Zero or Anti Baclash
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Transcript of Zero or Anti Baclash
8/3/2019 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:
To:
99765 68132 : Kumaresan
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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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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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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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