The Waiting Train Mechanism:


(Left click for larger view. Back click to return to the text)

I have tried to use the terminology of this original Gent's diagram: Please refer to this drawing if in doubt as to the exact details and movement parts mentioned below.

The Waiting Train mechanism is a work of electro-mechanical genius in my humble opinion. Once understood, its mechanical simplicity defies belief. Yet it was a breakthrough which had eluded clock designers for centuries. While the whole movement is known as a WT only the top left part of the movement consists of the Waiting Train mechanism itself.

The WT pendulum working alone is nothing more than a powerful, low speed motor for driving public clock hands. It already had its advanced servo control constantly reading the mechanical resistance and fine adjusting the power input required. (thanks to the amazingly simple but brilliantly ingenious Hipp Toggle and V-block described in the last chapter)

The motor pendulum itself would cheerfully ignore all timekeeping duties if left to its own devices. Providing it was connected to its 20Volts DC power supply the pendulum would keep swinging and the clock hands would keep on turning. It would continue to do this regardless of weather conditions outside the tower affecting the slow rotation of the clock hands.

Unfortunately the timekeeping would be absolutely dreadful! Totally unusable as a public clock. Yet the WT movement is capable of keeping time to one second per week without any temperature compensation of its pendulum. Pendulum design had been a vexing problem for clockmakers for centuries but the WT did completely without temperature control. The secret of the WT's success lies in the Waiting Train Mechanism. This allows the WT movement to be remotely controlled to a very high degree of timekeeping accuracy by a separate, electric master clock.

Gent's master clocks were (and still are) widely used to supply accurate time signals. The signals were used for clocking-in workforces, process timing and recording, to sound factory sirens, bells and whistles amongst many other things. The master clock could drive literally hundreds of clock dials of all sizes throughout a factory, school or office block via a thin wire. Anywhere where uniform timekeeping was required a master clock could provide that service. No more arguments between workers and the wages office over variations of timekeeping between clocking-in machines and their own watches or clocks. Uniformity of timekeeping and accuracy of process control, recording of production tasks and machine running time were all closely controlled or recorded via the master clock at the heart of the timekeeping system.

Right up to the beginning of the 20th Century public timekeeping was in the hands of tens of thousands of weight driven, mechanical turret clocks. Many churches and public buildings housed clocks and had external dials. Many of these dated back centuries and their timekeeping had become notoriously bad in some cases. The need for accurate public timekeeping to aid the industrial revolution and mass production became absolutely vital. The railways needed accurate timekeeping or their passengers could never rely on published timetables. Something had to be done about public clocks and their often wayward timekeeping. Many wealthy industrialists and financial empire builders wanted large, accurate clocks on their buildings as mark of their status and power over their workforces and the world they served.

The upper limit on the size of mechanically driven dials had been found in great clocks like "Big Ben" the Westminster Palace clock. The difficulty of turning such enormous hands on very exposed dials on very high towers was a mechanical nightmare. Requiring enormously heavy weights and vast and expensive clock movements. Such clocks needed a lot of space for the great weights to fall as they drove the enormous movement. The hands on these huge clock dials were often at the mercy of the elements. Onshore gales, flocks of of gulls or pigeons and winter snow and ice would resist the slow movement of the huge, highly exposed clock hands. Often causing the clock to stop or to run slowly and erratically unless provided with an over-abundance of driving power. Moreover, these larger clocks often needed constant supervision and exhausting rewinding at all too regular intervals.

Captioned image of the WT movement:

(Left click for a much closer view. Back click to return to the text)

The WT's time setting crank and its wooden handle has been removed for clarity in this particular image.

Gent's answer to the serious problem of limits of scale and expense lay in the Waiting Train movement and its Waiting Train mechanism in particular. The WT had an enormously powerful motor pendulum. This could be scaled up to match almost any power requirements. Gent's designers still had to find a way of controlling the pendulum's timekeeping to a high degree of accuracy. So they designed an ingenious and totally reliable mechanism which is blindingly obvious and amazingly simple once examined and understood. The genius lay in the design of such a simple mechanism in the first place. They chose to interrupt the drive to the clock hands very briefly without interfering with the powerful pendulum motor or losing control of the clock hands. Then releasing the mechanical "brake" with a small electromagnet precisely on time.

The hands of the WT driven clock dial are always connected to the wormwheel. (via lead-off rods and bevel gears) Wormwheels are always locked against free rotation by their matching worm. A worm is rather like a threaded rod or machine screw which rests against and rotates in the wormwheel's matching teeth. One turn of the worm advances the WT wormwheel by one tooth. A worm and wormwheel is a torque multiplier with a matching large reduction ratio of rotation. Assuming a 120 tooth wormwheel and worm of perfect efficiency the torque is increased by 120x and the speed of rotation reduced by 120 times. This is an ideal ratio for a public clock movement needing considerable torque to turn and control the huge hands but also needing slow rotation to show the time.

