Holroyd Tooth Form
The Holroyd Toothform corresponds to the recommendations of British Standard 721 but, in addition, has an exclusive feature which consists principally of important modifications to the worm thread and wheel tooth.
These alterations confer additional valuable properties to gear performance combined combined with the facility to manufacture by the least complicated methods, thereby minimising the risk of error.
The diagram shows a typical wormgear contact pattern at a given instant. Contact lines 1, 2 and 3 occur between 3 adjacents threads on the worm and their mating teeth on the wheel. As the worm rotates, the contact lines roll from the tip to the root of the gear tooth. Although the contact is theoretically line contact between the 2 surfaces the closeness with which the surfaces approach or envelope each other has a direct influence on the load capacity and life of the gears. Close fitting surfaces spread the load over a larger area, and so reduce pressure and wear to a minimum.
The 'Holroyd' thread and wheel tooth form is such as to ensure the closest possible fit.
Higher efficiency results have been obtained in authentic and independent tests on Holroyd worm gears than have been obtained or claimed for any other make in the world. A test at the UK National Physical Laboratory on a pair of 6 inch (152.44mm) 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). This remains a world record.
Renold Gears has developed the most accurate worm and wheel gear testing machine in the world, which aims to reduce variations to the lowest attainable minimum.
The company's Single Flank Error Testing Machine is designed to perform an all revealing test which is carried out on a worm and wheel as a 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 print-out indicating and quantifying variations in the transmission error. The print-out presents data in linear measurement form, which can easily be converted into angular measurements if required.