ROAD AND WORKSHOP by HANDYMAN
Page 28
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Gears Use and Abuse
RIGHT from the day the first horseless carriage rolled from its birthplace it has been necessary to transmit rotary motion, via gears, from one shaft to another, calling for a change of direction or speed, or both. Although when considering gears one thinks mainly of a speed difference, our industry would never have come so far had it not been possible to use a pair of gears for a change of speed and direction and to breed into those gears a strength and durability able to keep pace with ever increasing gross vehicle weights and horsepower.
It could be fair comment however, that from the operating side gears are only thought of when they eventually wear or fail. All too often a gearbox or differential is hurriedly opened up, defective parts renewed and just as hurriedly reassembled. Design or style of gears thus comes a bad second to ease of removal and assembly. Therefore, to the
younger fitters and mechanics a word on " cogs " in general may make the odd technical terms of transmission a little easier to understand.
I use the word cog because it represents the oldest form of gear and is understood by all. Put into standard engineering parlance this is a spur gear and will be found on parallel shafts with the teeth of each wheel cut in parallel with the shafts. Where a gear has the teeth cut out of parallel, this is called a helical spur gear, or simply helical.
Where helical pattern gears are used to connect non-parallel shafts, these are known as spiral gears. To drive shafts whose centre lines intersect, a bevel gear would be used. Standard bevel gears have straight teeth of diminishing size, meeting at the apex, and are termed straight bevel gears. In the helical bevel, better known in our trade as spiral bevel. the teeth are helically formed on a cone and, as with the standard bevel, the teeth
will diminish in size as the apex is aproached.
Where non-parallel or non-intersecting shafts are to be driven, the spiral gear is used. In its standard form, however, it has not a heavy load capacity, and for road transport we use the worm gear as a means to this end. This gear, with its smooth and noiseless action, can be found at work in one form driving the speedometer cable and in another, the heaviest job of all, the rear axle.