Linear Actuator Description
As for the worm gear actuator, the linear actuator is driven by a DC motor with a 1:5 planetary gearset reducer. The output shaft of the reducer is equipped with a cross pin driving the slotted nut connected to the actuator screw.
The screw is assembled to the nut with a 1.5 mm pin and is guided by a single ball bearing (purple) and two thrust bearings (salmon color). I only used one ball bearing which allows the shaft to slightly swivel in order to avoid unwanted efforts due to possible slight misalignments with the actuator piston.
The ball bearing is mounted into the 3D printed actuator housing. An aluminium machined ring is also fitted into the housing. It is blocked axially when the aluminium tube of the actuator is glued with epoxy into the housing.
The 2 thrust bearings are located on each side of the aluminium ring. A M6 nut with washer is tightened against the outside thust bearing to lock the slotted nut / actuator screw axially. The M6 nut must not be overtightened as this might damage the bearings and increase friction. I used loctite to prevent the M6 nut from unscrewing.
The piston is made from steel tubes with silver soldered ends. One end is M6 threaded and has a circular groove on which is inserted a slotted plastic ring. This plastic ring runs inside the actuator aluminium tube in order to minimize friction.
The other end of the piston is M5 threaded and is fitted with a swivel joint.
A slot is machined all along the piston and a 3 mm headless screw with a 2 mm nipple, crossing both the aluminium tube and an end plastic bearing, prevents the piston from rotating. A counter nut is tightened on this 3mm screw.
The piston is guided by both plastic ring and the plastic bearing.
The 3D printed housing of the actuator has a flange to connect the motoreducer.
View of linear actuators. Two actuators are use in parallel to control the ankle motion. Note the actuators housings are mounted into large swivel joints connected to the leg assembly through an aluminium plates + 3D printed support.