Application of the Month
Can You Top This? Only If It Fits!
October, 2009 - View all Application Examples
Backlash, torsion, and transmission error can all contribute to lost motion in a drive system that includes mechanical components. But this doesn’t mean you should eliminate these components, as the advantages of their use are significant. They just need to be monitored, analyzed and controlled.
A customer recently came to us with a problem that looked to include lost motion as one of the symptoms. This customer produced a line of packaging assembly machines, one series of which put tops on containers. Their typical drive design wasn’t working on a customized machine they had made and they weren’t sure why.
The difference in this container from others was that it had a much tighter fitting top than they had previously had to deal with. This meant that not only was the fitting tolerance much smaller than usual but that it required more force to close the container.
The package was designed with chamfered edges so the tops could pilot onto the container for easier assembly, but it still wasn’t working as some of the containers were being crushed. A second problem was the gearhead was making noise and the servo motor was getting hot. All were very perplexing problems that they hadn’t experienced in previous machines.
During the ensuing analysis, it was determined that the motor was pulling many more amps than expected. This likely meant that the torque required for the operation was higher than originally thought. But “why” was the question.
It seemed the torque required to close a tight fitting container was higher than for a loose fitting top. But so much?
As it turned out, the closing force was only part of the problem. The real culprit was the acceleration and deceleration torque required to maintain precise synchronization of the wheel to the incoming containers. Positioning tolerances were critical to avoid damaging the product so the feedback and control system had to work much harder. Why? Because it had to constantly adjust for lost motion.
The two things we began to focus on were system backlash and torsional rigidity. Backlash was an issue, but reducing it by itself wasn’t the Holy Grail. The rigidity problem was accentuating its negative effect.
Backlash is defined by clearance or “play” in a system. In gearboxes it’s manifested by the space between the gears. In a loaded system driving in one direction, backlash isn’t usually a concern. It’s much more important in reversing applications or in applications with intermittent loads.
Torsion is a twisting motion and the rigidity is the amount of twist for a given amount of torsion. The higher the rigidity, the less twist.
So, how were these issues affecting the positioning accurately? It had something to do with the inertia of the wheel and the need to have it at a precise position to place the top on the container. There was an adequate feedback system in place to tell the servo controlling the wheel exactly where the incoming containers were located. Another part of the system to verify the position of the wheel was also in place. Therefore, electronically everything was set up correctly. It was this dynamic control system that exposed the mechanical problem.
As positional errors were recognized, the control system had to accelerate or decelerate the wheel to get into the correct position. When accelerating, the inertia of the wheel created a resistance, which required an increase in motor torque. But the increased torque wasn’t instantaneously transmitted through to the wheel because a part was absorbed by the twisting of the connecting elements, which included the gearhead, the coupling and the wheel journal shaft. Hence, it didn’t get to where it needed to be, when it needed to be there.
When the wheel needed to be decelerated, the motor tried to slow it down. But because of the wheel’s rotary inertia, it continued to rotate and overdrive the gearhead, again not getting where it needed to be. As the motor tried harder and faster to compensate for positional errors, the torque required also expanded, taxing the gearhead above its capacity. Everything mechanical was going wrong. What to do?
Our suggestion started with offering our Dynabox Expert Servo Worm gearhead. The Dynabox series offered in backlash levels of 10, 5 and 1 arc minutes was designed specifically for right angle servo applications and available in single stage ratios from 5:1 to 90:1. The expert version, with less than 1 minute, would eliminate virtually all the system backlash, solving our problem.
The next suggestion was to increase the wheel journal shaft and eliminate the connecting coupling between the journal shaft and the gearhead. The rigidity of the existing design wasn’t adequate for the dynamic motion required. The Dynabox has a large output bore that allowed the gearbox to be shaft mounted directly on the larger journal shaft and then connected with a shrink disc. This provides zero backlash and higher rigidity, not only because the journal shaft has a larger diameter but also because it can now be shorter.
Because the customer wasn’t using his total motor speed, we also suggested increasing the gearbox ratio. This would reduce the reflected inertia back to the motor allowing faster acceleration with less motor torque.
With lower backlash and higher rigidity, the system was more responsive and required less dynamics to make the smaller adjustments. This stabilized the torque requirement allowing the gearbox to see less load variation, resulting in less wear and tear. Although some structure changes were necessary, the machine began to achieve the performance it was designed for.
While DieQua prefers to work with customers at the beginning of a design, sometimes we have to come in after the fact when things aren’t working as planned. That’s fine too. Very often, our application experience gives us insights into the direction our trouble shooting efforts should take.
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