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The structure and working principle of mechanical clocks

The structure and working principle of mechanical clocks

17 Aug 2020

There are many structural forms of mechanical clocks, but their working principles are basically the same. Clocks and watches are mainly composed of the prime mover system, transmission system, escapement governor, pointer system and winding needle system. The mechanical clock uses a clockwork as the motive force of the driving system, and drives the escapement governor to work through a transmission system composed of a set of gears, and the escapement governor in turn controls the speed of the transmission system. The drive train drives the pointer mechanism while pushing the escapement governor. The speed of the drive train is controlled by the escapement governor, so the pointer can indicate the time on the dial according to a certain rule.


The winding needle is a mechanism to wind up the mainspring or move the pointer. In addition, there are some additional mechanisms that can increase the functions of the clock, such as automatic winding mechanism, calendar (dual calendar) mechanism, alarm device, moon phase indicator, and measurement period mechanism.


The vibration period of the vibration system is multiplied by the number of vibrations in the process to be measured, and then the elapsed time of the process is obtained.That is time = vibration period × number of vibrations


1. Primitive system

The mechanism for storing and transmitting working energy is usually composed of barrel wheel, barrel cover, barrel shaft, mainspring and external hook of the mainspring. The mainspring is a spiral or S-shaped spring in the free state. It has a small hole on its inner end, which fits over the hook of the spool. Its outer end is hooked on the inner wall of the barrel wheel through the outer hook of the spring. When winding, the barrel shaft is rotated by the winding needle system to wind the barrel on the barrel shaft. The elastic action of the mainspring makes the barrel wheel rotate, thereby driving the transmission system.


2. Transmission system

The energy of the prime mover is transmitted to a set of transmission gears of the escapement governor. It is composed of two wheels (center wheel), three wheels (over wheel), four wheels (second wheel) and the gear shaft of the escape wheel. Among them, the wheel is the driving gear and the pinion is the driven gear. The formula for calculating the transmission ratio is that for a timepiece with a second hand device, the transmission ratio from the wheels of the second wheel to the pinion of the four wheels must be equal to 60. Most of the tooth profile of the clock drive train is a modified cycloid tooth profile made after correction based on the theoretical cycloid principle.


3. Escapement governor

It consists of two parts, the escapement and the vibration system. It relies on the periodic vibration of the vibration system (balance spring or pendulum) to keep the escapement in precise and regular intermittent motion, thereby achieving speed regulation. There are many types of escapement regulators, and the fork-type escapement is the most widely used escapement.


Schematic diagram of forklift escapement


It is composed of escape wheel, pallet fork, double disc and limit nails. Its function is to transfer the energy of the prime mover to the vibration system in order to maintain the vibration system in constant amplitude vibration, and to transfer the number of vibrations of the vibration system to the indicating mechanism to achieve the purpose of measuring time. The energy transfer function of the fork escapement is completed by the following two parts in cooperation: ① The energy obtained by the drive train of the escape wheel is transformed into impulse and transmitted to the pallet fork through the action of the teeth and fork. There are mainly 5 actions in the transmission process (Figure 6 [Energy transmission process of the fork escapement]), namely, locking, releasing, impact, falling and traction. ②The impulse is transferred to each other through the fork of the pallet fork and the disc nail of the double disc. The working process has two actions: release and impact.


The vibration system is composed of balance wheel, pendulum shaft, hairspring, movable outer pile ring, fast and slow needles, etc.



4. the balance spring vibration system


Vibration system


The inner and outer ends of the balance spring are respectively fixed on the swing shaft and the swing splint. When the balance wheel deviates from its equilibrium position by an external force and starts to swing, the hairspring is twisted to generate potential energy, usually called restoring torque. The escapement completes the aforementioned two parts of the process, that is, the process of the vibration system completing half of the vibration cycle. The latter will swing in the opposite direction under the action of the hairspring's potential energy to complete the other half of the vibration cycle. This is the principle of the continuous and repeated cycle of the escapement governor when the mechanical clock is running.


The winding needle system


The function of the winding needle system is the winding and the needle.


It consists of crown, stem, vertical wheel, clutch wheel, clutch lever, clutch lever spring, gear, compression spring, dial wheel, span wheel, hour wheel, minute wheel, large steel wheel, small steel wheel, pawl , Detent springs, etc. The winding and setting needles are realized by the crown part. When winding, the vertical wheel and the clutch wheel are in meshing state. When the crown is turned, the clutch wheel drives the vertical wheel, and the vertical wheel passes the small steel wheel and the large steel wheel to make the barrel wind up the mainspring. The pawl prevents the big steel wheel from reversing. When setting the needle, pull out the crown, the shifter rotates on the shift shaft and pushes the clutch lever to disengage the clutch wheel from the vertical wheel and engage with the needle wheel. At this time, turning the crown will drive the hour wheel and the minute wheel through the span wheel to correct the hour and minute hands.


Additional devices include automatic winding mechanism and calendar (dual calendar) mechanism.


