The pallet staff or pallet arbor, has the important role of supporting the pallet assembly and lever in a lever escapement. They run primarily in jewelled holes, except for in the cheapest of clocks and watches, and in the best clocks, often have an end stone supporting the lower pivot.
The presence of an endstone requires that the pivot is shaped differently, adding further complication to the staff, but this is to be ignored for the remainder of this post. Pivot design is covered in this separate post.
Pallet staffs are either press fit; as used in modern clocks and watches due to ease of manufacture. Or they are a screw fit; as is more common in older lever escapements.
Press fit pallet staffs are a simple cylindrical shape with a pivot at each end. The cylinder is often imperceptibly tapered to aid fitting.
A press fit, also called an interference fit, fastens two components by friction. In the case of a lever escapement, the forces involved in the daily running of the clock are low enough, that the fit needn’t be tight. In many cases, replacement parts are not made to strict measurements but are made to fit.
In mass production however, parts must be made to strict tolerances. Calculating the tolerance of an interference fit can be complicated; it is dependant on material, temperature at time of manufacture, operating temperature etc. but for the purposes of this basic example, we are looking at a shaft oversized in the range of about 0.01 – 0.005mm.
The press fit staff is typically fitted using a jewelling press, where micrometer adjustment can specify the depth to which it is pressed in, and therefore the height of the lever in reference to the escape wheel and balance. Some press fit models do have a shoulder for ease of assembly, in this case, the height cannot be adjusted.
The Screw fit type of pallet staff is a far more complicated design, as there needs to be shoulder, up to which the lever is screwed on. The thread must be cut all the way up to the shoulder, and of course be fitted correctly to the thread in the pallet frame. The shoulder determines the height at which the lever sits, and cannot be adjusted simply by pressing it up or down the staff.
As for the thread pitch etc used for this type of pallet staff, it clearly varies from clock to clock, and in these two examples, is very different.
I don’t know how a repairer is supposed to define the thread in order to cut a replacement, at this scale, thread gauges wouldn’t work. It is not a job I have had to do yet, but my approach would be to turn the outer diameter to match the original, then to pass it through the nearest available die. When the soft arbor is then screwed into the hardened pallet frame, the thread will be rubbed into the correct form. I would then proceed to harden and temper the pallet staff.
If you can provide any insight into how a repairer would define such a small thread, it would be greatly appreciated for you to add your thoughts to the comments below.
The balance staff is the axle on which the balance wheel is mounted. They come in several designs, but for the standard carriage clock is of the type seen above.
A balance staff whose length is perhaps the same as your little finger-nail, will have on average 12 critical dimensions, require perfect concentricity throughout, and must have perfectly shaped and finished pivots.
The critical dimensions are:
Lower pivot diameter
Lower pivot length (one third of which must be parallel)
Lower pivot to roller seat
Roller seat diameter (which must be a gentle taper)
Lower pivot to balance seat
Balance seat diameter
Balance seat height
Balance rivet height and undercut
Hairspring collet diameter
Top pivot diameter
Top pivot length
Between the roller seat and balance seat is the largest diameter, highest at the balance seat and tapering smaller towards the roller seat. This is the thirteenth critical dimension, but is not quite critical enough to make the list. This taper is often highly polished and adds decorative appeal to the staff, however the reason it is there is to provide clearance for the little dollop of shellac on the rear of the roller table which holds the impulse jewel. An obvious detail which somehow wasn’t apparent to me at first.
So why are these dimensions so critical?
The overall length of the staff must be such that it floats with barely perceivable endshake between the top and bottom endstones, but enough that it runs with absolute freedom. The pivot diameter, must of course fit the jewel holes with perfect side shake, and not a hair too much, pivot length allowing the pivot to rest upon the endstone without the cone of the pivot binding in the jewel hole. Still with me? Good.
The lower pivot to roller seat must position the roller high enough that it clears the lever and places the safety roller in line with the guard pin, and low enough that the impulse jewel enters the notch. The diameter of course must allow for a press fit of the roller onto the staff. The roller seat must the taper up to the balance seat.
The balance must sit high enough to clear the lever cock or bridge at all extremes of endshake, but low enough to leave room for the hairspring. It must fit the staff with no freedom and the rivet must protrude just enough, that when hammered lightly, it sits flush with the balance surface and hold it tight.
The hairspring collet diameter is less critical as the collet is split, but too tight a fit will distort the collet and make it hard to adjust beat, too loose will not hold the hairspring at all.
Staffs can now be easily produced to high tolerance using swiss CNC Lathes, and this is how it is done in mass production. However, in the small workshops of the clockmaker, or in independent watchmaking, they must be turned by hand.
