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Small Turntable 1

Tuesday, June 2nd, 2009

This post continues the Ideas section here on my blog. I want to point out that one of the main reasons I created this blog is to help me communicate these ideas and works-in-progress.

I’ve decided that I want to present my ideas (even the ones that don’t work out) and the stuff I’m currently developing. It takes a long time for me to finish a MOC and then present it on my website. While I’m building a typical MOC, I’ll go through many design iterations and mess around with many different ideas.

So, I’ll show them here. Maybe they’ll inspire some readers. Maybe some readers will help me out by leaving constructive comments. Anyway, here’s the next installment of my Ideas section:

Small Turntable 1

The classic Technic turntable is a fantastic part. It’s one of my favorite LEGO elements.

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However, it has its cons, especially when you attempt to build a large and heavy model with it. Specifically, there is a lot of friction generated within the turntable and it also pops apart when loaded by an overturning moment (think about a crane lifting a heavy load).

A better turntable design will use two things: 1) a thrust roller bearing, and 2) some means of “capturing” the top and bottom components so that they don’t separate under load.

The turntables I’ve built so far have been rather large, just because it takes a lot of space to incorporate the above two ideas into a functional assembly.

I’ve been wanting to build a turntable that is similar in size to the classic Technic turntable. Below is one, quick idea that incorporates rollers to provide a low friction bearing and a method of “capturing” the assembly so that it doesn’t pull apart under a high overturning moment.

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The double-stacked 40-toothed gears are captured by 8 rollers. The flat edge of the gears (on their sides) ride against the rollers.

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The “core” of the assembly is squeezed together with multiple threaded axles. The Technic threaded axle is a rare part and is no longer produced by LEGO. Note that this concept of a “core” made using threaded axles was used on my 4×4 telescopic crane. You can see that core design in this photo.

This assembly is not finished and is not functional. It’s basically just developed to a point where the major parts and geometry have been worked out.

What’s left to do is make a base that the “core” will attach to. It also needs a stronger centralizing support (the red 2×8 Technic plate) so it will rotate about a fixed center and also handle lateral loads applied to the turntable assembly.

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I’m not sure how this assembly will function. My gut feel is that the method of “capturing” the 40-toothed gears with rollers is a little flakey. They might slip off. They might grind too much under load and not rotate smoothly.

I’ve stopped development of this idea in favor of a new one (a different design for a compact turntable). I’ll present that one later.

Turntable for Crane Project, Phase 1

Thursday, May 28th, 2009

For the first phase of my recent crane project, I designed a new turntable. The concept of this turntable is based on having a thrust roller bearing and also having the whole assembly centered about a solid core.

I first developed a 12-segment ring for the round bearing.

base of turntable
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This ring is based on “perfect” spacing, i.e. every other segment pins into normally-spaced holes. This basically means that the “round” ring is affixed firmly to a square frame and can be easily integrated with the framework of the crane.

geometry of turntable ring
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The next step was to create a set of rollers. The ring of rollers shown below is actually a new concept, one that I didn’t actually use in Phase 1. This new one is the same roller ring as the original, except that the holders for the rollers are on the outside of the ring instead of the inside.

with roller ring
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My new idea for this turntable was to have the whole thing centered about a solid core made of stacked Technic turntables. The core would extend upwards through the superstructure and contain a “cap” with counter-rollers to keep the whole assembly pinned together.

core of turntable
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One of the primary loads on the turntable is weight (vertical, downward load), but it also has to resist an overturning moment. For example, when a crane picks up a load, the whole thing wants to tip over. Therefore the turntable has to hold the superstructure down against the base, so to speak.

partial assembly
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The photos above were taken after I broke the crane apart into its main components. Here is a photo of the crane when it was assembled:

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Here is a diagram showing the cross section of the turntable. Note the different sections and bearings (rollers).

cross section diagram of turntable
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One component I haven’t shown in the photos is the assembly that actually centralizes against the Technic turntables in the core. It is a fairly straightforward design that uses two sets of four 24-toothed pinion gears (spaced at 12, 3, 6, and 9 o’clock). Two of these gears are powered so that the turntable can be rotated by a motor.

Phase 1 of this project proved that this turntable functioned well but couldn’t handle much load. The rest of the crane was robust, much like my marine pedestal crane.

When lifting, the overturning moment on the turntable pried it apart greatly. I pushed it until I felt uncomfortable, but I didn’t break anything. The solid core in the middle is just not strong or stiff enough to keep the superstructure flat against the base. The other downside was that it kept throwing rollers. The 2×2 round bricks would frequently pop off due to the flexing of the whole assembly.

So, in Phase 2 of this crane project, I’m going to try to redesign it.

Crane Project, Phase 1 Overview

Friday, May 22nd, 2009

About a year ago, I got the itch to build another large crane. I started a model and finished it in about two months. The final MOC (my own creation) isn’t really a finalized piece, but rather a project to test various ideas.

What really inspired me to build this were several ideas I had for a large crane, and this project turned out to be more of a testing phase than a real, finished MOC.

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Specifically, I had some ideas about a new turntable design and also further development of using large plates to make beams for the framework (including joining them at an angle other than 90 degrees). I also spent the time to develop a compact winch package that could be reused on future projects.

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I call this “Phase 1″ because I’m going to contunue to work on it. I created a 4-legged pedestal base for this crane to test out my ideas of joining built-up beams (made from large plates) at different angles. In “Phase 2″, I’m going to explore other ideas I have for a base – a moveable base :-)

In the next phase, I’m also going to redesign the turntable. The original design worked okay, but had its flaws. The most serious flaw is that it lacked the strength I wanted. The crane would just barely support 20 pounds on the main hook, and I I’d like to see 50 pounds supported or more.

I have other projects in the way right now, so it will be a while before I continue on with this crane. But first I need to take close-up photos of the components of “Phase 1″ so that I can present the ideas here.

More to come soon… :-)