Friday

The WT Pendulum

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Let us start the detailed look at the WT movement with the motive power behind its remarkable accuracy. It should be said right away that the WT's pendulum does not directly control the accuracy of timekeeping. In fact the WT pendulum does not even have any adjustment for length to change its timekeeping. Nor any compensation for changes in temperature. The incredible accuracy of the WT comes from half minute signals received from a remote, and quite separate, precision master clock.

The heavy cast iron bob (seen below) is actually cast onto a short, bolt-on extension of the pendulum rod. This simple, but clever idea allows the bob to be removed and packed separately from the WT movement while on its way to an installation. Thus saving potential damage to the movement over long journeys, perhaps even overseas. When work is required on the WT movement the bob can be freed in moments simply by by removing two bolts. This small but important detail is typical of Gent's remarkable skill in design and fabrication. The advantage over a simple hook on the pendulum is that theft becomes much more difficult without immediate access to a suitable spanner.


For the moment let's deal with the almost unstoppable power of the Waiting Train movement. Regardless of snow and ice on the hands outside the high clock tower the WT can always cope. It brushes off fierce winter gales without a problem. The secret lies in the Hipp toggle and its all-important, matching Hipp V-block.


The Hipp toggle is a chisel-shaped piece of brass pivoted freely about half way down the WT's pendulum. The toggle is carried back and forth across its notched V-block by the swinging pendulum. Normally the toggle passes freely over the block in both directions with a rather hypnotic rattle. For a clock fan the Hipp toggle is priceless therapy. One can watch for hours. Just waiting ... and waiting. Until, at between 20-60 second intervals, the Hipp Toggle can no longer fall free when passing over the Hipp Block. The arc of the pendulum has imperceptibly reduced to the point where the toggle can no longer clear its notched block.

Then, with a powerful downward thrust, provided by the inertia of the heavy pendulum, the toggle is forced deep into the notch in the block. This pushes the attached bronze contact blade downwards and closes the heavy electrical contacts.

Click on the images for a close-up of the Hipp Toggle, V-Block and contacts. I made the entire missing contact assembly myself to match images and drawing provided by fellow WT owners. The bent bars are silver soldered together from yellow brass strip. The originals were cast but I had no access to any spares.

I really should have lacquered them deep gold by now to match the other, original brass parts. The screws also need shortening and tidying up. The contacts were made from solid silver rod bought from a local jeweller and soft soldered to riveted brass bases. Obtaining the necessary bronze spring strips was difficult but the material were finally found in the non-ferrous scrap bin of a local factory. The insulation between the parts is mostly model maker's plastic sheet. The holes were bored oversize and model aeroplane, fuel tubing slid over the long screws to ensure electrical isolation of all the various parts.


The moment the contacts are closed the 20 Volts of DC (Direct Current) electricity streams through the thousands of turns of insulated copper wire in the large electromagnets. The electromagnetic power in the coils is transferred to the soft iron cores. Which then attract the rocking armature with great force. The armature has a broad roller at the top which rises. Pressing upwards under the cast extension on the pendulum rod just as it swings past.

 Pendulum drive mechanism.

As the roller is forced upwards, to follow the curve of the cast hook, it gives a push to the pendulum.  Allowing it to continue swinging for another 20-60 seconds or even more under light loads. The method of pendulum drive impulsing is very similar to the gravity arm and roller impulse of many master clocks. Except that, instead of gravity, the impulse is applied upwards, in the WT, by the power of the drive electromagnets via the armature roller.

In this way the pendulum automatically calls on only enough power to manage the loads placed upon it. Just as a parent pushes a child on a swing only when the arc drops below a certain point and the child demands more height. Fortunately the WT does not get bored or tired and will continue pushing its pendulum for decades unless the electric power supply is interrupted.


Another view of the pendulum drive electromagnets, rocking armature, impulse curve and roller pallet.

