Technical Fundamentals


Fundamentally, gears are devices which transmit rotational motion from one axis to another.  Certain types of gears may also result in translational motion.  There are dozens of different types of gears in industry, only a few of which are replicated in LEGO.  The existence of LEGO gears actually predates the Technic or Expert Builder lines and goes back to the Samsonite gear wheels of the 1960's.  The different types of LEGO gears and their basic function and usage will be explained below.

Gear Ratios

In the simplest case, a set of gears results in the same rotation speed on both the input and the output axle.  This happens if both gears have the same number of teeth.  However, in most cases gears are used to change the speed and mechanical advantage between axles.  For simple gears like those used in LEGO, calculating this gear ratio is simply a matter of counting the number of teeth on each gear and comparing by dividing the number of output teeth by the number of input teeth.  For example, if the input gear has 8 teeth and the output gear has 24 teeth, then the gear ratio is 24/8 = 3.  The standard nomenclature for this is to use a colon and relate the gear ratio to one, for example 3:1.

So what does it mean to have a gear ratio of 3:1?  Firstly, this is the ratio of the rotational speeds of the axles.  The gear with fewer teeth always rotates faster, so in this example the 8 tooth gear rotates 3 times faster than the 24 tooth gear.  Secondly, the ratio of torque is the inverse of this ratio.  In this example, the 8 tooth gear has 3 times less torque than the 24 tooth gear.

When you use gears to make something rotate faster and produce less torque, it is called "gearing up".  You might use this to protect downstream components from the high torque of a motor.  When you use gears to make something rotate slower and produce more torque, it is called "gearing down".  You might use this to lift something heavy with a small motor or crank.  Sometimes you might not gear up or down, but simply use gears to get torque from one point to another with no change in speed or torque.

Spur Gears

Spur gears operate on axes which are parallel.  In the animation below, the gray 16 tooth gear on the green axle is driving the red 24 tooth gear on the yellow axle.  Note that the relative speeds are 3:2 which is the ratio of their number of teeth (24:16).  Note also that the contour of the gear teeth is not flat.  The profile used is called an involute and it allows the gear teeth to roll against each other rather than slide which minimizes friciton and maximizes efficiency.  Also note that each axle has two Technic bricks used as bearings for support.  In general, you want at least 2 bearings supporting each axle to balance the gear tooth loads, although it is possible to use just one axle pin with reduced efficiency.  The closer you can locate the bearing to the gear (like the black brick), the more efficiently it will provide support.  It is also generally better to locate bearings on either side of the gear (like the blue bricks) than to put them both on one side.  One side effect of a pair of spur gears is that the output axle rotates the opposite direction as the input axle, an effect which can be clearly seen in the animation.  Spur gears are the most common gear type in Technic. 
spur gears
There were some spur gears made before Technic and Expert Builder even existed in the 1960's and 1970's, but those are not covered by Technicopedia.  They were very large and ranged from 9 to 42 teeth.
old spurs

The image below shows the "standard" spur gears.  The three shown in light gray (8 tooth, 24 tooth, and 40 tooth) existed from the beginning in 1977 and are still in production.  The 16 tooth shown in tan came out in 1979.  The 24 tooth shown in dark gray was a stronger replacement for the older 24 tooth beginning in 1998.  The 24 tooth shown in white is a "clutch" gear which can slip once a certain torque is exceeded.  It is rare and was first seen in 1997.  All of these use the same tooth profile so any spur gear can be used with any other spur gear.  Note also that all tooth counts are a multiple of 8 which makes gear ratio calculations easy.

Finally, the "double bevel" gears can also be used as spur gears.  These will be explained further in the section on bevel gears.  They have wider teeth than the regular spur gears and can handle more torque.

Bevel Gears

Bevel gears operate on axes which are not parallel.  Bevel gears can be made specifically for axles at virtually any angle, but LEGO bevel gears are all made for perpendicular axles (90 degrees).  In the animation below, the red gear on the yellow axle is driving the blue gear on the green axle.  The axles are turning at the same speed because the gears have the same number of teeth.  Bevel gears have complex tooth shapes which also generate complex forces on the supporting axles.  Therefore, it is even more important than for spur gears that there are proper bearings for support of the axles.  In general, you want at least 2 bearings supporting each axle to balance the gear tooth loads, although it is possible to use just one axle pin with reduced efficiency.  The closer you can locate the bearing to the gear, the more efficiently it will provide support.  In the animation, a special gearbox is used specifically for this purpose.  Spur gears are the second most common gear type in Technic.
The image below shows the basic bevel gears.  The light gray 14 tooth bevel gear was introduced in 1980 and used for many years, including as part of the old differential assembly.  In 1995 it was replaced with the 12 tooth bevel gear shown in tan.  This gear has a web between the teeth and it is therefore much stronger and was used in the newer differential.   The uncommon 20 tooth bevel gear shown in dark gray was introduced in 1999.

The double bevel gears shown below are so titled because the teeth are beveled on both sides on the face of the gear.  This makes them somewhat more versatile than the regular bevel gears, but they are also twice as thick and therefore take up more space.  They come in 3 sizes: 12 tooth, 20 tooth, and 36 tooth and began to be introduced in 1999.

