A worm drive is a gear arrangement in which a worm (which is a gear in the form of a screw) meshes with a worm gear (which is similar in appearance to a spur gear, and is also called a worm wheel).
The terminology is often confused by imprecise use of the term worm gear to refer to the worm, the worm gear, or the worm drive as a unit.
Given a single start worm, a 20 tooth worm gear will reduce the speed by the ratio of 20:1.
There are three different types of gears that can be used in a worm drive.
The second are single-throated worm gears, in which the worm wheel is throated.
The final type are double-throated worm gears, which have both gears throated.
Small electric motors are generally high-speed and low-torque; the addition of a worm drive increases the range of applications that it may be suitable for, especially when the worm drive's compactness is considered.
The worm gear carries the differential gearing, which protects the vehicle against rollback.
A more recent exception to this is the Torsen differential, which uses worms and planetary worm gears in place of the bevel gearing of conventional open differentials.
Very heavy trucks, such as those used to carry aggregates, often use a worm gear differential for strength.
The worm drive is not as efficient as a hypoid gear, and such trucks invariably have a very large differential housing, with a correspondingly large volume of gear oil, to absorb and dissipate the heat created.
Plastic worm drives are often used on small battery-operated electric motors, to provide an output with a lower angular velocity (fewer revolutions per minute) than that of the motor, which operates best at a fairly high speed.
A worm drive is used on jubilee-type hose clamps or jubilee clamps; the tightening screw has a worm thread which engages with the slots on the clamp band.
Transmission :- an assembly of parts including the speed-changing gears and the propeller shaft by which the power is transmitted from an engine to a live axle.
Often transmission refers simply to the gearbox that uses gears and gear trains to provide speed and torque conversions from a rotating power source to another device.
In British English the term transmission refers to the whole drive train, including gearbox, clutch, prop shaft (for rear-wheel drive), differential and final drive shafts.
In U.S American English, however, the distinction is made that a gearbox is any device which converts speed and torque, whereas a transmission is a type of gearbox that can be "shifted" to dynamically change the speed:torque ratio, such as in a vehicle.
The simplest transmissions, often called gearboxes to reflect their simplicity (although complex systems are also called gearboxes in the vernacular), provide gear reduction (or, more rarely, an increase in speed), sometimes in conjunction with a right-angle change in direction of the shaft (typically in helicopters, see picture).
The mainshaft extends outside the case in both directions: the input shaft towards the engine, and the output shaft towards the rear axle (on rear wheel drive cars- front wheel drives generally have the engine and transmission mounted transversely, the differential being part of the transmission assembly.)
The gears and clutches ride on the mainshaft, the gears being free to turn relative to the mainshaft except when engaged by the clutches.
Attempts to improve the fuel efficiency of automatic transmissions include the use of torque converters which lock up beyond a certain speed, or in the higher gear ratios, eliminating power loss, and overdrive gears which automatically actuate above certain speeds; in older transmissions both technologies could sometimes become intrusive, when conditions are such that they repeatedly cut in and out as speed and such load factors as grade or wind vary slightly.
For certain applications, the slippage inherent in automatic transmissions can be advantageous; for instance, in drag racing, the automatic transmission allows the car to be stopped with the engine at a high rpm (the "stall speed") to allow for a very quick launch when the brakes are released; in fact, a common modification is to increase the stall speed of the transmission.
The Continuously Variable Transmission (CVT) is a transmission in which the ratio of the rotational speeds of two shafts, as the input shaft and output shaft of a vehicle or other machine, can be varied continuously within a given range, providing an infinite number of possible ratios.
Hydrodynamic transmissions are used in many passenger rail vehicles, those that are not using electrical transmissions.
A spiral bevel gear is a bevel gear with helical teeth.
A spiral bevel gear set should always be replaced in pairs i.e. both the left hand and right hand gears should be replaced together since the gears are manufactured and lapped in pairs.
A right hand spiral bevel gear is one in which the outer half of a tooth is inclined in the clockwise direction from the axial plane through the midpoint of the tooth as viewed by an observer looking at the face of the gear.
A left hand spiral bevel gear is one in which the outer half of a tooth is inclined in the counterclockwise direction from the axial plane through the midpoint of the tooth as viewed by an observer looking at the face of the gear.
A hypoid is a type of spiral bevel gear whose axis does not intersect with the axis of the meshing gear.
