Markus Kossman

Torque talk – the hidden power of gears

January 08
2014
Torque talk – the hidden power of gears

Think about this – LEGO(r) TECHNIC would not be very technical – or be much fun to build – without the humble cogwheel, an ingenious invention first used over 2300 years ago.

Cogwheels and gears, in their various modern interpretations, are not only a big part of LEGO(r) TECHNIC, they also play a major role in pretty much everything mechanical that you come into contact with. Whenever you use anything containing spinning parts you're almost certainly using gears, even if – as in LEGO(r) TECHNIC models – this isn't always entirely obvious!

So what is that gears do, in everything from power tools to Power Functions and clocks to mobile cranes?!? Read on for a brief introduction to the world of gears and to see all the incredible – and surprising – ways gears are used in the amazing Mobile Crane MKII.

Why do we need gears?

Gears do all sorts of vital mechanical jobs, the most important of which is known as gear reduction: a small motor can generate quite a lot of power, but often not enough to create a turning force, or torque. Reducing the motor output speed increases torque. For example, a small motor tool, like an electric screwdriver, would never work were it not for gears.

Along with decreasing (or increasing) rotation speed, gears are generally used for the following tasks:
  • To reverse the direction of rotation
  • To move rotational motion to a different axis
  • To keep the rotation of two axes synchronised

Many Technic models, not least those using Power Functions, rely heavily on multiple gears and transmission functions to work. Before we look at the how gears work on board the Mobile Crane MKII (that’s a lot of gears, by the way!) we need to consider a couple more things:
Gears come in all shapes and sizes!

spur gears

Spur gears are the most common type of gear. They have straight teeth, and are mounted on parallel shafts. Spur gears are used together to create very large gear reductions. Typical application: power tools, washing machines, clocks, you name it!

Helical Gears are very similar to spur gears except the teeth are at an angle to the face, giving helical gears more contact in the same area. Helical gears are smoother and quieter than spur gears. Typical application: car transmissions.

bevel gears

Bevel gears are useful when the direction of a shaft's rotation needs to be changed. They are usually mounted on shafts that are 90 degrees apart, but can be designed to work at other angles as well. Typical applications: drives in trains, planes and automobiles!

tandstang

Rack Gear is a straight gear used to transmit power and motion in a linear movement. Typical application: car steering

Gear ratios, types and … teeth!

The key to managing motion and power comes down to gear ratios. Put one big gear together with one small gear and rotate them - the speed at which they turn in relation to each other is known as the ratio. On any gear, the ratio is determined by the distances from the centre of the gear to the point of contact. For instance, in a device with two gears, if one gear is twice the diameter of the other, the ratio would be 2:1. If both gears had the same diameter, they would rotate at the same speed (1:1), and so on, up to hundreds even thousands to one!

1 1 gearing
1:1 Gearing, 8:8, 16:16, 24:24.

bevel gear ratio
Gearing 3:1-8:24, 1:1,67-12:20, 3:1-12:36. 

worm gear ratio

Worm gear gearing 1:8, 1:24 (the worm gear counts as one gear).

The importance of good teeth!

One last thing before we turn to the big crane – what are your teeth like? Straight, spiral or hypoid? Involute, perhaps? Hopefully they're straight (although spiral would be interesting)!

Gears and teeth (wooden pegs originally) go together like LEGO(c) and TECHIC. Different gears – and different requirements – require different types of teeth (with odd names, above). But perhaps the most important point about teeth and gears is shape. Gaps, or the wrong shape or form of teeth, mean constant changes of output speed. This is okay for some things (clocks, for example), but not for others (vehicles!). Engine gears need to work so everything fits together as smoothly as possible, and this is where the so-called involute gear profile comes in, with teeth designed to provide maximum contact between gears at all times.

Time to look at theory in practice!

To put all the above in some sort of context, we've asked Senior TECHNIC Designer Markus Kossmann to highlight the key gearing set-ups in the Mobile Crane MKII, so you can see how gears actually work in real life, or at least in in LEGO(c) TECHNIC.

Markus, over to you: The Mobile Crane MKII uses 127 gears to drive all its functions. This is the largest number of gears we have ever used in a Technic(r) model. Starting with the all important motor in the crane superstructure, I’ll be showing you how some of them work here:

step 1 powertrainTo manage torque, we chose a 12:20 ratio. The various crane functions need different speeds and torque. Finding the right balance of torque and speed is important if you don’t want to waste energy.