Worm Gears

A worm gear (or screw) can be thought of as a gear with a single tooth.   The LEGO worm gear operates on an axle which is perpendicular to a mating spur gear.  Worm gears have some special properties which make them differ from other gears.  Firstly, they can achieve very high gear reductions in a single stage.  Because the worm gear has only one tooth, the gear ratio is simply the number of teeth on the mating gear.  For example, a worm gear mated with a 40 tooth spur gear has a ratio of 40:1.  Secondly, worm gears have much higher friction (and lower efficiency) than the other gear types.  This is because the face of the worm gear's tooth is constantly sliding across the teeth of the mating gear.  This friction gets higher the more load is on the gear.  Finally, worm gears cannot (generally) be backdriven.  In the animation below, the worm gear on the green axle is driving the blue spur gear on the red axle.  But if you turn the red axle as an input, the worm gear will not turn.  This is useful for locking things in place like using a crank to raise and lower a lift gate.  One final thing to remember about worm gears is that there is force created which pushes the gear along the axle (green axle in the animation).  Something needs to be used to prevent this motion or the gear will slide away.  One possibility is the yellow gearbox in the animation.

The worm gear is a little less than 2 studs long.  Multiple worm gears can be put in a row to make a longer screw.  It is even possible to mate a worm gear with the rack gear, although the fit is not quite right.

Rack Gears

A rack gear is like a spur gear that has been unrolled to lie flat.  It is a means of transforming rotational motion from the mating spur or pinion gear to translational motion of the rack.  A the pinion rotates, it pushes the rack gear along as subsequent teeth mesh.  There is no gear ratio in the traditional sense, but the fewer the teeth on the driving gear the more power will be delivered to the rack.  The lateral movement of the rack is proportional to the number of teeth on the pinion.  These gears are most traditionally used for vehicle steering, but have many other uses including the extension of a telescoping boom on a crane.

The gear racks range widely in size.  The most common size is 4 studs long and attaches to a plate via studs.  These can be placed end to end to create a longer rack.  There are a series of even length racks (8, 10, 12, 14) which have pin holes at the end and are open underneath so they can slide along studs.  There is a small 2.4 stud length rack with ball joints in the front.  Finally, there is a 13 stud long rack with perpendicular axle joiners at the end (shown in black).

Differential Gears

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There have been 3 different differential gear carriers available over the years.  The one shown in light gray was released in 1980 and was typically used with the old 14 tooth bevel gears.  The ring gear had 28 teeth.  It was replaced by the one shown in old dark gray in 1994.  This was typically used with the 12 tooth bevel gears.  The large ring gear had 24 teeth at the large end and 16 teeth at the small end.  It also had a recess for a driving ring.  The last version shown in new dark gray was released in 2008.  It has a 28 bevel teeth on its ring gear and is used with the 12 tooth bevel gears.

Driving Rings

The driving ring, in combination with a pair of idler gears which do not turn with their axle of support, allow functions to be engaged or disengaged.  It slides over the ridged axle joiner which we first saw in 1993.  Small tabs on the driving ring allow it to lock along these ridges, but still slide with some extra force.  The driving ring grips the longitudinal grooves on the axle joiner causing them to rotate together.  A circumferential groove in the middle of the ring allows it to be pushed along the axle joiner in either direction.  A set of 4 driving dogs on either end then mate with a 16 tooth idler gear allowing the idler's rotation to be either synched with the axle or allowed to spin freely.

The animation shows how the new driving rings work to engage and disengage the clutch/idler gears.  The driving ring is shown in red.  The lower axles are joined with the gray axle joiner.  The driving ring rotates with the axles.  At first, the driving ring is disengaged so both the dark gray and green gears are not driven and slip on the axle.  The driving ring then engages the green gear and thus drives the blue gear.  Because the driving ring does not use gear teeth but rather uses four tapered driving dogs, there is considerable backlash between the driving ring and the gear.  The allows the driving ring to be engaged even while it and the mating idler gear are turning at different speeds.
A fairly small number of sets have contained these parts over the years.  It is generally used either in a transmission used for gear changes or in a gearbox used to select between multiple motorized functions.  The driving ring originally always light gray but has more recently been typically red.

Adjustable Pitch Rotor

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Ackerman Rack and Pinion Steering

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Trailing Arm Suspension

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Trailing Arm

Double Wishbone Suspension

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Many parts can be used to create the upper and lower control arms including simple beams and liftarms, but there are also a number of special purpose parts available.  The long Formula-1 style links shown in black first appeared in 2000, but these appeared in only a few sets.  The left dark gray control arm appeared in 1988 and was replaced with the one on the right in 2008.  The arm shown in yellow has appeared in a large number of sets since 1994.

A number of special hubs have appeared over the years with ball joints at the king pins and with additional provisions for a steering attachment.  The two in the center were only available in a single set each, a Supercar (8880 and 8865).  The others are easier to find and the yellow one on the left also supports connecting a driven axle to the hub with a constant velocity joint.

4-Bar Linkage

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Original (Square Piston)

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Revised (Round Piston)

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