Whether used as common speed reducers or exotic speed increasers, gearboxes face special challenges when rpms run high. Noise, vibration, and wind losses can diminish efficiency fast. Rapid engagements make for more friction and heat — which can ultimately lead to leakage. How to prevent these problems from hindering gearbox performance?
To shed additional light on the topic, we talked to industry experts, asking them what designers need to know to keep their gearboxes happy at high speed. Here's what they had to say.
What makes gearing high speed?
Tom/Mijno: Common non-servo electric motors using gear speed reducers are mostly 1,725 rpm input, with a few at 3,450 rpm. This lower input speed facilitates quietness. However, servomotors usually have continuous operating speeds that can reach 4,000 or even 6,000 rpm. This is where gearboxes must withstand faster operation.
Miriam/alpha gear: Several things make gearboxes a good design for high speed, right off the bat. For starters, they improve dynamic system response and amplify peak torque. This allows smaller motors to run at higher speeds, and operate near their ratings. Reduce reflected load inertia is another benefit; this allows for higher acceleration rates. Matching inertia increases system stability and simplifies servo tuning. This makes for higher gains and reduces position settling time, and allows for shorter cycle times.
Reduced size is another benefit of using gearboxes. Smaller motors and smaller amplifier motor/gearhead combinations mean weight is reduced. These systems are often more cost effective than large-motor systems. Too, smaller motors and amplifiers are more energy efficient.
George/Hamilton Gear: Generally speaking, a high-speed gearbox is a unit with pitch line velocity equal to or greater than 6,900 ft/min. or speeds greater than 4,500 rpm. Speed limits depend on many factors.
Miriam/alpha gear: With gearheads motor manufacturers can produce a few optimized servomotor designs, and then match their best speed/torque characteristics to various mechanical loads. The aim is to get full use out of a series of standard servomotors, avoiding the need to custom design a motor for each application. This keeps the motor, drive, cables, and other elements in the servo system reasonably sized. Gearboxes also address complicated design problems. With them, designers can turn a powering motor 90° or convert rotary motion to linear translation. And, additional bearings are eliminated.
The ratio for increasing torque is τout = τin × I
The ratio for reducing speed is nout = nin / I
The ratio for inertia reducing is jref = jload / I2
Packaging, paper converting, printing and semiconductor applications all require gearboxes to operate at very fast speeds. Few gearboxes can operate at these speeds continuously; those that do include features that decrease or eliminate the need for maintenance, with capabilities like dry running and long service life — sometimes to 30,000 hours.
What percentage of the time are gearboxes actually used to increase speed?
Miriam/alpha gear: There are gearboxes that connect to gear motors and increase the speed of the motor. These high-speed gear boxes push several limits, with acceleration torque reaching 42 to 485 Nm and input speed for continuous operation reaching 3,000 to 4,000 rpm.
Tom/Mijno: In our business, which is gearheads for servomotors, we are asked to size gearboxes for a large variety of industrial applications. However, my experience is that fewer than 3% of designers ask about speed increasers — and I suspect that few of those end up actually purchasing a speed increaser. In summary, I don't know of any general-purpose gearboxes marketed as speed increasers. For several reasons, geared speed increasers are usually custom items.
To put the subject in perspective: My experience is that most designers looking for speed increasers have 10,000 rpm and up in mind as an upper value. At this speed, centrifugal forces can be significant and precision balancing of components becomes important. In regular commercial gearboxes, components are not dynamically balanced.
Miriam/alpha gear: High-speed applications are common in the vertical heath and beauty markets. For example, say an OEM produces machines that make diapers. To become the market leader, the manufacturer wants all shelves stocked with their product. For this, they must increase their machine's output from 500 to 700 diapers an hour. One of two things must be increased: Motor speed or machine running time. Both are possible. But say the manufacturer decides to increase motor speed, which is already at 4,500 rpm. By attaching a high-speed gearbox onto that, depending on the ratio, the rpm can be increased substantially. In short: 4,500 rpm might be more than enough for a printing application, which is not continuously running or highly demanding; however, for the diaper application, this speed is simply not enough.
Tom/Mijno: Gears that transmit power at higher speeds require a few things: Precision-balanced rotating parts, lighter loads on contacting surfaces to reduce frictional heat, and shaft seals and lubricants that can withstand the heat that is generated.
As far as speed increasing goes: There are some types of gearing that simply cannot be used for speed increasing. Worm gearing that is self-locking is a primary example. Less well known are spur gearboxes incorporating long and short addendum gearing. This gearing type is fine for speed reducing, but does not perform well when back-driven as a speed increaser.
What design features in gearmotors most often limit speed?
George/Hamilton Gear: Some speeds are limited by driven equipment, support bases, and so on, while others are limited by gear design features and characteristics. For example, gears may be either single or double helical. The sum of teeth in a gearset must be of adequate proportions and geometry and must be able to transmit power for which it's designed — without excessive bending and torsional deflections. Teeth should be ground to AGMA quality levels of 12 (A5) to 14 (A3) or better. Also, all internal parts must be dynamically balanced within acceptable levels.
Miriam/alpha gear: When the load's inertia is greater than the motor's, controlling the system is problematic. This is because high-load inertia can increase instability. These applications are impossible without a gearbox. So, gearboxes increase the practical uses of servomotors.
Tom/Mijno: Gear housing must be rigid enough to withstand all rotational internal loads and stresses. The base mounting flanges must also be of rigid construction, to minimize deflections. Great care must be taken when mounting the unit on a baseplate.
What application factors can limit speed?
George/Hamilton Gear: One of the most important features that may limit gear speeds is the lubrication of bearings and gear meshes. The correct amount of lubricant with proper viscosity will form adequate elastohydrodynamic film thickness at the bearings and gearmesh. Forced-lubrication systems can be cooled by either water-to-oil or air-to-oil heat exchangers. Oil should be filtered and changed at regular intervals. Oil should be also checked for contamination and viscosity breakdown between regular oil changes.
Miriam/alpha gear: You could say that application requirements do not limit speed, but increase it. Designers often need to increase product output — and the only way of doing that is by increasing the machine's speed. After all, it is not very often that we can increase the amount of hours in a day. Many gearboxes today can run 24 hours a day, year-round.
What are the main attributes of a high-speed gearmotor?
Tom/Mijno: As for planetary gearheads, many designs can be adapted for use as speed increasers, but manufacturers usually stipulate only single-stage gearheads with a maximum ratio 10 to 1 or so, and reduced torque ratings. Here, maximum speed is around 6,000 rpm. The bearings in gearboxes designed for speed reduction also have to be evaluated. Cost-optimized designs may use lower-cost bearings on the low-speed side. If this low-speed side is switched to be the high-speed side, early failure can be expected.
George/Hamilton Gear: Vibration analysis of torsional, lateral, and axial modes is recommended to ensure that gear units do not exceed allowable limits. What can happen if vibration goes unchecked? Excessive vibration of internal components may result in additional dynamic loads for which the unit is not designed — and catastrophic failure may occur as a result.
Tom/Mijno: Still another design consideration is acceleration forces. Significant torque power may be needed to accelerate a load to a higher speed. In this case, a gearbox must accommodate torque-acceleration forces. For high-inertia loads, taking a longer time to accelerate can result in a lower-cost gearbox. Keeping the speed increase ratio low can also make the design easier.
MEET THE EXPERTS
alpha gear drives, Inc.
Mijno Precision Gearing
Park Ridge, Ill.