General Information about the Clock and Operation
5th Floor / No. 17
Explanation of the Clock Mechanism Operation
The Ptuj tower clock is an exceptional example of mechanical craftsmanship from the transition of the 16th to the 17th century. As one of the first mechanical devices to spread during this period, it represents a symbol of technological progress and social importance of its time. Clocks were valuable investments serving the entire community, enabling precise time measurement in an era when personal watches were not yet widespread.
The Ptuj clock operates on the principle of a pendulum and weight drive. The energy for the clock’s operation is stored in stone weights, which transmit movement to the gear system via steel cables and drive drums. The pendulum, based on Galileo Galilei’s discoveries regarding the isochronism of oscillation, regulates the uniformity of movement and ensures the clock’s accuracy. The heart of the mechanism is the deadbeat escapement, which allows for the controlled release of energy from the weights to the gears without friction.
The gears are made of steel and are hand-forged, turned, and filed, testifying to the exceptional skill of the watchmakers of that time.
The drive drums are made of acacia wood, chosen for its strength and resistance to wear. The clock is attached with forged brackets to the wooden beams at the height of the dial on the outside of the bell tower, to which it is connected via a shaft that rotates the hands on three sides of the tower.
This precise mechanism required regular maintenance and adjustment by a clockmaker, who had to ensure that the gears and pins were correctly set. Any irregularity could cause inaccurate chiming, which would confuse the town’s inhabitants.
Drum 1: 15-Minute Chime
The first drum is intended for chiming every fifteen minutes. More frequent chiming allowed inhabitants precise orientation in time. Every 15 minutes, the clock announced the passing time, thereby determining the rhythm of city life and helping craftsmen, merchants, and citizens with their daily tasks.
The drive drum is made of durable acacia wood, around which a steel cable connected to a stone weight is wound. This weight, of precisely calculated mass (150 kg), provides the necessary force to drive the mechanism.
The system of gears and pins is precisely manufactured; pins are arranged to trigger the chime at every 15-minute interval. As the drive drum rotates, pins on the gear wheel activate a trigger that releases the cable connected to the chiming hammer on the floor above. The number of bell strikes indicates the elapsed time: one strike for the first quarter of the hour, two for half past, and three for three-quarters of the hour.
Count Wheel for 15-Minute Chime
The count wheel for the 15-minute chime has a special shape. On the outer edge, it has 4 distinct notches that divide the wheel’s circumference into 4 uneven parts. The gear does not rotate continuously, as it is blocked most of the time by a wedge resting in one of the notches. The wedge is released every quarter of an hour by a lever activated by the clock’s drive drum. When the wedge is released, it allows the 15-minute chime drive drum to rotate. The drum simultaneously begins to drive the gear connected to it via a toothed transmission. As long as the wedge slides along the circumference, the drum can rotate freely, but when the wedge slips back into the next notch, the rotation of the drum, and with it the gear, stops.
The mechanism is designed so that the gear makes one revolution every full hour. During the drum’s rotation, the gear counts out 1, 2, 3, or 4 bell strikes, depending on the distance between the notches. This produces different chiming for the first quarter (one strike), half hour (two strikes), three-quarters (three strikes), and the full hour (four strikes). At the full hour, the 15-minute chime gear completes a full rotation and returns to the starting position, while simultaneously releasing the wedge of the hour chime drum via a beak-shaped lever, thus starting the hour counting cycle.
Drum 2: Timekeeping Mechanism and Rods for Hands on Three Sides
The second drum drives the main timekeeping mechanism, which moves the hands on three sides of the Ptuj tower clock. This innovative design allows time to be visible from different parts of the city, which was a technologically exceptional achievement for that period. The hands on the east, west, and south sides of the tower are synchronized and move uniformly, requiring exceptional precision of the mechanism.
The drive drum is connected to a stone weight that ensures a steady force for rotating the gear system. A complex system of gears reduces the rotation speed from the drive drum to a speed suitable for moving the hands. The gears are hand-forged, turned, and filed, and their precise manufacture is crucial for the correct operation of the clock.
Transmission of rotation from one shaft to three axes is enabled by bevel gears and cardan joints. Bevel gears transmit rotation at a 90-degree angle, while cardan joints allow for the transmission of rotation between axes that may not be perfectly aligned. This system ensures that the hands on all three sides of the tower move synchronously and display the same time.
Drum 3: Repetition (Custom Hour Chime)
The third drum is responsible for the repetition function, which allows the clock to chime the full hour again after a certain time interval. This function was extremely important at a time when chiming could be missed due to noise or distance. Repetition ensured that all inhabitants had the chance to hear what time it was, which was crucial for organizing daily life.
The drive drum with its own weight is connected to a gear wheel that has pins arranged to trigger chiming after a set interval following the full hour. The trigger mechanism, consisting of levers and arms, activates the chiming hammer via a cable.
