I've been asked to modify an existing zimmer/walking frame for a tall person. Bashing the bits of metal is the easy bit, just use tube to make longer legs to replace the original short removable adjustable ones, using tube that's actually thicker wall than the original.
The tricky bit is now they've asked me for a new weight rating, given that the frame is larger. The original frame was rated for use by people up to 180kg. It would be quite easy to say that as the new height is $x$, we should derate the weight by $x$, or by $x^2$, or by $x^3$, but justifying a PIDOOMA for the exponent or the weight is more difficult.
What are the expected failure modes of the frame? Here's a picture of it after fitting with the extended legs, the lowest 500mm.
I'm assuming two failure types are relevant, prompt failure resulting in the user ending up on the ground, and increased wear meaning a weakening after several years' use.
For prompt failure, if I assume that all joints are hinges, then the weakest appear to be on the cross member holding the two side frames together, which is why I guess there are corner braces there.
The moment on those hinges, should the user slump sideways on the frame, will be increased slightly by the longer legs, but will still be dominated by the width of the frame, so not even by a factor of $x$. By my reckoning, it's the smallest height of the triangle formed by the width and height of the walker, and a diagonal from one handle to the floor contact on the other side.
As the new legs are stronger than the original, and use the original spring pins into the same size holes, there should be no reason at all to expect any reduction in simple vertical strength.
The fretting moment at the area of tube overlap where the adjustable legs couple to the frame, held in place by spring pins, will increase directly in proportion to the adjustable leg length. However, the overlap region has been increased in length, more than proportionately, to reduce the contact force caused by this moment. So it may be no worse.
A colleague has suggested a simple reduction to a nice round 100kg, which would certainly be conservative. My concern is that I wouldn't be able to justify it, and that it may exclude other potential users for no good reason.
Are there industry standard tests for this sort of thing? How would the original manufacturer have rated it for 180kg users?