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Application of the Month

The Real Measure of Gearbox Quality
April, 2011 - View all Application Examples

With precision motion control required in many applications, for some reason the market has used backlash levels as the definition of gearbox quality. While it may be a contributing factor , predictable rotary motion is possibly even more important.

Backlash and torsional rigidity are two of the three main elements contributing to lost motion when using gearboxes or gearheads. In reversing applications backlash creates a lag when moving in the opposite direction by the amount of tooth clearance. In applications with fast acceleration or deceleration, torsion in gears, bearings, shafts, and even housings, can result in unexpected output position and rotational velocity.

The third element of lost motion is transmission error. This is the angular or velocity error experienced between the input shaft and output shaft during operation. It is the result of several factors but primarily the quality of the gear tooth form and positioning of the mating gears affects it the most.

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 this 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 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. 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.

The customer was using a standard industrial spiral bevel gearbox to drive the tooling that fed the material and created the folds. While adequate for his general power transmission and turning the corner he needed to it wasn’t giving him the predictable rotary motion he needed for his process.

Our solution was to offer our Tandler spiral bevel gearbox which, among other unique features, starts with higher quality spiral bevel gears that are then lapped as matched sets followed by hand fitting to get the optimal tooth flank contact. This assures that the designed mesh is achieved resulting in more predictable motion and reduced positional variance.

While the Tandler gearbox is already more accurate in this regard than any other in the market, we went one step further. With each tooth of each gear hand inspected, we were able to offer our option of a reduced transmission error gearbox through selecting the best gears then testing each completed gearbox.

The electronic test device measures the angular variance through an entire revolution. The printout, which is represented in the form of a sine wave, highlights the angular variance between the input and output shafts as well as within each tooth mesh. This shows the customer the maximum potential error he could possibly see out of the gearbox.

Once this customer put in our gearbox the quality of his bellows increased dramatically. Material feed and bellow folds were extremely repeatable and his product quality issues were over.

There are many other applications where 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.

Any application requiring a right angle configuration where you need to reliably know where the output is to a high degree relative to a known input, is a candidate application for the Tandler gearbox.

The concept of transmission error is not often considered when designing machine drive systems to achieve certain results. Oh sure, occasionally servo motor manufacturers will tout direct drive systems to eliminate gearing. But there is no need to forego the mechanical advantage of speed reducers when quality gear products are available. Providing gearboxes that can minimize this element of lost motion is one of the benefits of working with DieQua on your motion control applications.

So, when you have a demanding drive requirement whether for low backlash or low transmission error, give us a call. We have experience with thousands of applications to draw upon. It’s very likely we’ve already seen what you need and have the right solution for it.

Chris Popp
Director of Marketing

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