A new chain drive promises to improve the efficiency of electric bikes and extend their range. The drive was jointly developed by iwis Drive Systems and Clean Mobile, a firm that makes electric drives for two-wheeled vehicles. Both companies are based in Munich, Germany. The hybrid unit combines electric and human power to reportedly deliver effortless acceleration and excellent range both on and off-road.

Clean Mobile’s designers began with a small, 1,200-W electric motor that delivers 150 N-m of torque. Initial plans were to mount the motor near the pedals and run the motor at high speeds, even at slow riding speeds. It quickly became clear this setup couldn’t transmit power to the rear wheel and meet efficiency targets with conventional reduction gearing.

The company approached engineers at iwis, who recommended splitting the gearing into a primary and secondary transmission. Primary gearing would handle speed reduction at the pedal crank and use standard components for secondary power transmission to the wheel.

The result is the direct dual drive (DDD), which uses three chains and sprockets to connect the pedal crank and adjacent motor shaft, together yielding a 1:38 reduction ratio. Engineers selected chains with the required strength and fatigue life based on forces on the teeth of each sprocket and the resulting overall transmission ratio. The three-chain arrangement reduces motor speed from 3,600 rpm down to pedaling speed. A freewheel hub ensures that force exerted by the rider on the pedals goes only to the wheel, not the motor, should the battery ever fail. A second idle mechanism disengages the pedals from the rear wheel, as on conventional bicycles.

The chains are major contributors to the drive’s overall efficiency, explains Michael Frank, new business development project manager at iwis, because they only transmit tensile forces in the direction of travel. A spur wheel with helical gearing, in contrast, would induce additional, lateral forces and thereby reduce overall efficiency, says Frank.

Tests on the DDD by the Department of Drive, Control, and Actuator Technology at the German armed forces university (Universität der Bundeswehr München) in Neubiberg, near Munich, showed an efficiency of approximately 80% across a broad operating range. In contrast, typical electric bikes have efficiencies ranging from 25 to 50%, according to Clean Mobile officials.

An eSpire bicycle equipped with DDD, built by Munich cycleworks Third Element, won the first official E-bike world championship at last year’s Intermot in Cologne, Germany — the world’s largest bicycle and motorcycle show. Its electronic controls and torque sensors manage energy flow from the Li-ion batteries to the wheel, further enhancing efficiency. It has a top speed of 45 km/hr.

iwis is developing more-powerful units, as well as lower-rated versions that use plastic parts, to satisfy demand from builders of two, three, and four-wheel vehicles, says Frank.

© 2012 Penton Media, Inc.

Planetary gearheads are high-precision, motion-control devices that generate substantial torque for their size, have high torsional stiffness, and low backlash — making them suited for wide-ranging tasks. For instance, specific types of planetary gearheads:
• Run around the clock, seven days a week, for more than 30,000 hr in cartoning applications. The lubricated-for-life gearheads require no maintenance, and high torque-to-size ratios permit compact envelopes and small machine footprints.
• Help attain accuracy within a few ten-thousandths of an inch on plasma-cutting machines, thanks to exceptionally low backlash. Helical crowned gearing provides fast positioning and smooth movement, and sealed gearboxes keep out abrasive dust generated during cutting.
• Limit noise and vibration and meet strict backlash requirements in scanning tables for cardiovascular patients.
• Let food-processing equipment slice meat, bread, and frozen foods at speeds up to four slices/sec. Also available are slim, right-angle designs to fit within the machine envelope and provides quiet, smooth operation.

Planetary basics
A planetary gearhead takes a high-speed, low-torque input, say from an electric motor, then increases torque and reduces speed at the output by the gearhead ratio. This lets motors run at higher, more-efficient rpms in equipment that operates at low speeds. It also reduces inertia reflected back to the motor, increasing stability. And using a planetary gearhead often lets machine builders reduce the size and cost of motion-control hardware.

