Another fine mesh

In environments such as shipping and utilities, where higher ratios and greater torque are required, a planetary gearhead system seems to provide the answer, says Mark Venables.

Selecting the correct gearhead for a specific purpose means considering many requirements. The most obvious are the physical dimensions and torque capacity, with manufacturers placing great emphasis on the performance figures such as backlash. However, there are many more considerations.

The most important reason for using a gearhead is to increase torque.

Large electric motors are expensive, so it is cheaper to fit a gearbox to a fast rotating motor to produce greater torque than it is to increase the size of the motor. Also, gearheads can improve the dynamic response of a servo system if the inertia between the load and motor is correctly matched.

The first performance criterion considered is usually the backlash of the gearhead; this is the amount by which the width of a gear tooth exceeds the thickness of the engaging tooth, measured as the angle at the pitch circle of the gear.

For many applications, such as a car gearbox, backlash is of little concern and is in fact necessary to allow space for lubrication of the teeth. However, when it occurs in a servo system it will lead to positioning and repeatability errors unless compensated for, particularly if direction is reversed.

In a manufacturing process, or a machining application, such inaccuracies are unacceptable; the spacing of the teeth is a balancing act between maximum contact to ensure good power transfer and space for lubricant. Backlash is measured at the output shaft, with the input shaft fixed under minimal torque.

The backlash can be viewed as the ‘air’ in the system, the space between gear teeth, accounting for a lull between the input being rotated and the output beginning to move.

Gearhead manufacturers employ a number of methods to reduce backlash —the first is to preload the gearbox, the principle used in cyclic gearboxes.

A preloaded gearbox cuts backlash to a minimum, but the preloading makes them very inefficient because of the frictional losses introduced.

In the most basic gearboxes, spur gears are used. These are cheap to manufacture, which makes them cheaper to buy.

The disadvantage is that they typically have a contact ratio (defined as the number of teeth in mesh at any given time) of 1:5. By cutting the gears helically, the contact ratio can be increased to a typical ratio of 3:3 which is more than double that of a spur gear. This increases the effective torque capacity for a given frame size and makes the operation smoother, enabling the gearbox to operate at higher speed.

Using both crowned teeth and helically cut gears reduces wear, and the greater meshing increases the torque rating of the gearhead. A basic spur gearhead consisting of a pinion and a single spur has a shorter service life, requiring regular maintenance and lubrication.

To achieve higher ratios and greater torque capacity, a planetary gearhead is used. The environment also affects the choice of gearhead; for example any heat developed must be dissipated. This is particularly important in machines where space is at a premium, and a high-efficiency gearbox will reduce the dissipation requirement. In many cases this is academic because the servomotor will produce more heat than the gearbox, but correct inertia matching of the motor and the load will help reduce overall dissipation.

In corrosive environments, stainless steel gearheads can be used. These are more costly than standard heads, but are essential to ensure reliable operation and long service life in food, chemical, and pharmaceutical processes.

A planetary gearhead uses a more sophisticated gear arrangement, with several spur (typically three to five) gears (planet gears) rotating about a pinion (the sun gear). The planet gears orbit within an internal gear which is normally cut into the inner circumference of the gearbox.

This construction is rigid, increasing the torsion of the gearhead as a whole. Because the planet gears come into contact with the inner ring, a single drop of oil can effectively lubricate the whole gearhead.

The planet gears all share the load attached to the output shaft, which means that a planetary gearhead has a higher load capacity than a spur head for a given size of gearbox.

And because it is possible to have a number of gears within a confined space, very high ratios are possible. Ratios from 1:1 to 100:1 are typical, with ratios a high as 500:1 possible by employing multiple stages.

Thames Water, for instance, makes use of planetary boxes on a series of half-bridge scrapers at Maidenhead Sewage Treatment Works (STW). The company has recently fitted new Brevini planetary units that offer increased radial load and torque transmission ratings over their predecessors and were easily retrofitted to the existing platform.

The new Brevini planetary gearbox has increased ratings over its predecessor on radial load and torque transmission.

Shipping is another example of the use of planetary boxes. when the Queen Mary II completed her maiden voyage just over two years ago she manoeuvred smoothly and precisely into Fort Lauderdale Harbour, Florida thanks to a new podded propulsor system.

The liner’s Azimuth system, developed jointly by Rolls-Royce and Alstom Power Conversion, comprises four pods hung from the stern of the vessel.

It revolutionises marine propulsion by removing conventional shaft drives and replacing them with electric motors which directly drive the propellers. The system also offers additional benefits such as reduced fuel consumption and remarkably increased manoeuvrability.

The gearboxes are ideal for this application as they are remarkably compact and lightweight, and require little installation space. They are also able to deliver high reduction ratios in small packages, and to transmit several times the torque of similarly sized, conventional gear units.