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 balance wheel – the oscillating mass which controls many discrete aspects of a clock or watch.
The single or double roller – the assembly which carries the impulse jewel and the safety roller
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.