Overhead Door Springs

Ok... When we got new overhead doors in the garage a few years ago they put in all new hardware. Rails, springs, everything. I was busy
at the time, and had more money than time. Not so now, so please no Nigerian banking scams.
Anyway, there are two doors. One is an 8 x 9 and the other is 8 x 16. The smaller one has a single coil spring on a standard overhead door axle. The larger one has two springs. What is odd is that the are not the same size. One is about the same size as the one on the smaller door. The other is a good 14-18 inches longer.
Now if everything was working fine I wouldn't have even noticed, but the smaller of the two springs on the larger door broke. Makes sense since it would have less length to absorb the twist. My question I guess, is there any "legitimate" reason why they would two different size springs on a the same door axle?
I am not willing to just say, "Well they are the professional door company. They must know what they are doing." They do not. The rails, opener mount, etc were all out of square, and even the distance between the rails from the header to the motor was off. I called them back 3 times before I finally gave up and just fixed it all myself.
The unbroken spring on the larger door is much larger. I was wondering if it might not be adequate to handle the door by itself. I installed all the commercial doors on my shop (much heavier doors) and I still have a set of door spring tommy bars on the shelf.
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On Wed, 11 Jul 2012 16:00:07 -0700 (PDT), Bob La Londe

From the most excellent http://www.truetex.com/garage.htm
"You must use springs that are matched to the weight of the door. You cannot compensate for the wrong size spring by adjusting the number of winding turns."
"Unmatched or mismatched spring pair: You may find that you have a pair of springs that are different sizes. This mismatch may be a normal application, since the total torque on the torsion shaft is simply the sum of the torque contribution of each spring (indeed, very large doors can be lifted with 4 or more springs along the torsion shaft). The sum of the torque rates determine the lift; and dividing the torque among multiple springs does not change this. Some repair shops even apply mismatched pairs deliberately, since a few stock sizes of springs can be combined to fit a wider range of door weights than only matched pairs. For example, a technician may carry springs in increments of 20 lbs of lift, and when using pairs this allows a 20 lb increment in possible choices instead of 40 lb increments. Or, one spring from a pair may have broken and been replaced with a spring of equal torque rate but different size than the original.
Having a mismatched pair makes it difficult to specify the correct matched-pair replacements. To obtain replacement springs for a mismatched pair, you can either specify the same odd pair, try to calculate the equivalent matched pair sizes, or (this is the best method:) measure an accurate door weight and calculate the right spring size(s) "from scratch". The spring seller should be able to do the calculations from your accurate measurements of weight, height, and drum size; or you can attempt the calculations yourself using my engineering formulas below. "
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Thanks. I have installed a few OH doors over the years, but they all either had a single spring or a matched pair. I can see the thinking behind having mismatched springs for over all load capability and so they can carry fewer different springs in their service truck, but it still seems like the stress on the shorter spring would be greater over time.
I've done low head room rear frame springs. I've down high head room jack shaft setups with a "normal" top axle (usually solid for this application), and I have done regular hollow shaft normal height top spring installations. I've done some really heavy insulated commercial doors too. All had either single spring for light hollow doors or two matched springs for heavier doors.
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On 7/11/2012 10:03 PM, Bob La Londe wrote:

Could it be that they mixed up the springs? the single heavier spring for the single door, and the matched pair for the big door?
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On Wed, 11 Jul 2012 16:00:07 -0700 (PDT), Bob La Londe

Do you have Menards in you area? The have a real nice spring selection table base on door weight. Then you order what you need. They are color coded, does your good spring have a color code? And my large door has two different size springs.
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On 7/11/2012 4:00 PM, Bob La Londe wrote:

I just went thru this. Conventional wisdom is that springs have finite life and you should replace both springs. I'd recommend using the exact same ones you have now. In my case, they don't make 'em like they did 40 years ago. But there are charts that reliably translate from what you have to what you can get. I couldn't find anybody local who'd sell me springs. And the time to order them was longer than I could wait to get my car outa the garage. I couldn't lift the door. I paid the $100 over the mail order spring price to have it done the same day. After watching it happen, I think I coulda done it just fine. But a slip of the tool could cause some serious damage. I was afraid the guy was gonna have a heart attack when he was tightening the springs.
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I have no doubt about my ability to install a new spring (or springs). I have installed several overhead doors over the years. I even have a couple sets of Tommy bars in the shop for it.
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