Transferring rotation to the gear shift unit, located on the other side of the superstructure to provide more space, required a series of five, 16 tooth gears. Some functions require a lot of power, others less, so it was important to ensure that only one function could be engaged at a time. For easy access, the gear shift is located on a cross axle and has four separate positions to engage the functions individually.

step 2 powertrain Here you can see the gear shift unit. The dark-grey, 16 tooth gears rotate only when engaged.

powertrain step4

Here you can see the gearing needed for the boom extension function - it was quite a challenge to work out the relative placement of the rotating and stationary cross axles!

powertrain step9

Here you can see the boom gearing. The boom is hinged on a 12 cm cross axle fitted with a 20 tooth, free-running bevel gear. As the cross axle bears the entire weight of the boom, we chose a free-running bevel gear to keep friction to a minimum. Similarly, the 1:3 ratio clutched gear is also in place for safety reasons - without the clutch, forgetting to stop the worm gear used to extend the boom gearing would put a lot of stress on the structure and might even break certain elements! The worm gear stops the boom sliding down and helps reduce speed, thus preserving more power for the heavy work of extending the boom. A transfer of rotational power to linear power enables the use of tooth racks in the second stage of the boom extension.

powertrain step7

Here the winch is engaged, passing over two static axles and geared up 1.67 times, using 20 and 12 tooth gears for speed. The winch lock uses a worm gear together with a clutch gear. The worm gear prevents the winch spinning when the function is not engaged. The clutch gear prevents the lifting of too heavy loads, as well as stopping the hook from jamming in the winch.

powertrain step 8

Here you can see the boom lifting function engaged. As this requires a lot of torque we chose a 1:3 down gearing ratio, using 8 and 24 tooth gears. We used four rotating axles to transfer power. No clutch gear is required here as the linear actuator has one built-in.

powertrain step 6

Here you can see the outrigger function engaged. It is linked via a static axle to a 16 module cross axle with two, 12 tooth, 1:1 ratio bevel gears linking to the turntable on the crane chassis. We added a specially designed brace element to ensure that the two gears run well together.

chassis side view

Side on, the chassis looks completely packed. It is built up in four layers to ensure that everything is located in the right place.

chassis side view motor stroke

The bottom layer features the piston engine drivetrain. Rotation is transferred using a series of five, 16 tooth gears.

chassis side view steering

The second layer features the steering, centrally placed along the entire length of the chassis - here the tricky part was transferring power to the outrigger modules.

chassis side view outriggers stroke
The third layer features the outrigger controllers, which also uses the second layer gearing to extend the outrigger modules.

chassis side view feet stroke

The fourth layer is used to lift and lower the outrigger feet.

chassis 1 1

Here you can see a 3D view of the chassis - multiple functions run down the middle, including the steering, which is controlled from the rear of the model. With the outriggers placed in the front and the rear of the model, designing a functional yet strong chassis was a huge challenge.

chassis step 2 gearbox

The power is transferred through the turntable into the chassis using a combination of three bevel gears and a 90 degree rotation. To integrate the clutch gear, rotation had to be directed to the center and then back again using 8 and 24 tooth gears.

In early versions, as the small linear actuators have a built-in clutch function, we placed the clutch gear in the powertrain used for extending the outriggers. This used too much power, so we had to move the clutch gear in front of the gear shifting units, so that is now works for both functions.

Another problem we faced was how to synchronise the outriggers. We solved this by pushing both outriggers into driving position and then sliding one of the 16 tooth gears into place to lock the synchronisation - this turned out to be quite tricky, as we had to do it twice!

chassis step 3 outtriggers 2

chassis step 3 outriggers detail stoke

As the outriggers extend from both sides of model the gears have to turn in different directions. To manage this, we placed two 16 tooth gears one module away from center (so they counter rotate!) and then transferred the rotation to the 20 tooth gears that drive the 12 tooth gears, which move the outriggers in and out.

chassis step 4 feet 2

The outriggers are controlled using a newly-developed, frictionless 8 tooth gear wheel that slides on the cross axles when the outriggers are extended - a really cool function, which we will use again in future models.

 
chassis step 5 steering stroke 2

Developing the steering for such a long model is is very demanding, as all the axles require their own individual steering angle. To keep the model simple enough to build, we used a single axle with adjustable levers, where long levers turn the wheels less than shorter levers. The axles in front of the middle axle have to turn counter to the back axles. This challenge was resolved by locating the gear rack on the opposite side of the axle.

chassis step 6 motor strokeThe piston drive was perhaps the easiest to function to develop. The differential drives the axle all the way to the front of the crane chassis, where power is transferred to the top of the chassis using a series of five, 16 tooth gears.

And that’s your lot! Thanks Markus! Don’t hesitate to get in touch If you have any questions re. gearing in the Mobile Crane MKII or anything else LEGO(r) Technic related - and remember to check back here to read our next all-action blog!