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Mechanical backlash: how much is too much?

31st August 2017

Posted By Paul Boughton


High quality precision couplings can be balanced for speeds of up to 80,000rpm
For heavier duty applications where a zero-backlash design is critical, R+W ST series torque limiting couplings are available in sizes from 2,000Nm to 165,000Nm
Space saving servo gearheads with reduced backlash to less than 1 arcminute
Design features that set PowerGear Miniature apart from other gearboxes include a case-hardened bevel gear tooth system, a friction locked shaft and bevel gear fit and an extremely lightweight aluminium housing. This ensures low backlash and high transmission accuracy alongside an energy efficiency of 98%

It is probably a natural reaction to assume that backlash is a bad thing which needs to be designed out of every engineered system. The reality isn’t so simple, as Ian Carr explains

The first thing to say about backlash is that it is often confused with wind up, so let’s clear that up first.

Wind up relates to torsional rigidity and may, most-simply, be described as a rotational spring effect – if you take a strip of metal and twist it axially, it will spring back to its original flat shape as soon as the twisting force is removed.

Backlash is ‘play’ or looseness in a rotary system that results in lost motion, ie the inability of a system to return to its exact starting point if its motion is reversed.

Clearly there are similarities between backlash and wind up and, in truth, they can often be treated as the same thing.

However, in some cases – usually relating to precision motion engineered systems – there is a need to consider them in great detail, recognise that they are different and possibly account for the effects of both individually.

Backlash can be defined as ‘the distance or angle through which a component in a mechanical system can move without acting upon its neighbouring part’.

A rather obvious example of this is the relative motion between railway carriages when a train reverses direction, expressed as clattering noises and shaking of the carriages (and passengers).

The same effect is present between the parts of a screw jack, gearbox or even a precision gearhead, although on a rather different scale.

Although many servo and precision motion systems are described as zero-backlash, they nearly all have some tiny amount of backlash that can, in practical terms, be ignored.

The reasons for the backlash are the spaces between the contact surfaces of the various components.

In some instances these spaces are required to allow lubrication, in others they are caused by manufacturing tolerance, deflection under load, thermal expansion, etc.

Engineers often associate backlash with couplings. While there is backlash throughout a system, it can often be thought of as ‘accumulating’ in couplings. Thus, by specifying an appropriate coupling, the ‘ideal amount’ of backlash can be designed into a system.

Systems that require extreme precision tend to need backlash minimised to a very high degree so that there is virtually no lost motion. These include robot installations, instrument systems and many servo type applications.

Often with such cases expert advice is invaluable; the chances are that engineers at companies like Drive Lines have seen projects similar to yours several times and so can bring experience as well as technical expertise to bear.

Drive Lines divides it coupling offering into two ranges, industrial and precision, all of the latter being recognised as zero backlash.

As mentioned earlier, in a literal sense every component will have some degree of backlash, even if it’s at a molecular level.

For reference, many of the engineers working on servo applications that Drive Lines supplies will specify ‘zero backlash’ as 0.5 to 5 arc min.

However, this level of precision is not always required. In the real world many applications can tolerate a degree of backlash. This is particularly true in light load and general light duty tasks, where super-accurate positioning is not required an in relatively simple installations and where there are cost constraints.

Often such systems will be self- compensating for errors caused by backlash; for instance when a packaging machine is used to position products into pockets in a presentation tray.

Here the pockets will probably have slightly flared or rounded edges that will guide the products to their final position.

Elsewhere in the engineering world a bit a backlash is actually a good thing, because it allows the accommodation of misalignment, shock loading, wind up etc.

In fact, elastomer couplings are designed specifically to add wind up into systems. They are likely to be used on manufacturing machines, in processing lines, high speed packaging systems etc. where motion has to be accurate and repeatable but where there is a notable likelihood of shock loads, jams, blockages and other ‘real world’ incidents.

Taking this a step further, some drive systems require little in the way of precision; typically they are at the simpler end of engineering, and have to be more forgiving towards unexpected loads and forces.

Such drive systems are designed to be robust and may, for instance, include a tyre coupling to provide protection against relatively large amounts of backlash – plus another near-relative shock load.

Typical uses could be on compressor drives, agricultural power take offs and other heavy duty applications.

Ultimately, many engineering mechanisms, such as valve tappets and leadscrews need backlash to work properly – but they need precisely the right amount.

So the best advice is probably that design engineers should consider how their application will react to backlash and then speak to an expert to source a coupling and other components which best displays the performance requirements of the application.

Engineers at Drive Lines supply the world class R+W couplings that range from industrial to high precision.

Ian Carr is Managing Director of Drive Line Technologies.









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