This is the 2nd part of the new TECHNIC Linear Actuators (LAs) review, which started here about three weeks ago.
This 2nd part is intended to focus on,
- LAs operation limits.
- Working load examples with different motor types.
- Some possible configurations with singlte and multiple LAs.
However, let’s start with a few pictures missed at the review 1st part, where we can see how the two different available brackets, directly attach to the LA.
1. Internal clutching
Another complementary information to part I of this review, regards LAs internal clutching.
I revised the initial information to say that LAs design, include in fact some internal clutching mechanism, for part protection sake in opposition to what initially wrote.
Didier went even further than I, disassembling one LA to see exactly that, as illustrated below (images gently provided by Didier).
It becomes obvious from the pictures, that when load goes above a certain limit, internal force increases and the clutch mechanism enters in action, allowing the orange part to rotate against the white tube. It makes a characteristic noise as you will be able to ear on a video demonstration in the next section (below).
This is intended to protect actuator from getting damaged and at the same time allows LAs to continue working normally as soon the driving source changes direction.
2. Load tests
I’ve made a setup to measure how far we can go stressing a LA, before the internal clutch starts to actuate.
The tests performed were simple Push Load tests, as you can see on the videos below.
The conclusion is that LA extends till it finds a resistance about 3,5-3,6 Kg, when the clutch gets to absorb all the driving movement.
I’ve found the readings to be just a bit higher when the PF XL-Motor is used, if compared to the PF M-Motor. However I’ve also observed the measures to slightly increase each time the experience was repeated, probably due to some permanent effect caused by letting the LA clutch slipping for sometime. This fact prevents a definite conclusion about this small different behavior between M- and XL-motors, without more systematic measures.
As found on some applications below, there are other motors that stall under certain load conditions, preventing them to rotate at all and by consequence to make the LAs to extend or retract. So it means the required resistance to actuate the clutch is not even reached. Despite this was observed with multiple LAs assemblies, it rests to measure the maximum admissible force for these motors like the LEGO micromotor in these setups.
It is also possible to make some Pull Load tests, which were however not yet conducted, because they require a more complex and less reliable setup. Also I do not expect significant measurement differences, between Push and Pull measurements.
3. Functional tests
In order to evaluate the LAs capabilities under significant load conditions and some advantages over LEGO pneumatic cylinders, I’ve prepared a demo using two LAs in a back-to-back (B2B) configuration to raise/lower the boom on the heavy TECHNIC Mobile Crane (8421).
This set is known for the “weakness” of its twin pneumatic solution, which has two main problems:
- The pneumatics tend to leak some air under the pressure of the heavy boom when raised (especially if extended too), which progressively gets down in some hours due to the air leaks.
- When the pneumatic switch position is turned to lower the boom, it goes down abruptly if not manually sustained.
In detail below, some photos for the 8421 adaptation performed as well as a comparative image between the back-to-back mountings, with pneumatic cylinders and LAs.
As it can be seen on the video below, the setup with LAs behaves reasonably well in almost every condition.
Just when the boom is fully extended and almost totally lowered (in fact an unrealistic situation), we can hear the gears slipping for a while, since the torque should be very high.
This is the situation where a small motor like an hypothetical future PF S-Motor would have a nice looking fit. However it was not possible to move the boom when using smaller and less powerful motors than the XL. Not even the M-Motor was able to do the job, this not speak about smaller motors like the micromotor or the custom motor from Mauricio/iLegos .
As seen in the previous section, both M- and XL-Motors perform in a similar way during the load tests, however in this particular boom position and forces configuration, it seems the stress is put more on the motor than on the LAs clutch mechanism which should explain why some motors stall and don't even start to rotate (note that also the LAs clutch doesn't actuate while the boom is raising/lowering).
Gearing down the motor output could have helped, but it would enlarge the design which is also not intended for this specific solution.
4. Single configurations
Now, some examples of simple methods that can be used to drive a LA in a single setup.
4.1. Direct motor attachment
4.2. Geared transmission
Some configurations with single and double bevel gears (from left to right: B12t + B20t; DB12t + B12t; B12t + B12t).
Some other configurations with gears (from left to right: DB12t + DB20t; z16 + z16; z8 + z24).
