Floating balance movements began appearing in mechanical clocks in the early 1950s. The floating balance has distinct advantages over pendulum clocks. It is robust, dependable and not susceptible to disturbance. It will tolerate being out of level unlike pendulum clocks which must be on a level surface.
This Hermle movement was taken out of a Dugena 1960s vintage Mantel clock and is otherwise unmarked. The seller advised that it loses time and might just require a simple adjustment. This is code for, “it’s worn out”. The clock has been completely disassembled, cleaned, reassembled, oiled and tested. My hope is that I have eliminated friction loses in the trains.
To function correctly the balance wheel rotates back and forth and maintains the steady beat of the clock through a pallet fork assembly. There is a speed adjustment and after adjusting it enough it becomes clear that the problem is within the balance wheel itself since it continues to run slowly.
The floating balance assembly consists of a balance wheel, 2 jeweled bearings on each end of a hollow tube riding on a pivot wire supported in a u-shaped frame.
The most common reasons for sluggish balance performance are dirt, rust/pitting/roughness of the wire, bent or weak wire or excess oil on the wire & jewels.
Removal is simple – take out two screws, tilt the balance to avoid bending the pallet fork and pull out. Here is the assembly removed from the movement. The bad news is that the helical hairspring appears to be distorted. Ideally it should enclose the balance shaft perfectly.
The balance wheel should float on the wire that goes through the cylindrical shaft like the next photo. While in this position a push on the balance wheel should allow it to rotate freely for a minute or more. No more than a few seconds for this one, however.
The thin wire is actually resting on the banking forks (or base). If the shaft rests on the base, additional friction slows the rotation of the balance wheel and will eventually slow the clock.
In this case either the jewels are cracked or the spring has lost its strength. The solution is to either find a new helical torsion spring or find a replacement balance wheel assembly.
Given what I paid for it, it is probably not worth spending more money.
Call it a learning experience. Into the drawer it goes and perhaps someday it will provide parts for a better running movement.