Slow and Steady Wins the Race

 Merry Christmas! I spent some time working out my idea for the mud gearing. I want to reduce the  ratio between the the pedals and the tires, so that a full revolution at the pedals only turns the wheels by a fraction. This also increases the force produced at the tires by the same fraction. If, for example, I reduced the ratio by 2. The tires would spin at half the rate of the pedals, but would have twice as much force turning the wheels. 

The common axle in the center is driven by the freewheels. The gears drop down 1:2 four times for a 16x increase in torque. The two sets of gears move at the same rate until the sprockets at the ends. All the gears in are equidistant, so we'd use the same sized belt at every junction.

As I was working on this, I thought about switching modes. Maybe the switch could happen just by loosening the set screws on the drive axle? Only one set of pulleys at a time would spin and switching modes only needs an Allen wrench. I think it could work. But I still want a more elegant solution. 

Also, I got these in the mail yesterday.

They're adapters that let you mount freewheels directly on shafts. We definitely could've used this on Big Fish.

Rudy December 25, 2022

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How Do Manual Transmissions Work, Anyway?

 I'm exploring using a transmission to switch between drive modes. My current idea is to mount two sprockets next to each other and use a rear derailleur to move a roller chain between them, like a manual transmission but with sprockets instead of gears. I've also contemplated switching modes manually, but the weight reduction would be minor so I want to try the transmission.

I wanted to see the internal workings of a manual transmission, but I wasn't quite finding what I was looking for. Instead, I looked for a LEGO manual transmission. I found this fascinating video.

Rudy December 21, 2022

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The Drive Might Shred Itself

Cleaning out my garage of the summer projects has turned into finally organizing all my landscaping tools. So it's taking longer that I wanted, but it will ultimately be better. Hopefully, I'll finish my chore this week so I can work on the lawnmower disassembly next week.

Meanwhile, I've been working on figuring out the drive system. First thing was to figure out the gearing between the front and rear wheels. I had already arbitrarily selected a 36-tooth sprocket (choice was between 36 and 44 for the bore diameter I wanted). This turned out to a good choice. In order for the rear wheel to rotate at the same rate as the front wheels, the rear sprocket should have 28 teeth--a standard sprocket size--to the front's 36 tooth.

The basics of the drive system so far is: the pedals (w/ front derailleurs) are chained to freewheels (w/ rear derailleurs). The freewheels are coupled to a common shaft that would drive the main transmission chain. This is where things get a little fuzzy.

I want the drive to have two modes, road and mud, with an easy way to switch between them. The road mode will initially be geared 1 to 1 to the freewheels shaft with the final ratio determined after actual testing. The mud mode would be significantly geared down to increase torque output at the wheels. 

To keep the weight down, I want to do this with timing pulleys and belts and use sprockets and roller chain only to interface to the wheels.

Rudy December 21, 2022

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Free Lawnmower!

 I acquired a free cub cadet lawnmower today, thanks to builder, Jay Hungate​'s, tip. 

At the very least, there is a differential on the rear axle. Finding one at an affordable price has been difficult. Building one from scratch is also expensive, so I hope this one works out.

I'll most likely need to make a container for the differential, but I should theoretically be able to use the rear axle to drive the front wheels on the sculpture. 

A fun bonus is the possible existence of a large ring gear/small pinion bevels that should attached to the differential. 

The ring gear will be replaced  with a sprocket so we can attach it to cranks. The bevels will be key to how I want to do the water propulsion.

Rudy December 15, 2022

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Not All Universal Joints Are Equal

 I'm waiting on the sprocket that will attach to the front differential. In the meantime, I've been working on the other end of the wheel axles. I need universal joints to connect between the differential and the front wheels.

The problem with your typical universal joint is that the rotational speed of the joint changes based on the angle. The steeper the angle, the weirder the spin.

To bypass that problem, I have to use constant velocity (CV) joints. These are only made for AWD vehicles. I found ones for ATVs that aren't super expensive.

I was expecting an assembly that had joints on both ends, but when I got them there was only one. I put a pin in that issue. In the meantime, I've been exploring how the fat tires could interface with the end of the CV joint. The last few nights of working on my laptop in front of the TV has produced this assembly.

Collars sandwiched on either side of the wheel are bolted to a cylinder using 6 bolts on the inside and 9 on the outside. 6 on the inside because that matches up with the 44mm bolt pattern for disc brakes. 9 on the outside because the wheel has 9 natural openings where a bolt will fit between the spokes.

The tube at the center of the assembly would fit over the spline shaft of the CV joint and bolted tight with one giant bolt. The ring around the tube is a thrust bearing, so the whole thing will still spin when it's bolted very tightly together. I also worked on the base that this assembly attaches to.

The wheel assembly bolts to the CV joint through the base. I really tried to think about this from the point of view of fabricating the part. I started with a square part, but I switch to using round stock and cutting the top/bottom flat because I think that will be easier to machine.

Insanity? 😅

Rudy December 07, 2022

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The Theoretical Differential

We need a front differential to maintain speed around corners and keep the wheels from sliding on the pavement. After calculating the ratio between the bevel gears that we need based on the length of the wheel base, I modeled the basic design of a "perfect" front differential for the new base.

I haven't been able to add a sprocket for the ring gear, because the sprockets I've found online do not have dimensions for the mounting holes. I ordered one so I could measure it directly. 

I have the same issue with the universal (CV) joints available. They are made for specific vehicles, so the only "dimensions" available are make and model. I ordered CV axles for ATVs that I found at a deep discount so I could measure the shafts directly.

Shaft diameter may change the size of the bevel gears in the differential, which would change the overall dimensions of the differential. This is when I really appreciate Fusion 360's parametric timeline. I always start a design by laying out the purchased parts and anchoring dimensions from those parts. It makes it much easier to change parts and have the change ripple through the timeline.

Rudy December 02, 2022

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Working on the (Rebel) Base

I've been working on the new base. It's still going to be a reverse trike, because I really like the look. But it complicates things because I want to both steer and drive from the front.

I've worked out the steering, but to get drive in the front I'll need a differential axle. 

The internet is vast and wondrous! I found a paper online called "Design and Development of Open Differential for Transmission System of Quad Bike". It's a great explanation of an open differential and they show their work!

For my differential, the crown gear will be a sprocket and driven by a chain. The front wheels are 20"D. Now I need to measure the distance between the front wheels and between the front and rear wheels, so I can calculate the ratio for the bevel gears. 

I calculated the ratio at 1.4:1. The closest ratio they make is 1.5:1. Those are not as common and are pricey. Maybe I can extend the wheel width to make the turn ratio 2:1, but I think that might be too wide.

Rudy November 29, 2022

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A New Hope

I applied for a cultural grant to help fund my next sculpture for the Lowell Kinetic Sculpture Race. It was supposed to be a complete rebuild of Big Fish.  By the time I applied for the grant, I decided that I wanted to move on from that sculpture and build something new. My grant application reflected that. 

Last year, we wanted to create a Star Wars themed sculpture but we started so late that we quickly shifted to making Big Fish. Now that we have a race under our belt, it's time to get into the design of the our next kinetic sculpture, The Aluminum Falcon! 

I've already started designing it. Hopefully a year is plenty of time to finish it. Here's a drawing of my concept:

 

The pie in the sky idea is a 2-person reverse tricycle with all wheel drive, front wheel steering, and separate modes for the road and mud--made mostly out of aluminum. May the force be with me.

Rudy October 28, 2022

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