D.De Carle's book on clock repairing

[Michael1962]

I am asking some questions as I go through the book to make sure that I understand correctly what I am reading.

There are two types of drop on an escapement. An entry drop and an exit drop. Because there are two pallets, we have four drops in total. The drop being the distance between the front of an escapement wheel tooth and the front side of the pallets and between the back side of an escapement wheel tooth and the back side of the pallets, when the escapement wheel is just about to lock or just after the impulse movement.

Yes/no?

[oldhippy]

Just got this from the net. This explains it in a simpler way.

Each back and forth movement of the balance wheel from and back to its center position corresponds to a drop of one tooth (called a beat). A typical watch lever escapement beats at 18,000 or more beats per hour. Each beat gives the balance wheel an impulse, so there are two impulses per cycle.

[rodabod]

I presume you want the definition of “drop”. 
 

It’s usually defined as two separate things: inside drop, and outside drop. 
 

It can be measured absolutely in distance, but is often measured as an angle of rotation instead as this more closely takes the rest of the geometry into account. 
 

To keep things as simple as possible, watch an animated drawing of a recoil/anchor escapement. The “outside drop” angle is how far the escape wheel can freely run unhindered before it is arrested by the outside pallet face (so, the entry pallet) and the inside drop is (you guessed it) how far the escape wheel can freely run before it is arrested by the inside pallet face. There are only two faces in total which contact the escape wheel teeth. 
 

Excessive drop is considered unwanted as it is effectively wasted energy which could have been driven into the oscillator, and it can potentially lead to accelerated wear when advanced because the escape wheel accelerates to a much greater speed before being stopped “dead” by the pallet. You can sometimes hear excessive drop on very worn clocks.