Product category:
Drives, motors and power transmission, couplings, clutches
News Release from: Holroyd | Subject: Worm gears
Edited by the Manufacturingtalk Editorial
Team on 24 October 2003
Worm gears: a mature technology still
evolving
A number of initiatives in the evolution of wormgears are leading both to higher accuracy and also greater efficiency in gear performance, as this article from Holroyd explains
The principle of worm gears dates back as far as the Archimedes screw and was also the subject of sketches produced in the 15th century by Leonardo da Vinci Like so many of Leonardo's ideas that were ahead of their time, it wasn't until much later - the early 20th century - that worm gears really started to be used on a large scale, by the automotive industry as rear axle drives
This article was originally published on Manufacturingtalk on 2 Dec 2004 at 8.00am (UK)
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As a power transmission medium, worm gears are generally viewed as "mature" technology.
However, this analysis is quickly disproved if one considers the range of precision tasks that contemporary worm gear systems are required to perform.
Modern dual lead worm gears are used in telescopes that scour the heavens and in dicing machines cut the latest in silicon chip wafers, They are also increasingly the first choice for use in machine tool positioning tables, cutter drives on milling machines, robotic arms and roller drives on printing machines.
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Holroyd's unrivalled expertise in the manufacture of wormgear sets has been enhanced with the development of the Wormgear Contact Analysis Program.
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What has made the evolution of wormgears possible to perform these precision tasks is the continuous improvement in production methods leading both to higher accuracy and also greater efficiency in gear performance.
British company, Holroyd, has taken the lead in this improvement process with a number of initiatives including its own exclusive gear toothform, a new Wormgear Contact Analysis program that revolutionizes the production of large centre-distance wormgears, and Dual Lead wormgears that mininise backlash.
In addition, Holroyd has developed the most accurate worm and wheel gear- testing machine in the world, which aims to reduce variations on worm gears to the lowest attainable minimum.
The unique Holroyd toothform corresponds to the recommendations of British Standard 721 but, in addition, has an exclusive feature, which involves significant modifications to the worm thread and wheel tooth.
These alterations bring additional important properties to gear performance, as well as providing the facility to manufacture by the least complicated methods, thereby minimising risk of error.
The 'Holroyd' thread and wheel tooth form is such as to ensure the closest possible fit.
That this has been achieved is evidenced by the results of independent tests.
These revealed that higher efficiency results were obtained with Holroyd worm gears than with any other make in the world.
At the UK's National Physical Laboratory, one test on a pair of Holroyd 6 inch (152.4mm) centres, 4:1 ratio gears yielded a range of efficiencies from 97.5 to 98.2% at speeds of 1000 and 1500 revs/min with loads up to 48kW (64hp).
Not surprisingly, this achievement constituted a world record, and one that still stands to this day.
The second major factor that has enabled Holroyd to optimize worm gear performance is the company's Wormgear Contact Analysis program.
This both reduces the margin for error in the manufacture of precision, made-to-order gearsets and enables dramatic reductions in lead times to be achieved.
The program also ensures greater consistency in manufacture and longer gear life by providing benefits, such as improved controllable contact spread, enhanced lubrication and the ability to cut wormgears off-load to offset distortion due to loading during use.
Developed at Holroyd's Rochdale production plant over a four-year period, the analysis program has been proven with hundreds of hours of theoretical, synthesised and actual marking patterns, logged on wormwheel sizes from 3" (76.2mm) to 33" (838.2mrn).
In addition to delivering approximately 95% mirror image accuracy between predicted and actual marking patterns, crucially, Holroyd's Wormgear Contact Analysis Program also predicts contact movement, affected by worm/wheel distortion or centre line/centre distance misalignment.
This enables Holroyd to use information regarding the predicted worm/wheel distortion imposed by a given load, to cut wormgears off-load, whose carefully calculated contact pattern will automatically compensate for the inevitable distortion imposed by the load to which the gears will be subjected during use.
In its continuing development of worm gears Holroyd has also addressed the problem of systems that require backlash to be kept to an absolute minimum.
The design that has made this possible is the Dual Lead wormgear, a kinematically correct gear, which can run in either direction of rotation and on which the backlash is infinitely adjustable from an acceptable maximum to zero.
Dual lead wormgears are constructed with two leads.
One flank of the worm thread and its mating sides of the wheel teeth are manufactured with one lead, and the other flank and its mating wheel teeth to a slightly different lead.
As far as the worm is concerned, this has the effect of producing a worm thread, which increases in thickness from one end of the worm to the other.
The advantage of this design is that, if the worm is moved in an axial direction when in mesh with the wormwheel, backlash between the pair can be adjusted to the required amount.
In practice the gears are manufactured so that the initial backlash is achieved at a certain distance relative to a datum plane on the worm.
The worm can then be adjusted from this point as and when necessary during the life of the gears.
Complementing Holroyd's considerable investment in worm gear production technology is a similar investment in testing equipment, necessary in order to validate the manufacturing improvements achieved.
As part of this investment Holroyd has developed the most accurate worm and wheel gear-testing machine in the world, which aims to reduce variations in the production of wormgears to the lowest attainable minimum.
The Single Flank Transmission Error Testing Machine is designed to perform an all-encompassing test, which is carried out on a worm and wheel set in mesh, just as they would be in an application.
This 'single flank test' provides actual operating data about both the worm and wheel in simulated operating conditions in both directions.
With a worm and wheel mounted in mesh, the machine works by slowly driving the worm at a constant angular velocity until the wheel has made one revolution, in this way variations in the position and speed of the wheel are revealed, this is known as the transmission error.
Information is then fed to a microprocessor which produces a graphic/numeric printout indicating and quantifying variations in the transmission error.
The printout presents data in linear measurement form, which can easily be converted into angular measurements if required.
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