Out With The Old And In With The New
A policy of replacing rather than repairing old gearboxes on conveyor drives can help quarries to cut energy bills and reduce other operating costs, says Renold Gears’ product manager, Simon Chipchase
To echo the slogan of a well known supermarket chain, ‘every little helps’ in the drive to reduce energy costs in Britain’s quarries. Every effort should be made to find ways of reducing energy consumption and operating costs if the industry is to stay healthy and remain in profit. One area in which savings can be gained is in the development of more energy-efficient drives for the big conveyor systems that move material over huge distances. Quite often these conveyors are several miles long, perhaps running heavy stone from a quarry face to a processing plant or terminal where it may be loaded on to lorries, ships or trains for delivery to market.
These big conveyor systems account for a large part of a quarry’s power consumption, so cutting costs here can have a significant impact on the total energy bill for the year. Motor manufacturers have rallied to the cause with a range of energy-efficient products that are helping to combat rising energy prices, but there is another component in the power transmission system that can make a significant contribution too, and it is one that is often overlooked — the gearbox.
Industrial gearboxes have a reputation for being reliable, rugged components that will run for decades before they need to be replaced. When they do begin to wear out there is often a temptation to have them repaired on an emergency basis to extend their life rather than purchasing a brand new unit. A repair will undoubtedly cost less than a new gearbox, but any short-term saving is a false economy in the longer term.
It is worth remembering that an old gearbox coming to the end of its working life will almost certainly have been manufactured in an era before modern computer-aided design and finite element analysis were in common use. It is easy to assume that power transmission is a mature technology, and indeed it is, but what has changed is the precision with which gears are designed and manufactured compared with similar products made decades ago. Modern gearboxes are also manufactured with advanced materials, improved bearings and better lubricants than were previously available. An old gearbox is inefficient at transmitting power compared with a modern one, and although a repair will extend the life of an old gearbox, the real cost will show up in the energy bill.
Consider the typical running costs of a conveyor system in a quarry with a 45kW motor running 16h a day, 365 days a year. At current rates the bill to drive this one motor alone will come to over £11,000 a year, and every 1% of efficiency loss in transmitting power from the motor to the conveyor will cost £110. Not too much you might think, but when you consider the fact that older gearboxes may be up to 30% inefficient things begin to look a little different. The old gearbox is now adding around £3,300 a year to the quarry’s energy bill.
Conversely, modern gearboxes are now so efficient that only a few percent of motor power is lost, dramatically reducing running costs. When you calculate the potential savings for all the gearboxes fitted on a quarry’s conveyor drives the potential savings run into serious figures.
The accompanying graph (fig. 1) shows the efficiencies of three different gearboxes: a triple-reduction helical unit and two single-reduction wormgear units. All three are physically the same size but the two wormgear units compared here have low and high ratios. The helical unit has a constant efficiency of 97% across all ratios and input speeds but the wormgear units’ efficiencies are significantly different and are dependent on the ratio and input speed. At the higher speed ratio the efficiency reaches 95%, but drops to 80% at the lower speed ratio.
Additionally, because modern gearboxes are so efficient, it is often possible to reduce the size of the gearbox and motor to a size smaller than the units being replaced. A typical comparison of the output ratings across the ratio range between the same size wormgear and helical gearbox is shown in figure 1.
When replacing an old gearbox and motor, it is worth remembering that the original design of the motor will have taken into account the high efficiency loss of the old gearbox. For instance, consider a drive with a 4,900Nm output torque requirement at a speed of 30 rev/min. If the motor is connected to a 70% efficient gearbox it can be calculated that a 22kW motor is required. Since most replacement drives are sized according to the installed motor power, and the original torque calculated by the OEM machine builder is not known, an oversized motor and gearbox would be fitted.
However, having established the new gearbox efficiency from the gearbox supplier it is possible to calculate the output torque. If the drive is recalculated with a 97% efficient gearbox a smaller 18.5kW motor can be selected to transmit the same torque, thus reducing the size and purchase cost of the motor.
In addition, there are other savings that can be made with a modern drive system, over and above reduced energy consumption and the reduced cost of purchasing a smaller gearbox and motor than the units being replaced. For example, older power transmission systems were often configured with a foot-mounted motor and gearbox on a sturdy metal base-plate. The motor and gearbox would have been connected using a flexible coupling — a component that requires regular maintenance and more frequent replacement than either the gearbox or the motor.
Modern designs allow for flange mounting the electric motor directly on to the drive shaft, eradicating the need for the flexible coupling and base-plate structure. Therefore, the initial purchase cost of the coupling and base-plate and the ongoing coupling maintenance and replacement costs, as well as the headache of how to guard the whole arrangement, can be eliminated.
Even though the age of the old gearboxes still operating in British quarries is testament to the quality of manufacturing at the time, modern gearboxes will last even longer. Older gearboxes were fitted with lower load capacity bearings, based on older Imperial ball-race design technology, than their modern equivalents. Modern bearings have no effect on the efficiency of the gearbox but they do have a much longer operating life, which cuts maintenance costs and extends the life of the gearbox still further.
Repairing old gearboxes to get a few years extra life out of them may seem tempting, but in today’s world of rising energy costs it is worth considering replacement as an alternative in order to start benefiting sooner rather than later from the energy savings of an efficient drive system.