Hand turning to these tolerances is incredibly difficult and requires a lot of practice. I am no expert as I often outsource this work due to time restraints, however it takes me roughly four hours of uninterrupted concentration to produce a well finished staff, a more experienced turner would take a third of this time.
For perfect concentricity of the staff, it should be turned between centres on the lathe or turns (using the turns, a primitive bow driven lathe, is still taught by many watchmaking schools) or should be turned in once piece, without ever removing the piece from the lathe collet.
Many prefer to turn the staff in two ‘chuckings’, or, by turning half of the staff, then turning it around in the collet and turning the other half. Whilst there are some concentricity errors with this method, the affect on timekeeping is minimal, and only noticeable in the best of clocks and watches.
They are made of steel, hardened and tempered to blue. Blue steel rod is available from suppliers and is used almost universally for staff turning. This eliminates any possible distortion from hardening after the workpiece is finished.
The pivots need to be burnished to perfect their surface, and their ends must be rounded over. The shape of the pivot end is very important to reduce friction on the endstone. To do this a special tool is used, the ‘jacot tool’. I can be seen using one in my youtube video ‘repivoting a platform escape arbor’.
As this isn’t an instructional, but merely an informative post, let’s move on to the variants of staff available.
In carriage clocks with the lever escapement, the staff is almost certainly going to be as in the above example. The most likely variation you will meet will be a balance wheel which is a press fit, rather than riveted.
Another type of balance staff you will encounter will be of the alarm clock style. These are often much simplified in design, a necessity of economics, which unfortunately is extended to their quality and function. The staff is a simple rod with ground conical pivots, the safety roller is instead a milled slot in the staff, the impulse jewel, a steel pin protruding from the balance wheel. Others may have a roller table instead of the milled groove.
Of course there are special cases, (Chronometers, Duplex escapements) but these are outside of the scope of this post.
However in watchwork especially, many variants exist.The Ronda balance staff book from 1968 lists a total of 22 staff variants. Many variations are due to the changing ways the balance wheel is mounted, roller design, or pivot shape. This too, is outside the scope of a clock based post.
This concludes my introduction to balance staff design and manufacture. I will in the future post an informative ‘how to’ turning article. If you can provide any further insight, please do so in the comments below.
In the mid eighteenth century, English Clockmaker Thomas Mudge invented the lever escapement. Although it lacked certain important features, to be added by subsequent makers, its genius was in the way the escapement was detached from the balance wheel for the majority of its vibration. This allowed the balance to oscillate with little external influence; the situation in which a balance performs at its best.
The modern-day English lever escapement, derived from Mudges original design, and developed by several notable horologists such as; Josiah Emery, Peter Litherland, and George Savage, has remained largely unchanged in design since the late eighteenth century. After further refinements in the mid nineteenth century,the Swiss lever escapement was developed along side it. Both types of escapement continued to be made, although eventually the swiss lever won.
The Swiss lever escapement, due to its superior design, ease of manufacture, damage resistance etc. is now the most common form of escapement in any mechanical watch, or travelling clock.
The English Lever
The characteristics of the English lever are
Fine, pointed escape wheel teeth.
The lever set tangentially to the escape wheel.
The impulse surface is wholly on the pallet stone.
The tangentially set lever of this escapement allows it a smaller footprint than the Swiss lever escapement, but the fine pointed teeth (cut in a brass) are susceptible to damage, and have high wear characteristics at the tooth tips.
The lever escapement is composed of three major parts
The escape wheel.
The balance wheel.
The escapement has two main jobs
To provide impulse to the balance wheel.
To release the escape wheel teeth one at a time, or regulate the running of the gear train.
Each of the three major components can be broken down into further minor components.
The balance complete.
This is the colloquial name for the entire balance assembly, and is composed of the following:
The balance staff – the shaft on which the components are arranged. There are as many as thirteen critical dimensions on this tiny shaft.
The impulse jewel – these come in many shapes and sizes, although the ‘D’ shaped jewel has become the standard.
The hairspring – or balance spring. The dimensions of this spring actually determine the rate of the clock. By alerting its length via the index (discussed here) we can speed up or slow down the clock.
I have written a separate post for everything listed which covers their design, development and manufacture in more detail. Just click the blue text to Jump to that page.
The English lever itself is made of 5 components plus the guard pin, each having their own complications.
The pallet staff – the shaft upon which everything is assembled. They are either a press fit into the pallet frame, or are threaded and screwed into place.
The pallet frame – the steel frame which holds the pallet jewels. A slot is cut into the frame to accept the jewels.
The pallet stones/jewels – there are two of these, the entrance pallet and the exit pallet. Each has a locking corner, a locking face, an impulse face and an exit corner; they are attached to the pallet frame with shellac. English lever pallet stones are often curved on the impulse face to reduce friction.
