OK, I'll admit it. I'm an axle support fanatic. But I have my reasons. For starters, gears under load generate forces that, if unchecked, would force their axles apart. That's a very real problem in the LEGOŽ realm, because the bending resistances offered by the axles in a LEGOŽ gear train are often the weakest links in the mechanical power chain from motor to external load. Then there's the wiggle. Because LEGOŽ axles are meant to turn easily without lubrication, they can't fit tightly in pin holes, their most common bearings. The required tolerance is enough to allow an axle mounted in a pin hole 1 LU long to wiggle by as much as 2 ° about the hole's centerline.
Allowing your gear train axles to bend or wiggle under torque is a recipe for trouble for several reasons -- especially in large or heavily stressed MOCs. (i) These unwanted motions waste power that could otherwise have gone into, say, the wheels of your truck or the propellers of your boat. (Most notably, it takes energy to bend a stationary axle and power -- the rate of energy expenditure -- to maintain the same bend in a rotating axle.) (ii) Bending and wiggling allow the gears involved to skip under lower torques than would otherwise be the case, and that can also limit the torque or mechanical power your motor can ultimately to your MOC's external load. (iii) Bending and wiggling increase total friction by forcing or allowing axles to bind on the edges of their pin holes. None of this is good.
Minimizing axle bending and wiggling under torque is what axle support is all about. The following rules of thumb for supporting the axles associated with a meshing gear pair are easily generalized to more complex situations. No need to be fastidious with lightly loaded axles here, but if you hear the clicking of skipping gears, the fixes may well be on this list.
The more points of support along the axles, the better -- regardless of the location of the gears.
The closer the supports to the gears, the better.
Supports on both sides of the gear pair are more effective than supports on one side only.
The more rigid the supports, the better.
The deeper the supports in the axle direction, the better.
Several of the photos on my spur gear tutorial page show gears mounted on axle pins for illustration purposes only. I usually avoid that in practice, even in low-torque settings, because the axle has only one support 1 LU deep. Hence, there's little to stop the axle pin from wiggling or even bending away from a heavily loaded meshing gear. In fact, it's not uncommon for heavily loaded gear-bearing axle pins to pop right out of their pin holes. The photo below comes from that page.
The next photo shows several well-supported axles, each with 2 points of support outboard of all the gears. Some of the axles are a bit too long for my taste, especially the one at far right, but at least they've been reinforced. The frame providing the support isn't all that rigid, but it might do under light to moderate loads.
The next photo shows a heavy-duty 2-speed transmission for a large tracked vehicle with all-out axle support to suppress skipping in high gear, when the torque within the transmission was particularly high.
The leftmost example in the next photo shows what good axle support looks like. The partially hidden 3x3 T-liftarms at both ends of the frame on the left add rigidity and maintain proper spacing between the axle supports. The other support arrangements get progressively less effective to the right. Cantilevering gears away from their nearest support on long axles, as seen at the far right, should be avoided whenever possible.
LEGOŽ axles resist torsion (twisting under load) better than they resist bending, but torsion can become an issue in high-torque settings -- especially with very long axles. The simplest defense against unwanted torsion is to use the shortest axles you can. Failing that, you can reinforce long axles by cladding them with bushes. Another way to limit torsion is to build long axles up from shorter ones using axle joiners, as shown in the photo below. Either way, you're forcing the applied torque to deform more plastic, or to focus a lot more deformation on shorter lengths of axle, to achieve the same total twist angle.
I lump these 2 approaches under axle reinforcement. Axle reinforcement also reduces bending and often reduces wiggle as well. All the axles in the 4th photo back have been reinforced with axle joiners and bushes.
These reinforcers also serve to keep gears from drifting out of alignment along their axles. That's less important for spurs than for crowns and bevels used as such, but bear in mind that spurs are also prone to exploit any available avenue of escape from very heavy loads. Cladding axles with pin joiners can also be useful in holding gears in place, but it doesn't provide effective reinforcement.
Great work, I will also add that I think 1/2 bushes are the best to use for reinforcing axles.
I like it
March 20, 2014
Genius Jeremy! I always assumed that the main problem with the gears was the axle flexibility. Once it flexes out of position, it slams the adjoining teeth. Granted, straight cut gears are prone to that; however, seeing that Lego hasn't created synchromesh gears, this is the most viable solution. Love the idea of using the connector tubes for support along the length. Great stuff!
I lost track of how many axles I broke as a child back in the early 80s, although it my case it wasn't due to loading as such. I built open-wheel racecars and skidded them around the room crashing into furniture, often snapping the wheel axle. On the bright side, I managed to create some new unique parts, like axles of length 3, 5 and 7 (which didn't exist back then).