The WT pendulum at 70cm effective length was designed to run faster than one which would keep perfect time over a half minute. On each swing to the right the pendulum drags the worm round via a strong 16 tooth ratchet-toothed escape wheel and heavy driving pawl. The ratchet (or escape) wheel makes one turn in 27-28 seconds and is fixed on the same shaft as the worm. The worm drives the wormwheel which drives the clock hands via the bevel gears and lead off rods.

Here is an image of the WT section during normal running. The D-shaped pin has just passed its lifting position. The electromagnet has been energised allowing the right angle lever to drop back to its normal position resting against its back stop. (presently covered in a length of black plastic tube which should ideally be replaced by an eccentric tubular backstop like the original)

The masking pawl has dropped out of the way. Allowing the driving pawl back into contact with the teeth of the escape wheel. The D-shaped pin will make a full revolution with the escape wheel before it arrives back at the position where it will again lift the masking pawl. (Left click for larger image. Back click to return to the text)

At about 27-28 second intervals the masking pawl is lifted by a D-shaped pin which is bolted to the escape wheel. A high tooth precedes the D-shaped pin ensuring the wheel is safely gathered as the lifting face begins to push upwards on the driving pawl. Once lifted, the driving pawl can only slide back and forth on the top surface of the masking pawl as the heavy gathering hook (pawl) can no longer reach the escape wheel teeth. The hands are still securely held by the wormwheel and worm. Which keep the hands safely locked against any imbalances caused by ice or uneven pressures from the wind. A right angle extension of the masking pawl is locked into a step in the relay electromagnet's armature.

Here is the locked and lifted position of the WT mechanism. The right angle lever is locked in the step of the relay electromagnet's armature. The driving pawl slides helplessly back and forth on top of the masking pawl unable to reach the teeth of the escape wheel. The high tooth above the D-shaped pin has allowed the driving pawl to pull the wheel to the locked position. Without the high tooth the driving pawl might not be able to safely draw the escape wheel round by the last tooth. Thereby entirely stopping the drive to the clock hands. The high tooth ensures a positive gather as the masking pawl rises. Forcing the driving pawl out of contact with the normal height teeth on the escape wheel. (Left click for larger image. Back click to return to the text)

At exactly 30 second intervals (with a very high degree of accuracy) a short, clean, low voltage electrical impulse comes down the wire from the master clock. The release (or relay) electromagnet is energised and snaps its armature to its core. The right angle lever is instantly released from the locking step in the armature. Dropping the masking pawl out of the way. Which allows the driving pawl back into normal contact with the teeth of the escape wheel. The driving pawl starts to gather teeth normally again. The ratchet wheel and the worm begin to rotate driving the wormwheel. The wormwheel shaft rotates the bevel gears and the hands move on again. Without the slight pause ever being noticed by the public on the street far below.

A gear "train" is a series of connected gear wheels. The waiting train mechanism literally keeps the train of wheels (which drive the clock hands) waiting for about a second or two every half minute. Because the ratchet (or escape) wheel rotates once every half minute the worm is rotated twice in one minute. The 120 tooth wormwheel thus rotates once per hour. This speed perfectly matches the required rotation of the minute hand on a clock dial. A simple gear train (called motion work) further reduces this speed by 12:1 to drive the hour hands.

Another view of the Waiting Train mechanism.

An image of the rear of the movement: The worm is on the same shaft as the ratchet-toothed escape wheel. The back of the silver grey ratchet wheel can be seen on the right of the picture. The worm (which looks like a section of threaded rod or a screw is above the wormwheel and in close contact with the wormwheel's teeth. The worm locks its wormwheel against free rotation so it can only rotate as a result of the worm's own slow rotation. (left click for larger image. Backclick to return to the text)

In summary: The heavy pendulum's powerful swing is converted from linear into rotational energy by the driving pallet, ratchet wheel and worm. The worm and its wormwheel greatly increase the torque available to drive the minute hands of the external clock dial(s).

All the parts are very heavily built to cope for decades with the enormous strain placed on the movement by bad weather on the exposed hands of the clock dial. The materials are of the highest quality and the parts precision made. Gents were one of the first British companies to use hard chrome plating. This is used on the worm, the driving pawl, the escape wheel and the masking pawl. To ensure low friction and very long life despite sometimes indifferent lubrication. The plating also protects the steel parts from rust. Many of the movement parts are gunmetal or bronze for very long life in difficult, salty or damp environments. The quality of the lacquering speaks for itself. It is almost flawless after 70 years.

It is remarkable to think that this small movement could drive the hands of 4 dials of 6 feet diameter or two of 8 feet diameter. A weight driven clock movement would have to be far larger to manage this feat. The WT movement could also serve as a powerful and reliable motor for anything which required very slow and steady rotation.


Click on any image for an enlargement. Back click to return to the text.



Neil said...

Great site Chris, and very nice photographs. I maintain two tower clocks in the city in which I live, but neither is a WT unfortunately. One is weigth driven, built in Melbourne Aust in 1939 from memory, & the other is a gents.

Chrisbee said...

Thankyou, Neil.

I'm sorry I missed your comment until now.

I ought to invest some more time in this project. The blog is still very much an experimental format for sharing this kind of special/narrow interest material. I have been monitoring the visitor numbers to assess interest levels before spending more time on further developing this material.