Capstan Differential Break Actuator (FreeCad)

thingiverse

This is a model of a differential brake actuator with a capstan that allows one to drive multiple capstans from a single shaft. A capstan is actuated by slowing down a free spinning differential gear via a brake, and pulley and brake locations can be interchanged. The right gear can be used for input, the middle gears for breaking action, and the left gear for output. The actuator is backdrivable, which means it can be used to drive a robot's single joint directly instead of driving a capstan. In direct drive mode, the actuator's ability to hold a joint's position needs to be investigated. A disk brake is used in this model to slow down the gear, and a caliper is not provided - users are advised to use their fingers to create one. The actuator design is not limited to disk brakes; MR fluid brakes, piezoelectric brakes, band brakes, electromagnetic/solenoid brakes, or eddy current brakes can be used instead. It's possible to make an eddy current brake out of 3.5-inch hard drive plates by printing the eddyGear and mounting a 3.5-inch HD plate on it if desired. The transferPattern4EddyGear can be used to mark holes on an HD plate. To assemble this actuator, users need six 608 bearings, which require two gear608bearing copies to be printed. All gears should be printed upside down except for the breakGear, which needs to be printed right side up with supports. One breakGear or eddyGear can be used; they both perform the same function. To assemble the actuator, users need to insert bearings into all of the gears and mount two gear608bearing onto the outer ring shafts. An inner ring should be placed onto the outer ring shafts, and a bearing inserted into it. An 8mm shaft can then be threaded through the inner ring bearing and mounted with the break gear onto it. The shaft is then inserted into the base of the actuator. The driveGear should be threaded onto the shaft while it's in the base, and two bearings inserted into the base to support the shaft. A handle can then be placed on top. It may be necessary to glue the outer ring shafts to the inner ring while assembling. The main shaft can be driven by a geared DC motor at a few hundred RPM, which could lead to a design without bearings and possibly a 100% 3D printable actuator design. For now, a disk brake can be squeezed via a gripper actuated by a small servo. If users are planning to use disk brakes in their designs, they should be made from metal since plastic disk breaks will melt if too much pressure is applied. Users are encouraged to take a look at https://www.thingiverse.com/thing:4397460 for a bidirectional capstan/pulley design. This actuator design has been placed into the public domain, and FreeCad FCStd files are included for users to modify as desired.