Note how the under surface of the "hook" is curved to almost follow the radius from the pendulum suspension pivot point. The difference in the curve from a true radius allows the roller to apply a push to the pendulum without jarring. Were it a true radius it would be a "dead" surface and no push would be applied. Such dead surfaces are used in some clock escapements to avoid variations in friction between sliding surfaces. The "corner" is the point at which the roller is freed from its upward pressure against the impulse surface. The Hipp toggle contacts will then open and the armature will fall back again due to gravity.

Under still conditions the pendulum may swing for up to a minute before taking another power impulse from the electromagnets. When, however, there is a roaring wind and ice forming on the exposed clock hands the loads on the movement increase enormously. Because of the increased resistance to rotation, felt through the lead-off rods, the pendulum loses its arc much more quickly. The moment the arc falls below the minimum set by the Hipp toggle the pendulum is rewarded with another push from the drive electromagnets via the armature, roller and impulse pallet.

As weather conditions improve the power impulses grow less and less often. When bad weather demands it the impulses can be as often as every other swing. An increase in power of up to 30 times, or more, compared with normal power impulsing. This clever power demand compensation was a remarkable breakthrough.

Most mechanical clocks have no answer to greater loads on the clock hands. The drive weights are simply made heavy enough to overcome any likely resistance. Which is wasteful and applies too much pressure when the clock is lightly loaded. If the weights are reduced the clock risks stopping in a storm. Some clock keepers increase the weights in winter and reduce them in summer.

Note the wax insulation on the electromagnet coils in the picture above. This was part of Gent's protection for the movement against the foul conditions often experienced by turret clocks. Notice the heavy build of all the parts to help the movement survive for many decades. Often under difficult environmental conditions abroad. Where skilled repairers were often completely absent. Gent's knew their market and built equipment to match the worst conditions imaginable. Despite the need for reliable function many of Gent's products still have an honest, workmanlike appearance. Gent's were were often at the leading edge of technical and materials use and development.


Here is an image of the hand wheel and pendulum support bearing housing at the top of the pendulum. This is another of Gent's breaks with tradition. The flat spring, which had supported pendulums of all kinds for centuries, is discarded in favour of sealed ball bearing races. (journal bearings) This was a bold move and shows Gent's care in their designs. The completely enclosed bearings could be greased in the factory and then be relied upon for several decades without skilled attention.  Being invisible, there was no temptation to add potentially damaging lubricants.

The main advantage was the lack of  a fragile pendulum suspension spring. Which could be broken on the journey out to some distant site of installation. Or broken during clumsy installation by unskilled labourers. The twin ball bearings themselves are housed in the silver-coloured castings. The top of the pendulum is bifurcated to allow it it be supported equally either side of the bearing assembly. In warmer climes no doubt the grease would be better distributed than in colder, northern environments.


The hand wheel has a very clear message on its face: Turning the handle occasionally, at random redistributes the grease and avoids localised wear on the concealed bearing races.



This image shows a lead-off rod, with universal joint, fitted into one of the four forked driver/expansion couplings on this WT movement. The lead-off rods eventually reach the backs of the clock dials. Where a similar forked coupling  would join the motion work. (A simple reduction gearing of 12:1) The motion work gearing reduces the rotational speed of the minute hand from one revolution per hour to once every twelve hours. The hour (hand) is fixed to a pipe which surrounds the solid minute hand arbor. (or shaft) The direct drive from the lead off rods drives the minute hand. While the hour hand is driven at 1/12 the rotational speed by the motionwork gearing. 

Lead-off rods can be of almost any length required to reach the clock dials. Abrupt changes of direction would be handled by bevel gears. This WT movement could drive the hands of four dials simultaneously. The simple universal, coupling joints are slotted to allow the lead-off rods to expand and contract with changing temperature without binding. Simple plain strap bearings were usually used to keep the lead-off rods in place. The longer the rods, the more twist in the system, over the length from the clock movement to the minute hand.

Where a clock refuses to run it is sometimes these simple metal to metal bearings which have dried up over time. A little oil may help if the strap bearing cannot be easily removed for cleaning and lubrication. Trying to push the lead-off rods lengthways in their bearings will sometimes indicate where they are sticking. The expansion joints should allow a bit of freedom along the length of the rod. So a stuck bearing will prevent this normal free play.


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