Planetary units with helical gears, rather than spur gears, have a larger contact ratio. The contact ratio is the number of teeth in mesh at any given moment. While typical spur gearing has a 1.5 contact ratio, helical gearing more than doubles it to 3.3. Benefits of higher contact ratios include:

• 30 to 50% more torque capacity than equivalent spur-type planetary gearing.
• Better load sharing, which increases life.
• Smoother and quieter operation.
• Backlash reduced by as much as 2 arc-min.

The gearhead’s helix angle also has a significant impact on performance because the greater the angle, the more teeth in the mesh at any one time. So increasing the helix angle from the typical 12° up to 15° raises torque capacity by 17 to 20%; and by as much as 40% over straight-cut spur gears. Gears with a 15° helix angle also emit less noise.

Helical-gear teeth generate axial loads on the motor shaft. Gearhead bearings must compensate for these loads. Helical gearheads using ball bearings with little or no axial load capabilities can suffer premature motor-bearing or gear failure. A better approach uses tapered roller bearings, such as in Micron Helical gearheads, to completely compensate for axial loads.

Single-stage planetary gearhead ratios range from 3:1 to 10:1. Gear ratios cannot exceed 10:1 because pinion gears can be made only so small. Gear ratios greater than 10:1 are possible with an additional planetary stage, although this normally increases length and cost. Planetary designs also cannot have ratios less than 3:1 because then the pinion and outer ring gear would need to be nearly the same size, leaving no room for the planet gears. Ratios between 4:1 and 8:1 provide the best combination of pinion and planet-gear size, performance, and life.

Crowning involves slightly modifying the gear-tooth profile to improve gear mesh alignment, increasing torque capacity and reducing noise. It also improves load distribution on the tooth flank, thereby minimizing high-stress regions that can cause surface pitting.

Some clearance is needed for a planetary gearhead to work effectively. Clearance prevents excessive heat and gear wear and ensures good lubrication. But the small gap between gear teeth leads to lost motion. Real-world gearheads also cannot have infinite torsional stiffness, so windup (flexing) in the gearhead generates additional lost motion.

Understanding how different manufacturers measure backlash is important when choosing a gearhead. There are no strict standards regulating how to measure backlash. This can lead to confusion and misconceptions. Some manufacturers measure and average four or more points on the output shaft to produce a backlash specification. Using this method, a unit with backlash measurements of 4, 6, 10, and 12 arc-min would have a rating of 8 arc-min. Thomson engineers believe backlash should be based on the largest measurement on the output shaft, so the above example would yield a 12-arc-min rating.

In addition, some manufacturers apply 2% of the rated torque to generate backlash ratings, while others apply less. The latter produces lower backlash measurements and doesn’t provide true backlash ratings over the life of a product.

Backlash will increase over time. A planetary gearhead might have 8 arc-min of backlash out of the box but 15 arc-min after six months of use, for example. So how well a planetary gearhead maintains accuracy over its life is an important consideration for most users.

Sizing and selection
Choosing the right gearhead and accurately sizing it is critical to long and reliable life. As a starting point, designers can approximate required gearhead size from:

T_r = T_m × r × e

where T_r = application torque, T_m = continuous torque, r = ratio, and e = efficiency.

To precisely size a gearhead, however, engineers must consider the complete motion profile, including speed, torque, acceleration, deceleration, and cycle rate. And they should apply a derating factor for high-cycling conditions. (Typical values are shown in the table.)

Nonstop, continuous-duty applications do not require derating factors. In those cases, the most common problem is overheating that breaks down the lubricant and causes gear failure. High-performance gearheads, such as Micron EverTrue, are designed to run 24/7, operate under 140°F, and last more than 30,000 hr.

Online selection and sizing tools can save time by letting engineers find and compare planetary gearheads that fit a particular application. (See the accompanying sidebar for more details.)

Troubleshooting
Several problems can crop up in gearheads that aren’t sized and installed properly:

Noise. Inappropriate input speed, gearhead ratio, output torque, radial and axial loads, and mounting errors can all contribute to gearhead noise. But proper mounting is critical to minimizing noise and maximizing performance. Many gearheads must be mounted to the servomotor while positioned vertically. This lets the motor shaft center the gearhead. After mounting to the motor, the gearhead can be used in any orientation.

