SHOCK ABSORBER SYSTEMS
Figure 1: Plan and section view of a shock absorber system
Figure 2: Behavior of the shock absorber during axial shocks
Figure 3: Behavior of the shock absorber during radial shocks
History
Because of the high inertia of balance wheels and the extremely fine pivots of their staff, pivot breakage due to impact was common until the 19th century. In 1806, Abraham-Louis Breguet patented the first shock protection system, which he named the “pare-chute”.
Description
A shock protection system is essentially an enhanced cap jewel bearing designed to absorb shocks and prevent the breakage of delicate pivots. It is used mainly for the balance staff bearings, and more rarely for the escape wheel (see cap jewel bearings).
Construction
The bearing jewel is securely fitted into the lower section of the setting and pierced to allow the passage of the pivot. The cap jewel, which is flat and unpierced, rests freely in the upper, removable part of the setting. Its diameter is slightly smaller than its housing (Figure 1), allowing it to move vertically along the pivot axis (Figure 2) and tilt slightly in any direction (Figure 3) during an impact.
The setting itself is mounted with a free fit inside the shock setting, which is pressed into the bridge or mainplate. The setting is held in place by the lyre spring, whose flexible blades press the setting against the base (Figure 1). During an impact, the lyre spring allows the setting to move laterally and vertically, while the cap jewel simultaneously shifts vertically relative to the setting, thereby absorbing the shock (Figures 2 & 3).
Lubrication
Most shock protection systems can be opened for cleaning and lubrication. The lyre spring can be lifted or removed, allowing the setting and both jewels to be easily extracted. The shock setting itself remains pressed into the bridge or mainplate. Once disassembled, the jewels can be cleaned and reassembled individually.
Once cleaned, a shock protection system can be lubricated in two different ways:
By opening the system and applying oil directly onto the flat surface of the cap jewel using a traditional oiler. The cap jewel is then carefully positioned on the setting, ensuring the quantity and centring of the oil are correct. Finally, the setting is placed back, and the assembly is closed with the lyre spring.
It is also possible to assemble the entire system dry (without oil) directly on the bridge or mainplate and lubricate the assembled shock protection system using an automatic oiler, applying the oil through the hole in the bearing jewel.
Applications
Shock protection systems are used primarily for the balance staff pivots, where the risk of breakage is greatest, and occasionally for escape wheel pivots.