zero or anti baclash

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NANDHA ENGINEERING COLLEGE , ERODE – 638 052.

PAPER ON

ZERO OR ANTI BACKLASHZERO OR ANTI BACKLASH

SUBMITED TO

BANNARI AMMAN INSTITUTE OF TECHNOLOGY,SATHY.

AUTHORS:

S. ADHITHYAN (080111306001)

A.K.PARTHIBAN (080111306030)

PRE-FINAL, MECHANICAL ENGINEERING.

EMAIL:

[email protected]

[email protected]

To:

[email protected]

99765 68132 : Kumaresan

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mailto:[email protected]








ANTI OR ZERO BACKLASH

Introduction:

In mechanical engineering, 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 gears, backlash is the amount of clearance

between mated gear teeth. 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 neglected), and may or may not be desirable, depending on the

application. Some backlash is required to allow for lubrication, manufacturing

errors, deflection under load and differential expansion between the gears and the

housing.

• Gear couplings 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

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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

ballscrews instead of lead screws, 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.

Mathematics

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:

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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:

b c = 2(Δc )tanφ

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 pinion(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.

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http://www.worldlingo.com/ma/enwiki/en/Angle_of_pressure

http://www.worldlingo.com/ma/enwiki/en/Angle_of_pressure



Antibacklash gear designs

Order by Catalog Number

When ordering AMGA Quality 12, please supply the catalog number and add Q12.

Gear

Class

Total

Composite

Tolerance

Tooth to Tooth

Composite

Tolerance

BorePitch

Diameter

Outside

Diameter

Q10 0.001 0.0004 +0.0005 -0.001 -0.002

Q12 0.0005 0.0003 +0.0003 -0.0007 -0.0015

In certain applications, backlash is an undesirable characteristic and should be

minimized; for example, a radio 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

spring 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.

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High-precision main drives and positioning drives of CNC machine tools use duplex

worm 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.

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Minimum Backlash

Minimum backlash is the minimum transverse backlash at the operating pitch circle

allowable when the gear tooth with the greatest allowable functional tooththickness 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.

Servo Zero Backlash Gearsets

Many Cone Drive products are available with Absolute Zero Backlash gearsets

ideal for applications in which extremely accurate positioning is required.

Absolute zero backlash worm gearsets have a two-piece worm, quilled together at

the axial center of the worm thread. One segment of the worm is fixed in its

bearing set, while the other segment is positioned laterally by adjustable-forcepre-load disks, maintaining a consistent clamping force on both sides of the gear

thus removing all backlash.

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In this self-adjusting design, half of the power input worm contacts the drive side

of the gear while the other half of the worm makes continuous contact with the

opposing side of the gear. The result of this design innovation is absolute zero

backlash, ideal for applications in which extremely accurate positioning is required.

No-load chatter and free-travel shock in reversing applications are eliminated by

the pre-loading characteristics of this design.

Absolute ZERO

• Absolute zero backlash precision for extremely accurate positioning and

repeatability

• Self adjusting design to insure zero backlash

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• Eliminates no-load chatter and free travel shock in reversing applications

• Available as loose gearset or as an assembled single stage or multiple stage

gear unit

Product Specifications

•

Center Distance:2 inch through 32 inch (Special sizes available)• Gear Ratios: Available in standard ratios through 70:1 in single stage

(Special ratios available)

Backlash can be defined as a rotational arc clearance formed between a pair of

mounted gears. A backlash (clearance) is crucial to any Gear set for checking the

damages caused by the gear tooth interference. Lack of backlash causes the

following problems:

• Noise

• Overloading

• Overheating of gears and bearings

• Seizing and failure

Backlash amount gets measured by limiting the rotation of one member, that

generally is the pinion, while measuring the other component's rotational movement

at a reference radius. Backlash is usually defined in arc degrees (min/sec). The arc

movement measurement thus obtained is converted to degrees by applying the

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following equation:

Backlash (degrees) = Arc Movement (inches) x 57.296/Radius (inches)

The following table gives the American Gear Manufacturer's Association specified

Backlash range for Coarse pitch Spur, Helical and Herringbone type of gears.

Applications

Gear couplings use backlash to allow for angular misalignment.

Backlash is undesirable in precision positioning applications such as machine tool

tables. It can be minimized by tighter design features such as ball screws instead

of leadscrews, 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.

Conclusion:

There can be significant backlash in unsynchronized transmissions because of the

intentional gap between dog gears (also known as dog clutches). The gap is

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http://en.wikipedia.org/wiki/Ball_screw

http://en.wikipedia.org/wiki/Leadscrew

http://en.wikipedia.org/wiki/Dog_clutch

http://en.wikipedia.org/wiki/Ball_screw

http://en.wikipedia.org/wiki/Leadscrew

http://en.wikipedia.org/wiki/Dog_clutch



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 small

clearance, it would be nearly impossible to engage the gears because the teeth

would interfere with each other in most configurations. In synchronized

transmissions, sychromesh solves this problem.

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zero or anti baclash

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