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A fellow clock enthusiast has kindly sent me some images of his Gents' bell striker with permission to use them on my blog. The following is an attempt to describe the function of each part of the Bell Striker mechanism from examining these images. I base my description on my experience with weight driven clocks which also used count wheel striking. Any errors or omissions are entirely my own and will be corrected as soon as they are discovered.
As is typical of Gent's genius they used electricity to avoid the use of heavy weights normal to almost all clock making practice up until that time. The use of electricity allowed the bell striker to be remarkably compact. Completely avoiding the need for heavy weights to be lowered over a very long fall down through the building which housed the clock. Which would have been the typical requirement for any striking turret clock. Moreover, the strike train usually requires much heavier weights than the going (timekeeping or ticking) train in a clock. The extra weight ensured reliable striking even when a clock and its weight cables, shafts and gears were neglected. (wheels and pinions are the terms used by horologists for large and small gears respectively)
The secret to applying electrical power to any mechanism is to drive it at the fastest end of the gear train. Any reduction in speed greatly multiplies the power (torque) available further down the gear train. Conversely, weight-driven devices usually do all their work at the slow end using massive weights. Then use the vagaries of a long gear train to greatly reduce the power via friction, to almost nothing, at the faster, escapement, or air brake end.
Without falling weights there is no need for a clock winder, minder and oiler. An electrical clock system can be fitted and (almost) forgotten in some very inaccessible places. Allowing the fitting of dials and bells in structures which could never cope with a weight driven system. Examples include slender towers, even chimneys and war memorials. However, this places great demands on the reliability of the mechanisms to avoid periodic breakdown through simple neglect. Inaccessibility has its price.
Gents constantly worked to improve their equipment. Often using very advanced materials for the time. Stainless steels, hard chrome plating and aluminium were certainly not commonplace materials in the heyday of these cutting edge, electromechanical clock systems. Ironically, a simple (and cheap) wooden box to cover these mechanisms would avoid all the falling dirt associated with their usual situations. This is also true of weight-driven clocks of course.
When any clock is asked to strike the hours there are three distinct tasks to be performed: The first is to lift and drop a hammer to strike a suitable bell. The second is to accurately and reliably count out the number of hours to be struck. The third is to start striking exactly on the hour. This is so that the bell will keep the same time as that shown on the clock dial.
Despite their usual size and position, public clock dials are not always easily visible. Particularly in the narrow streets of many older cities. Or when the viewer is separated from the clock by distance or obscuring trees. It is known that the very earliest clocks had no external dial but did have audible striking. The attentive and interested listener pricks up his, or her, ears and counts the number of hammer blows on the bell. Then knows the time, with some confidence, without ever having seen the clock dial.
Here I have labelled the main components of the bell striking mechanism. There are four rotating shafts (arbors or axles) and two long, pivoted levers. There are also two, speed-reducing worms and their matching worm wheels. One large and one smaller one. Plus two small bevel gears used to 'turn the corner' between the cam shaft and the count wheel, worm drive shaft. Note that the mechanism is resting on a trolley which is not part of the device. This trolley is only to facilitate movement of the very heavy mechanism. The painted, black, cast iron baseplate dictates the major dimensions of the C54 Bell striker.
This Gents Pulsynetic C54 Bell Striker both counts the hours and lifts and drops the bell hammer. All driven by quite a small, mains, electric motor. This particular Bell Striker mechanism measures 615mm x 320mm with an overall height of about 280mm. Or roughly 24" x 12.5" x 11" high, in old money. It weighs all of 60kg! Or 132lbs! This weight is mostly due to the heavy, cast iron baseplate. The mechanism shown here is described as 'early' due to the overall use of black paint and design details. Later examples would be painted blue-grey or (later still) even plain, industrial grey. See the bottom of this page for fine examples of later Pulsynetic Bell Strikers.
In the image above, the electric drive motor is on the right. This is the prime mover for all four shafts via the three pairs of gears. The motor is coupled to a long, rotating shaft which runs the length of the striking mechanism. Over to the left (in this view) can be seen the larger worm wheel. This is driven from beneath by a matching worm (a short, coarse-threaded, screwed rod) on the extended motor shaft. The worm and its worm wheel reduce the speed of the motor shaft by an amount dictated by the number of teeth on the worm wheel. The torque provided increases at the same rate as the speed reduction with only a slight loss due to friction. The worm wheel itself drives the cam shaft at right angles to the motor shaft. The cam shaft holds the cam near its centre. There is also a long, latch release detent (finger) fixed alongside it on the cam shaft.
