In
mechanical engineeringMechanical Engineering is an engineering discipline that was developed from the application of principles from physics and materials science. Mechanical engineering involves the analysis, design, manufacturing, and maintenance of various systems...
,
backlash, sometimes called
lash or
play, is clearance between mating components, sometimes described as the amount of lost motion due to clearance or slackness when movement is reversed and contact is re-established. For example, in a pair of
gearA gear is a component within a transmission device that transmits rotational torque by applying a force to the teeth of another gear or device. A gear is different from a pulley in that a gear is a round wheel that has linkages that mesh with other gear teeth, allowing force to be fully...
s, backlash is the amount of clearance between mated gear teeth. In other words it is the difference between the tooth space and the tooth thickness, as measured along the pitch circle.
In
mechanical engineeringMechanical Engineering is an engineering discipline that was developed from the application of principles from physics and materials science. Mechanical engineering involves the analysis, design, manufacturing, and maintenance of various systems...
,
backlash, sometimes called
lash or
play, is clearance between mating components, sometimes described as the amount of lost motion due to clearance or slackness when movement is reversed and contact is re-established. For example, in a pair of
gearA gear is a component within a transmission device that transmits rotational torque by applying a force to the teeth of another gear or device. A gear is different from a pulley in that a gear is a round wheel that has linkages that mesh with other gear teeth, allowing force to be fully...
s, backlash is the amount of clearance between mated gear teeth. In other words it is the difference between the tooth space and the tooth thickness, as measured along the pitch circle. Theoretically, the backlash should be zero, but in actual practice some backlash must be allowed to prevent jamming of the teeth due to tooth errors and thermal expansion. This gap means that when a gear-train is reversed the driving gear must be turned a short distance before all the driven gears start to rotate. A similar effect is the taking up of slack when a train starts to pull away at the station and each car bumps as its link becomes tight. At low power outputs, backlash results in inaccurate calculation from the small errors introduced at each change of direction; at large power outputs backlash sends shocks through the whole system and can damage teeth and other components.
Backlash is an unavoidable property of all reversing mechanical couplings (although its effects can be negated), and may or may not be desirable, depending on the application. Some backlash is required to allow for
lubricationLubrication is the process, or technique employed to reduce wear of one or both surfaces in close proximity, and moving relative to each another, by interposing a substance called lubricant between the surfaces to carry or to help carry the load between the opposing surfaces...
, manufacturing errors,
deflectionDeflection or deflexion may refer to:*Deflection *Deflection *Deflection *Electrostatic deflection*Deflection...
under load and differential expansion between the gears and the housing.
Gear couplingA gear coupling is a mechanical device for transmitting torque between two shafts that are not colinear. It consists of a flexible joint fixed to each shaft. The two joints are connected by a third shaft, called the spindle....
s use backlash to allow for angular misalignment. The looseness of railroad car couplings is another type of backlash which is deliberate; when starting a train from a dead stop, the engine starts each following car rolling in turn as the couplings engage, instead of having to start all the cars at once, thus efficiently overcoming the initial inertia and static friction with less force over a longer time.
Backlash is undesirable in precision positioning applications such as machine tool tables. It can be minimized by tighter design features such as
ballscrewsA ball screw is a mechanical device for translating rotational motion to linear motion with little friction. A threaded shaft provides a spiral raceway for ball bearings which act as a precision screw. As well as being able to apply or withstand high thrust loads they can do so with minimum...
instead of
leadscrewA leadscrew , also known as a power screw or translation screw, is a screw designed to translate radial motion into linear motion...
s, and by using preloaded bearings. A preloaded bearing uses a spring or other compressive force to maintain bearing surfaces in contact despite reversal of direction.
There can be significant backlash in unsynchronized transmissions because of the intentional gap between dog gears (also known as
dog clutchesA dog clutch is a type of clutch that couples two rotating shafts or other rotating components not by friction but by interference. The two parts of the clutch are designed such that one will push the other, causing both to rotate at the same speed and will never slip.Dog clutches are used where...
