Escapement mechanisms control the release or transfer of mechanical energy to produce regulated or intermittent motion. They are best known for regulating mechanical clocks and watches, but escapement principles can also be used in counters, timers, indexing equipment and machinery.
Different escapements perform significantly different functions. Some periodically release energy from an independently loaded mechanism, while others actively drive an output in discrete steps. The distinction is important when selecting an escapement for an engineering application.
Escapements take many forms and are often designed specifically around the required timing, indexing or energy-release function.
In a mechanical clock, an escapement controls the release of stored energy from a spring or suspended weight. A pendulum, balance wheel or other mechanical oscillator determines the timing, while the escapement permits the gear train to advance in regulated increments and supplies sufficient energy to sustain the oscillator.
| Type | Principal Function |
|---|---|
| Inverse escapement | Actively drives an output in discrete increments |
| True escapement | Periodically releases energy from a separately loaded mechanism |
| Runaway escapement | Controls output speed through repeated interaction between an escape wheel and oscillating pallet |
| Tuned escapement | Uses a mechanical oscillator to establish the operating rate |
| Machine escapement | Produces controlled intermittent movement in machinery, counters and indexing systems |
An inverse escapement can be regarded as an arrangement in which the normal energy relationship of an escapement is reversed. Rather than simply releasing an independently driven wheel, the escapement member actively drives and indexes the output.
In the simple arrangement illustrated below, cycling a solenoid causes a verge to index a ratchet-like wheel. With suitable geometry, the wheel remains under control of the verge throughout the indexing operation and the driving member locates against a tooth at the end of the stroke.
This can provide good repeatability because wear of the tooth surfaces does not necessarily produce the same direct loss of angular location that may occur in a conventional pawl ratchet.
For correct operation, one driving tooth must withdraw sufficiently before the opposite tooth engages the wheel. The resulting lost motion allows the driver to acquire velocity before engagement, so inexpensive inverse escapements may operate through a sequence of impacts rather than smooth impulses.
The original RoyMech reference reports applications in electromechanical counters operating at rates up to approximately 3000 cycles per minute. This should be regarded as an example of achieved performance rather than a general operating limit; permissible speed depends on the particular geometry, inertia, actuator and impact loading.
The solenoid input can be replaced by a cam. Appropriate cam design gives greater control over the velocity of the driving member as it contacts the escape wheel and can therefore reduce engagement impact.
A further development uses a divided driving arm controlled by a two-level input cam. The two driving teeth can then be controlled independently, allowing engagement to occur with substantially reduced impact.
In the following arrangement, one side of the driving arm is replaced by a detent arm. The detent must contribute to indexing as well as locating the wheel; otherwise the powered arm would repeatedly engage the same wheel tooth.
A true escapement periodically releases a force or torque that is already trying to move the load. The main energy source therefore acts on the driven system independently of the relatively small control action required to operate the escapement.
This distinguishes a true escapement from mechanisms such as many ratchets and Geneva drives, where a substantial part of the transmitted power passes through the intermittent-motion mechanism itself.
The control components of an escapement can consequently operate at relatively low power while controlling the release of energy from a much more powerful source.
Horological escapements are highly developed examples. Although normally associated with clocks and watches, their underlying principles can be applied where accurately regulated intermittent motion is required.
High-precision escapements can be deceptively difficult to design. Small changes in geometry, friction, inertia or impulse can significantly alter the behaviour of the oscillator and therefore the timing performance.
In the runaway escapement illustrated below, torque applied to the escape wheel causes a tooth to contact one arm of an oscillating pallet. The resulting pallet movement releases that tooth while bringing the opposite pallet arm into a position where it intercepts a subsequent tooth.
The wheel and pallet centres are arranged so that one pallet arm repeatedly interferes with the wheel. Rotation therefore occurs through a succession of releases and arrests as the pallet oscillates.
Increasing the applied torque gives a stronger impulse to the pallet and tends to increase its oscillating speed, allowing the escape wheel to rotate more rapidly. Reducing the torque produces the opposite effect.
The original reference describes the resulting behaviour as broadly analogous to damping and reports an approximate relationship in which output speed varies with the square root of applied torque. This is a characteristic description of the particular escapement behaviour rather than a universal design equation.
Runaway escapements have historically been useful where a rugged mechanical timing or speed-controlling device is required. Their operating rate remains dependent on applied torque, however, so they are unsuitable where high timing accuracy is required without good control of the driving torque.
A tuned escapement uses a mechanical oscillator to establish its operating rate. Typical oscillators include a pendulum or a balance wheel and spring. Unlike a runaway escapement, the timing is primarily determined by the oscillator rather than directly by the applied driving torque.
The interaction is more complex than simply releasing the escape wheel. During each cycle the escapement also supplies a small impulse to the oscillator to replace energy lost through friction, air resistance and other dissipative effects.
The impulse should sustain the oscillator while disturbing its natural motion as little as practicable. Excessive or poorly timed impulse can introduce timing errors.
Two important traditional clock arrangements are the recoil escapement and deadbeat escapement.
In a recoil escapement, interaction with the pallet causes the escape wheel to move backwards slightly during part of the oscillation. In a deadbeat escapement, the escape wheel remains stationary during the locking portion rather than recoiling.