At its remote end, the cam shaft has a small bevel gear fixed to it. This drives a matching bevel gear to turn another worm on the same shaft. This second worm drives the second, smaller, worm wheel. Which drives the short (fourth) shaft. Which also carries the vital strike count wheel.
In addition to the four rotating shafts there are two long, rocking arms: A black steel arm for pulling the bell hammer wire is pivoted just behind the motor. There is also a long, counterbalanced, aluminium, control arm. Which has electrical switching duties and also engages with the count wheel via a detent. (i.e. A steel tongue formed on a bracket fixed to the control arm)
The extended motor shaft continues to the extreme left. Where a spring and solenoid-activated disk brake is fitted. The brake is important because the inertia of the motor cannot be brought to an abrupt halt with a pin and (air brake) fan like a typical weight driven striking clock. If the electric motor were to free-wheel on, until friction eventually brought everything to a halt, this would leave all four shafts at random positions. The random effect might even be cumulative. Leading to all sorts of problems. Without the brake an overrun would occur each time the motor was switched off. Leading to a chaotic and random situation between important components.
The rest positions of the four shafts after striking are not just accidental. The hammer lifting cam must be brought to a definite rotational position after striking. So that it is positioned accurately and ready for the next strike. It is a cardinal rule in clock work that striking starts without any load on a hammer lifting device. This requires that the cam is at its minimum lift position at rest. The system thus has a chance to get up to full speed, before the greatest hammer lifting load falls on the drive system. Maximum torque for greatest lifting power can then be reliably achieved. If the cam was already lifting the bell hammer at start up it might even stall the motor. Or draw excessive current with consequent damage to the motor and its switching contacts.
Here is a picture of the hammer lifting cam and large worm wheel seen from the other side. The cam works against a roller on its underside. Pushing down the long, black bell striking lever to pull on the bell hammer wire.
The bronze bearings for the motor shaft extension are visible on either side of the worm wheel. Each has a grease or oil cap. The worm is hidden behind the cam in this view. The long, upright lever, next to the cam, is the unlocking detent for the sprung latch when it is holding down the control arm.
The motor runs on standard mains electricity at 250Volts 50Hz AC.
It runs at 1425rpm and drives a 72 tooth worm wheel via single start worm:
1425 /72 would provide ~20rpm on the cam shaft. Or one strike every 3 seconds. Which will sound the bell with proper authority and suitable pondus. Perfect!
A faster strike always sounds completely lame. Even worse if a clock fly (fan) is loose on its arbor (axle) and the speed of striking rapidly increases throughout the strike! The friction springs which hold the fly against its shaft on the striking train of the clock should really be adjusted. If only to avoid a strike train runaway as happened with "Big Ben"! (Westminster Clock) However, the Gents' bell striker has no need of an air brake fan. It has an electromagnetically released , spring-operated brake instead.
Here is the notched strike count wheel (or count plate as they are also known). You will notice that each 'land' between the deep notches on the rim has a number stamped there. Each 'land' is longer than the one before it as it rotates clockwise when seen face on. One o'clock is just one wide notch. These numbers represent the number of strikes of the bell for that hour. This is standard clock practice where count wheel striking is fitted. In a clock a lever tries to fall at each blow of the hammer but rests briefly on the 'land' as the count wheel slowly rotates. When the count wheel turns far enough the next deep notch arrives at the detent. The lever can then drop into the notch and locks the strike system until the next hour. In the case of the Gents Bell Striker the lever lifts into the notch. Which amounts to much the same thing. Since it stops the (electrical) power going to the motor by releasing the microswitch.
Notice the small black bracket and its detent (tongue) which sits deep in the count wheel notch between 4 and 5 o'clock in the image above. This bracket and its detent are fitted to a long pivoted and counterbalanced control arm of bright metal. Probably of lightweight aluminium alloy. Its lightness is to make counterbalancing easier. When the tongue (detent) is sitting in the notch the striking is locked. Not by the tongue itself but by the solenoid operated brake on the extended motor shaft. Which is actuated by the control arm rising away from the motor controlling microswitch.
Here the count wheel (or count plate) can be seen sitting on its own short shaft with its driving worm wheel. Both are pivoted on a raised extension of the main base casting. To the right of the count wheel in the foreground can be seen two bevel gears. These provide a right-angle drive to a second worm hidden under the worm wheel which drives the count wheel alongside it. A spring is used to avoid damage should the system suffer a mechanical blockage. This spring and its drive pin can be seen through the count wheel spokes. This spring may be to avoid damage to the count wheel rim if the detent does not clear properly. The notch walls are also biased at an angle to help the detent to enter and exit. A jammed condition might occur if the solenoid-activated, motor brake failed. Perhaps allowing the motor to run on for too long. Even causing the locking detent to jam against the side of a notch.
