Rolling a wheel over a threshold

Forum|Forum|13 years ago
September 24, 2012
1 reply
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Does anyone know the math for estimating if a four wheel cart with certain mass, wheel diameter, and speed will cross over a 20mm threshold? This is for ISO 60601-1, 9.4.2.4.3 safety test.

In fact, I already have the cart. I need to solve for minimum wheel diameter.

Andy

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M

MichaelH

1-Visitor

Andy Nicoll wrote:
Does anyone know the math for estimating if a four wheel cart with certain mass, wheel diameter, and speed will cross over a 20mm threshold? This is for ISO 60601-1, 9.4.2.4.3 safety test.

As far as I can tell from a quick search on the internet, the ISO standard deals with the safety and stability of crossing noted threshold (a risk assessment), not whether or not the cart can physically cross it.

Any wheel diameter greater than 4 cm should technically be able to go over the 20 mm threshold if (1/2)v^2 > g*h, where v is the speed of the cart, h = 20 mm, and g is 9.8 m/sec^2.

I think there are other criteria you are missing for your ISO question.

A

Anic

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That's where I started, too Michael.

According to the spec, the test set up is a speed of 0.4m/s. Given your constraint, that kinetic energy must be greater than (or equal to)potential energy, (1/2)V^2 > g*h, and re-arranging we find an impossible situation, (1/(2*h)v^2 >g --> 4 (m/s^2) > 9.8(m/s^2).

Looking at the problem again, the obstruction, 20mm high x 80mm wide, only has to lift one end of the cart at a time. The total mass isn't involved in the potential energy term. That modifies the equation to (1/2)V^2 > (b/c)g*h where b is the distance from the wheels on the opposite end of the cart to the center of mass (gravity) and c is the distance from the front wheels to the back wheels.

Solving for b/c, b/c < (v^2)/(2g*h) --> b/c < ~0.41 (unitless). That implies that the distance from the front wheels to the center of gravity must be greater than ~0.59 * distance-between-the-wheels and !!the center of mass can't be in the middle of the cart!!(??).

As for wheel radius, when the wheel hits the obstruction, it does so at an angle. Not all of the force is vertical. This is the tricky bit I'm having trouble with. It has to help or wheels would be useless. Any clues?

You're right about the current regulation (version 2) discussing things in terms of causing hazards. However, version 3 takes effect in July 2013 and is very specific about hazards caused by tipping (overbalancing), speed, and thresholds including test setup and pass/fail conditions. For example, a cart is launched at the thrshold obsticle at 0.4m/s. If it tips over, fails to cross, or causes a hazard in any other way, it fails the test.

Andy