The absence of recoil can reduce disturbance of the oscillator and is one reason the deadbeat principle is associated with precision clock mechanisms. Actual timekeeping performance, however, depends on the complete oscillator and escapement design rather than this feature alone.
Deadbeat Escapement |
Recoil Escapement |
Precision mechanical timekeeping systems have also used separate master and slave oscillators. The master oscillator is allowed to run with minimal disturbance, while a slave system operates the display, counter or switching mechanism.
The master oscillator can be sensed and the slave periodically corrected to remain synchronised with it. This reduces the mechanical load imposed directly on the precision oscillator.
The original RoyMech material records historical systems achieving timing errors of approximately 0.92 seconds per day. This is retained as historical performance information rather than as a general comparison with modern time standards or electronic oscillators.
Watch escapements can be divided into arrangements including friction-rest and detached types. The mechanism below illustrates a cylinder-type friction-rest escapement.
The escape wheel remains in contact with the oscillating member during a significant part of each cycle. A hairspring, not shown in the illustration, maintains the restoring torque of the balance assembly. The escape-wheel teeth are shaped to provide an impulse that replaces oscillator losses while minimising disturbance of the balance motion.
In the detached lever arrangement shown below, a hairspring and balance wheel form the mechanical oscillator. As the balance passes through the central portion of its motion, it moves the lever between its two operating positions.
The interaction occurs close to the region where the balance has high kinetic energy, reducing the relative disturbance caused by the work required to operate the lever.
After the impulse and unlocking operation, the balance is substantially free from the escapement for most of its oscillation. The escape-wheel teeth and lever are arranged so that a small amount of energy is returned to the balance to sustain its motion.
Escapement principles can also be used in machinery where the requirement is controlled intermittent movement rather than timekeeping.
The required stepping rate may be fixed or adjustable, and the energy used to move the load can be supplied continuously and released intermittently.
The arrangement below uses an electric motor driving an escape wheel through a slipping clutch. The escape wheel remains stationary until a solenoid moves the pallet.
As one pallet tooth releases the wheel, the opposite tooth moves into position to arrest it. The angular increment is therefore determined principally by the mechanism geometry rather than by the duration of the electrical operating pulse.
Instead of driving the escape wheel directly, a motor can progressively wind a spring. The stored spring energy is then released through the escapement when an output movement is required.
This is useful where the machine requires short bursts of relatively high output power separated by longer dwell periods. The motor then needs to supply the required energy over the complete cycle rather than provide the entire instantaneous output during each short movement.
The original reference describes machine escapement developments operating at approximately 1200 steps/min, with experimental expectations of considerably higher rates. These figures are retained as examples of particular designs rather than general escapement speed ratings.
Another arrangement uses the input motion to wind a spring and then releases the stored energy by triggering an escapement. The output is therefore intermittent even though the input may be continuous.
This type of mechanism isolates the input drive from the sudden change in load associated with rapid output movement.
The following machine escapement has a continuously driven input while the escape member is loaded by a stalled motor, spring or slipping clutch. A holding ring and plate arrangement alternately restrains and releases the output.
Linkages can be combined with other machine elements to generate intermittent motion. Four-bar linkages, for example, can be arranged so that useful output movement occurs during selected portions of the input cycle.
Linkage geometry provides considerable freedom to tailor displacement, velocity and acceleration characteristics. Such arrangements have historically been used for applications including film and punched-tape indexing and can be combined with other locating or intermittent-motion elements.
Electrically assisted mechanical escapements should be distinguished from purely electronic timers. The timing or indexing function remains mechanical, while electrical or electromagnetic means provide sensing, impulse energy or actuation.
Some arrangements resemble lever escapements but sense and sustain the balance wheel electromagnetically. In other designs the oscillator itself supplies the mechanical movement to the escape wheel while electrical energy replaces the losses in the oscillator.
Where adjustable intermittent motion is required, a clutch and brake can be used instead of a traditional escapement. Electrically controlled clutch-brake systems allow the periods of drive and dwell to be changed without altering the mechanical geometry.
Available power, indexing rate and stopping accuracy depend on the motor, clutch, brake, load inertia and control system.
| Consideration | Engineering Significance |
|---|---|
| Function | Determine whether the escapement must regulate timing, release stored energy, or actively drive an intermittent output. |
| Oscillator | For tuned systems, oscillator period, inertia, damping and disturbance from the escapement determine timing performance. |
| Input energy | Establish whether energy is supplied directly, through a spring, weight, motor, slipping clutch or another storage mechanism. |
| Index increment | Tooth count and mechanism geometry determine the movement produced during each release. |
| Impact | Repeated engagement and arrest can produce high local forces, vibration, noise and wear. |
| Inertia | Higher driven inertia increases the energy that must be accelerated and arrested during each indexing operation. |
| Friction | Friction affects energy loss, oscillator disturbance, wear and repeatability. |
| Accuracy | Clearance, wear, tooth geometry and dynamic overtravel can affect the final indexed position. |
| Operating rate | Higher cycle rates increase dynamic loading and may require more controlled engagement and release. |
| Life and maintenance | Materials, lubrication, contact stresses and replaceable wear components should suit the required operating life. |