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Mechanism general notes

Ratchet Mechanisms

More details on Ratchet Mechanisms are to be found in the links below the table


There are many forms of ratchets often requiring some ingenuity in their derivation.   Ratchets are used widely in lifting equipment to lock the motion and prevent reverse rotation when the input force is removed.    Ratchets can also be used to drive a motion in one direction and allow free-wheeling in the reverse direction.  The best example of this application is the bicycle chain drive

Pawl Ratchet

Sprag Clutch

T The pawl ratchet as shown above is a very simple toothed wheel with a pawl allowing free rotation in only one direction.   The Sprag clutch is a modern variation working using a number of sprags between concentric rings.  When the outer ring rotates in one direction relative to the innner rings the sprags all pivot a little away from the outer wall allowing free movement.   If the outer ring reverses the sprags all pivot the other way and their geometry is such that they lock up the rings together..

Pawl Ratchet Applications

TThe pawl ratchet is simple, versatile, inexpensive and reasonably accurate.   It is used in all sizes, shapes and power capacities. The action of a ratchet can be either harsh or smooth depending on the configuration of teeth & pawls, accuracy of design and manufacture and the method used to drive the pawls. When a solenoid or spring is used as in the mechanism below the drive pawl strikes the ratchet wheel with an impact and the wheel will jump ahead if the load is sufficiently light.    This is one of the principal drawbacks of the plain ratchet.   A brake or detent, or careful attention to load will ensure accurate indexing.  The unit shown below includes a second set of teeth which engage a projection on the pawl arm to prevent overtravel.

Most ratchets incorporate a device to prevent the drive pawl from pulling the ratchet wheel backwards when the pawl backs up to take another bite.   In the figure a holding pawl is used for this purpose .    In some cases the load is sufficient to hold the wheel..

Spring Blade Ratchet

TThe ratchet shown below uses flat springs as drive and holding pawls. with proper heat-treatment this type of drive will give long life under light loading.    Indexing rates can be as high as several a second and the wheel can be indexed with accuracies of about one-tenth of a degree.    Spring-blade ratchets are typically used to wind springs and drive printed-circuit commutators or timing cams..

Cam Driven Ratchet

TIn this type of ratchet as shown below the cam can be design so that the drive pawl has very little kinetic energy when it strikes the wheel: this reduces impact & overtravel.    The unit shown below illustrates another variation of cam input. The ratchet wheel is indexed in a clockwise direction for either clockwise or counterclockwise rotation of the input shaft.  Although the ratchets shown are normally used in small counters, the same configuration can be used to drive loads of kW size.    Permissible indexing rates depend on the design strength of the various parts...

Friction Ratchet

TIn addition to the more common types of ratchets i.e pawl and toothed-wheel mechanisms other types of indexing drives can be classed as ratchets. The main feature is that an oscillating member works a one way clutch to index a wheel. One typical system is shown below. A reciprocating linkage drives the output shaft by means of a one-way spring clutch. Many other types of clutches can be used in a similar arrangement.

Magnetic Ratchet

TThe figure below shows a fully magnetic ratchet. When the solenoid is energised the soft-arm pole piece fastens to a soft iron tooth on the wheel and indexes the wheel. Wehn the solenoid is turned off, a compression spring raises the pole to meet the next tooth , whilst a permanent magnet holds the wheel. this method is useful for driving a wheel inside a hermetically sealed housing..

Links to Mechanism Design
  1. Brock Eng. (Virtual mechanisms)...Excellent Notes and Graphics
  2. D& T Online ..A review of various types of mechanisms
  3. Mechanisms ..A site with lots of useful moving graphics for simple mechanisms
  4. Introduction to Mechanisms ..Lots of very useful reference notes

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Mechanism general notes

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Last Updated 23/01/2013