Automatic winding mechanism: Watches with automatic winding mechanism are called automatic watches.


automatic winding mechanism


It is a kind of automatic mechanism. It is composed of heavy hammer, heavy hammer support, eccentric shaft, ball, automatic rocking plate, ratchet wheel, pawl, and automatic splint. When the watch is worn on the wrist, with the random movement of the human arm, the automatic hammer automatically winds up the clockwork under the action of inertial force and static moment. The automatic winding mechanism can be roughly divided into swing type one-way or two-way winding and rotary one-way or two-way winding. The former is called semi-automatic and the latter is called fully automatic. People have different opinions on the one-way and two-way winding performance of automatic watches, and it is generally believed that the one-way winding automatic mechanism performs better.


Calendar (dual calendar) mechanism: Watches with calendar (dual calendar) mechanism are called calendar (dual calendar) watches.


Calendar organization


It is a kind of calendar organization. It is composed of a calendar ring, a calendar positioning rod, a calendar positioning rod spring, a date wheel, a day cross-wheel component, a head and a calendar cover, etc., and is equipped with a toggle mechanism or a quick dial mechanism for date adjustment. Its basic working principle is to drive a date wheel by the hand wheel system, and the transmission ratio between the date wheel and the hour wheel must be 1:2. Then the dial head is driven by the date wheel to make the calendar ring with the date mark act once every 24 hours. The dual-calendar mechanism also rotates the weekly calendar wheel under the coordination of the positioning components through the shift head, so that the week can be replaced. According to the length of time required to change the date, the calendar mechanism can be divided into three types: slow climbing, fast climbing and instant jump. The slow-climbing type requires 1 to 3 hours to change the day, the fast-climbing type generally does not exceed 30 minutes, and the instantaneous jumping type changes the date at zero o'clock every day.


Time accuracy of mechanical clocks


The regularity and accuracy of timekeeping. Clocks and watches require accurate timekeeping, stability and reliability. However, some internal factors and external environmental conditions will affect the accuracy of timekeeping.


Internal factors include the structural design, work performance, selection of materials, processing technology and assembly quality of each component system. For example, the stability of the clockwork torque, the smoothness of the transmission system, and the accuracy of the escapement governor all affect the travel time accuracy.


The external environmental conditions include temperature, magnetic field, humidity, air pressure, vibration, collision, use location, etc. For example, changes in temperature will cause changes in the performance of lubricating oil and balance springs in watches and clocks, thereby causing changes in travel time performance; when the environmental magnetic field strength is greater than 60 oersteds (Oe), some parts will be magnetized and slow down ; Humidity will cause oxidation and corrosion of some parts.


There are 5 parameters commonly used to indicate travel time accuracy:


① Indication difference: The difference between the indicated time of any instant of the clock and the standard time. It can be positive or negative.


② Daily difference: Subtract the indicated difference between clocks and watches for 24 hours.


③ Position difference: the difference between the maximum value and the minimum value of the instantaneous daily difference measured at each position of the spring.


④ Equal time difference: Under other conditions unchanged, the maximum value of the instantaneous daily difference at each corresponding position at the time of full spring and full 24 hours.


⑤ Daily variation: the daily difference between two adjacent days.


Brief explanation


Mechanical watches are composed of exterior parts and movement. Appearance parts refer to the parts that can be seen directly such as the case, dial, hands, etc.: main drive train, escapement speed control system, pointer drive train and winding needle mechanism, etc. Mechanical watch parts are composed of upper, middle, lower, and three-layer splints. The lower splint is the main deck and the basic part. The upper splint includes strip splint, swing splint and upper splint. The middle plywood is composed of the Shen plywood and the fork plywood. When assembling, mechanical watches are positioned by the position nails and the position nail tube of the main splint. As long as we install the parts of the escapement speed control system and the transmission gear system of the mechanical watch on the corresponding positions of the splint, and assemble the central plates and the main splint with screws, the accuracy of the mechanical parts can be ensured. To facilitate the disassembly and assembly of the splint, a tweezers opening is milled on the main splint. All these movement parts are installed inside the watch and are usually invisible.


The working principle of a mechanical watch is to use a vibration system to control it to produce a stable period. Multiply the period by the number of vibrations in a process to get the time experienced by the process. That is: time = vibration period x number of losses.


During the entire operation of mechanical watch parts, due to the unavoidable movement resistance of the vibration system during work, such as bearing friction, air resistance, and friction between elastic parts, the amplitude of vibration is gradually attenuated. In order to prevent this attenuation and make the vibration system work continuously, it is necessary to periodically supplement the mechanical watch with the energy consumed due to resistance. The mechanical energy of the watch is stored by the clockwork device. That is to say, the force required for the transmission and vibration of the body comes from the clockwork. Winding up the mainspring will periodically supply the machine with the motive power to supplement its mechanical energy consumption and prevent the occurrence of the aforementioned attenuation phenomenon.


Its formula is: Vibration-consumption of two energy supplements. These three processes continue to cycle, so that the movement continues to work. In order to travel accurately, the escapement speed control system plays a role in controlling the speed. By the way, the winding needle setting mechanism has two functions: one is to store energy for winding, and the other is to set the time. It is an indispensable mechanism for watch-type timing mechanisms.

 
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