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Escapement Mechanisms

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Escapement Mechanisms

More details on Escapement Mechanism design are to be found in the links below the table


There are many forms of escapements often requiring some ingenuity in their derivation.   Escapements are gemerally used in clocks with mechanical motions.   When used with clocks the escapement controls the spring driven clock mechanism such that it moves in regulated steps controlled by a pendulum or an oscillating arm.

Inverse Escapements
Inverse escapement are really cam systems with the follower driving the cam.   They are called inverse escapements because although they resemble true escapements, they drive the load rather than merely releasing it.

The mechanism can be very simple and inexpensive. In the typical example shown below, as the solenoid is cycled the ratchet-like wheel is indexed by the verge.   With proper design, the wheel is always under control of the verge, so that overtravel is impossible.   Indexing accuracy is generally good since the driver is moving towards the centre of the wheel and stops. when it bottoms in a tooth.   Even with considerable tooth wear , the wheel still stops close to the original design position. For comparison, even a little tooth wear in a ratchett reduces positioning accuracy.

For the mechanism to operate correctly,one drive tooth must be at least partially retracted from the ratchet wheel before the other drive tooth strikes the wheel.    This lost motion is undesireable since it means that the driver will acquire some kinetic energy before it starts to move the wheel.   In an inexpensive inverse escapement, and low cost is one of the principle attractions of the device, indexing is accomplished by a series of blows or impacts rather than by a series of smooth impulses.   Nevertheless, this is a useful mechanism for light and moderate power levels.   Inverse escapements are used in many electomechanical counters, and may be operated at speeds of 3000 cpm.   Variations of the inverse escapement are illustrated below .

In the figure below the solenoid has been replaced by a cam. Proper design of the cam can reduce the impact of the driver on the wheels but the cost is higher.

In the figure below the cam driver has been carried a step further by dividing the cam driver into two parts and using a double-level input cam.   Each drive tooth can be controlled independantly of the other so tha impact is practically eliminated.

In the figure below one half of the drive arm has been replaced by a detent arm.    This detent must index the wheel as well as position it,since if it did not move the wheel the powered arm would keep hitting the same wheel tooth and there would be no resulting motion

True Escapements

True escapements periodically release a force which drives a load.   The force is always trying to drive the load , but cannot do so until the escapement allows.   The main flow of power in ratchets and genevas, for example, is through the intermittent motion mechanism: on the other hand an escapement, which is a control device is operated at low power levels even when it is releasing energy from a high-powered source.

Usually horological escapements are not considered for intermittent motion in applications other than watches and clocks, but the need not be limited to these applications. They are the most accurate of all indexing mechanism, furthermore they do not necessarily have to be tiny , low powered devices.   Escapements that control steeple clocks have controlled forces of hundred of kN's.   Even the watch escapements should not be considered as delicate delays. Fuse bombs timers have been built to withstand 30,000 to 40,000 g's

High precision horological escapement are not easy to design or build for a slight change in configuration can cause a large change in performance.   Design of high precision horological escapements requires a great deal of experience.

Runaway Escapement

In the type of escapement shown below torque is applied to the scape wheel causing it to rotate. One tooth of the scape wheel encounters on arm of the pallet and the pallet is pushed aside. Rotation of the pallet in turn , frees this tooth, simultaneously bringing the alternate pallet arm into interference position with a second tooth.   The pallet and scape wheel are mounted on close centres so that one arm or the pallet will always interfere with the motion of the scape wheel.   As the wheel rotates its movement is arrested by repeated (periodic) impact with the pallet. Thus the wheel can rotate only when and as the pallet is free to oscillate.

As input torque increases , the scape wheel imparts a stronger impulse to the pallet.    The resulting increase in pallet speed allows the scape wheel to move more rapidly.   Similarly, as torque is reduced the wheel slows down. Resulting overall performance resembles that of a viscous damper, except that output speed is roughly proportional to the square root of applied torque in a true viscous damper speed is proportional to the torque. This viscous like action is a useful feature of the runaway escapement. Also performance varies much less with temperature and age than does with that of conventional liquid dampers. The inherent ruggedness and relative lack of environmental sensitivity of runaway escapements makes them suitable for many severe timing applications .   For accurate timekeeping, of course, input torque levels must be carefully controlled.

Tuned escapements

Most contemporary timers are tuned escapements instead of runaways. The runaway escapement output speed varies with applied torque but in a tuned escapement speed is determined by a mechanical oscillator - a pendulum or an oscillating spring-mass assembly .   Input torque has little effect on performance.

Although tuned escapements can look fairly simple the action is quite complex. The oscillating escapement periodically releases energy from a power source to drive a load. As it does this however , the escapement receives energy enough energy to overcome internal losses caused by friction and impact. If the escapement does not get this small but essential input the mechanical oscillator will be disturbed and timing errors are introduced.

There are two basic types of clock escapement- recoil and dead beat. A typical recoil escapement is shown below. It is called a recoil escapement because the scape wheel backs up slightly as it turns. a typical deadbeat escapement is also shown below. The scape wheel does not reverse motion so this type in theory is considered more accurate than the recoil type. In practice however the recoil escapement is often more suitable.

