Wormgear predictions by computer

Highly accurate data vital to the success of wormgear design can now be provided by computer, putting important aspects of production on a much more scientific basis and giving a number of advantages over existing procedures with their heavy reliance on costly empirical trials.

The new program can give rise to useful savings in time, cost, and resources and is compatible with any computer running MS-DOS or Windows.

The development is the result of a study into wormgear transmission carried out by Michael Fish, of Huddersfield University’s mechanical engineering department. The research has the backing of Holroyd and four other British gear manufacturers. The consortium was established with support from the Department of Trade and Industry, and the Gear Research Foundation Committee of the British Gear Association.

Holroyd has now appointed Dr Fish as a full-time research engineer with the company, where he will continue this work. At Holroyd the system is now being used as a practical design and development tool.

A key element is newly devised software for analysing wormgear contact. Known design parameters and manufacturing settings are fed into the program, which then calculates the clearance between engaging tooth flanks, and the positioning accuracy (or transmission error) resulting from a given specification. The analysis can also include influences that will result from the wormgear manufacturing process and from how the set will be used in the individual application – so factors such as machining quality, alignment of components, and deformation of the contacting surfaces under load can also be taken into account.

The software generates accurate representations of the final off-load contact conditions that will be achieved using the given parameters. An exact contact marking pattern illustrating this information can be generated `up front’.

An example using a sample gearset is shown. Figure 1 shows the theoretical central contact pattern and indicates the areas on a geartooth which would be marked in a standard `blue dye’ test. Figure 2 is the synthesised pattern generated by computer which includes error sources found during the manufacturing process defined as deviations from the theoretical conditions.

The close correlation of the synthesised and recorded marking pattern confirm the accuracy of the new method.

The software can calculate, to an accuracy as low as 2-3 micro m, the transmission error for any gear design including simulated error sources anticipated by the operator in contact conditions.

The graphs in Figures 3, 4 and 5 show the theoretical, synthesised and measured transmission error graphs for a specimen manufactured wheel design over several tooth engagements. The synthesised calculations are far more representative of actual contact conditions.

The new system enables the required contact conditions to be achieved more quickly than with existing iterative processes which involve cutting then marking the worm and wheel set, followed by inspection and assessment.

Used as an integral part of manufacturing and servicing processes, the software can allow the operator to give fast analysis of theoretical designs to find optimum contact condition, compensate for the effects of manufacturing tolerances on theoretical contact and assess the tolerance of a design to operating conditions.

The software has a `trouble-shooting’ role too. It can model the changes in linear and angular shaft alignments which happen during assembly, or which sometimes take the form of slight deflections under load in operating conditions. The gearset design can be modified to compensate for this effect as a result. This gives the option of investigating and removing sources of unacceptable operating errors in existing sets.

Holroyd is aware that the new program represents a significant tool with which to quickly release the expertise held within the company. The operator can draw on existing working design and manufacturing knowledge which recognises criteria necessary for any intended application, and then apply this through the software to produce an optimum design.

Holroyd is playing a role in the development of a radical new method for predicting contact patterns and other details vital to the successful design and manufacture of wormgears. Design Engineering investigates.

Information: Holroyd Tel: 01706 526590