[time-nuts] Pendulum clock suspension

[iovane at inwind.it]

This puzzled me since yesterday as I discovered how the suspension of pendulum clocks is made, that is a springy plate.
I thought that:
-the suspension point (i.e. the point the pendulum moves about) cannot be considered fixed,
-following the above, the lenght of the pendulum varies during a swing,
-the spring contributes to the oscillation (it cumulates and releases energy), i.e. not only gravity at work..
Ok that we can neglect the above yet having an extremely good approximation, but is it conceptually right?
Thanks for the answers.
ant