The worm drive bevel gears are well seen in the foreground in this image. The number of teeth on the second worm wheel must be 78. 1+2+3+4 up to 12. Of course the count wheel is not rotating continuously. It only turns when the hours are being struck and it is being driven by the electric motor. Everything stops and rests in between striking sequences.
On a striking clock a bar would fall onto the outside of a ring fitted to a second wheel in the gear train. This ring would have a single notch. When the bar falls into the notch the striking is brought to an abrupt halt. So a flat-bladed fan is used in most clocks to reduce and steady the gear train speed and the shock of stopping the strike so suddenly. (Though torque is usually very low at the fast end of a clock train) The fan also helps to even out the pauses between each single, hammer blow on the bell. Instead of an air-resistance fan the Gents' unit uses a solenoid operated brake. This brake is on the end of the extended motor shaft. Its normal condition is brake locked on.
Though it is difficult to see from this angle the opposed brake disks are on the right behind the black painted metal shield. When no current is flowing to the solenoid the disks are pressed firmly together by the large coil spring on the left in this image. The disk attached to the solenoid is fixed. So this slows the motor shaft by friction without an abrupt shock to the entire system. The driver motor has already been switched off, as the brake is re-applied by simultaneously cutting power to both the motor and the brake solenoid. The solenoid coil has a measured resistance of 236 Ohms suggesting mains operation. The brake solenoid coil is only energised during striking. It will draw 1 Amp = ~244 Watts.
The bell hammer, strike lever's pivot is just alongside. As is that of the control arm.
Here are the two control arm electromagnets. These sit under their armature which is fixed to the control arm. The many copper windings are bandaged and lacquered for extra insulation and protection. The armature is attracted to the poles when electricity is applied on the hour to the two coils. The armature firmly pulls down the control arm against the counterbalancing weight. These electromagnets are only activated by the timed impulse from a remote contact maker on the hour. No electricity flows through these coils at any other time. They have a combined resistance of 33 Ohms. An applied DC voltage of around 33 Volts would draw a current of 1 Amp during the short impulse required to pull down and latch the contacting/strike control arm.
The control arm catch is labelled sticking up above the control arm in this view. This catch holds down the control arm on a step cut into it. The brass nut near the end of the control arm holds a square peg which fits the catch step.
An overview image of the cam (right) with its roller underneath, the bevel gears (top left) and the aluminium control arm (left) and its catch. (bottom left)
A long extension on the catch reaches backward and bends sharply to point towards the cam. The long steel 'finger' on the cam shaft is the unlatching detent. This rotates with the cam and pushes the latch free of the control arm peg.
The cam roller is boxed within a strong bracket to provide firm support from both sides. The roller carries the downward force of the snail cam's rotation into the hammer lever and thence to the bell hammer wire. The rolling action avoids excessive friction and wear on the cam during striking.
An adjustment screw in a brass bracket (centre left) limits how high the counterbalanced control arm can rise.
The catch is pivoted at the bottom on a cast tab projecting from the base. There is a flat leaf spring visible resting on the base which provides a return spring for the catch.
Here is a close-up, cropped from a larger original image. It helps to clarify the shape and function of the L-shaped latch. Which I have 'painted' in bright orange with PhotoFiltre image handling software. The latch was rather lost in the smaller image I used above. Once recognised it is much easier to see what is lost in the detail.
Anthony has kindly supplied an image of the latch removed from the mechanism. The latch is even more complex than I had imagined. Its leaf spring lies alongside. Thanks to this new image I believe I can now follow the entire sequence of actions.
Bell striker sequence of operations:
The system sits motionless with the motor shaft brake applied by its spring.
The strike locking detent sits deep in a notch in the count wheel rim.
The aluminium control arm is raised and unlatched.
The microswitch contacts are open. No power is flowing.
On the hour, a low voltage, impulse comes from the impulse timer. (Contact device or programmable bell ringer mechanism?)
The electromagnets pull down the control arm against its opposing counterweight.
The control arm falls, is latched down and closes the electrical contacts in the microswitch.
The microswitch energises a relay in the associated black box.
The relay takes responsibility for switching the mains electricity to the motor and brake release solenoid.
Current flows simultaneously to the drive motor and brake solenoid pulling off the brake and starting the motor.
