Before servos, before steppers, before variable-frequency drives and even before dc drives, motor speeds were controlled by a variety of mechanical means. Nowadays, in most machine and robot applications, the machine builder will tell you that electronic speed controls have proven to be a superior alternative.
But in a surprisingly large number of instances, including perhaps yours, mechanical speed control is the best option.
Mechanical Solution Superior
Lantech (www.lantech.com) produces a full line of stretch-packaging, shrink-packaging, and case-erecting machines. The company was challenged by a Fortune 100 client to produce a stretch-wrap machine (Figure 1) that could wrap and unitize up to 180 loads per hour of a very soft product.
To achieve this high rate of production and not damage the product, Lantech needed to rethink the way film is applied to the load. "We tried our traditional control systems, including load cells, dc motor control, ac frequency drives and torque control," says Brian Limberg, product innovation director at Lantech. "But none of these electronic systems were able to run at the high speed required without damaging the product."
Lantech's machine required some sort of feedback from the film to determine how fast to pay film out of the film delivery system to keep from damaging the load. The problem was that at a rate of one revolution per second, the electronic feedback systems couldn't keep up and loads were being damaged or not wrapped tightly enough to survive transport.
Why Use Mechanical Speed Control?
- High starting torque required
- Better speed control needed
- Better suited to hostile environments
- No programming required
- No need to periodically upgrade software
- Simpler feedback required
Mechanical Speed Control Limitations
- High purchase costs
- Complex sizing and selection required
- High maintenance needed
- Large footprint
- Low efficiency
- No remote speed adjustment
The challenge in using a mechanical drive was linking it in a way that ensured a robust design and provided maximum uptime in such a high-speed environment, Limburg says. "We achieved this by using a combination of timing belts and flat-style drive belts, including a unique 90° belt drive arrangement. We used belts everywhere possible to eliminate the need for regular maintenance and lubrication."
A typical feedback system for an electronically controlled film delivery system includes dancer bars, springs and sensors connected to a controller of some sort to control the speed at which the film is delivered to the load being wrapped. With the mechanical solution, Lantech eliminates all the variables and delays included in the electronic version along with all the expensive hardware needed to make the feedback loop work. All that was required was a way to tell the drive to engage or disengage, and Lantech accomplished that with a simple pivoting roller.
"The beauty of the mechanical drive system is that it doesn't have to interface with the machine controls for speed regulation," Limberg says. "All the controller needs to know is whether the film is being delivered or not, and that simple feedback was accomplished using photoelectric sensors, and a flag on the rotating film delivery system that actuates only when material isn't being dispensed."
Besides the technical challenges involved, there was also the need to change the mindset of Lantech and its customers. "Traditional control systems require and provide some sort of feedback, and the industry had become accustomed to this information requirement, but it simply wasn't needed or available with our purely mechanical system," Limberg explains. "So we had to shift our way of thinking, and we found that the ability to dial a machine in can lead to variances in machine performance and make the machine tweaky and less operator-friendly."
Malcolm Irwin, vice president at CAD software provider Aucotec (www.aucotec.com), previously worked for printing press manufacturers and plastic extrusion machine manufacturers, both of which employ electronic and mechanical gearbox designs to control web tension. He recalls a particular application in which a hybrid mechanical/electronic solution provided the best performance.
"On the chill rolls of web offset presses, the speed of the chill roll has to be extremely precise and synchronized with the line shaft that turns the press, but with the ability to compensate for the change in web tension due to the cooling of the web as it exits the dryer," Irwin explains. "The web has to remain at a constant tension throughout the whole process—not an easy task, considering it could be traveling at speeds up to 2,500 feet per minute. The solution was to mechanically gear the line shaft to a harmonic gearbox. A servo drive system was connected to the harmonic gearbox to effect minute offsets of input vs. output speeds of the harmonic drive."
As shown in the figure, a harmonic gearbox has three main parts: an outer circular spline, an inner flexible spline, and a wave generator. These gearboxes are extremely compact, offer very high turndowns, and exhibit zero backlash. In the chill roll application, the outer circular spline was driven from the press line shaft to set the input speed, and the inner spline drove the chill roll at the output speed. The wave generator changed the ratio of input to output speed.
