Reply by Doss

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Posted on Awesome new hardware. Question....

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779 posts in 2231 days

#1 posted 08-02-2012 05:15 PM

First, keep in mind I’m working (at work) so I’m doing this fast and loose with little checking or fancy calculus and larger static force calculations.

These are pretty slick. The thermoplastic ones seem plenty strong for your application. And you’re right, it’ll clean up that mechanism substantially.Bertha

There will be no force added to the load on the hinges besides from the weight of the person leaning back on them. With 2 hinges, they will (apparently) take about 90lbs of force 10” from the hinge. This is probably borderline sufficient. Not sure what the typical force exerted on a backrest is. If upgraded to the metal ones, it’d be rated at around 250lbs of force at 10” out. That is plenty strong.dakremer

Incorrect. About 87.5 lbs 10” out.

If they are adequate for a normal size person, you could always put a disclaimer on the chair (Not to exceed 180lbs etc.,)Bertha

Not true. Disclaimer won’t save you from standards (BIFMA in this case… though I’m not sure if they apply specifically to your type of chair).

This hinge is not sufficient. Doubling that hinge (the math may not work out correctly):
450 lb-in = 37.5 lb-ft x 2 = 75 lb-ft

Commercial-grade furniture is usually tested at (for chair backs) 150 lbs for 1 minute @ 16” distributed via foam block and rope which is followed by a 200 lbs proof load test (can break but cannot catastrophically fail).

Translating that quickly,
16 in = 1.333 ft
150 lbs x 1.333 ft = ~200 lb-ft at the fulcrum (the hinge)

or using your 10” number:
10 in = .8333 ft
150 lbs x .8333 ft = 124.95 lbs at the hinge (I don’t know if you can scale the lbs down due to shorter distance, but it would make sense so: 125 lbs x .8333 ft = 104 lb-ft at the hinge)

I don’t know if your specific chair has to come anywhere close to that standard, but it’s something to consider.

Also, for the proof load:
200 lbs x 1.333 ft = 266 lb-ft at the hinge
Your number: 200 lbs x .8333 ft = 167 lb-ft at the hinge

These are just quick, simplified calculations and may not apply to what you are doing, but I don’t think those hinges are going to work.

If you’re going to risk it, do so with the aluminum or stainless steel ones. If you don’t want any risk, use the steel ones (not stainless). They give you over double the capacity.

-- "Well, at least we can still use it as firewood... maybe." - Doss

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