Application of the Month
Pressing Mechanical Advantage Over Direct Drive
July, 2010 - View all Application Examples
A trend these days when timing two machine elements running at different speeds is to direct drive one of the elements with a servo motor and electronically control the speed of the second in relation to the first. But this often is not the best or most cost effective solution.
We had a customer call us recently to discuss the rebuild of his coating station on his printing press. The infeed was mechanically linked to the press but required a speed variation of +/- 10% to match the substrate linear speed over differing roll diameters.
The present design included a PIV unit, which is a mechanical variable speed device with two adjustable pulleys. As the pulley halves converge or separate, a fixed length chain rides up or down the pulleys at different pitch diameters, changing the internal input to output ratio. Significant speed changes of 2:1 or 3:1 are possible and allow for relatively easy speed adjustments.
But there is a downside to this type of transmission. Because of the steel chain and mechanism required to adjust the pulley distances, accuracy is sometimes compromised. Small variations in the chain tolerances and angles of the pulley can create some unwanted speed variance inaccuracy. This usually is not critical when it is used as a main drive in some process machine, but when speed matching precise elements the variations can affect the final product.
Another issue appears when a specific speed setting is in place for a long period of time and then a very small speed change is desired. Any slight wear pattern in the pulley has a tendency to capture the chain and make a small pitch change difficult to maintain.
This customer wanted a more reliable and precise way of maintaining web speed over similar but not the same roll diameter. This concept is also relevant in other tension control applications where material thickness changes may cause unacceptable tension variations, requiring one roll to slightly speed up or slow down to avoid web breakage.
If presented this situation the typical system integrator would immediately try to uncouple the mechanical driveline and direct drive the roll requiring adjustment with a servo motor. This customer thought of that too. But then he thought, “We have all the power we need with the main drive motor. Why are we adding another large power source?". Their original design of mechanically adjusting speed utilized the main machine power source so it made sense to look at an alternative that could do the same, but better.
He had some of our shaft phasing gearboxes, or speed correction drives as we call them, on some of his other printing presses and finishing equipment. They were using them for registration control. He wondered if that technology couldn’t be incorporated. In fact, we use this type of gearbox all the time for narrow range speed and tension control.
The design of our speed correction drive is essentially a planetary reducer with a movable ring gear. The ring is rotated by a worm gear. The most popular use is in maintaining register between multiple mechanically linked rotating bodies. A small secondary power source rotates the worm in either direction advancing or retarding the position of the output shaft relative to a fixed input. Therefore, a single station can be adjusted compared to others, while running.
However, by continually rotating the worm, additional or reduced speed can be achieved on the output shaft relative to the input. This allows for very precise, narrow range speed control. I say narrow range because there is a very high reduction ratio in the “trim” shaft. This allows the use of a very small motor because of the tremendous mechanical advantage and small incremental speed changes of a few percent that are relatively easy to control. In this case, the customer wanted up to 10% variation. Therefore we had to consider our additional lower ratio worms as options to achieve this variation range.
One important issue to point out and clarify is that this is not a speed variator or friction drive where you turn a dial and you get a new ratio. To achieve a speed difference from the standard fixed gearbox ratio you have to continually rotate the worm trim shaft. This means some level of control is necessary, especially during acceleration and deceleration of the machine. To maintain a specific percentage change, the trim shaft has to accelerate and decelerate at the same rate as the main drive.
This is a relatively easy function to control. By monitoring line shaft speed or web tension, a controller can quickly calculate the required trim shaft speed and instruct it to follow accordingly to maintain the proper speed variation between rolls. This allows a constant linear velocity of two different roll diameters that are mechanically linked.
There are several advantages to using this system over a direct drive servo, even if a gearhead is used. For one, these gearboxes are extremely precise so predictable rotary motion is inherent and assured. A servo has to constantly adjust to maintain a velocity match. Two, load variations in individual stations don’t affect the system. The main power source along with the inertia of the entire mechanically linked system just powers through them. And third, adding or subtracting speed can be achieved with a motor a small fraction in size of a direct drive motor, as the vast majority of the torque required to drive the roll is supplied by the main machine drive motor. This reduces power consumption and motor and controller costs.
Now, these types of gearboxes are not cheap. But when compared to a much larger servomotor, possibly a gearhead, and sophisticated controls, the costs are either lower or very comparable and very often the entire system operates more reliably.
While servo systems do have advantages in certain applications, this was not one of them. We helped the customer select the proper gearbox size, ratio and configuration as well as calculated for him the required ratio, torque and speed of the trim shaft. He took care of setting up the simple control function. The result was a straightforward and reliable drive line using proven technology that achieved his desired performance criteria.
DieQua Corporation works closely with their customers to find the best gearbox technology to meet their specific needs. We have the widest range of gearboxes for both power transmission and motion control available from a single source. Many of these are typical designs, but we also have a variety of specialty and customized models for unique applications.
We invite you to give us a call with your next gear drive challenge. You may be pleasantly surprised at what we come up with.
Director of Marketing
Did you know that DieQua publishes a monthly email newsletter that features interesting articles just like this one?
Our newsletter was designed to be a little different than other manufacturer's newsletters that are primarily product centric. While products and services available from DieQua are highlighted, other elements revolve around the personal and professional development of the design engineer.
To receive this newsletter, please visit the registration page and fill out your information.