How do you measure the pressure a person puts on a walking pad?
I received the pressure measurement foil and tested it right away.
Pressure measurement film is a sheet that changes colour under load. The harder something presses on it, and the smaller the area it presses on, the darker the mark. You walk on it, and afterwards you can see exactly where the force went.
The comparison
You can see the result of me walking on the Office Walker versus the test rig in the photos. Both at the same weight and the same number of steps.
I even exaggerated my walking quite a bit, stomping with my heels as much as I could.
It is hard to quantify the pressure absolutely, but relatively it looks like easily double the load.
Also you can see that the load is very focused on a small area with the rig, while it is spread much more with a real person on it.
Why spread matters more than weight
This is the thing I had not thought about properly before.
A 150 kg person and a 65 kg machine are not comparable just because one number is bigger. What damages a belt is force per square centimetre, not force in total. A heavy person walking spreads their weight across the whole surface of a foot. A steel wheel concentrates it into a strip a few centimetres wide.
So a lighter machine can be far more punishing than a heavier human, and that is exactly what the film showed.
It also explains why the dents only appeared in three specific spots. The rig hits the same places every rotation. A person never does.
What happens next
The plan now is to get someone at the maximum user weight to test it and see the result on the pressure foil. That should give us a good idea of the expected load.
This is a huge relief to be honest. I have not slept well the past nights because I was anxious about this problem. I still have to test this further, but the first results look promising and point more towards the test rig than the Office Walker.
One more thing to check
I also realised that the belt gets warm on the test rig, which likely increases the problem. So I will create some recordings with a thermal camera comparing the test rig to a human.
If the temperature difference is as large as I suspect, that is the second half of the explanation.