Tuesday, September 9, 2008

Load Cell Actuator, Part 1

The load cell which produces the numbers for braking force is the heart of this design, and is more than anything else its reason for being.

The load cell is fairly large, and its dimensions to some degree determine the length of the platform on which it sits, and to which the pedal shaft is attached. This in turn determines the length of the platforms holding the other pedals, and the spacing between the rods which will support all three pedal platforms.

Since Todd's production design differs in the way it mounts the load cell from his DIY design, and we had decided to copy the production design, we had to work out the layout and dimensions of all the components on the brake pedal platform before we could finalize the dimensions of the other pedal platforms and the floor mount.

This meant that designing and building the brake pedal assembly, with load cell and actuator assembly, came next in the process.

A key component of the load cell actuation assembly is a right angled part which will pivot around a bolt mounted to a bracket which is in turn bolted to the pedal platform. The brake spring pushrod will project through the vertical section of this part (we're calling it the load cell actuator) and one end of the spring will rest on the rear face of this section.

When the brake pedal is pressed, pressure on the spring will tend to push the vertical section of the load cell actuator toward the front of the assembly, which will cause the actuator to want to rotate around its pivot bolt.

This in turn will produce a lifting force on the horizontal section of the load cell actuator, and this lifting force will be transferred via a couple of other parts to the end of the load cell, subjecting the load cell to a bending force.

In Todd's production design, the load cell actuator is a weldment, but although we had access to both a MIG welder and a gas torch, I didn't want to weld anything on this project because that would have meant painting it, and because of my chemical injury, I don't want to have freshly painted things in my apartment.

So after some thought and debate Amos and I came up with a design for this part which is made from a piece of galvanized steel angle and a small section of the rectangular aluminum tubing that we were planning to use for part of the floor stand.

Making this piece required some serious precision work with the drill press in order to put small rivets into places in the two parts where they wouldn't interfere with either the pivot bolt or the spring, pushrod, and stop washers.

Amos did a brilliant job with this. Note the closely spaced rivets, in just the right place to attach the two pieces together without interfering with the moving parts.

The actuator isn't quite finished; we will need to cut a slot in its horizontal surface for the attachment of another part, which we're calling the load cell actuator fulcrum. We can't cut the slot until we've determined the dimensions of the fulcrum, so this is it for now.

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