A Measure Of Gearbox Quality
By: Chris Popp
Director of Sales and Marketing
With precision motion control required in many machinery design applications the necessity of quality in the mechanical drive components is a given, especially in the gearing and gearboxes. But what does quality in a gearbox mean?
For some reason the motion market has gravitated toward using the amount of backlash in the gears as the definition of gearbox quality. Perhaps this is because it’s an easy concept to understand and one that can readily be measured and compared to competitive models. However, while it may be one measure of quality it isn’t the only one. Predictable rotary motion is possibly even more important.
When discussing accuracy and precision in motion control, a key problem affecting predictability is lost motion. In gearboxes, backlash and torsional rigidity are two of the three primary elements contributing to lost motion. Backlash is defined as the actual space between gear teeth. In reversing applications backlash creates a lag when moving in the opposite direction by the amount of tooth clearance. It can also affect final positioning of a driven component as the output could rock back and forth between the front flank of one gear tooth and the back flank of the previous or next tooth. Or, as a running load varies in speed it could actually float between gear teeth.
Torsional rigidity is the amount of component bending when torque or force is applied. In applications with fast acceleration or deceleration of a driven mass, high torque levels are required to overcome inertia and/or friction. Bending of the material in gear teeth, bearings, shafts, and even housings, can result in unexpected output position and rotational velocity errors. Usually it results in a lag during acceleration, an over run during deceleration and oscillations after quick stops.
A third less well known element of lost motion is transmission error. Given consistent input motion, transmission error is the angular or velocity error between the expected and actual output shaft rotation during operation. It is the result of several factors but the quality of the gear tooth form and positioning of the mating gears affect it the most. Poor quality gears have gear teeth that engage and disengage in unpredictable ways. Shaft and bearing runout can also be the culprits to causing transmission error.
In the lower quality gears used in most industrial gear assemblies there is actually a velocity variance as the gear tooth enters and exits the mesh with a mating gear tooth as well as when it moves from tooth to tooth. This slight speeding up and slowing down means the output shaft is not exactly where it is expected to be relative to the input shaft.
The easiest example to illustrate transmission error is in a 1:1 ratio. The input and output shafts will always return to the exact same position after one full revolution because of the accuracy of the gear diameter. But, nowhere else within the revolution will they be tracking in the same position. In some places the output will be a few angular minutes of an arc behind and sometimes a few ahead.
While it’s true these small variances due to transmission error don’t impact most applications there are many, many others where it is critical. These are applications where highly predictable rotary motion is required to produce a quality product.
In a past article we have already discussed one customer’s issue as a result of transmission error. In short, a manufacturer of composite and fabric bellows contacted us with a problem he was having controlling his folding process. The process included multiple folds within a single output revolution of his tool. His issue was the material length between each fold was varying which negatively affected the bellow when it was compressed. It kinked slightly instead of collapsing accurately and he couldn’t figure out why that was. His motion within the revolution was varying.
There are many other applications where lost motion, and specifically transmission error, is a critical determinate. One is in printing. When laying down multiple colors at multiple spots on a product, whether that is a magazine page, a wine bottle label, or a cereal box, knowing exactly where the die is going to be is of utmost importance. We’ve all seen poor quality printing where colors overlap resulting in a muddy image.
Another would be in film production or coating. You can imagine if the gearbox output is not accurately tracking the highly controlled input that the accuracy of the film or coating thickness would vary, sometimes too thin, sometimes too thick. In either case the quality of the product is compromised.
Another would be a rotary table where angular actuation places product in very specific positions, with minimal variance. Think machine tools or assembly machines.
Whether due to backlash, torsional rigidity or transmission error, lost motion is a fact of life when using mechanical components. That is one of the rallying cries of servo motor manufacturers who tout the elimination of gearing in motion applications. But don’t be swayed. Running servos at lower speeds than optimal results in other problems such as higher reflected inertias, reduction in resolution, tuning problems and higher motor and power costs.
The action plan is not to eliminate mechanical components but to understand what is necessary for the application and select gear products that can produce the required motion profile. That is where Diequa comes in. With the widest variety of gear technologies available for controlling motion and our 36 point application characteristic checklist, we can help select the best gearbox for the situation.
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