Backlash (engineering) - Gears - Anti-backlash Designs - Leadscrews Where Positioning and Power Are Both Important

Leadscrews Where Positioning and Power Are Both Important

Another area where backlash matters is in leadscrews. Again, as with the gear train example, the culprit is lost motion when reversing a mechanism that is supposed to transmit motion accurately. Instead of gear teeth, the context is screw threads. The linear sliding axes (machine slides) of machine tools are an example application.

Most machine slides for many decades, and many even today, were simple-but-accurate cast iron linear bearing surfaces, such as a dovetail slide or box slide, with an Acme leadscrew drive. With just a simple nut, some backlash is inevitable. On manual (non-CNC) machine tools, the way that machinists compensate for the effect of backlash is to approach all precise positions using the same direction of travel. This means that if they have been dialing left, and now they want to move to a rightward point, they move rightward all the way past it and then dial leftward back to it. The setups, tool approaches, and toolpaths are designed around this constraint.

The next step up from the simple nut is a split nut, whose halves can be adjusted and locked with screws so that one side rides leftward thread faces, and the other side rides rightward faces. Notice the analogy here with the radio dial example using split gears, where the split halves are pushed in opposing directions. Unlike in the radio dial example, the spring tension idea is not useful here, because machine tools taking a cut put too much force against the screw. Any spring light enough to allow slide movement at all would allow cutter chatter at best and slide movement at worst. These screw-adjusted split-nut-on-an-Acme-leadscrew designs cannot eliminate all backlash on a machine slide unless they are adjusted so tight that the travel starts to bind. Therefore this idea can't totally obviate the always-approach-from-the-same-direction concept; but backlash can be held to a small amount (1 or 2 thousandths of an inch), which is more convenient and in some non-precise work is enough to allow one to ignore the backlash (i.e., act as if there weren't any).

CNCs can be programmed to use the always-approach-from-the-same-direction concept, but that is not the normal way they are used today, because hydraulic anti-backlash split nuts and newer forms of leadscrew other than Acme/trapezoidal, such as recirculating ball screws or duplex worm gear sets, effectively eliminate the backlash. The axis can move in either direction without the go-past-and-come-back motion.

The simplest CNCs, such as microlathes or manual-to-CNC conversions, use just the simple old nut-and-Acme-screw drive. The controls can be programmed with a parameter value entered for the total backlash on each axis, and the machine will automatically add that much to the program's distance-to-go when it changes directions. This "backlash compensation", as it's called, is a useful trick for capital-frugal applications. "Professional-grade" CNCs, though, use the more expensive backlash-eliminating drives mentioned above. This allows them to do 3D contouring with a ball-nosed endmill, for example, where the endmill travels around in many directions with ease and constant rigidity.

In mechanical computers a more complex solution is required, namely a frontlash gearbox. This works by turning slightly faster when the direction is reversed to 'use up' the backlash slack.

Some motion controllers include backlash compensation. Compensation may be achieved by simply adding extra compensating motion (as described earlier) or by sensing the load's position in a closed loop control scheme. The dynamic response of backlash itself, essentially a delay, makes the position loop less stable and prone to oscillation.