The clockmaker could adjust the intervals and the number of chime repetitions according to the community’s needs, adding a unique character to the clock.
Drum 4: Hour Chime
The fourth drum manages the main hour chime, which announces full hours with the corresponding number of strikes. The pin wheel has pins arranged so that each pin corresponds to one strike of the bell. At the full hour, the pins trigger the release mechanism, which activates the chiming hammer. The number of strikes matches the number of full hours (e.g., 12 strikes at 12 o’clock).
Count Wheel for Hour Chime
The count wheel for the hour chime also has a special shape. It has 78 teeth on the inside, and on its outer edge, 11 notches can be observed. Since the distances between the notches increase, they divide the circumference into 11 uneven parts. Each notch is slightly further from the previous one—exactly by the length of one (1) tooth on the inner gear. This gear does not rotate continuously like the clock’s drive gear, but only when the chime drive is released. When at rest, movement is prevented by a mechanical stop (wedge) resting in one of the notches on the gear’s circumference. The wedge is released by a lever activated by the clock’s drive drum exactly at every full hour. When the wedge is released, the hour chime drive drum begins to move. The number of bell strikes is determined by the distance between two consecutive notches on the circumference of the count wheel. The wedge slides along the circumference until it falls into the next notch; during sliding, each tooth on the inside of the gear counts down exactly 1 strike of the hour bell. The only exception is the first notch, which is slightly wider; this represents one o’clock, when the bell strikes only once. The gear is designed to make two full rotations each day. Since the distances between the notches increase, we get twelve different chiming lengths for the 12 hours of the day. The longest distance between notches will thus trigger 12 bell strikes—for the 12th hour of the day. Then the cycle ends, the gear returns to the starting position, and the countdown begins again.
Escapement Mechanism and Force Transmission from Pendulum to Clock Drum
The deadbeat escapement mechanism is the heart of the Ptuj tower clock. This mechanism allows the conversion of the pendulum’s steady oscillation into controlled gear movements, ensuring precise time measurement.
The pendulum (a rod with a stone weight at the end) swings at even intervals. Its oscillation is based on the laws of physics discovered by Galileo Galilei and is independent of the amplitude of the swing. The pendulum is connected to an anchor with two pallets. These pallets alternately catch the teeth of the escape wheel, which is a specially designed wheel with precisely shaped teeth. As the pendulum swings, the anchor controls the release of the escape wheel. Each movement of the anchor allows the wheel to advance by one tooth, transferring energy to the gear system and driving the clock hands.
The accuracy of this mechanism depends on the exceptional precision of the manufacture of the teeth and anchor; even small deviations can cause irregularities in the clock’s operation.
The use of quality materials, such as brass for the escape wheel and steel for the anchor, along with regular lubrication, is crucial for reducing friction and wear. This mechanism was a technological breakthrough in the 17th century and enabled the development of precise mechanical clocks.
Fans on Bell Drums and Their Function
Fans, also called fly governors, are a crucial part of the chiming mechanism on the Ptuj tower clock. Their task is to control the rotation speed of the drums during chiming, ensuring an even and pleasant chime and preventing damage to the mechanism due to excessive speeds.
The fan consists of wings or vanes attached to the shaft of the chiming mechanism. When the mechanism is triggered and the weight begins to rotate the drum, the fan wings also rotate. The rotation of the wings creates air resistance, which acts against the direction of rotation and slows down the mechanism. This ensures that the bell strikes are evenly spaced in time.
The clockmaker can adjust the angle and surface area of the fan wings to fine-tune the chiming speed according to the community’s needs or preferences. The wings must be balanced to prevent vibrations and excessive wear on the mechanism. The materials from which the wings are made (light metal) are chosen for optimal performance and resistance to weather conditions.
Regular maintenance of the fans, including checking for any damage or deformation and lubricating the bearings, is crucial for their effective operation. The fans are an indispensable part of the mechanism, ensuring the longevity of the clock and the quality of the chiming.
Clockmakers, Their Work and Signatures on the Hut Around the Clock
Clockmakers were masters of their trade, combining knowledge of mechanics, metalworking, carpentry, and art. Their work was crucial for the manufacture, maintenance, and improvement of the tower clock. They often left their signatures or marks on parts of the mechanism or in the vicinity of the clock, giving us today an insight into the history of the clock and the people who cared for it.
One of the most famous clockmakers associated with the Ptuj clock was Primož Okrogelnik from Celje. In 1812, he thoroughly renovated the clock, repairing the chiming mechanism (“Schlag- und Hammerwerk”). His signature is preserved on the repair invoice.
Clockmakers were often organized in guilds, which ensured high standards for their work. The apprenticeship period lasted between 4 and 6 years, ensuring that clockmakers were highly skilled. Their role was not only to maintain the accuracy of the clock but also to introduce innovations and adaptations to the mechanism according to the community’s needs.