Belt Tensioning

Open-Source Leg v2.0

Tensioning the OSL is a sensitive process and requires a clear understanding of each so it can be completed successfully. The tensioning mechanism of the OSL has increments of 5 degrees, which means that for every 5 degrees of rotation there will be 2 shaft cap holes, 180 degrees across each other, aligned with 2 housing holes. The required torque on each shaft cap is 0.4Nm.

Before tensioning (while loose) the shaft caps must be aligned to have the same angular position using the help of the tiny, machined notch on the shaft caps as an indicator. Once aligned, both shaft caps can be rotated at the same time in the direction shown in Fig.1A (for ankle) and Fig.1B (for knee) using the 3D printed tool (shown in Fig. 2). This tool is designed specifically with a beam bending mechanism to match the required torque needed on both shaft caps together (sum of 0.8Nm for both shaft caps). If this special tool is not available, a dial torque wrench could be used simultaneously with a ratchet on the shaft cap of the opposite side to tension one shaft cap up to the required torque while rotating the other shaft cap by the same amount so the entire tensioner sub-assembly stays aligned and the torque reading is accurate. If the two shaft caps are not rotated simultaneously, the tensioner sub-assembly could jam and as a result, the torque reading will not reflect the true belt tension and instead will give a fault torque value.

Once the desired torque is reached on one side, that shaft cap should be fastened down by the two appropriate screws (180 degrees apart) as shown in Fig.2. The second shaft cap should be fastened down at exactly the same angular position as the first shaft cap to avoid misalignment in the tensioner sub-assembly. A tiny, machined notch on the shaft cap can be used to indicate the specific angle of each shaft cap and to sync the angular position of the second shaft cap according to the first (properly torqued) one.

Note: Unless the result is unsatisfactory, this tensioning process should be done only once to figure out the appropriate angular position of the OSL shaft caps. After that, the shaft caps can be restored to that position every time the OSL is dismantled and assembled again.

Fig.1A: Side view of the OSL ankle showing the rotation direction for tensioning the belt transmission.

Fig.1B: Side view of the OSL knee showing the rotation direction for tensioning the belt transmission.

Fig.2: Side view of the OSL ankle showing the 3D printed tool in action and the shaft caps being fastened down by the appropriate screws.

Note: As tensioning starts, the operator will notice that each set of two holes (180 degrees apart) on the shaft cap aligns with two holes on the OSL housing. If the rotation continues and those specific shaft cap holes are no longer aligned with the housing holes, the next set of two shaft cap holes will align with two new holes from the OSL housing. It could be tricky to notice that each set of holes on the shaft cap is only aligned once every 20 degrees and that the 4 sets of holes on the shaft cap are what makes this specific design have the 5-degree increment (the shaft cap-to-housing hole aliment alternate between the 4 sets of shaft cap holes every 20 degrees). The idea behind this design is inspired by the alignment of teeth of a stepper motor (which is a creative way to increase the rotational increments of a mechanical system with minimal space).

Tools needed:

How to print the tensioning tool?

The design and stress analysis of this tool was based on what we have found to be the 3D printing material with best repeatability in terms of testing. The material is called Tough 2000 using SLA printer (SLA T2000). The tool was printed in this following orientation due to our small printing bed size (printing settings are included in the screen shot below):