Gear selection shouldn’t be like pulling teeth
Although there appears to be a myriad of different gear types available in the marketplace, Darren Reynolds, Operations Manager at R. A. Rodriguez (U.K.) Limited, outlines how choosing the right gear for the application need not be as challenging as it sounds.
The first step to correct selection is getting familiar with the principal gear types. Spur gears, for instance, are arguably the most common and cost effective type of gear. Often meshed with linear racks, spur gears are designed to transmit motion and power between parallel shafts that rotate in opposite directions.
Spur gears have high power transmission efficiency, are easy to install, transmit large amounts of power, offer constant velocity ratio and, unlike belt drives, have no slip. As a result, applications include everything from machine tools, marine engines and steel mills, through to washing machines, pumps and materials handling equipment.
One application where spur gears are sometimes not the most suitable is where high speeds are involved. Here, the quieter running of helical gears typically prove a better choice. This is because the teeth of helical gears engage a little at a time rather than the entire face at once, thus generating quieter power transfer. Furthermore, for the same tooth size and equivalent width, helical gears can handle more load than spur gears because the gear tooth is effectively larger (since it is positioned diagonally). With all this in mind, helical gears are often first choice for transmission and powertrain applications, although further potential uses include textile machinery, food industry equipment, elevators, conveyors and compressors, to list but a few.
Worm gears, a long-established gear type, essentially comprise of a screw butted up against what looks like a standard spur gear with slightly angled and curved teeth. They are also known for their quiet and smooth running, while further advantages can sometimes include self-locking capabilities, good meshing effectiveness and the ability to reduce speed and increase torque. High velocity ratios can also be achieved. Typical applications range from heavy machinery through to vehicle differentials.
Another common gear type, bevel gears, are used typically where the axes of two shafts intersect. They feature conically-shaped tooth-bearing faces and come in straight or spiral gear profiles, and are most often mounted on shafts that are 90° apart, but can also be designed to work at other angles.
Mitre gears, which again come in a straight tooth or spiral gear profile, are essentially a special type of bevel gear designed to operate in pairs with identical numbers of teeth and diametral pitches (the number of teeth per mm of gear pitch circle diameter – PCD).
Bevel and mitre gears offer specific advantages. For instance, this type of gear makes it possible to change the operating angle, while differing the number of teeth on each wheel allows the mechanical advantage to be adjusted. With this flexibility in mind, bevel gears are the driving force behind many commonly used machines and instruments. They can be found in vehicles, printing presses, watches, drills and can openers.
Once a design engineer knows what type of gear to select, he or she can then focus on sourcing a suitable standard gear or, if the application dictates, a modified or custom gear.
If cost, lead-time or ease-of-maintenance are among the primary considerations, then a standard, off-the-shelf gear will always prove best. Moreover, design engineers will find no shortage of choice when it comes to factors such as material choice, configurations, number of teeth and gear module. The module number of a gear indicates the tooth size and is the number of mm of PCD per tooth. For gears to mesh, their modules must be equal.
Inevitably there are applications where a standard gear cannot be sourced, driving demand for modified gears, or even custom-manufactured gears. Among the considerations that can lead to the requirement for modified or custom gears include specific technical performance demands, geometric constraints or the need for non-standard gear ratios.
Reputable suppliers of gears will be able to offer modifications in line with customer requirements, addressing factors such as special bore diameters/tolerances, keyways, special mountings, thinner gears or bosses, smaller boss diameters, counterbores, drilled and tapped bosses, hardening or stress relieving of steel gears, and anodising of aluminium gears.
Of course, once the design engineer enters the realm of custom gears, there are almost no limits to what can be specified. In all cases however, if a design engineer is unsure regarding anything concerning standard, modified or custom gears, the message is always the same … seek advice from a reputed supplier.