4.3. U-joint transmission
As seen in these pictures, former 4L U-joint (left) won’t work here, if you intend to use the capability of this bracket for an articulated setup with the bottom axle holes. However the new 3L design (right) will make a perfect fit.
4.4. Znap flex axle transmission
(however limited by only one length available, 26L with 2L and 5L axle ends).
5. Twin configurations
Below some examples of possible configurations where two LAs get used in conjunction.
5.1. Back-to-back synchronous setup - symmetrical
This setup uses a mixed LA bracket configuration, to make it smaller or more compact.
As seen from the pictures above, several type of motors can be used, depending on the force and speeds required.
Below another possible setup for the same effect. Yes,… it resulted from a failure attempting to perform the next asymmetrical configuration (see ahead)...
However in opposition to the example above, it shows an alternative how to build this setup in a non mixed bracket configuration.
Some care was taken with gears setup, so both LAs rotate at the same speed. This way both pistons extend and retract at the same velocity, also reaching the same limit positions.
And below two other designs. On the left one variant using z16 to avoid to care about different sized cogs. On the right a bit larger but another design for the same result.
5.2. Back-to-back synchronous setup - asymmetrical
The design below was achieved based on the similar symmetrical setup (one of the designs above), where one B12t just changed position to modify the symmetry behavior.
And again, a variant based on the correspondent symmetrical design (find it at the section above), where one B12t just changed position to modify the symmetry behavior.
5.3. Parallel synchronous setup
This is for instance a typical configuration to mimic the boom lifting mechanism as used on real excavators.
In the designs here presented, it turns obvious that gear geometry is much faster than worm geometry. However, of course there are other designs possible and this may be the desired effect also at some applications.
5.4. Parallel synchronous alternate setup
With a small change to the geared version of the previous “Parallel synchronous setup”, we woul obtain this variation performing an alternate motion.
6. Other multiple LA configurations
Later, I'll post here at TBs also some synchronous configurations, for 3 (equilateral triangle geometry) and 4 (square geometry) LAs.
7. Final considerations
Since the first references here to the new LAs, they have been compared with the older LEGO pneumatic cylinders.
They offer advantages and disadvantages. For a comparative, refer to this TBs previous post from Alexandre (see both post and respective comments).
Also for a physical comparative of both, take a look at this article in “La Mounière” blog (although in French), despite we have a different understandings for the parts measured lengths.
From above you can compare both pneumatics and LAs, while retracted and extended.
Pneumatic Cylinder (retracted: 6L*; extended: 9,5L*; extends: 3,5L; course: 28mm)
LA (retracted: 10L*; extended: 15L*; extends: 5L ; course: 40mm)
*) measured between axle holes center
Also interesting at this post are the considerations and comparations (pneumatic cylinder vs LA) about how pistons flex when extended. An effect that with some attention, we could also observe from the video above in this post, where functional tests on the '8421 Mobile Crane' are conducted.
The conclusion is that the piston from pneumatic cylinders is much more resistant to flexion than the correspondent from LAs.
The LA extra piston length (5L vs 4L) justifies part of this, but it is a consequence mainly from the internal design differences among both parts.
The new LAs allow for unprecedented and accurate linear measures in what concerns LEGO parts.
As Sariel explained in his LAs analysis at Klocki, while traditional LEGO worm gears need 6 revolutions to extend 2L, LAs take 26,5 full revolutions to extend or retract 5L.
This means we have just got the most accurate LEGO linear measuring solution, with 5,3 revs/L accuracy, against the 3 revs/L for the regular LEGO worm gears (almost doubled).
Despite we have seen above that in load situations small motors do not perform for the needs, these still have a valid use in many other situations.
The release of the new LAs, has also increased the need of a new PF S-Motor for compact designs, although not very demanding in terms of torque.
If such motor will ever see the daylight, it would be useful they would have a versatile design, so they can be easily setup for longitudinal or perpendicular uses.
Hope you have found useful, the information here collected.
And lets hope too, TECHNIC Designers come up in 2009, with another hype like they did with the LAs in 2008.
Added a few more examples at 'Geared transmission' (Single configurations).
Added images for a few more B2B variants.
Last Update: 2008.Aug.06 14:43 GMT