The best modern clocks use two oscillators. One allowed to run nearly free is the master oscillator. The other which drives the output counter or clock dial escapement or electrical switches is called the slave. Motion of the master is sensed electrically and energy losses are sometimes supplied electrically although mechanical devices are more common. The slave is periodically stopped or speeded up to keep it synchronised with the master. Accuracies of 0.92 sec per day have been obtained with some of these systems, they are more accurate than crystal oscillators over long periods of time.

Deadbeat escapement

Recoil Escapement

Watch escapments also can be divided into two classifications, friction rest and detached. The figure below illustrates the friction rest type.   It is considered a type of cylinder escapement, because the escape member on the pallet is a tiny cylinder and is classed as a friction rest-escapement because the scape wheel drages on the oscillating mass (balance arm) during most of each cycle.   A spiral hair spring not show keeps the balance arm oscillating .   Teeth of the scape wheel are carefully shaped to deliver a slight impact to the balance arm at every cycle to sustain oscillation while disturbing the balance arm motion as little as possible...

The detached pin lever escapement is shown below is a more sophisticated arrangement.   A hairspring coupled to the balance wheel keeps it oscillating. The balance wheel periodically pushes the pin lever ( connecting the scape and balance wheels ) from one side to the other. This push occurs when the balance wheel is passing through the point of maximum kinetic energy. At this point the oscillator is best able to perform work without being distrubed. After the balance wheel has moved the lever however the wheel is free to continue in motion. It generally oscillates about 360 degrees. As a result, the balance wheel oscillator is detached from all restraint during most of its cycle , so the timing accuracy is good.

Scape wheel teeth are shaped to impart no torque to the lever until the balance wheel moves the lever. then a small amount of energy is transmitted from the scape wheel to balance wheel to sustain oscillation.

This mechanism is not difficlt to make and is found in many inexpensive watches. Versions with jeweled pallets in place of the pins are used in very accurate watches..

In many machine applications it is desirable to be able to vary the stepping rate. The adjustable rate escapement shown in below resembles the inverse escapement in ****. In the inverse device however the arm is the driver. In the unit shown an electric motor is driving a scape wheel through a slipping clutch. The scape wheel remains at rest until the solenoid is actuated. As one tooth on the pallet arm frees the scape wheel, the other pallet tooth moves to engage the wheel. As a result, the amount of motion is controlled and is independent of the length of the electrical pulse delivered to the solenoid. This mechanism is part of a heavy duty counter. An alternate method of loading the escapement is shown in fig 30. A motor winds a clock sspring from the centre . The outer end of the spring engages a drive pin on the scape wheel which is driven by the spring whenever the solenoid actuates the escapement.

The motor can be a normally stalled torque motor, continuously energised. Alternatively the motor can be turned on only when sensing switches indicate that the clock spring has run down (the switches sense the increase in spring dia as it unwinds.)

Spring arrangements such as this are useful if the load requires short bursts of pulses, followed by a long delay or dwell.    The motor need not be large enough to accelerate the load at the maximum pulse but only large enough to supply energy to the spring at an average pulse rate.    This can be quite low if the dwell time is a significant portion of the total cycle.

The escapement shown below can produce very high speed intermittent rotary motion.. One tape transport mechism has been designed to run at speeds of 1200 steps /min .   Indications were that 30,000 steps/min could be obtained with further refinement.

In another type of spring loaded escapement shown below input can be either continuous as shown or intermittent.   Output is intermittent and occurs once per revolution of the input cam.    In operation, the input winds a spring : continued rotation then releases the spring by triggering an escapement.   This is called a "load of fire" escapement and isolates the input from sudden changes in load.M

The figure below shows another common machine escapement , input to the escape cam is continuous. The scape wheel is loaded by a stalled motor spring or slip clutch.   the type of escapement is really just a holding ring-held plate combination & can be similar to the holding elements used with mutilated gears .

Impact shock and high acceleration are characteristic of any escapement since the load is released and stopped suddenly.   Noise & wear can result.   However the devices are frequently used whenever high indexing speed and /or precision are required.

Linkage escapement

Linkages are often used with other machine components to produce intermittent motion. The example below is a typical application using a four bar linkage.

A wide rang range of dynamic characteristics is available with four bar linkages.    Input can be varied easily, and accurately.  Linkages are often used to index movie films or punched tape,. they are less common in rotary drives.   In most cases, linkages are used with other elements to produce intermittent motion..

Electronic escapements

Electronic escapements are used mostly in clocks and watches.  They are electrically driven  mechanical escapements and should not be confused with electronic timers.

many electronic escapements resemble the pin lever mechanism shown above except that motion of the balance wheel is sensed (and maintained) electro-magnetically. In some cheaper clocks, the balance wheel is the main source of mechanical energy and it drives the scape wheel instead of just releasing it.   Batteries replace the losses in the balance wheel.

Clutch Brake Systems

A clutch brake mechanism is often used to produce controllable intermittent motion; some manufacturers supply motors with built in clutch brakes. If the clutch and brake are themselves controlled by electrical signals as in the system shown below, the duration of drive and dwell can be easily varied. Power capacity and indexing speed depend of course on motor size and the capacity of the clutch and brake.

Links to Escapement Mechanism Design
  1. ClokckWatch... A comprehensive set of pictures devoted to escapements
  2. Brock Eng. (Virtual mechanisms)...Excellent Notes and Graphics
  3. D& T Online ..A review of various types of mechanisms
  4. Chapter 11- Watch and Clock Escapement ..Informative download
  5. Introduction to Mechanisms ..Lots of very useful reference notes