The Drive

Project update: I am waiting for some quotes on the cost of steel for this thing. I work with these vendors regularly so I have some handle on what things should cost, but if it comes out at 2x what I expect I may have to scrap this project.

How do we make it go?

A up-close view of the drive assembly is shown below. From right to left we have the motor, coupler, and a right angle gearbox. Behind the gearbox you can see a sprocket on a shaft, the shaft is driven by the gearbox and the wall panels ride on a chain driven by the sprocket.

The motor does look relatively tiny. The idea is that by using a non-backdrivable gearbox (worm gear) the climbers weight will drive the wall and the motor will simply act as a governor, controlling the wall speed. So we don’t need a big motor, just big enough.

Wall Drive System

In my experience walls that are driven by climber weight alone can feel “squishy”. When you really push with your legs, say for a dyno, the wall accelerates robbing energy from your movement, changing how you climb. A wall running at a constant speed doesn’t have this same feeling of give. This is the main reason I was inclined to go with a motor-driven wall.

It would be nice to have the wall driven by climber weight ’cause hey free energy. Ideally, we would simply have a brake that could maintain a constant speed regardless of load, but the worm gear seems like a cost effective solution. I am open to other ideas!

Using a stepper motor here should be ideal due to the high low-speed torque and ease of control. With the gearbox and the coupler, the climber won’t be able to feel the individual steps.

The flexible coupler lets you get away with the shaft of the motor and gearbox not being perfectly aligned. This means we can use a piece of formed steel to mount the two components easily without, potentially, destroying the bearings in the motor. For the extra 5$ it is likely worth using this over a rigid coupler.

Issues: We are just about maxing out the gearbox I dug up as far as it’s torque rating goes for a 200lb climber. It is still within its spec but if anyone has a lead on high torque (~300 in*lb), low speed (60rpm output), cheap (<200$usd) gear boxes let me know! Friction from the wood panels sliding in the channels will help reduce torque on the gear box. So if we can keep the friction high in the channels without adding undo wear to the channels or the wood that would be awesome. (I am still working on getting some science behind the wear in the channels)

How do we change the angle?

Based on our survey results (all 8 of them) I am inclined to leave the range of angles available right where they were in post 1, so around 0 to 30° overhanging. There are two options for how the angle can be adjusted. There is the cheap and simple method, where the wall frame is simply pinned in place in one of a series of holes as shown below. This is the approach I will be taking with the initial prototype.

The easy way. Pin the wall in place.

There is also the deluxe option which I have been designing. In this design instead of the wall being pinned in place its angle is maintained by a small gear that runs on the curved plate.

Wall Angle Drive Gear

This gear is driven by a gear box tucked inside the wall frame behind the wood. This gearbox is just a smaller version of the one driving the wall. So when the motor on this gearbox isn’t running the wall will be locked in place. The gearbox and motor arrangement are shown below with the mounting bracket hidden (it hides the gearbox).

Wall angle gearbox and motor

This is kind of an expensive upgrade, at around 300$US. But it would be cool to have the wall angle change automatically. You could have programs for different routes, that change angle, a program for interval training, or other options. I was thinking it would also be neat to make the controller program open-source so it could be readily “hacked”.


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