PPRuNe Forums - View Single Post - Check your brakes or it could get expensive!
Old 10th Jan 2018, 22:44
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MurphyWasRight
 
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but as the brakes cool down they shrink. With a Hydraulic system which applies pressure, even when turned off via an accumulator, the pressure keeps the brakes clamped. An electric brake works by movement, so when the electrics are turned off, there needs to be some sort of adjustment to compensate for the shrinking brake pack.
I do not know specific details but I would be very surprised if there is no deliberate "compliance" in the parking brake mechanics.
Compliance of course being an obfuscating a word for the technical term 'springy bits'.

As the brakes are applied the spring (in whatever form) deflects 'storing' reserve force in the spring. Without this the driving motor would come to a hard stop with risk of mechanical damage.

This provides continued pressure as the brakes cool, will drop slightly depending on the spring constant of system.

The accumulator provides the analogous spring function for hydraulic brakes.
Even without leakage the pressure will drop (possibly imperceptibly) as the brakes cool and slightly more fluid is required to maintain contact.

On self locking:

A better model than pushing a car in fourth gear (which while hard is theoretically possible) is a scissors car jack.

Turning the threaded rod moves the jack up or down but the geometry is such that the weight of car cannot turn the rod.
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