A duplex worm or dual lead worm is a worm gear set where the two flanks are manufactured with slightly different modules and/or diameter quotients. As a result of this, different lead angles on both tooth profiles are obtained, so that the tooth thickness is continuously increasing all over the worm length, while the gap between two threads is decreasing. This allows control of backlash.
In mechanical engineering, backlash, sometimes called lash or play, is a clearance or lost motion in a mechanism caused by gaps between the parts. It can be defined as "the maximum distance or angle through which any part of a mechanical system may be moved in one direction without applying appreciable force or motion to the next part in mechanical sequence".p. 1-8 An example, in the context of gears and gear trains, is the amount of clearance between mated gear teeth. It can be seen when the direction of movement is reversed and the slack or lost motion is taken up before the reversal of motion is complete. It can be heard from the railway couplings when a train reverses direction. Another example is in a valve train with mechanical tappets, where a certain range of lash is necessary for the valves to work properly.
At the worm wheel, the different modules result in different addendum modification coefficients and rolling circle diameters at both flanks. Because of this the profiles are different at the front and at the rear flank. The thickness of each tooth and the tooth gaps remain constant at the circumference of the wheel.
Backlash adjustment is done by shifting the worm axially, so that the section of the worm with the needed tooth thickness will be in contact with the wheel, giving the desired backlash (fig. 1).
This way, backlash can be easily adjusted to any desired value when mounting the gear, and even worn gears can be readjusted at any time delicately and continuously, without modifying the tooth contact or creating meshing interference.
Other possibilities of backlash adjustment
fig.2
Besides the above explained duplex method, there are various possibilities to adjust the backlash of worm gears:
variation of center distance, by rotating an eccentric hub, in which the worm shaft and/or the worm gear wheel are cradled
axial shifting of a conical worm (fig. 2a)
division of the worm in two halves (fig. 2b), to be rotated or shifted relatively to each other. (system Ott)
division of the wheel in two disks (fig. 2c), to be rotated relatively to each other.
In mechanical engineering, an eccentric is a circular disk solidly fixed to a rotating axle with its centre offset from that of the axle.
However all these methods demonstrate substantial disadvantages:
adjustments and/or readjustments are interfering with the geometrically accurate meshing.
they shift the contact profile zone and change its form and size.
with this they decrease the load-carrying capacity and deteriorate the efficiency.
each adjustment causes a tremendous amount of start-up wear.
the dangers of wrong assembly and destruction of the worm gear set are tremendous.
fig.3 part of a duplex worm shaft in a rotary table
Duplex gearings do not create these kind of problems.
They permit an always geometrically accurate teeth contact and beyond that, very delicate backlash adjustment. Neither the evolved contact area, the load-carrying capacity nor the actual efficiency are affected. In addition as duplex teeth are executed as involute gear they are insensitive in regards to modifications of the center distance, e.g. caused by worm shaft deflections.
The involute gear profile is the most commonly used system for gearing today, with cycloid gearing still used for some specialties such as clocks. In an involute gear, the profiles of the teeth are involutes of a circle.
Setting of backlash
Installing and resetting of a duplex worm wheelset is typically done as follows:
Mount worm shaft and wheel sequentially. The arrows for positioning on both parts have to point in the same direction. (Fig. 1)
The shaft has to be mounted with bearings in an axial position before position zero. In this position the shaft has still backlash against the wheel.
The wheel is mounted in its housing, but is still axially displaceable, i.e. transverse to the worm shaft axis.
Both flanks of the worm shaft will now be coated with spotting color. Then rotate the shaft by hand in both directions by at least 2 x 360°, so that the marking ink is transferred from the worm flanks to the flanks of the teeth of the wheel. This creates the load pattern in the contact region.
The wheel has to be displaced axially, up to an optimal position where the load pattern on front and back flank are approximately equally distant to the center of the wheel.
In this optimal position the wheel has to be fixed axially.
The shaft can now be screwed in to the zero position (notch in height of the wheel axle) or until another desired backlash.
Now, the shaft is axially secured in this position.
Applications
Duplex gears are mainly utilized where any backlash is unwanted or can be harmful, to maintain repeated high precision positioning in both directions, to prevent impulse loaded damage, and when the contact flanks are alternating. Common applications include: rotary and tilting tables, milling machines, and presses.
A rotary table is a precision work positioning device used in metalworking. It enables the operator to drill or cut work at exact intervals around a fixed axis. Some rotary tables allow the use of index plates for indexing operations, and some can also be fitted with dividing plates that enable regular work positioning at divisions for which indexing plates are not available. A rotary fixture used in this fashion is more appropriately called a dividing head.
A forming press, commonly shortened to press, is a machine tool that changes the shape of a workpiece by the application of pressure. The operator of a forming press is known as a press-tool setter, often shortened to toolsetter.
External links
Wikimedia Commons has media related to Duplex worm.
This page is based on this Wikipedia article Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.
