan might be most needed when say towing up a steep hill at low
speed.

Hence most decent ones being thermostatically controlled for many a year,
rather than one which simply limits the fan speed.


How do you thermostatically control a purely mechanical device?

One obvious way is with something close to a bimetallic strip.
Another is to use something that melts with higher temps
like with wax etc.