TechArchive

Original article date: January 1999

Gearbox catalogues present information in different ways making the life of an engineer more difficult. TONY FLETCHER, JBJ’s technical specialist for gearboxes explains why and what to do.

Imagine the situation…you have your design on the board (or screen!) and the final drive required could not easily be achieved by application of standard electric or hydraulic motors. The chances are you’ll turn to a pile of gearbox catalogues for your solution.

As with any product on the market gearboxes come in all shapes and sizes. Most manufacturers go to great lengths to provide data sheets and catalogues for their range. So where do you start?

Your design will dictate the final drive speed and torque. There may be a preference for an electric or hydraulic drive. So given your output speed and the input drive speed you can specify a ratio. Easy!

Now you have power speed torque ratio and hence can specify the configuration of the input and output to suit your motor and final drive shaft.

The application can be put out to tender to several gearbox suppliers and you wait for the response. The result will probably be a wide range of gearbox sizes along with an equally wide range of prices.

As a supplier gearbox enquiries arrive on our doorstep in several forms. The drive outlined above is a typical example. But when we contact the customer we discover that when other manufacturers have put forward their idea of what they think you want some will err on the large side just to cover any unknowns. Others will offer much smaller units because they make different assumptions for the drive parameters.

Another form of enquiry would be for engineers and buyers to ask us to give a competitive quote based on the gearbox part number of an existing drive. From a supplier point of view this is the least desirable method. The supplier can never know what factors have been taken into account when sizing the original unit. Therefore we must select our equivalent based on the strongest attribute of the gearbox in question. It may turn out that given the original specification we CAN offer a smaller gearbox to do the same job.

Take the extra time to assemble as much information as possible. It will save you money!

Imagine now that you are at the stage where the table starts to creek with all the quotations and catalogues showered on you by we suppliers. How do you compare like for like when all the manufacturers present their data in different ways?

To tackle this question it pays to know a little about the way a gearbox is selected for a particular application. Catalogues list gearbox ratios along with their acceptable torque capacity. So for a given torque capacity you can look down the list of ratios to find the gearbox you need.

This range will have a common outer case and will use gears of the same tooth profile. Varying the number of teeth on the internal gears creates different ratios. For example take the JBJ RES 2000 range of 20 000Nm planetary gears.

Now look more closely at the figures quoted. The torque figures differ according to the ratio even though they share the same tooth profile.

The gear sets are all the same strength and will transmit the same torque but this is before you take its life into consideration. The torque figures listed in the tables have been adjusted so all the gearboxes will transmit the torque figure for the same ISO L10 life. The surface durability of the gear material becomes the limiting factor for the gearbox as the number of cycles – and hence the number of tooth engagements – increases.

What all this boils down to is that for two gears in mesh the small gear wears faster than the large gear. For a planetary gearbox the size of the sun and planet gears will ultimately determine the life of the gearbox not because of gear strength but because of the speed that they wear.

The ratio that offers the best life will therefore always be the one that has similarly sized sun and planetary gears. Out list in Table 1 bears this out: we show the strongest gear set at ratio 4:1 which has a sun gear and a planetary gear of approximately the same size. As you get further from the ideal the value of torque falls away as one or other of the gears gets smaller.

Beware that some manufacturers list torque values calculated for the sun gear only. For small ratios the planet gear is smaller than the sun gear and will wear more quickly. It may be that some exotic material has been chosen for this one small gear to offset the effects of increased wear but how likely is this in a standard range?

If the assumption is correct and the gearbox fails at some reduced life a possible response is “Yes but look at the good condition of the sun gear”. This is misleading when you consider the gearbox as a whole. Imagine buying a car that is advertised as good for 100 000 miles only to find the wheels drop off at 50 000 miles.

The example labours the point but it brings to light hidden issues that lie behind the data. We have come across similar examples of catalogues making exaggerated claims to peak torque loading only to find the actual more realistic values tucked away in the small print. It can be quite a task to work your way round all these differing approaches. Most importantly you need to establish certain facts before you can attempt a like for like evaluation of alternative gearboxes (see panel).

After all this information has been evaluated only then are you in a position to assess how the gearbox specifications compare and how easy each will be to fit. You may find that you have to go through the whole process several times. The actual ratio of the gearbox may require a rethink on the input or output. The gearbox may be oversize to accommodate high radial loads. A minor modification to the machine design could allow a two-stage gearbox instead of a three-stage one. Do not rely on your supplier to tell you if they have a cheaper gearbox that just fails to meet your initial design when with minor modifications to the machine design you can use it!

Get your suppliers to do the leg work! Give as much information as possible about your application so when you are faced with several quotes they are all of a similar tyoe and size and you can make the final evaluation on cost versus particular strengths and weaknesses.

• Specify the duty cycle of the transmitted torque as fully as possible including peak torque
• Specify the expected life of the machine
• Specify the duty cycle of any radial or axial loads
• State the type of machine
• What is the mounting position?
• What are the ambient conditions?

JBJ has developed an unique tool to assist with the selection process and which is designed to put the designer back in control. Distributed free of charge it comes on CD-Rom. The selector program allows you to quickly establish the correct gearbox and also shows any close match that could be used with a design modification.

Users input the drive details and the selector will list all gearboxes that match in addition to any which are a close match and could be used with a design modification. “What if?” scenarios can be performed quickly without having to pick up the phone. The designer can now change service factors speeds ratios and optimise the whole drive to make most efficient use of the gearbox.

• < b>JBJ Techniques
• 01737 767493
• Mike Davis

January 1999