Related Research Articles
A gear or cogwheel is a rotating machine part having cut teeth, or in the case of a cogwheel, inserted teeth, which mesh with another toothed part to transmit torque. Geared devices can change the speed, torque, and direction of a power source. Gears almost always produce a change in torque, creating a mechanical advantage, through their gear ratio, and thus may be considered a simple machine. The teeth on the two meshing gears all have the same shape. Two or more meshing gears, working in a sequence, are called a gear train or a transmission. A gear can mesh with a linear toothed part, called a rack, producing translation instead of rotation.
Hobbing is a machining process for gear cutting, cutting splines, and cutting sprockets on a hobbing machine, which is a special type of milling machine. The teeth or splines of the gear are progressively cut into the material by a series of cuts made by a cutting tool called a hob. Compared to other gear forming processes it is relatively inexpensive but still quite accurate, thus it is used for a broad range of parts and quantities.
A continuously variable transmission (CVT), also known as a shiftless transmission, single-speed transmission, stepless transmission, pulley transmission, or, in case of motorcycles, a 'twist-and-go', is an automatic transmission that can change seamlessly through a continuous range of effective gear ratios. This contrasts with other mechanical transmissions that offer a fixed number of gear ratios. The flexibility of a CVT with suitable control may allow the input shaft to maintain a constant angular velocity even as the output speed varies.
A manual transmission, also known as a manual gearbox, a standard transmission or colloquially in some countries as a stick shift, is a type of transmission used in motor vehicle applications. It uses a driver-operated clutch, usually engaged and disengaged by a foot pedal or hand lever, for regulating torque transfer from the engine to the transmission; and a gear selector that can be operated by hand or foot.
A gear train is a mechanical system formed by mounting gears on a frame so the teeth of the gears engage.
A worm drive is a gear arrangement in which a worm meshes with a worm gear. The two elements are also called the worm screw and 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.
Bevel gears are gears where the axes of the two shafts intersect and the tooth-bearing faces of the gears themselves are conically shaped.
Bevel gears are most often mounted on shafts that are 90 degrees apart, but can be designed to work at other angles as well. The pitch surface of bevel gears is a cone.
A reduction drive is a mechanical device to shift rotational speed. A planetary reduction drive is a small scale version using ball bearings in an epicyclic arrangement instead of toothed gears.
Gear manufacturing refers to the making of gears. Gears can be manufactured by a variety of processes, including casting, forging, extrusion, powder metallurgy, and blanking. As a general rule, however, machining is applied to achieve the final dimensions, shape and surface finish in the gear. The initial operations that produce a semifinishing part ready for gear machining as referred to as blanking operations; the starting product in gear machining is called a gear blank.
Hydraulic pumps are used in hydraulic drive systems and can be hydrostatic or hydrodynamic. A hydraulic pump is a mechanical source of power that converts mechanical power into hydraulic energy. It generates flow with enough power to overcome pressure induced by the load at the pump outlet. When a hydraulic pump operates, it creates a vacuum at the pump inlet, which forces liquid from the reservoir into the inlet line to the pump and by mechanical action delivers this liquid to the pump outlet and forces it into the hydraulic system.
Hydrostatic pumps are positive displacement pumps while hydrodynamic pumps can be fixed displacement pumps, in which the displacement cannot be adjusted, or variable displacement pumps, which have a more complicated construction that allows the displacement to be adjusted. Hydrodynamic pumps are more frequent in day-to-day life. Hydrostatic pumps of various types all work on the principle of Pascal's law.
A Hirth joint or Hirth coupling is a type of mechanical connection named after its developer Albert Hirth. It is used to connect two pieces of a shaft together and is characterized by tapered teeth that mesh together on the end faces of each half shaft.
A spiral bevel gear is a bevel gear with helical teeth. The main application of this is in a vehicle differential, where the direction of drive from the drive shaft must be turned 90 degrees to drive the wheels. The helical design produces less vibration and noise than conventional straight-cut or spur-cut gear with straight teeth.
In horology, a wheel train is the gear train of a mechanical watch or clock. Although the term is used for other types of gear trains, the long history of mechanical timepieces has created a traditional terminology for their gear trains which is not used in other applications of gears.
Gear shaping is a machining process for creating teeth on a gear using a cutter. Gear shaping is a convenient and versatile method of gear cutting. It involves continuous, same-plane rotational cutting of gear.
A roller screw, also known as a planetary roller screw or satellite roller screw, is a low-friction precision screw-type actuator, a mechanical device for converting rotational motion to linear motion, or vice versa. Planetary roller screws are used as the actuating mechanism in many electro-mechanical linear actuators. Due to its complexity the roller screw is a relatively expensive actuator, but may be suitable for high-precision, high-speed, heavy-load, long-life and heavy-use applications.
The slewing drive is a gearbox that can safely hold radial and axial loads, as well as transmit a torque for rotating. The rotation can be in a single axis, or in multiple axes together. Slewing drives are made by manufacturing gearing, bearings, seals, housing, motor and other auxiliary components and assembling them into a finished gearbox.
A motorcycle transmission is a transmission created specifically for motorcycle applications. They may also be found in use on other light vehicles such as motor tricycles and quadbikes, go-karts offroad buggies, auto rickshaws, mowers and other utility vehicles, microcars, and even some superlight racing cars.