The lever – this is often riveted to the pallet frame by two tiny pins. It has formed into it; the horns, the guard pin or its solid equivalent, and a counter weight to balance the pallets mechanically and aesthetically. The lever also interacts with the banking pins which are part of the platform plate.
The escape wheel.
The escape wheel is cut from hardened brass, has fine pointed teeth, usually fifteen of them, and is riveted to the escape pinion to take drive from the gear train. The escape wheel has no parts to list, and its physical attributes such as thickness and diameter may differ from clock to clock.
It is important that the escape wheel is light and well-balanced, as it accelerates and decelerates at great speed, so you will notice that the crossings are delicate and the rim of the wheel is very fine.
The Swiss Lever
The characteristics of the swiss lever are
‘Club toothed’ escape wheel made of hard steel.
The lever, escape wheel and balance set in a straight line.
The impulse surface is split between the pallets and the especially shaped tooth tips.
The advantages of the swiss lever are many. The escape wheel by design is more robust and provides a more efficient impulse to the balance wheel. The lever is made of one piece, therefore easier to manufacture, especially with today’s wire EDM machines which are able to cut out a lever ready for assembly.
Its only misfortunes are the slightly larger footprint due to the layout and the added complication to manufacturing the escape wheel.
The escapement is composed of the same three major components, with the balance complete being of the same design; the escapement also has the same two jobs, to deliver impulse, and regulate the gear train.
The swiss lever is made of just four parts, plus the guard pin.
The pallet staff – as before, these can be a screw fit, but are in most cases just pressed into the pallet frame.
The pallet frame – a simple 2D profile which compliments simple machining practices and efficient design.
The pallet stones – Both an entrance and exit jewel as before, the design differs, but not in the way they function.
The escape wheel.
The escape wheels of the swiss lever escapement are an interesting design. The tips of the teeth, instead of being pointed, have a locking corner and an impulse face. This works with the locking and impulse faces on the pallet stones to increase the efficiency of impulse, providing better timekeeping. When viewed from the correct angle, you will also notice that the tips of the escape teeth are thinned to reduce frictional contact with the lever; a feature only available due to the inherently stronger nature of the design.
The escape wheel is made of polished hardened steel, easily manufactured with modern techniques, but a very difficult thing to replicate with hand methods. The teeth need to be cut while the steel is in its annealed form, the wheel ‘crossed out’ prior to this, as the sawing may distort the blank disk. When the completed wheel is finished, it must be hardened and tempered without any distortion, and then polished to a mirror finish. Bare in mind, these wheels can be as small as a few millimeters these days.
With all of that said, it should be clear that the workings of the escapements are very similar, and only need to be described once. I have described the working of a Swiss lever escapement below, but the description applies equally to the English version.
How does it work?
For clarity, we will look at the escapement from three perspectives; the interaction of the balance and the lever, the interaction of the escape wheel and the pallets, and the interaction of the guard pin and roller.
There are many things happening at once in the lever escapement, so it is one of the more difficult escapements to understand, and by far one of the most difficult to competently describe in text.
Firstly, let’s assume that the balance wheel is already in motion. This is a self-starting escapement if properly set up, so it is a safe assumption, but we could equally blow gently on the balance wheel to set it oscillating.
As the balance wheel rotates, it carries with it the impulse jewel and safety roller. The impulse jewel is set to enter the notch of
the lever as it passes by. The notch is the cut out or slot at the end of the lever, between the horns.
The lever is resting at one extreme of its motion, as determined by the banking pins. As the balance rotates and the impulse jewel enters the notch, it gathers up the lever and begins to lift it away from the baking pin. The rotary motion of the balance wheel is transferred to the lever, which rotates around the pallet staff until it reaches the other banking pin.
The impulse jewel exits the lever notch and continues rotating with the balance, until the hairspring coils up, slows the balance, and changes its direction of rotation.
On its return swing, the impulse jewel enters the notch of the lever, gathers it up, and knocks it back over to the banking on in which it started, and this cycle continues endlessly.
Stop here and reread this section until you can picture the process in your head, it’s about to get complicated.
You see, what we have just described is a very simplified version of the impulse jewel moving the lever back and forth between the banking pins, but this is only half-true.
As the impulse jewel starts to move the lever, and therefore the pallets, the escape wheel passes the locking corner of the pallet and starts to provide impulse, pushing the lever (we’ll explain in more depth later, for now, keep thinking about the notch).
As the impulse pin begins to move the lever, it is pushed by the by the escape wheel at its other end, and accelerates faster than the impulse pin. At this point, with the impulse pin still within the notch of the lever, the lever actually pushes the impulse pin along, accelerating it and pushing the balance wheel.