The spiral angle in a spiral bevel gear is the angle between the tooth trace and an element of the pitch cone, and corresponds to the helix angle in helical teeth.
In older automotive designs, hypoid gears were typically used in rear-drive automobile drivetrains, but modern designs have tended to substitute spiral bevel gears to increase driving efficiency.
For practical purposes, it is often impossible to replace low efficiency hypoid gears with more efficient spiral bevel gears in automotive use because the spiral bevel gear would need a much larger diameter to transmit the same torque.
A herringbone gear, also known as a double helical gear, is a special type of gear which is a side to side (not face to face) combination of two helical gears of opposite hands.
Precision herringbone gears are more difficult to manufacture than equivalent spur or helical gears and consequently are more expensive.
Where the oppositely angled teeth meet in the middle of a herringbone gear, the alignment may be such that tooth tip meets tooth tip, or the alignment may be staggered, so that tooth tip meets tooth trough.
Solutions to this have included assembling small gears by stacking two helical gears together, cutting the gears with a central groove to provide clearance, and (particularly in the early days) by casting the gears to an accurate pattern and without further machining.
With the older method of fabrication, herringbone gears had a central channel separating the two oppositely-angled courses of teeth.
The logo of the car maker Citroën is a graphic representation of a herringbone gear, it comes from André Citroën's earlier involvement in the manufacture of these gears.
Epicyclic gearing or planetary gearing is a gear system consisting of one or more outer gears, or planet gears, revolving about a central, or sun gear.
Epicyclic gearing systems also incorporate the use of an outer ring gear or annulus, which meshes with the planet gears.
Planetary gears (or epicyclic gears) are typically classified as simple and compound planetary gears.
The three basic components of the epicyclic gear are: In many epicyclic gearing systems, one of these three basic components is held stationary; one of the two remaining components is an input, providing power to the system, while the last component is an output, receiving power from the system.
In one arrangement, the planetary carrier (green) is held stationary, and the sun gear (yellow) is used as input.
For instance, if the sun gear has 24 teeth, and each planet has 16 teeth, then the ratio is -24/16, or -3/2; this means that one clockwise turn of the sun gear produces 1.5 counterclockwise turns of each of the planet gear(s) about its axis.
Extending this case from the one above: So, with the planetary carrier locked, one turn of the sun gear results in turns of the annulus.
The annulus may also be held fixed, with input provided to the planetary gear carrier; output rotation is then produced from the sun gear.
These are all described by the equation: where n is the form factor of the planetary gear, defined by: If the annulus is held stationary and the sun gear is used as the input, the planet carrier will be the output.
More planet and sun gear units can be placed in series in the same annulus housing (where the output shaft of the first stage becomes the input shaft of the next stage) providing a larger (or smaller) gear ratio.
Therefore, several turns of the "sun" gear made the "planet" gears complete a single revolution, which in turn made the rotating annular gear rotate by a single tooth.
Advantages of planetary gears over parallel axis gears include high power density, large reduction in a small volume, multiple kinematic combinations, pure torsional reactions, and coaxial shafting.
The sun and planet gear (also called the planet and sun gear) was a method of converting reciprocal motion to rotary motion and was utilised in a reciprocating steam engine.
Note the planet-gear is fixed to the pumping rod and thus does not rotate around its own axis.The axis of the planet gear is connected to the axle of the sun gear, and therefore the flywheel, by a linkage that freely rotates around the axis of the sun gear and keeps the planet-gear engaged with the sun-gear.
A rack and pinion is a type of linear actuator that comprises a pair of gears which convert rotational motion into linear motion.
A circular gear called "the pinion" engages teeth on a linear "gear" bar called "the rack"; rotational motion applied to the pinion causes the rack to move, thereby translating the rotational motion of the pinion into the linear motion of the rack.
The rack and pinion arrangement is commonly found in the steering mechanism of cars or other wheeled, steered vehicles.
The use of a variable rack (still using a normal pinion) was invented by Arthur Ernest Bishop, so as to improve vehicle response and steering "feel" especially at high speeds, and that has been fitted to many new vehicles, after he created a specialised version of a net-shape warm press forging process to manufacture the racks to their final form, thus eliminating any subsequent need to machine the gear teeth.
This basic rack is the profile of the conjugate gear of infinite pitch radius.
