Miniature gearing and planetary gearing put to good use

Gears function in an array of power-transmission applications. They are the natural complement to electric motors in industrial and consumer designs. Consider some examples: Amusement rides to consumer-grade home printers make use of spur gears. These roll through meshing for up to 98% or higher effi ciency per reduction. The only caveat is that they exhibit tooth sliding and noise due to initial tooth-to-tooth contact and audible shock loads.

Helical gear reducers are costlier than spur sets but work in designs needing high horsepower and efficiency. Textile machinery, conveyor drives, rolling mills, and elevators all use helical gearing to engage gradually over tooth faces for smooth operation and high load capacity. The only caveat is that the machine setup must include framing or supports to resolve thrust loads originating from the gears.

Non-parallel and right-angle gears go into motion applications for material handling, aerospace and defense, packaging equipment and food-processing machinery. Input and output shafts protrude in different directions; gear teeth are worm, hypoid, bevel (straight, spiral or zerol) as well as skew or crossed-axis helical. Common bevel gearsets often go into material-handling and packaging equipment.

Hypoid gears (useful for high-toque applications) are like spiral- bevel gearsets, but output and input shaft axes don’t intersect — and that simplifies integration of supports. Common in aerospace, zerol gearsets have curved teeth that align with the shaft to minimize thrust.

Speed reducers work with an array of motors. These gearsets alter the torque of a motor — usually as an increase proportional to rpm reduction.

Common in material-handling setups, shaft-mounted gear reducers come in designs that use special couplings to address reactionary torque. Other shaft-mounted reducers actually mount to the machine housing (especially in machine setups with surrounding enclosures) so the input shaft doesn’t support the reducer.

This subassembly is from a maxon GPX 22UP X-series 22-mm diameter planetary gearhead. Notice how the planet gears spin on precision needle bearings instead of higher-friction plain bearings. This and other features give the GPX UP planetary gearbox higher continuous and peak torque capabilities than comparable designs — for top efficiency, service life, and smoothness during back-driving … even in multistage iterations. Image courtesy of maxon

The American Gear Manufacturers Association (AGMA) defines the term speed reducer as sets with pitch-line velocities not exceeding 5,000 fpm or pinion speeds not exceeding 3,600 rpm.

Consider worm-gear reducers. These typically go into low to moderate-horsepower motion applications because they have high ratios and output torque but are cost effective and compact. Most worm gears are cylindrical with teeth of consistent size; some worm-based reducers have double-enveloping tooth geometry (in which pitch diameter is deep and short and then deep again) to boost tooth engagement.

Metal cutting and forming machinery, construction equipment, and packaging machinery all benefit from the low backlash of this gearing type.

To illustrate, printing-press rolls hold tight print registration at high speeds thanks to the ability of double-enveloping worm gearing to withstand shock and extreme acceleration. The gearing’s low inertia also lets presses start and stop more quickly than those with multistage gearing.

Gearheads, much like gear reducers, are useful where applications call for high torque at low speed. They reduce a load’s reflected mass inertia, so ease the acceleration of big loads ... which in some cases, even lets machines run off smaller motors. Gearheads range from basic spur gearheads to complex planetary gearheads and harmonic gearheads, all exceling in select applications.

The latter (also called strain-wave gearing) is for special speedreducing applications. This gearing excels in robot-arm articulation, medical equipment, and offshore drilling. Strain-wave gearing helps designs with zero backlash and high torque density.

One force-feedback application is that of a surgery robot. Here, maxon GPX UP gearboxes cleanly pass torque either from the input or output side … resulting in a superior haptic system and surgical result. Image courtesy of mason

Much machinery integrates servogears into application-specific electromechanical arrangements, and several of these arrangements are common enough to have their own labels. Gearmotors (most useful in machines that move heavy loads) include a gear reducer integrated with an ac or dc electric motor. Gearboxes are contained gear trains; planetary gears are a common form. Planetary gears often go in servo systems. Usually, the planet gears mount on a movable arm that rotates relative to a sun gear. In most applications, an outer annulus meshes with planet gears.

Planetary gearsets offer applications advantages over other gearsets, including multiple kinematic combinations, power density, big reductions from compact setups, and pure torsional reactions. Planetary gearboxes also boost overall design efficiency. Losses never exceed 3% per stage, so the sets transmit most energy for productive motion output. In applications driven by servo systems, gearboxes also reduce settling time … which is otherwise a problem when load inertia is high compared to motor inertia.

