Peristaltic pumps are widely used by the water industry because of their ability to transfer media with up to 80% solids without contacting the moving parts of the pump. However, there are specific requirements when choosing a suitable gearbox for the drive train. Vibration, unbalanced loading and the need for high reduction ratios often means that traditional transmissions are too large, inefficient or prone to damage, particularly on the larger pumps, an example of which is illustrated. One solution to this problem is to fit a planetary gearbox, which offers low maintenance, a compact envelope and the possibility of very high reduction ratios (as high as 15,000:1 in a multi-stage configuration).
Energy efficiency is also important, especially when running a pump continuously. Planetary gearboxes provide close to 98% energy efficiency per stage, and as the configuration is three planetary gears inside a toothed outer ring, the load is spread among many contact points. This means less point pressure and less internal friction and hence the greater efficiency. Alternative high-reduction transmissions may have as low as 50% efficiency; the difference often means that a smaller motor may be installed, which results in lower running costs for the pump overall.
In larger pumps and other water treatment processes it may be necessary to move a large shaft at low speed, a task that requires high torque handling capability. Some planetary gearboxes can transmit torque of up to 800kNm, and as there are more contact points, the system is capable of maintaining power without slipping. While this level of torque is easily transmitted via other gear systems, they will be up to twice the size and require a far larger motor to run them. And then there is the question of reliability.
As an example, Yorkshire Water’s Rawcliffe sewage treatment works can process up to 23,600m³ of water per day. However, until planetary gearboxes were installed at the site on aerator duties, this maximum rate was difficult to attain due to frequent breakdowns. Brevini planetary gearboxes have now been successfully installed on duties at this plant. The gearboxes feature twin taper roller bearings, which can withstand extremely high radial loads. They are also fitted with a double seal arrangement that accommodates higher pressures; thus, when the hose needs replacing, the lubricant in the pump does not enter the gearbox.
The planetary design also offers a high degree of flexibility for re-fit projects that are in any way ‘non-standard’. For example, Brevini proved the only gearbox supplier able to meet Thames Water’s ‘no engineering’ rule when an old gearbox finally had to be replaced at one of its sites, saving both downtime and money. This flexibility is to the peristaltic pump user’s advantage, as each unit is likely to have its own set of specifications. For example, the planetary system is able to run clockwise and anti-clockwise and any standard motor can be directly mounted.
Too cumbersome
Many users of large peristaltic pumps find that conventional gearboxes are far too cumbersome to be used effectively, requiring big support structures to house them. Planetary gearboxes can be up to half the size and 60% lighter than conventional ‘worm and wheel’ gears, so a smaller support structure can be used, which is ultimately easier to handle. The concept that ‘small and light’ can also mean ‘strong’ isn’t the sole province of water processing; other sectors, including F1 motor racing, are realising this and using it to their advantage.