The striking system now runs under the power of the motor.
The large snail cam rotates to lift the bell hammer by depressing the long black lever and pull wire.
The unlatching detent (finger) releases the control arm latch on every revolution.
The count wheel rotates but the intervening 'land' won't allow the arm to rise again despite its counterweight.
The locking detent rubs gently along the rim of the count wheel.
Striking continues until the locking detent is finally allowed to rise again with the arrival of the next deep notch in the count wheel rim.
The microswitch is released as the control arm rises with the help of its counterweight.
Power is switched off as the microswitch contacts open and release the relay switching contacts.
Without power, the motor slows but would run on due to momentum.
The moment the brake solenoid is switched off its powerful spring re-applies the brake.
This quickly stops the motor shaft from turning.
The system sits quietly and waits for next hourly impulse from the remote impulse timer. (Contact device or programmable bell ringer)
The main disadvantage of count wheel strike control is maintaining a match between bell and dial. Unless the count wheel turns then night silencing will interrupt the count wheel from counting the hours in numerical sequence. During night/weekend silencing the clock moves on but not the bell striker! Then, when the strike action is restarted the count wheel will continue from its last struck hour. Regardless of which hour, that might have been, it will strike the next hour in sequence. By timing the strike silence to exactly 13 hours this problem can be overcome.
I am indebted to Anthony Roberts for his kindness in supplying the images above and many technical details. Hopefully they may allow the guardian, or owner, or even the discoverer of other bell strikers to make sense of their own mechanism. Thus avoiding it standing idle. Or even worse; being scrapped as unrepairable through wilful ignorance!
Below is an image of a later model of C54 Bell striking machine in wonderful condition. Of a slightly different layout to others shown here. The earlier microswitch seems to have given way to a pair of heavy contacts. The motor shaft brake and other details seem much more complex and robust. Perhaps it is a larger model to strike on a heavier bell? The previously overhanging brake assembly has been considerably modified and brought well inboard on these later models. The counterweight on the rocking arm has also been massively increased in size. Though the latch and its peg are still present. Note the use of a long coil spring on the hammer pull lever to limit any potential damage due to a stuck pull wire.
This image is borrowed from Hans Vrolijk's excellent website: http://www.vrolijk-clocks.nl/page/gent-motor-driven-striking-gear One of only two C54 bell strikers found in a general Google search.
My only interest in publishing my blog is to share details of these fascinating mechanisms with a wider audience and to make them as easy as possible to find online. Hopefully to avoid their early redundancy. I receive no financial reward for my efforts towards that end. If a single device can be saved from the scrap heap then my countless hours of typing, editing and research will have been worthwhile. Otherwise what is the point of documenting this robust, but gently fading technology?
Here is another fascinating website dealing with a working Gents' system. Now with a cleaned and beautifully restored (even later model) Gents C54 Bell Striker: Note the changed layout compared with the earlier examples above. The count wheel and cam have been moved even closer to the motor and the whole mechanism simplified.
Electric Impulse Clock Systems
Unfortunately Martin's email address bounces and he gives no surname. If Martin would like to get in touch I would be grateful for permission to use his image. Or even to have a larger image to show sharper details.
Anything to help those clambering about on their knees, in a dark and rickety roof space. Who find themselves staring at a begrimed mechanism. They need help to more easily recognise their unexpected find. With luck they will find the Pulsynetic label and start searching for further clues on their computer, smart phone or tablet.
What is the point of a "learned" (charitable status) horological society which does not openly publish all its technical papers online? How is the amateur or professional industrial archaeologist to recognise his exciting finds? How is the auction house peruser to determine historical value from a mere pile of grubby scrap? Isn't it our job to produce a set of "Haynes manuals" for all these fascinating clock systems and their components? Hopefully before it is too late and they are all gone?
Answers on a postcard to chris.b at smilemail.dk Or leave a comment.
A link follows to a website offering a multitude of images of electric clocks and related mechanisms:
This link leads directly to striking mechanisms:
My thanks go again to Anthony and Donald for pointing out the more likely sequence of operations of the Gents' C54 bell striker. My original description required a very long impulse to hold down the control arm via the electromagnets. Their alternative (and much more sensible) suggestions were for a short timing impulse on the hour to re-latch the control arm. The latching ensures that the notch can be rotated clear of the detent without the need for an extended timing impulse. The latch is then released automatically by the next rotation of long "finger" on the camshaft during striking.
Anthony must also be thanked for kindly providing these excellent images. Any errors and failure to understand the exact sequence of actions of the bell strikers are entirely my own.