Gearboxes 101

HISTORY

The first industrial mills appeared in Medieval Europe between 770-1443 AD. Grist mills, flax, saw and cotton mills all employed gears to convert input energy from waterwheels, wind blades or work animals into specific work.

The first patent for gear hobbing, the process of cutting gear teeth, was granted to the English inventor John Whitworth in 1835.

Throughout the industrial revolution and up to the present, gears have continued to serve a vital role in developing humankind’s methodologies for accomplishing tasks, and they continue to evolve and improve in the way they provide brute force as well as delicate positioning (Figures 2 and 3).

TODAY’S GEARS

Some of the forms of gears today include spur, helical, double helical, bevel, worm, epicyclic, rack and pinion. All of these gears are used for transmitting either rotary or linear motion, for multiplying force (torque being a rotary force), for increasing or decreasing rotational speed or for intricate position synchronization.

By definition, a gearbox is a set of gears within a casing. In the realm of actuation, that definition fits: a box (casing) contains gearing with the typical purpose of increasing the amount of input torque applied as an output to impart rotational movement.

Changes in rotational direction between the input and output of the gearbox and the number of rotations between the input and output are functions of the configuration of the gears and applied gear ratios. Products that require actuation and typically employ one form of gearbox or another include, but are not limited to: valves, slide gates, drum gates, dampers, skim troughs, and any other form of equipment that requires a rotational input to perform its intended function.

The advantage of coupling a gearbox with an electric actuator is an increase in output torque because of the mechanical advantage of the gearbox. This allows a cost-effective solution through use of a smaller electric actuator as compared to a stand-alone electric actuator of comparable torque output.

This type of gearbox also can incorporate a base to allow the acceptance of thrust that results from the actuation in a threaded rising stem design valve or gate. The threaded stem of the valve or gate mates with a removable output drive nut, which is machined to the same thread of the stem.

When the input shaft of the gearbox is turned, either manually or by electric actuation, the output drive nut is rotated in the same direction as the input rotation (as viewed from the top of the gearbox). This imparts linear movement of the threaded stem. The drive nut is housed in a hollow shaft of the gearbox, which is open to the atmosphere, allowing the threaded stem to pass through the gearbox for long linear stroke applications.

A worm gearbox accepts a multi-turn input and provides a partial turn output, typically a nominal 90-degree rotation. This type of gearbox is intended for quarter-turn valves such as butterfly, plug or ball valves, or dampers. The output connection is typically a removable splined coupling, bored and keyed to match the stem of the valve or damper.

A spline connection of the coupling within the gearbox allows available positioning increments of the gearbox based on the number of splines provided in the 360-degree connection. A spline connection is a -manufacturer-specific feature—not all gearboxes offer this method of connection to the operating stem. This eliminates the need for machining a special keyway position in the gearbox to accommodate the directional rotation needed between the open and closed positions of the valve. It also eliminates the mounting orientation of the gearbox needed when the stem bore and keyway have to be machined directly in the worm wheel of the gearbox. Integral position indication is provided by an adjustable pointer cover for the full-open and full-closed positions.

Gearboxes are constructed to provide a mechanical advantage between the input and output force. For manual operation, this would be taking an achievable human-derived handwheel rim pull and increasing the force to an output torque capable of operating the intended device.

Over its life cycle, a gearbox is expected to face numerous harsh conditions without failure of the gears or compromise of the enclosure case. This is because failure of either of these components is simply unacceptable.

INHERENT COMMONALITIES

A number of common inherent properties of gearboxes are addressed differently by different manufacturers and can be considered when choosing the best overall gearbox for an application. All these properties are manufacturer specific.

Backlash is an inherent property of gears—in new gears, this results from the manufacturing process, and in existing gears, this results from wear. The amount of clearance existing between a gear tooth space and a mating gear tooth width represents the amount of backlash present. This is the amount that either of the mating gears can rotate while its mate is stationary. The greater the backlash, the less the ability to maintain a position. Manufacturers incorporate precise machining (hobbing) and tight tolerances to minimize gear backlash.

A desirable feature for gearboxes is a self-locking ability. This is the inability of the gear to be driven from the rotational output end of the gearbox. In other words, torque applied to the output of the gearbox will not result in operation of the gearbox, an action accomplished by the geometry of the gears.

Gearbox efficiencies or mechanical advantage in simplest form is the gearbox output torque divided by the gearbox input torque (with torque in the same units). As an example, in a gearbox incorporating two gears, the first gear would be a small diameter. This would result in a high transmission force at the gear teeth as Force=Input Torque divided by the gear pitch radius. A small radius results in larger force. The second gear would be a larger diameter than the first. Accepting the high transmission force from the first gear at the gear teeth results in force times the second gear pitch radius, which equals output torque. A large radius increases the force by multiplication, resulting in an overall increase of torque from input to output.

The number of turns is the required amount of gearbox input revolutions to equal one output revolution. Because the function of a gearbox is to increase the amount of input torque as an output to impart rotational movement, the employed gears typically follow the gearbox efficiency configuration mentioned above. By this configuration, the first gear, accepting the input, would be a small diameter and the second, providing the output, would be a larger diameter. A smaller input gear mating with a larger output gear results in an increased number of turns required at the input to provide one complete turn of the output. Typically, as the efficiency of a gearbox increases, so does the required number of input turns to equal one output turn. This results from further varying gear diameters or the addition of additional gears to provide an increase of the output torque while the input torque remains constant.

STANDARDS AND SPECS

Various standards exist for different industries governing the construction and testing of gearboxes. This ensures the purchaser of a gearbox that the manufacturer meets a certain level of quality and aides in alleviating possible uncertainties for the basic integrity of a unit.

A full alloy bronze worm gear (Figure 8) can provide optimal operation, corrosion resistance and four times the life span of a segmented gear by having the ability to use four quadrants of the gear before requisitioning a replacement expenditure for a worm gear or complete replacement unit. The adjustable 90-degree stops are integral mechanisms activated by the turning of the worm shaft to be able to incorporate a full 360-degree worm wheel (this is a manufacturer specific design).

CONCLUSION

It’s up to the purchaser to define specifications and exact requirements. Careful consideration should be afforded to determine the best overall configuration for the application.

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MARK BUJALSKI is an application specialist for AUMA Actuators Inc. Reach him at Mark.Bujalski@auma-usa.com.