The passing of energy from the lever to the balance like this is called impulse, and it is what keeps the balance oscillating for as long as the clock is wound. Without impulse, the balance would slowly lose energy due to friction, and stop. Think of it like pushing a child on a swing, if you stop pushing, they slowly come to a stop.
To summarise so far;
the impulse pin starts the motion of the lever, which soon takes over and gives the impulse pin a little nudge, passing energy to the balance wheel. The lever then locks against the opposite banking pin, while it waits for the balance to change direction and repeat the process.
We’re now going to look at what happens at the other end of the lever, between the pallets and the escape wheel.
We start as before, with the lever locked against one of the banking pins. In this position, one of the escape wheel teeth is resting against the locking face of one of the pallet stones. Let’s start with the entrance pallet.
The wheel is trying to rotate, but the pallet stone is stopping it.
Imagine yourself pushing against a wall which is able to move only up or down . The wall is at a ninety degree angle to the force you are applying, all of the forces are balanced, and there is no movement. Now imagine the wall is at an angle so that the top of that wall is leaning towards you. If you continue to push in the same direction, the force you apply will now push the wall upwards. If the top of the wall is leaning away from you, then the force you apply will be pushing the wall down.
In the above scenerio, you are the escape tooth and the wall is the locking face of the pallet stone. By pushing the pallet stone downwards, we rotate the lever around its axis in such a way that it is drawn in hard against the banking pin. This is a good thing, because if the clock is knocked and the lever jumps out of position, the pushing force of the escape tooth will draw the lever back to its preferred position against the banking pin, rather than leaving it to rub against the balance staff.
This is called draw and is what was missing from Mudges original escapement.
As the impulse jewel enters the notch (as described earlier), the lever rotates around its axis, lifting the pallet stone out of the escape wheels path. The draw angle causes the escape wheel to rotate ever so slightly in the reverse direction, until the tip of the tooth reaches the locking corner of the pallet stone.
At this moment, the ‘angle of the wall’ changes from negative to positive, and the escape wheel is able to accelerate. As it does so, the tooth tip slides along the impulse face of the pallet stone, pushing it up and out of its way. This is the moment when the lever outpaces the impulse pin, and gives the balance wheel a push to keep it going. In the above picture, the escape wheel is in the impulse position on the entrance pallet, as described.
Eventually the tip of the tooth runs out of pallet face and reaches what is known as the exit corner, and the escape wheel is free of all contact with the pallets and lever. It rotates freely for a moment, until another tooth further around the wheel comes into contact with the other pallet stone, the exit pallet.
The new tooth is caught safely by the locking face of the exit pallet and the wheel decelerates suddenly, its remaining force is put to work drawing the lever against the opposite banking pin. When the banking pin is reached, the entire operation stops, and waits patiently for the balance wheel to change its direction and start the process over again.
We mentioned earlier, that draw is a safeguard to return the lever to the correct banking pin if the clock or watch is knocked. But if the clock is knocked hard and the lever moves far enough, it would be possible that the escape tooth will reach the locking corner and begin to provide impulse to the lever.
If this happens and the balance wheel is not in the right position, then the lever will jump to the opposite banking pin. When the balance wheel returns to gather the lever up, it will crash into the outside of the lever frame, stopping the clock instantly.
To stop this, we need to ensure that the lever, if knocked, is not able to move far enough to unlock the escapement. This distance is controlled with a simple pin, sticking up or off the lever, and a roller mounted on the balance staff. If the clock is knocked, the lever moves until the guard pin contacts the safety roller (briefly disturbing the free motion on the balance) at which point, the draw angle swiftly pulls the lever back into position.
When it comes time that the impulse jewel enters the notch, and it is necessary for the lever to move without the safety action from preventing it, then a simple crescent shape in the safety roller allows the pin to pass freely by.
For the brief period of time where the crescent is presented to the lever, but the impulse jewel is not yet in the notch, the horns of the lever are so shaped that they will contact the impulse jewel and provide a simple safety action of their own.
There are many avenues of discussion we can go down with this escapement;
What is the best draw angle to reduce recoil whilst providing a safe action
What is the most efficient impulse jewel shape
How long should the lever be
The depth of locking
How many teeth should the pallets embrace
Servicing and repairs
Some of these I intend to answer in future posts, most of which have already been answered by the test of time.
If you have any questions, answers, or more detail to add to any particular aspect of the lever escapement, please leave a comment below.
The best way to understand the escapement is to study one. Take it apart, inspect it, rebuild it. Flick the lever back and forth with a little power applied, and see the effects of impulse and draw for yourself. A YouTube video of me doing this exact demonstration can be seen here.