With the wave generator held stationary, the output speed will be slightly faster than the input speed because the flexible spline has a couple of teeth less than the circular spine. If the wave generator is rotated, the result will be either a slight gain in output speed or a slight reduction, depending on which direction the shaft is rotated. A servo drive controls the position of the wave generator, and the result is a virtually infinite number of small adjustments, both positively and negatively, to the draw or tension on the web.
The hybrid system was able to change speeds in the order of 0.001% over the surface of the chill roll, proportionate to line shaft speed, Irwin says. "This degree of accuracy just wasn't possible with purely electronic drive systems. There are many other examples in web control and web offset printing where speed following is achieved with mechanical drives but speed offset is handled with electronic adjustment into the mechanical system."
This isn't new stuff. "We developed this chill roll system back in the late 1980s at the Baker Perkins Printing Machine Co. in Peterborough, U.K.," Irwin says. "However, it's not common knowledge that these harmonic gearboxes exist, and I'm sure there are lots of applications which could benefit from their use."
Although mechanical or hybrid speed control actually can offer better performance than purely electronic systems, there are other more prosaic reasons to control machine and robot speeds mechanically.
More Torque, More Rugged
"We still see mechanical variable speed in mixing applications such as seed treating," says Steve Greene, regional sales manager at Lenze Americas (www.lenzeamericas.com). "A lot of these types of applications need high starting torque, and in many cases, mechanical variable speed can handle the shocks from starting and operation much better than electronic systems."
There are other reasons for using mechanical speed control. "If the customer only has mechanical tech support, the machine can be serviced and repaired on site," says Alex Himmelberg with Lenze Americas' brake and clutch products group. "Most of these locations are now in underdeveloped markets and remote areas. Disco planetary wheel drives can be used in explosion-proof areas, and the availability of explosion-proof VFDs is very limited and expensive. Disco drives are also good for extremely dirty and wet environments. Belt drives increase the amount of rotating energy stored in the drive system, and ride through momentary overloads up to the motor pull-out torque."
Another supplier seconds some of Himmelberg's thoughts, and offers additional reasons to use mechanical drives. "Any application requiring high torque at low speeds is a good fit for a mechanical speed-changing product," says Rick Stewart, Baldor's product manager for Dodge torque-arm gearing (www.dodge-pt.com). "Our company typically uses helical gearing, worm gearing, v-belts or synchronous belts to reduce speeds. In some applications such as conveyors, we use a combination of gear reducers and belts to optimize the driven speed and torque to deliver high performance with low cost."
The result of a mechanical speed-changing device in a speed-reduction application is that torque is increased while horsepower remains constant, Stewart says. "This allows the use of a small motor while producing large amounts of torque," he explains. "Common applications would be conveyance, mixing, processing, pumping and air handling."
When a customer requires speed adjustment as well as variable-speed control, Stewart recommends using mechanical speed products such as gear reducers combined with standard induction motors driven by off-the-shelf inverter drives. "This allows maximum torque by use of the mechanical product combined with active speed adjustability, without having to add the complexity or cost of a servo type of product," he explains.
Know Your Limitations
"With mechanical variable speed, remote control is difficult and expensive, so the drive usually needs to be adjusted locally and mechanically," Himmelberg reminds. "Mechanical drives take up more space than electronic solutions, and often have limited speed ranges."
Other disadvantages are noted by Rich Mintz, marketing development manager for mechanical drives at Siemens Industry (www.industry.siemens.com). "The sizing and selection of mechanical speed-control systems isn't very straightforward as these systems don't simply adjust speed, but instead work by effectively changing the ratio of the connection between the motor and the reducer," Mintz says. "So you're not just adjusting speed; you're adjusting available torque."
VFDs are often more cost-effective up front, Mintz adds, and are virtually maintenance-free. "With mechanical speed control systems, belts or discs wear over time and must be replaced, and these items can be relatively expensive as their use becomes less widespread. Mechanical components can be very inefficient, especially belts and friction discs."
In summary, it's generally best to use electronic speed control when it can meet the performance and operating requirements of the machine or robot. But designers should keep in mind that in certain specialized applications, mechanical speed control can be the best option. When combined with electronic speed control in a hybrid solution, mechanical speed controls can offer very precise speed control.