Consider the applications of one miniature line of planetary gearing. maxon’s new gear planetary X-series Ultra Performance (GPX UP) is now a standard product offering for configurable X-drive assemblies. So now the GPX variations include:

• the GPX A (with metal planetary pins standard)

• the GPX C (ceramic planetary pins for increased torque and life)

• the GPX LN (for reduced noise) and the GPX LZ (for reduced backlash)

• the GPX HP (for high power and increased torque capacity)

• the GPX UP (for ultra performance in the form of smooth back-drivability and top efficiency

GPX UP planetary gearing already works in Mars rovers and Formula 1 racecars to deliver high torque, ruggedness, and efficiency. Normally miniature gear assemblies rotate the planet wheels and gears on greased steel or ceramic pins (which serve as simple plain bearings).

In contrast, GPX UP gear assemblies actually include tiny needle bearings at these rotary axes — for rolling instead of sliding friction (and less friction and heat generation).

This 22-mm diameter GPX UP planetary gearbox was designed for an aerospace application. Notice how the planetary gears spin on needle bearings and not plain bearings. Image courtesy of maxon

GPX32 UP three-stage assemblies get efficiencies to 90% — far better than the 70% or so of comparable designs. So if fitted with an otherwise identical motor and controller, the entire system can output 30% more torque and power at output … or need 23% less input to generate a given output. If the motor happens to be a brushed motor, that extends the brush and overall system life. Plus more efficient gearing lets the motor operate at a higher location on the efficiency versus torque curve … or allow use of a smaller and lighter motor with control electronics. Such efficiency rom such a small component is particularly useful in battery-powered motion designs.

Having a needle bearing (and not a plain bearing) at the planetary-wheel ID axle means the assembly runs more coolly as well … which is helpful on handheld medical instruments.

Cooler operation also extends the life of the gear assembly’s lubricant — with less thermal stress and abrasive behavior through abraded metal particles and lubricant agglutination … so the GPX UP gearbox lasts 11 times longer than comparable competitor models.

The GPX UP also excels in haptic (force-feedback) applications because its smooth-turning planet gears allow repeatable back-drivability — that turning the shaft from the output side of the gearbox. So torque feedback via the GPX UP to the motor on the gearing’s input is consistently proportional to the load.

Traditionally-built planetary gearheads can’t guarantee smooth back-drivability or consistent load interpretation through the gearbox because of fluctuating efficiency as well as variable tolerance stackups between gearbox internals and the potential cocking of planet gears.

Here are various mason GPX planetary-gear offerings.

TWO HAPTICS APPLICATION EXAMPLES

Consider a specific haptic application — that of electronic fly-by-wire flight control. In these systems, a pilot’s steering movements aren’t transferred via conventional mechanical or hydraulic actuators. Instead, electronic transmission occurs via a feel (force feedback) motor assembly in a joystick control column.

Such haptics-generating motor assemblies benefit from gearboxes capable smooth back-drivability and simultaneously generating torque and force based on joystick position.

Another haptics application is that of surgical robots for minimally invasive micro and telesurgeries. Doctors performing surgery through these robots dynamically and instantaneously feel forces equivalent to those exerted by the robot end effectors. Here, gearboxes with smooth back-drivability are essential.

GPX UP gear assemblies deliver on this parameter even on axes needing gear ratios that necessitate two or three planetary stages. Traditional gearing exhibiting uneven or sticky back-drivability is mechanically noisy and unusable in these robots.

Gearing and other motion in extrusion equipment

Parts fabricator and machine builder Meyer Enterprises recently built a machine that turns PTFE tubing into medical-grade heat-shrink tubing. Key to the design is a puller assembly capable of pushing and pulling tube through the equipment during processing. In fact, two Versa Caterpillar Feed C-22 non-motorized puller assemblies tug material through the heat process — one in and the other out. Optional pneumatic operation of the belt booms along a constant centerline allows control of the pressure applied to the product being pulled.

In contrast, machine builder Simonds Industries uses motorized Versa PM-22 pinch-roll pullers to manufacture bandsaw blades and cutting tools. The pullers take bandsaw blade material off a 300-ft coil and pay it off to the required lengths for cutting. The motorized PM-22 puller assembly includes the motor, drive, control, and gear reducers … modified to allow feeds to 350 ft/min.

The Versa C-22 caterpillar feed applies even clamping force over product without deformation — key when manufacturing medical-grade plastic products.