TechArchive

Original article date: May 2000

Selecting the correct gearbox can prove crucial to the success of a servo application. JOHN DURRANT of Rockwell Automation and ALAN FEINSTEIN of Bayside Motion Group discuss the issues involved.

To get the best out of a servomotor it needs to run at a fairly high speed. Unfortunately many applications require input speeds which are fairly low and, if the motor is directly coupled, it would run very inefficiently. Also higher torque motors are inevitably larger and more costly.

Another common problem is that the inertia of the load will almost certainly be much greater than that of the motor. This presents practical problems with stability and performance, particularly if the transmission system has backlash or is compliant.

Both of these problems can be solved by using a speed reducer – a gearbox – so that the motor can run at a more efficient speed and with a better inertia match.

When choosing a gearbox, one of the first steps is to eliminate the risk of radial and axial loads on the input shaft of the motor to avoid damaging it. While a straight-cut pinion mounted directly on a motor shaft can cause additional radial loads on the motor bearings, a helically-cut pinion can also cause an unwanted thrust on the motor shaft. This is why the the planetary gearbox is the most popular type for servo duty, because of the way it distributes the load on its three input pinions in such a way as to prevent heavy bearing loads. However, unless it is well made the load sharing is not perfect and a cyclic load can be applied to the motor shaft, which can cause even more problems. Typically gearbox losses are expressed as an efficiency. Generally the efficiency will be quoted at a high torque, but it may not be quoted at a high speed, although it is at speed where most losses occur.

Measurements show that a quadratic approximation can be used to calculate speed-dependant losses and a direct proportionality to calculate torque dependent loss. This produces a three-dimensional family of curves usually similar to those shown in the diagram. However, obtaining the necessary data from the gearbox manufacturer may not be easy.

Shock loads drastically affect the life of a gearbox. Unfortunately these are typical in servomotor applications. For example, an apparently small change to a production process may lead to a slightly more aggressive motion profile with a surprisingly large increase in the peak torque. In an application with a 40% increase in peak torque, the life of the gearbox could be reduced to 200 hours instead of 20,000 – and soon failures will begin to occur.

A new gearbox has minimum backlash and the teeth are covered in lubricant, damping out any remaining backlash, so that the system can be tuned as if the load was solidly coupled to the motor. After a period the teeth bed in, the lubricant disperses and, as backlash develops, the load becomes momentarily disconnected from the motor during reversals. It is likely that the system now becomes momentarily unstable due to the increased gain, and that promotes rapid wear and premature failure.

Making the selection

In order to select a gearbox manually you first need to find a ratio that lets you to use the smallest motor. This is not intuitive since a low ratio increases copper losses, but a high ratio increases iron losses and the optimum may be one that gives an intermediate motor speed.

Once a ratio is chosen, a real gearbox compatible with the motor must be selected and checked to ensure it is large enough to drive the machine without breaking, and also to be sure it will have an adequate life. The extra inertial load and friction losses must now be added to the motor sizing calculations, to ensure the additional load will not take the motor over its limit. If it does, the whole process will have to be repeated. This is very time consuming and requires repeated similar calculations, so an automatic software selection tool is clearly a big time saver. Automatic selection of a gearbox in motion control applications requires three key information elements: a valid gearbox model; a gearbox library (database); and a selection algorithm.

Rockwell Automation’s Quick Size and Motion Analyzer sizing tools contain these elements and support automatic selection of gearboxes, with data supplied by leading gearbox manufacturers such as Bayside.

Accurate selection of gearboxes for use with servomotors is not a trivial task, and it is necessary to have a lot of extra data over and above that which is normally available. Even when the correct selection has been made, it is important to ensure the system is properly fitted, properly tuned and is not programmed so as to impose greater loads on the gearbox than were envisaged when sizing and selecting the system.

• Rockwell Automation
• Bayside Motion Group

May 2000