Friction. Too much grease, out-of-tolerance components, and poor gear or bearing quality can cause excessive friction and drag. Look for gearhead manufacturers that test every gearhead for input drag before shipment. Each size and ratio has an acceptable range for drag, and peak levels should be measured in both directions.

Sealing. For applications that require protection against dust, dirt, and water, be aware that combining an IP65 motor and an IP65 gearhead does not always provide IP65 protection. Look closely at how the interface between the motor and gearhead is sealed. The best approach is to use O-ring seals between all housings for IP65 protection on the full assembly.

A new type of planetary gearhead, the Micron AquaTrue, meets IP67 requirements for food and beverage handling, packaging, and dispensing, thanks to a round stainless-steel housing with no external seams. Such gearheads can withstand caustic cleaning chemicals and high-pressure wash downs, giving engineers the flexibility to mount it without the added cost and complexity of components such as enclosures, shielding, and mechanical transmissions.

Lubrication. Oil or grease can effectively lubricate planetary gearheads. Grease has the advantage of providing lubrication for the life of the gearhead, eliminating a lot of maintenance. Grease permits mounting in any orientation and eliminates concerns about leakage.

Oil requires maintenance and relubrication, usually every few thousand hours. And leaks are always a concern with oil lubrication. Orientation with oil lubrication is usually restricted, must be specified when ordered, and usually cannot be changed. A common misconception is that oil-filled units always run cooler than grease-lubricated gears. Actually, the sealing required for an oil-filled gearhead often generates more heat than the oil saves.

© 2012 Penton Media, Inc.

Variable frequency drives have an Achilles Heel. Even though the VFD itself is protected against damaging electrical conditions, the drive's power cables can fail when subjected to heat or voltage levels greater than their insulating layers can tolerate. Power cables can also be irreparably damaged by heavy mechanical loads or corrosive environmental conditions. Fortunately, it is easy to avoid VFD cable failures--and the ensuing downtime--by paying attention to cable design. This technical paper from Lapp outlines the key design features engineers should consider when picking reliable cables for VFD applications.

In the past, rack-and-pinion drives were generally developed for planetary-gear units, with pinions fitted to the output shafts and matching gear racks. But the rack-and-pinion drive would put an overhung load on the gear unit that limited the size of the drive. Thus, the drives could only use 40 to 60% of the gear units’ torque.

Engineers at Stober, Maysville, Ky. wanted to solve this problem, so they developed the ZTRS drive, a compact, highly efficient rack-and-pinion drive. They developed the drive together with Atlanta, a gear-rack manufacturer based in Farmingdale, N. J.

The ZTRS drives are based on the company’s PH and PHA planetary-gear units, which are torsionally stiff and generate high torque. Ratios range from 4:1 to 121:1, and drives are available in right-angle versions and with built-in motors. But the ZTRS can be attached directly to any servomotor.

The output-bearing housing supports the rack-and-pinion drive, which relieves the load on the flange-mounted planetary gear’s bearings. This reduces the tilting moment and frees engineers to match the drive to the torque needed for their application. In general, this lets engineers use gear units a size smaller than those used in the past. The device also uses comparatively small-diameter pinions, so it takes more speed to get the required feed rate. This means the gear ratio is lower and, in many cases, a single-stage planetary gear unit can be used instead of a two-stage device.

The ZTRS reduces linear backlash by 50% to 3 arc-min, and increases linear stiffness by up to 100% compared to standard PH and PHA units. The new mounting configuration lets it use pinions with small pitch diameters.

Apart from improving linear stiffness and linear backlash, the pinion concentricity was adjusted to less than 0.01 mm. The drive is lubricated using a felt pinion built into the output-bearing housing. It can be manually relubed or hooked to an automated lubrication subsystem.

The flange-mounted planetary-gear unit offers a choice between helical PH and PHA versions. And the ZTRS-PH(A) rack-and-pinion drive comes in sizes 7, 8, and 9, and modules 2 to 6, with gearing quality 5.

© 2011 Penton Media, Inc.