Language is a funny thing. For example, the word ‘backlash’ might evoke images of some political uprising when used in a historical context, but in mechanical engineering parlance, we’re usually referring to something a little less dramatic. In power transmission systems, an engineer is more likely to be talking about the clearance or play between meshing gears, not some oncoming revolution.

To be clear, engineers analyze methods used for addressing gearing errors when power transmission systems are the topic of conversation. Those methods include the use of gear manufacturing methods with higher production tolerances, adding some form of backlash compensation mechanism, and incorporating a modified design that applies a constant force or preload to the gears, thus eliminating the play between the gear teeth causing the backlash effect.

The peculiarities of the English language aside, let’s rewind the gears of time a little — no backlash in evidence — to add fine detail to the effects of backlash in power transmission systems. Believe it or not, backlash is a desirable gearing feature when used judiciously.

What is backlash in power transmission mechanics?

It’s the tiny gap between gear teeth. It’s desirable because an absolute match between two sets of meshing gears would cause binding. The fit would be so precise, so high-tolerance, that they’d lock together with nowhere to go. On the other hand, a large backlash would see the system destabilize and produce end-system errors.

For the latter design condition, imagine a precision-based CNC machine, its backlash set too high. A ‘Dead Zone’ forms; there’s a fraction of a second when the gear doesn’t move at all. If the machinery needs to move along another axis, this gap causes a positional flaw. Not that we need to state the obvious, but CNC equipment can’t afford any degree of play. Left in this state, cuts are less precise and machine holes take on misshapen dimensions, compromising manufacturing tolerances.

Nomad Desktop CNC Machine. Image Source: Carbide 3D

What causes backlash?

Like any other power transmission system, finite errors multiply as kinetic energies are carried through a powertrain. A low tolerance production process could be responsible, or there’s the possibility of heat treatment expansion or operational thermal effects as well. The general wear and tear of gear teeth also introduces more play, so backlash increases over time.

Too little play between gears can introduce a whole new set of problems. The incorrect compensating spring is introduced or a preloading mechanism set too high. The result is gear locking and a reduction in lubrication because there’s nowhere for the oil to go. What’s needed is a backlash solution that doesn’t push the clearance between teeth too high or too low. This delicate balancing act is achieved through precise manufacturing, appropriate material selection, and careful gear system design

Mitigating powertrain backlash

Engineers can enhance machine performance and component longevity by offsetting gear play. This Plant Engineer’s Handbook describes one of the more common solutions, that of cutting each gear tooth slightly thinner. By making each gear tooth thinner by half the required backlash dimension, the mating gears achieve the required backlash. This is called ‘Gear Splitting,’ the deliberately engineered act of dividing backlash between two mating gears. However, designed gear play reduction solutions don’t necessarily account for power transients and wear problems that accumulate over time.

What if the backlash has grown during the operational cycle, turning a once smooth meshing power transmission system into a noisy, inefficient mass of dissonant parts that no longer reflect the harmonious whole? Active backlash compensation is a modern solution to this age-old problem. Springs and preloading mechanisms play pivotal roles, coming to the fore to provide adjustable compensation. The tension on the spring can be adjusted, preload values changed, and optimal teeth meshing restored to its full former glory.

Spring-loaded nuts are products we see often, used to apply constant force. This Nomad 3 Desktop CNC Mill mentions anti-backlash nuts on every axis, compensating for precision attenuating parts looseness. Elastomeric materials and compensatory compression springs do the same job, offsetting backlash looseness. Special gears, actually inserted in the powertrain, work effectively as well, splitting the preload and reducing slack. A screw mechanism adjusts the value, bringing the amount of play under control.

Finally, the finest machines, those that have precision-imbued meshing, can use software controls to offset this issue. Algorithms take account of the play and adjust motor movements in real time. This is a growing solution to a common problem, as transient loading effects and thermal operational events can introduce unpredictable backlash issues. Taking this strategy to its logical conclusion, modern devices are now trending in the direction of installing special micro-controllers, which then use backlash feedback as a control signal.

Bringing backlash under control

Proof of engineers being on top of this power transmission hindering complication can be found pretty much everywhere. A precision-imbued Sankyo Automation RollerDrive CNC machine features zero-backlash engineering, using a preloaded ‘rollerdrive’ mechanism in place of geared teeth. The roller removes gear play while introducing power transmission tension. The end result is teeth-less mechanical energy transmission in all axial planes.

A peek into the world of software compensation reveals an even more elegant solution, one that can handle backlash in real time. For example, Mach4 CNC software uses software algorithms to manage its ‘Motor Tuning’ capabilities. A deeper dive into the PDF did illustrate yet another interesting point, though, that software compensation is something of a stopgap measure. The only way to properly correct the issue is to make corrections in the system mechanics.

This fact pushed a deeper look into the backlash conundrum, which is why industrial robots have been used to tie up all of the issues we’ve raised. Think of the coming robotics revolution, with walking AI informed machines walking out of the factory and into the home. Accuracy is paramount in such systems if we’re to develop trust in their capabilities. Secondary feedback-controlled microcontrollers are the way forward, taking the measured backlash signals and using those signals to enhance power transmission accuracy, minus gear play.