Gears, Gears and More Gears
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16-02-2011, 12:41 PM
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Gears, Gears and More Gears
What is a gear?
A gear is a wheel with teeth that mesh together with other gears.
Gears change the
• torque (rot. force)
• direction of rotating axles.
Different types of gears
Belts & Pulleys
Most “common” type of gear, a wheel with teeth
Spur gears do three things.
1. Change rot. speed
2. Change torque
3. Change direction
Make sure there isn’t too much friction between the
gears and the beam. The gears should spin easily
The gear ratio is the ratio of the number of teeth on one gear to the number of teeth on the other gear
Gear ratio = 40 to 8 or, simplifying, 5 to 1.
That means it takes 5 revolutions of the smaller gear to get 1 revolution of the larger gear
The gear ratio tells you the change in speed and torqueof the rotating axles.If it takes 5 turns of the 8 tooth gear for every 1 turn of the 40 tooth gear, that means the 40 tooth gear will rotate 5 times slower than the 8 tooth gear.BUT, it also means the 40 tooth gear’s axle has 5 times the torque (rotational force) as the 8 tooth gear’s axle
An idler gear is a gear that is inserted between 2 other gears Build the following. Add another
8 tooth gear to the right of the40 tooth gear.
How many turns of the 8 toothgear on the left does it take to make 1 turn of the new 8 toothgear on the right?
Answer: 1! It’s as if the 8 tooth gears are meshed together.
Idler gears DO NOT change the gear ratio.
Idler gears DO…
• make both 8 tooth gears rotate in the same direction, add spacing between gears
Bevel gears are spur gears that mesh at a 90 degree angle. The gear ratio rules remain the same, but the axles are perpendicular to one another
Worm gears have some special properties
1: The axles are perpen-dicular, like bevel gears.
2: How many rotations of theworm gear does it take for 1rotation of the spur gear?
The worm gear acts like a gear with 1 tooth! This givesvery large gear ratios.
Worm gears are not back - driveable You can turn the worm gear’s axle, but you can’t turn the spur gear’s axle.This property is used as a locking mechanism
Belts & Pulleys
Belts & pulleys are related to gears. They change speed and torque, but with a few differences...
Pulleys transfer their force by the friction of the belts, rather than direct contact with the teeth of gears.
This can cause the belts to slip.Unlike gears, the pulleysrotate in the same direction. Belts can transfer force across long distances Like gears, however, belts and pulleys do have a “gearratio.” It is the ratio of the diameters of the pulleys
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Joined: Feb 2011
18-03-2011, 10:02 AM
Gear1.docx (Size: 1.16 MB / Downloads: 65)
Introduction to Gear
A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine. Geared devices can change the speed, magnitude, and direction of a power source. The most common situation is for a gear to mesh with another gear, however a gear can also mesh a non-rotating toothed part, called a rack, thereby producing translation instead of rotation.
The gears in a transmission are analogous to the wheels in a pulley. An advantage of gears is that the teeth of a gear prevent slipping.
When two gears of unequal number of teeth are combined a mechanical advantage is produced, with both the rotational speeds and the torques of the two gears differing in a simple relationship.
In transmissions which offer multiple gear ratios, such as bicycles and cars, the term gear, as in first gear, refers to a gear ratio rather than an actual physical gear. The term is used to describe similar devices even when gear ratio is continuous rather than discrete, or when the device does not actually contain any gears, as in a continuously variable transmission
Comparison with other drive mechanisms
The definite velocity ratio which results from having teeth gives gears an advantage over other drives (such as traction drives and V-belts) in precision machines such as watches that depend upon an exact velocity ratio. In cases where driver and follower are in close proximity gears also have an advantage over other drives in the reduced number of parts required; the downside is that gears are more expensive to manufacture and their lubrication requirements may impose a higher operating cost.
The automobile transmission allows selection between gears to give various mechanical advantages.
External vs. internal gears
An external gear is one with the teeth formed on the outer surface of a cylinder or cone. Conversely, an internal gear is one with the teeth formed on the inner surface of a cylinder or cone. For bevel gears, an internal gear is one with the pitch angle exceeding 90 degrees. Internal gears do not cause direction reversal.
Spur gears or straight-cut gears are the simplest type of gear. They consist of a cylinder or disk with the teeth project and implimentationing radially, and although they are not straight-sided in form, the edge of each tooth is straight and aligned parallel to the axis of rotation. These gears can be meshed together correctly only if they are fitted to parallel axles.
Top: parallel configuration
Bottom: crossed configuration
Helical gears offer a refinement over spur gears. The leading edges of the teeth are not parallel to the axis of rotation, but are set at an angle. Since the gear is curved, this angling causes the tooth shape to be a segment of a helix. Helical gears can be meshed in a parallel or crossed orientations. The former refers to when the shafts are parallel to each other; this is the most common orientation. In the latter, the shafts are non-parallel.
The angled teeth engage more gradually than do spur gear teeth causing them to run more smoothly and quietly. With parallel helical gears, each pair of teeth first make contact at a single point at one side of the gear wheel; a moving curve of contact then grows gradually across the tooth face to a maximum then recedes until the teeth break contact at a single point on the opposite side. In spur gears teeth suddenly meet at a line contact across their entire width causing stress and noise. Spur gears make a characteristic whine at high speeds and can not take as much torque as helical gears. Whereas spur gears are used for low speed applications and those situations where noise control is not a problem, the use of helical gears is indicated when the application involves high speeds, large power transmission, or where noise abatement is important. The speed is considered to be high when the pitch line velocity exceeds 25 m/s.
A disadvantage of helical gears is a resultant thrust along the axis of the gear, which needs to be accommodated by appropriate thrust bearings, and a greater degree of sliding friction between the meshing teeth, often addressed with additives in the lubricant.
For a crossed configuration the gears must have the same pressure angle and normal pitch, however the helix angle and handedness can be different. The relationship between the two shafts is actually defined by the helix angle(s) of the two shafts and the handedness, as defined:
E = β1 + β2 for gears of the same handedness
E = β1 − β2 for gears of opposite handedness
Where β is the helix angle for the gear. The crossed configuration is less mechanically sound because there is only a point contact between the gears, whereas in the parallel configuration there is a line contact.
Quite commonly helical gears are used with the helix angle of one having the negative of the helix angle of the other; such a pair might also be referred to as having a right-handed helix and a left-handed helix of equal angles. The two equal but opposite angles add to zero: the angle between shafts is zero – that is, the shafts are parallel. Where the sum or the difference (as described in the equations above) is not zero the shafts are crossed. For shafts crossed at right angles the helix angles are of the same hand because they must add to 90 degrees.
• 3D Animation of helical gears (parallel axis)
• 3D Animation of helical gears (crossed axis)