Worm cost away gear down to a planetary solution

Conventional industrial gearboxes are traditionally specified by working from tables in the manufacturer’s catalogue and using just power, speed and a service factor. Yet working solely from a table won’t highlight such key variables as the non-linear relationship between torque and gearbox life which indicates that at certain critical points on the graph, a minor increase in torque imposes a substantial reduction in service life.

Nor will it necessarily take account of the split life of a typical gearbox, with higher loads, say 30,000Nm, required on start-up and stopping, to overcome the machine’s inertia, separated by hours running on a light load, say just, 10,000Nm. Nor will a straightforward power/speed table enable design engineers to fully exploit the superior efficiency, robust construction, precision engineering and modular nature of planetary gearing.

In fact, it is impractical to construct a table containing all the parameters for correct gearbox specification. Instead, it is far better to consult the gearbox manufacturer at the earliest possible design stage and be prepared to give more than just those two basic pieces of information. Because planetary gearing in particular can then return substantial savings, which will increase your machine’s market competitiveness, without sacrificing performance and reliability.

So let us examine, through a worked example, how savings can be achieved and more competitive OEM price structures facilitated by giving the gearbox manufacturer access to more detailed power transmission data.

We take as our start point, an initial enquiry for an industrial mixer drive let’s say it mixes treacle in a food processing factory. In line with the catalogue approach to gearbox specifying, we are given just two selection parameters: 55kW installed power and an output speed of 10rpm. The buyer is looking for a quotation there and then, so we assume correctly that the installed power is an electric motor and convert those figures to a torque rating, at the same time applying a conservative 90% efficiency rating for a planetary gearbox. This gives us an output torque of 48,000Nm.

In the absence of any other information, and obviously wanting to avoid gearbox failure, we have to allow for starting torque or possible overload, so we double the torque rating to give us a potential maximum output of 96,000Nm. This enables us to select a gearbox at a base price of say £6000.

DIGGING DEEPER

Then we probe a little more and discover that data comes from a similar mixer that had been designed earlier, which was fitted with a double reduction worm box. This arrangement would be at best 80% efficient, so from 55kW installed power the actual torque would not exceed 42,000Nm. It then transpires that the design torque limit is actually 40,000Nm, but the relative inefficiency of the worm box meant that the smallest motor for achieving that is 55kW, bearing in mind electric motors are only available in incremental sizes.

Thus by exploiting the inherent superior efficiency of a planetary box, we can recommend moving down a size of motor to 45kW. This will give the required 40,000Nm output, which when multiplied by a factor of two, to allow for start-up, gives a maximum rating of 80,000Nm, which enables us to drop a complete gearbox frame size, at a cost of say £4700. So just by looking a little further, we’ve already saved some 20% cost.

Now we’re allowed to talk to the engineer who reveals that 40,000Nm is actually the starting torque needed in the morning, so we don’t need the high start up rating. Although the motor has an inherent overload capacity, it is quite likely that the drive will be controlled by an inverter. So we can move down a size again, perhaps to a cost of £3500, saving 40% on the first quotation.

On talking to the engineer further we discover that the mixer needs virtually nothing to drive it except for the first 5 or 10 minutes in the morning, when the treacle is stiff and sticky. Thus, over the life of the gearbox, for only 5% of the time is it running at 40,000Nm; for the remaining 95% less than half that torque is actually needed.

DUTY CYCLE

This is the machine’s duty cycle and in my experience, it is one of the more difficult pieces of information to obtain about an application, but potentially the most useful. Because, employing a simple formula, it is possible to calculate the mean torque, which in this example is 26,000Nm. This enables us to drop yet another gearbox frame size and reduce the base price to £2500 which is 60% less than the first figure.

In addition to the obvious economies introduced, there are weight and space savings to be gained also. The original worm box in our example would have weighed over 2tonnes, whereas the first planetary box quoted would have been half the size and weighed just 800kg. Moving down the different frame sizes , this would have been reduced in stages to only 250kg, a massive 70% reduction in weight.

A power transmission expert will also be able to interpret the graph of gearbox life against torque and identify where there is scope to manoeuvre without dramatically shortening the life of the drive, the objective being to size the gearbox to last the life of the machine. Where the curve of the graph rises steeply, a minor increase in torque imposes a disproportionate reduction in gearbox life, and vice versa; thus at certain points, a mere 4% reduction in torque will introduce a 30% increase in theoretical life and a 10% decrease will bring an almost 100% increase.

To return to our example the engineer identified an output torque of 40,000Nm for the mixer. But was it really 40,000Nm or was that just a nice round figure? Was it actually 37.000Nm? A 3000Nm reduction in torque would impact significantly on gearbox life. So it’s worth carrying out an initial study with the gearbox manufacturer, in order to get the right drive.