). The gap is necessary so that the driver or electronics can engage the gears easily while synchronizing the engine speed with the driveshaft speed. If there was a tight tolerance, it would be nearly impossible to engage the gears because the teeth would interfere with each other in most configurations. In synchronized transmissions, synchromesh solves this problem.
Gear Tooth Backlash
Backlash is created when the tooth thickness of either gear is less than the tooth thickness of an ideal gear, or the zero backlash tooth thickness. Additional backlash is created when the operating center distance of the gear pair is less than that for two ideal gears. The total backlash is defined as:
where:
| |
|
= total backlash, |
| |
= backlash due to tooth thickness modifications |
| |
= backlash due to operating center distance modifications |
Backlash due to tooth thickness changes is typically measured along the pitch circle and is defined by:
where:
| |
|
= tooth thickness on the pitch circle for ideal gearing (no backlash) |
| |
= actual tooth thickness |
Backlash, measured on the pitch circle, due to operating center modifications is defined by:
where:
| |
|
= difference between actual and ideal operating center distances |
| |
= pressure angle |
Standard practice is to make allowance for half the backlash in the tooth thickness of each gear. However, there are applications where this may not be advisable. For example, if the
pinionA pinion is a round gear used in several applications:*usually the smallest gear in a gear drive train. In many cases, such as remote controlled toys, the pinion is also the drive gear, although in the case of John Blenkinsop's The Salamanca, the pinion was rather large.*the smaller gear that...
(the smaller of the two gears) is too small or has few teeth, the engineer may elect to not modify the pinion teeth and instead, take the total backlash out of the gear teeth.
Antibacklash gear designs
In certain applications, backlash is an undesirable characteristic and should be minimized; for example, a
radioRadio is the transmission of signals by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
tuningTuning can refer to:*Musical tuning**Guitar tunings**Piano tuning*Radio tuning: see tuner*Tuning properties of neurons: see neuronal tuning*Car tuning**Engine tuning*Database tuning*Self-tuning...
dial where one may make precise tuning movements both forwards and backwards. Specialised gear designs allow this. One of the more common designs splits the gear into two gears, each half the thickness of the original. One half of the gear is fixed to its shaft while the other half of the gear is allowed to turn on the shaft, but pre-loaded in rotation by small
coil springA spring is an elastic object used to store mechanical energy. Springs are usually made out of hardened steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...
s that rotate the free gear relative to the fixed gear. In this way, the spring tension rotates the free gear until all of the backlash in the system has been taken out; the teeth of the fixed gear press against one side of the teeth of the pinion while the teeth of the free gear press against the other side of the teeth on the pinion. Loads smaller than the force of the springs do not compress the springs and with no gaps between the teeth to be taken up, backlash is eliminated.
High-precision main drives and positioning drives of CNC
machine toolA machine tool is a powered mechanical device, typically used to fabricate metal components of machines by machining, which is the selective removal of metal. The term machine tool is usually reserved for tools that used a power source other than human movement, but they can be powered by people if...
s use
duplex wormContrary to standard toothing of worm gear sets, the two flanks of duplex or dual lead worms and wheels are manufactured with slightly different modules and/or diameter quotients....
gear sets for backlash adjustment.
In mechanical computers a more complex solution is required, namely a frontlash gearbox. This works by turning slightly faster when the direction is reversed to 'use up' the backlash slack.
Some motion controllers include backlash compensation. Compensation may be achieved by simply adding extra compensating motion or by sensing the load's position in a closed loop control scheme.The dynamic response of backlash itself, essentially a delay, makes the position loop less stable and prone to oscillation.
Minimum Backlash
Minimum backlash is the minimum transverse backlash at the operating pitch circle allowable when the gear tooth with the greatest allowable functional tooth thickness is in mesh with the pinion tooth having its greatest allowable functional tooth thickness, at the tightest allowable center distance, under static conditions.
Difference between the maximum and minimum backlash occurring in a whole revolution of the larger of a pair of mating gears.